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Soil organic nitrogen (N) mineralization not only supports ecosystem productivity but also weakens carbon and N accumulation in soils. Recalcitrant (mainly mineral‐associated organic matter) and labile (mainly particulate organic matter) organic materials differ dramatically in nature. Yet, the patterns and drivers of recalcitrant (MNrec) and labile (MNlab) organic N mineralization rates and their consequences on ecosystem N retention are still unclear. By collecting MNrec (299 observations) and MNlab (299 observations) from 57 15N tracing studies, we found that soil pH and total N were the master factors controlling MNrec and MNlab, respectively. This was consistent with the significantly higher rates of MNrec in alkaline soils and of MNlab in natural ecosystems. Interestingly, our analysis revealed that MNrec directly stimulated microbial N immobilization and plant N uptake, while MNlab stimulated the soil gross autotrophic nitrification which discouraged ammonium immobilization and accelerated nitrate production. We also noted that MNrec was more efficient at lower precipitation and higher temperatures due to increased soil pH. In contrast, MNlab was more efficient at higher precipitation and lower temperatures due to increased soil total N. Overall, we suggest that increasing MNrec may lead to a conservative N cycle, improving the ecosystem services and functions, while increasing MNlab may stimulate the potential risk of soil N loss. [ABSTRACT FROM AUTHOR]

Summary: High‐latitude ecosystems are warming faster than other biomes and are often dominated by a ground layer of Ericaceous shrubs, which can respond positively to warming. The carbon‐for‐nitrogen (C‐for‐N) exchange between Ericaceous shrubs and root‐associated fungi may underlie shrub responses to warming, but has been understudied.In a glasshouse setting, we examined the effects of warming on the C‐for‐N exchange between the Ericaceous shrub Empetrum nigrum ssp. hermaphroditum and its root‐associated fungi. We applied different 13C and 15N isotope labels, including a simple organic N form (glycine) and a complex organic N form (moss litter) and quantified their assimilation into soil, plant biomass, and root fungal biomass pools.We found that warming lowered the amount of 13C partitioned to root‐associated fungi per unit of glycine 15N assimilated by E. nigrum, but only in the short term. By contrast, warming increased the amount of 13C partitioned to root‐associated fungi per unit of moss 15N assimilated by E. nigrum.Our study suggests that climate warming affects the short‐term exchange of C and N between a widespread Ericaceous shrub and root‐associated fungi. Furthermore, while most isotope tracing studies use labile N sources, we demonstrate that a ubiquitous recalcitrant N source may produce contrasting results. [ABSTRACT FROM AUTHOR]

Boreal forests harbor as much carbon (C) as the atmosphere and significant amounts of organic nitrogen (N), the nutrient most likely to limit plant productivity in high‐latitude ecosystems. In the boreal biome, the primary disturbance is wildfire, which consumes plant biomass and soil material, emits greenhouse gasses, and influences long‐term C and N cycling. Climate warming and drying is increasing wildfire severity and frequency and is combusting more soil organic matter (SOM). Combustion of surface SOM exposes deeper older layers of accumulated soil material that previously escaped combustion during past fires, here termed legacy SOM. Postfire SOM decomposition and nutrient availability are determined by these layers, but the drivers of legacy SOM decomposition are unknown. We collected soils from plots after the largest fire year on record in the Northwest Territories, Canada, in 2014. We used radiocarbon dating to measure Δ14C (soil age index), soil extractions to quantify N pools and microbial biomass, and a 90‐day laboratory incubation to measure the potential rate of element mineralization and understand patterns and drivers of legacy SOM C decomposition and N availability. We discovered that bulk soil C age predicted C decomposition, where cumulatively, older soil (approximately −450.0‰) produced 230% less C during the incubation than younger soil (~0.0‰). Soil age also predicted C turnover times, with old soil turnover 10 times slower than young soil. We found respired C was younger than bulk soil C, indicating most C enters and leaves relatively quickly, while the older portion remains a stable C sink. Soil age and other indices were unrelated to N availability, but microbial biomass influenced N availability, with more microbial biomass immobilizing soil N pools. Our results stress the importance of legacy SOM as a stable C sink and highlight that soil age drives the pace and magnitude of soil C contributions to the atmosphere between wildfires. [ABSTRACT FROM AUTHOR]

1.Experimental studies across biomes demonstrate that herbivores can have significant effects on ecosystem functioning. Herbivore effects, however, can be highly variable with studies demonstrating positive, neutral or negative relationships between herbivore presence and different components of ecosystems. Mixed effects are especially likely in the soil, where herbivore effects are largely indirect mediated through effects on plants.2.We conducted a long‐term experiment to disentangle the effects of non‐native moose in boreal forests on plant communities, nutrient cycling, soil composition and soil organism communities.3.To explore the effect of moose on soils, we conduct separate analyses on the soil organic and mineral horizons. Our data come from 11 paired exclosure‐control plots in eastern and central Newfoundland, Canada that provide insight into 22–25 years of moose herbivory. We fit piecewise structural equations models (SEM) to data for the organic and mineral soil horizons to test different pathways linking moose to above‐ground and below‐ground functioning.4.The SEMs revealed that moose exclusion had direct positive impacts on adult tree count and an indirect negative impact on shrub percent cover mediated by adult tree count. We detected no significant impact of moose on soil microbial C:N ratio or net nitrogen mineralization in the organic or mineral soil horizon. Soil temperature and moisture, however, was more than twice as variable in the presence (i.e. control) than absence (i.e. exclosure) of moose. Overall, we observed clear impacts of moose on above‐ground forest components with limited indirect effects below‐ground. Even after 22–25 years of exclusion, we did not find any evidence of moose impacts on soil microbial C:N ratio and net nitrogen mineralization.5.Our long‐term study and mechanistic path analysis demonstrates that soils can be resilient to ungulate herbivore effects despite evidence of strong effects above‐ground. Long‐term studies and analyses such as this one are relatively rare yet critical for reconciling some of the context‐dependency observed across studies of ungulates effects on ecosystem functions. Such studies may be particularly valuable in ecosystems with short growing seasons such as the boreal forest. [ABSTRACT FROM AUTHOR]

Afforestation on degraded lands is an effective measure to control desertification and soil erosion, but these functions are often constrained by soil nitrogen (N) availability. Moreover, soil N availability usually shifts along plantation development, yet we have a limited understanding of factors driving this shift. Here, we examined dynamics of soil N mineralization and nitrification rates along plantation development, and explored mechanisms behind these dynamics from the perspective of N‐cycling microbes. We measured needle litter quality, N‐hydrolyzing enzyme activity, biomass and community composition of soil microbes (phospholipid fatty acid analysis) and ammonia oxidizers (real‐time quantitative PCR and high‐throughput sequencing), and net N mineralization and nitrification rates along a chronosequence of Pinus sylvestris var. mongolica stands with six age classes ranging from 15‐ to 61‐year‐old in the Three‐North region of China. Results showed that stand development increased net N mineralization rate by 54%, nitrate‐N concentration by 106%, and nitrate‐N:ammonium‐N ratio from 1.09 to 2.63. Gram‐positive:gram‐negative bacterial ratio and β‐N‐acetylglucosaminidase activity initially increased and subsequently decreased along stand development with the highest values in 40‐year‐old stands, while fungal:bacterial ratio showed the opposite pattern with the lowest value in 40‐year‐old stands. These shifts in soil microbial properties were associated with age‐related changes in needle litter C:N ratio and soil pH. Net N mineralization rate was positively related to β‐N‐acetylglucosaminidase activities, but not to soil microbial biomass and community composition. Net nitrification rate was negatively correlated with ammonia‐oxidizing archaeal abundances and positively with ammonia‐oxidizing bacterial abundances. Collectively, our results indicate that soil N availability increases and N cycling accelerates with plantation growth, and suggest that microbial N recycling driven by β‐N‐acetylglucosaminidase is tightly related to soil N dynamics along stand development. [ABSTRACT FROM AUTHOR]

Nitrogen (N) is a key limiting nutrient in terrestrial ecosystems, but there remain critical gaps in our ability to predict and model controls on soil N cycling. This may be in part due to lack of standardized sampling across broad spatial–temporal scales. Here, we introduce a continentally distributed, publicly available data set collected by the National Ecological Observatory Network (NEON) that can help fill these gaps. First, we detail the sampling design and methods used to collect and analyze soil inorganic N pool and net flux rate data from 47 terrestrial sites. We address methodological challenges in generating a standardized data set, even for a network using uniform protocols. Then, we evaluate sources of variation within the sampling design and compare measured net N mineralization to simulated fluxes from the Community Earth System Model 2 (CESM2). We observed wide spatiotemporal variation in inorganic N pool sizes and net transformation rates. Site explained the most variation in NEON's stratified sampling design, followed by plots within sites. Organic horizons had larger pools and net N transformation rates than mineral horizons on a sample weight basis. The majority of sites showed some degree of seasonality in N dynamics, but overall these temporal patterns were not matched by CESM2, leading to poor correspondence between observed and modeled data. Looking forward, these data can reveal new insights into controls on soil N cycling, especially in the context of other environmental data sets provided by NEON, and should be leveraged to improve predictive modeling of the soil N cycle. Plain Language Summary: Nitrogen (N) is not only a key limiting nutrient that often constrains plant growth but also a major pollutant in places where supply exceeds demand. However, we lack the ability to accurately predict and model rates of soil N cycling. This paper introduces a first of its kind, standardized and publicly available data set of soil inorganic N pools and net transformation rates spanning the United States. We describe the data set in detail, then examine spatiotemporal trends in N pools and fluxes and how those measurements compare to predictions from an Earth system model. Key Points: The National Ecological Observatory Network (NEON) measures soil inorganic nitrogen (N) pools and net flux rates at continental scaleThe most N cycle variation is observed at the site level, followed by plots within sites. Organic horizons have high N pools and flux ratesThe Community Earth System Model 2 poorly predicts temporal trends in net N mineralization. NEON data can be used to improve predictive power [ABSTRACT FROM AUTHOR]

Given that global change is predicted to increase the frequency and severity of drought in temperate forests, it is critical to understand the degree to which plant belowground responses cascade through the soil system to drive ecosystem responses to water stress. While most research has focused on plant and microbial responses independently of each other, a gap in our understanding lies in the integrated response of plant‐microbial interactions to water stress. We investigated the extent to which divergent belowground responses to reduced precipitation between sugar maple trees (Acer saccharum) versus oak trees (Oak spp.) may influence microbial activity via throughfall exclusion in the field. Evidence that oak trees send carbon belowground to prime microbial activity more than maples under ambient conditions and in response to water stress suggests there is the potential for corresponding impacts of reduced precipitation on microbial activity. As such, we tested the hypothesis that differences in belowground C allocation between oaks and maples would stimulate microbial activity in the oak treatment soils and reduce microbial activity in in the sugar maple treatment soils compared to their respective controls. We found that the treatment led to declines in N mineralization, soil respiration, and oxidative enzyme activity in the sugar maple treatment plot. These declines may be due to sugar maple trees reducing root C transfers to the soil. By contrast, the reduced precipitation treatment enhanced soil respiration, as well as rates of N mineralization and peroxidase activity in the oak rhizosphere. This enhanced activity suggests that oak roots provided optimal rhizosphere conditions during water stress to prime microbial activity to support net primary production. With future changes in precipitation predicted for forests in the Eastern US, we show that the strength of plant‐microbial interactions drives the degree to which reduced precipitation impacts soil C and nutrient cycling. [ABSTRACT FROM AUTHOR]

Insect herbivores play important roles in shaping many ecosystem processes, but how climate change will alter the effects of insect herbivory are poorly understood. To address this knowledge gap, we quantified for the first time how insect frass and cadavers affected leaf litter decomposition rates and nutrient release along a highly constrained 4.3°C mean annual temperature (MAT) gradient in a Hawaiian montane tropical wet forest. We constructed litterbags of standardized locally sourced leaf litter, with some amended with insect frass + cadavers to produce treatments designed to simulate ambient (Control = no amendment), moderate (Amended‐Low = 2 × Control level), or severe (Amended‐High = 11 × Control level) insect outbreak events. Multiple sets of these litterbags were deployed across the MAT gradient, with individual litterbags collected periodically over one year to assess how rising MAT altered the effects of insect deposits on litter decomposition rates and nitrogen (N) release. Increased MAT and insect inputs additively increased litter decomposition rates and N immobilization rates, with effects being stronger for Amended‐High litterbags. However, the apparent temperature sensitivity (Q10) of litter decomposition was not clearly affected by amendments. The effects of adding insect deposits in this study operated differently than the slower litter decomposition and greater N mobilization rates often observed in experiments which use chemical fertilizers (e.g., urea, ammonium nitrate). Further research is required to understand mechanistic differences between amendment types. Potential increases in outbreak‐related herbivore deposits coupled with climate warming will accelerate litter decomposition and nutrient cycling rates with short‐term consequences for nutrient cycling and carbon storage in tropical montane wet forests. [ABSTRACT FROM AUTHOR]

Afforestation on cultivated farmlands causes major shifts in the nitrogen (N) cycle. The consequences of large‐scale Zanthoxylum bungeanum afforestation on spatio‐temporal patterns of soil N mineralization and inorganic N (ION) availability have not been reported. Moreover, the regulatory roles of microbial biomass and soluble organic N (MBN and SON) in the N cycle are poorly characterized in soils below 20 cm. To investigate the long‐term effects of afforestation on the governing mechanism of vertical N dynamics, a 0–100 cm soil profile was collected from a chronosequence of Z. bungeanum plantations aged 8‐year (H8), 15‐year (H15), 20‐year (H20) and 28‐year (H28), as well as adjacent farmland and abandoned‐land (28‐year) as controls in an arid valley in Southwest China. With increasing stand age, conversion of farmland to Z. bungeanum plantations significantly improved soil organic carbon, available nutrients, and all N forms. These impacts were more evident in the topsoil (0–20 cm) than in the subsoil layers. MBN and SON contents improved by 1.42‐fold and 1.34‐fold in H28, respectively, compared to H8. Net N mineralization (1.31‐fold), net nitrification rates (1.30‐fold), and total ION (1.31‐fold) content followed similar trends of increase along with the stand age. Correlation and redundancy analysis also established a positive relationship and demonstrated that increased ION availability is due to improved MBN, SON, and urease activity with the plantations age. Although the nitrate‐N content was highest in abandoned‐land, its content also increased steadily in Z. bungeanum plantations with the stand age. A relatively low ammonium/nitrate ratio (0.331) in H28 advocated improved N supply via nitrification as well as low N leaching risks from plantations. The spatial investigation provided novel insights into controls of N cycle and suggested converting farmland to Z. bungeanum plantations is a suitable approach for restoring soil N. [ABSTRACT FROM AUTHOR]

Interactions between nitrogen (N) and phosphorus (P) are important for plant growth and ecosystem carbon (C) sequestration. While effects of N supply on P dynamics have been much studied, much less is known about the opposite (P‐effect on N). We conducted a meta‐analysis by compiling a total of 1,734 individual experimental observations from 116 peer‐reviewed publications to assess P‐addition effects on soil N dynamics. Globally, P additions increased the soil total nitrogen (TN) pool, potentially as a result of enhanced plant and microbial immobilization and reduced N losses, with a stronger effect detected under longer duration of P addition (≥5 yr). A coupled increase in soil organic C with TN signifies the fundamental role of exogenous P supply in enhancing soil C sequestration. Phosphorus addition accelerated some of the soil N cycling processes including gross N mineralization, gross nitrification, and denitrification, with the effect sizes varying among ecosystem types and increasing with P addition rates. Our results indicate the fundamental role of P in affecting soil N pools and processes, and highlight the efficacy of P supply in sequestering soil C and mitigating global C emission. Plain Language Summary: Interactions between nitrogen (N) and phosphorus (P) are important for plant growth and carbon (C) sequestration. While effects of N supply on P dynamics have been much studied, much less is known about the opposite (P‐effect on N). We conducted a meta‐analysis by compiling a total of 1,734 individual experimental observations from 116 peer‐reviewed publications to assess P‐addition effects on soil N dynamics. Globally, P additions increased soil total nitrogen (TN) pool, potentially as a result of enhanced plant and microbial immobilization and reduced N losses, with a stronger effect detected under longer duration of P addition ({greater than or equal to} 5 yr). A coupled increase in soil organic C with TN signifies the fundamental role of exogenous phosphorus supply in enhancing soil C sequestration. Phosphorus addition accelerated some of the soil N cycling processes including gross N mineralization, gross nitrification, and denitrification, with the effect sizes varying among ecosystem types and increasing with P addition rates. Our results indicate the fundamental role of P in affecting soil N pools and processes, and highlight the efficacy of P supply in sequestering soil C under global warming. This work is valuable for the discipline of Ecology in understanding N biogeochemistry under exogenous P supply. Key Points: We conducted a meta‐analysis by compiling 1,734 individual experimental observations to assess phosphorus (P)‐addition effects on soil nitrogen (N) dynamicsSoil total N increased with P addition, especially with longer duration of treatment due to higher biological N retention and lower N lossAccrual of soil total N was coupled with an increase in carbon (C) pool, demonstrating the fundamental role of P supply in promoting C sequestration [ABSTRACT FROM AUTHOR]

Interactions between herbivores and detritivores are common in greenhouse and laboratory experiments. Such interactions are thought to cause feedbacks in real ecosystems where the combined actions of these animals create either high or low nutrient cycling rates. There is limited evidence from factorial field experiments to support these expectations.We present the results of a 3‐year experiment wherein we factorially manipulated grasshopper herbivores and earthworm detritivores in an old‐field ecosystem and tested for significant interaction effects on plants, nitrogen mineralization and micro‐organisms. Then, we used a dynamical system model built and parameterized for the study ecosystem to test the theoretical strength of these interactions. We predicted that grasshoppers and earthworms would have a positive interaction effect on plant growth and nitrogen cycling by driving plant community change.We found neither evidence for interaction effects on any of the variables we measured nor a consistent change in the composition of the plant community even though the individual effects of grasshoppers and earthworms were as expected. Our dynamical system model made the same prediction across a broad section of parameter space (e.g. feeding rates, death rates, etc) and after longer term simulations.Our results suggest that interactions between herbivores and detritivores are only likely in situ when animals have exceptionally high individual effects on ecosystems and where the exogenous forces driving plant community change and soil biogeochemical fluxes are weak. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

The loss of aboveground plant diversity alters belowground ecosystem function; yet, the mechanisms underpinning this relationship and the degree to which plant community structure and climate mediate the effects of plant species loss remain unclear. Here, we explored how plant species loss through experimental removal shaped belowground function in ecosystems characterized by different climatic regimes and edaphic properties. We measured plant community composition as well as potential carbon (C) and nitrogen (N) mineralization and microbial extracellular enzyme activity in soils collected from four unique plant removal experiments located along an elevational gradient in Colorado, USA. We found that, regardless of the identity of the removed species or the climate at each site, plant removal decreased the absolute variation in potential N mineralization rates and marginally reduced the magnitude of N mineralization rates. While plant species removal also marginally reduced C mineralization rates, C mineralization, unlike N mineralization, displayed sensitivity to the climatic and edaphic differences among sites, where C mineralization was greatest at the high elevation site that receives the most precipitation annually and contains the largest soil total C pool. Plant removal had little impact on soil enzyme activity. Removal effects were not contingent on the amount of biomass removed annually, and shifts in mineralization rates occurred despite only marginal shifts in plant community structure following plant species removal. Our results present a surprisingly simple and consistent pattern of belowground response to the loss of dominant plant species across an elevational gradient with different climatic and edaphic properties, suggesting a common response of belowground ecosystem function to plant species loss regardless of which plant species are lost or the broader climatic context. [ABSTRACT FROM AUTHOR]

Advancing conservation agriculture depends on understanding nutrient dynamics of organic matter amendments (OMA) on no‐till soil. This field incubation study compared surface‐applied composted dairy manure (CM), green waste compost (GWC) and an unamended control from March to September in 2015 and 2016 using a RCBD in a California almond (Prunus dulcis) orchard. Measurements included OMA nutrient release rates, changes in soil organic carbon (SOC), total N (TN) and inorganic N, P and K availability using in‐season soil sampling and collection of ion exchange resin (IER) membranes from 0 to 10 cm depth, and cumulative N and P availability using soil IER cores from 0 to 50 cm depth. We hypothesized OMA sources with a lower initial C:N increase soil N availability, greater soluble phosphorus (P) and potassium (K) concentrations increase P and K availability, and all OMA sources increase SOC with the greatest N recovery in the TN pool. No differences were observed in C, N and P release rates, while the K release rate was the greatest. In‐season N availability showed no effect but P and K availability differed as evidenced by greater IER adsorption and soil extractable P and K. Both OMA sources significantly increased in SOC and TN. Net N mineralization from OMA sources ranged from 0.7% to 8.0% of applied N and total N recovery in TN and inorganic N pools increased based on the initial C:N. These results advance our understanding of nutrient dynamics while conserving the soil due to the no‐till practice of surface‐applied OMA. [ABSTRACT FROM AUTHOR]

Despite ever‐increasing availability of detailed information about microbial community structure, relationships of microbial diversity with ecosystem functioning remain unclear. We investigated these relationships at the Coweeta Hydrologic Laboratory, where past forest disturbances (e.g., clear‐cut) have altered both ecosystem processes (e.g., increased N export) and microbial communities (e.g., increased bacterial diversity). We sampled soils from disturbed and adjacent reference forests, characterized resident microbial communities, and measured several microbial C‐cycle and N‐cycle process rates. Microbial communities from historically disturbed soils exhibited altered ecosystem functioning, including generally higher rates of C‐ and N‐cycle processes. Disturbed soil microbial communities also exhibited altered ecosystem multifunctionality, a composite variable consisting of all measured process rates as well as extracellular enzyme activities. Although we found few relationships between ecosystem functions and microbial alpha diversity, all functions were correlated with microbial community composition metrics, particularly r:K strategist ratios of bacterial phyla. Additionally, for both ecosystem multifunctionality and specific processes (i.e., C‐ and N‐mineralization), microbial metrics significantly improved models seeking to explain variation in process rates. Our work sheds light on the links between microbial communities and ecosystem functioning and identifies specific microbial metrics important for modeling ecosystem responses to environmental change. [ABSTRACT FROM AUTHOR]

Soil food web models are a powerful tool for calculating the carbon and nitrogen mineralized by different soil organisms. Two limitations of the current models are that they use (a) fixed parameters for determining the relative efficiency of carbon and nitrogen conversion and (b) a generic set of trophic species (i.e. nodes).We propose a new method for analysing soil food webs that varies production efficiency and diet mechanistically with resource stoichiometry. Then, we calculated how the lumping or splitting of trophic species affects carbon and nitrogen mineralization for both net mineralization rates as well as the distribution of mineralization (i.e. sources of mineralization). Our models with additional stoichiometric details better represent organisms that consume basal resources with high C:N ratios.This is important because we show that lumping together trophic species at low trophic levels, which often differ in C:N ratios (i.e. microbial taxa), causes the largest deviations in estimates of carbon and nitrogen mineralization. One reason for the large effect of C:N ratio is that it impacts diet and production efficiency in our stoichiometric model. Conversely, lumping together species at higher trophic levels causes the largest errors when those species have different production and assimilation efficiencies. Finally, we demonstrate that differences in death rate and biomass are mostly important when lumping species that have different diets.We suggest that C:N ratios are especially important when grouping microbial taxa because microbial C:N ratios vary widely and microbes feed at low trophic levels where differences in C:N ratio mattered the most. Conversely lumping higher consumers will be most problematic when they have differences in the conversion efficiencies. We provide an approach to calculate structural error and report it with parameter uncertainty. Then, we demonstrate, using oribatid mites as an example, that considering the life history of different taxa can help us group organisms more efficiently into trophic species. In doing so, we provide a way to reduce and report the uncertainty in soil food web models and to stimulate future empirical work measuring key life history parameters. A free Plain language summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Lignin's role in litter and soil organic carbon (SOC) decomposition remains contentious. Lignin decomposition was traditionally thought to increase during midstage litter decomposition, when cellulose occlusion by lignin began to limit mass loss. Alternatively, lignin decomposition could be greatest in fresh litter as a consequence of co‐metabolism, and lignin might decompose faster than bulk SOC. To test these competing hypotheses, we incubated 10 forest soils with C4 grass litter (amended with 13C‐labeled or unlabeled lignin) over 2 yr and measured soil respiration and its isotope composition. Early lignin decomposition was greatest in 5 of 10 soils, consistent with the co‐metabolism hypothesis. However, lignin decomposition peaked 6–24 months later in the other five soils, consistent with the substrate‐limitation hypothesis; these soils were highly acidic. Rates of lignin, litter, and SOC decomposition tended to converge over time. Cumulative lignin decomposition was never greater than SOC decomposition; lignin decomposition was significantly lower than SOC decomposition in six soils. Net nitrogen mineralization predicted lignin decomposition ratios relative to litter and SOC. Although the onset of lignin decomposition can indeed be rapid, lignin still presents a likely bottleneck in litter and SOC decomposition, meriting a reconsideration of lignin's role in modern decomposition paradigms. [ABSTRACT FROM AUTHOR]

Soil freeze–thaw can alter the soil nitrogen (N) mineralization rate by increasing root litter. However, the mechanism remains unclear. We compared the effect of adding root litter from two plant species on soil NH4+ production rate and soil NO3− production rate under low‐temperature conditions in laboratory incubation. Root litter of oak (Quercus crispula) or dwarf bamboo, Sasa (Sasa nipponica) was added to soil and incubated under nonfrost and soil freeze–thaw treatments. We monitored temporal change of net/gross NH4+ production rate and NO3− production rate throughout the soil frost period and the following melting period. We measured dissolved organic N (DON) in root litter leachate under the two temperature treatments. The soil N mineralization with Sasa root litter was higher than that with oak root litter in the two temperature treatments. The DON in root litter leachate of Sasa was larger than that of oak. The effect of root litter addition on net NH4+ production rate was larger than that on net NO3− production rate. The higher net NH4+ production rate of soil with Sasa was considered to be caused by higher DON in leachate from root litter of Sasa. The higher inducement of N mineralization of Sasa root litter than oak root litter was related to quantity of organic N in the root litter leachate. [ABSTRACT FROM AUTHOR]

Increasing agroecosystem biodiversity with cover crops can restore many ecosystem functions that are lost with simplified crop rotations. Mixtures of species with complementary plant traits, such as legumes and grasses, may increase multiple functions at once, including soil nutrient supply and retention, which depend on microbial decomposition dynamics. Litter mixtures can stimulate decomposition compared to individual species, and decomposition also varies with soil properties. However, the interactive effect of cover crop functional type and soil fertility status on decomposition are not known. Here, we tested for mixture effects with a legume–grass litter in soils with lower and higher levels of fertility. We also identified specific soil properties that are associated with those effects. We incubated hairy vetch, cereal rye, and vetch‐rye litter treatments for 360 d in two soils from fields with contrasting management histories and fertility levels, as defined by biological indicators such as particulate organic matter (POM) pools. We measured decomposition dynamics through respired CO2, microbial biomass, microbial extracellular enzyme activity, and inorganic nitrogen (N) mineralization. With no litter addition, the soil with larger POM pools had twofold greater microbial biomass C, seven times more net N mineralization, and respired 58% more CO2. Across both soils, after 30 d microbial biomass C increased by 58–208% following litter addition, and litter addition significantly increased CO2 production compared to the no‐litter control. However, there was no difference in the magnitude of CO2 production among cover crop treatments and soils after litter addition. The lower fertility soil had a greater response to the litter C input for CO2 production and enzyme activities in soil. Furthermore, the size and N content of free and intra‐aggregate POM pools were associated with differences in the microbial response to litter addition. Our results demonstrate that cover crop litter affects microbial decomposition dynamics differently in soils with distinct soil fertility levels and suggests that new C inputs have larger effects in lower fertility soils. Understanding how soils with different fertility levels respond to diverse cover crops, including mixtures, will inform management of agroecosystems for sustainability. [ABSTRACT FROM AUTHOR]

The nitrogen (N) release from composted and un‐composted biosolids and plant available N (PAN) of the biosolids were quantified to evaluate if composting can contribute to stabilize biosolids N and reduce the nitrate (NO3-) leaching potential in biosolids‐amended soil. Biosolids were composted at >55°C for 21 days after mixing the biosolids with yard waste at 1:1 (w/w) ratio. In the N release study, we installed field lysimeters filled with soil (sand and clay) amended with composted and un‐composted biosolids at two rates (30 and 150 dry Mg/ha) and measured the inorganic N in leachate after each rainfall and soil inorganic N monthly. The N released from composted biosolids during the two‐year study period were lower (6% of organic N added for clay and 11% for sandy loam soil) as compared to un‐composted biosolids (14% of organic N added for clay and 21% for sandy soils). Composted biosolids showed a lower N release rate constant k value of 0.0014 and 0.0027 month−1 for clay and sandy soil, respectively, compared to corresponding values of 0.0035 and 0.0068 month−1 for un‐composted biosolids. We used greenhouse bioassay with corn (Zea mays), ryegrass (Lolium perenne), and Miscanthus (Miscanthus giganteus) as test plants grown for six months with reference to N chemical fertilizer ranging from 0, 75, 150 to 300 kg N/ha to evaluate the PAN of the biosolids. Based on our study, plant growth was not affected by using either composted or un‐composted biosolids but the PAN was lower in composted biosolids (4.0%–5.9%) than un‐composted biosolids (11.4%–13.6%). Composting results in higher N‐retention efficiency in biosolids and composted biosolids are a valuable source of N to support the plant growth with lower N released to the environment. Thus, the potential of N leaching would still be low in the situations where a high rate of biosolids needs to be applied for land reclamation or landscaping soil reconstruction. Practitioner points: Composting enhances N‐retention efficiency in biosolids and composted biosolids are a valuable source of N to support the plant growth with lower N released to the environment.Potential of N leaching would still be low in the situations where a high rate of biosolids needs to be applied for land reclamation or landscaping soil reconstruction.N released from composted and un‐composted biosolids can be adequately described by first‐order kinetic model. [ABSTRACT FROM AUTHOR]

Understanding how soil nitrogen (N) mineralization (Nmin) responds to environmental changes is critical for improving ecosystem management, especially in a resource‐constrained region. Intensive land exploitation in arid land has profound influences on soil ecosystems and thus on soil Nmin. A local‐scale field investigation was conducted to reveal the temporal dynamics of Nmin under land‐use change from desert to farmland, and to verify the mechanisms controlling Nmin change during this process in a typical desert oasis region. The results showed that Nmin ranged from −0.14 to 2.69 mg N kg−1 day−1, with an average value of 0.74 mg N kg−1 day−1. Nmin in old oasis farmland (OOF) was significantly higher than that in GCF (Gobi desert conversion farmland) and SCF (sandy desert conversion farmland), and the average change rates of Nmin were 0.036 and 0.032 mg N kg−1 day−1 year−1 in GCF and SCF, respectively. Structural equation modelling (SEM) was used to test whether the measured variables affected Nmin, and the results showed that soil organic matter (SOM), bulk density (BD) and sand content were the main soil factors affecting Nmin. These soil factors, together with farmland type and cropping time, can explain 31% of the variation in Nmin. Our observations revealed that Nmin changed substantially under the land conversion process from desert to farmland, and our findings will help with assessments and predictions of future N cycles in desert oasis regions in response to land‐use change. Highlights: We used Nmin as an observed variable to evaluate the dynamics of the soil evolution process under a land‐use change from desert to farmland.Cropping year was identified by using map image data to reveal temporal trend of Nmin.Nmin was primarily affected by soil organic matter, bulk density and sandy content. Intensive land exploitation in arid land profoundly influences soil Nmin. [ABSTRACT FROM AUTHOR]

Soil net nitrogen mineralization rate (Nmin), which is critical for soil nitrogen availability and plant growth, is thought to be primarily controlled by climate and soil physical and/or chemical properties. However, the role of microbes on regulating soil Nmin has not been evaluated on the global scale. By compiling 1565 observational data points of potential net Nmin from 198 published studies across terrestrial ecosystems, we found that Nmin significantly increased with soil microbial biomass, total nitrogen, and mean annual precipitation, but decreased with soil pH. The variation of Nmin was ascribed predominantly to soil microbial biomass on global and biome scales. Mean annual precipitation, soil pH, and total soil nitrogen significantly influenced Nmin through soil microbes. The structural equation models (SEM) showed that soil substrates were the main factors controlling Nmin when microbial biomass was excluded. Microbe became the primary driver when it was included in SEM analysis. SEM with soil microbial biomass improved the Nmin prediction by 19% in comparison with that devoid of soil microbial biomass. The changes in Nmin contributed the most to global soil NH4+‐N variations in contrast to climate and soil properties. This study reveals the complex interactions of climate, soil properties, and microbes on Nmin and highlights the importance of soil microbial biomass in determining Nmin and nitrogen availability across the globe. The findings necessitate accurate representation of microbes in Earth system models to better predict nitrogen cycle under global change. This study provides a comprehensive evaluation of the determinants of global Nmin with a focus on soil microbial biomass. The results demonstrated that soil microbial biomass predominantly controlled the variability of Nmin at a global scale. Climate, soil properties, and substrates influenced Nmin via their impacts on soil microbial biomass. The study highlights the importance of microbial biomass in determining soil nitrogen cycling, which challenges the conventional view that climate and soil properties are the dominant drivers of soil Nmin and advances our current understanding on the global patterns of nitrogen cycle. The findings suggest that changes in soil microbial biomass under global change would result in profound consequences on ecosystem processes by changing soil Nmin. [ABSTRACT FROM AUTHOR]

Successful conservation of ecosystems and ecosystem services requires understanding the structural–functional relationships underlying resilience to disturbance pressures. State‐and‐transition models (STMs) are box‐and‐arrow diagrams conceptualizing system resilience, but many STMs lack functional descriptions. For a cattle‐grazed sagebrush steppe ecosystem in northwestern Colorado, we aimed to update an existing STM by incorporating functional descriptions of the states to inform future hypotheses. In this STM, a diverse set of native perennial understory plants define a Diverse state, but long‐term exclusion of disturbances to sagebrush results in a shift to a Depauperate state with increased bare ground interspaces and sagebrush dominance. We hypothesized that herbaceous species were inferior competitors to sagebrush over the long term for soil nitrogen. To examine evidence for this hypothesis, develop functional descriptions of the Diverse and Depauperate states, and identify more specific hypotheses for future studies, we measured the following: the mass, C:N, and distribution of the litter layer; nitrogen mineralization rates; nitrate pools; and soil attributes on plots classified as Diverse (n = 5) or Depauperate (n = 5). Plots in the Depauperate state had ~20% higher soil bulk density, half the clay and soil moisture concentrations, and 30% lower percent soil nitrogen compared to Diverse state plots; litter layer patchiness was nearly three times greater on the Depauperate plots than on the Diverse plots, but there was no significant difference between states in mean litter layer mass or C:N. We detected no significant differences between states in nitrogen mineralization rates or nitrate pools. This may be due to our sampling design, which examined nitrogen transformation at the plot level rather than at microsites within the plots, or it may be that another resource such as soil moisture is more limiting than nitrogen. Differences in soil texture between plots in the Diverse and Depauperate states may indirectly influence the competitive interactions between sagebrush and native herbaceous understory by controlling available soil resources. These results suggest that the vulnerability of sagebrush steppe ecosystems to state changes may be dependent on soil texture, which is highly variable across this broadly distributed ecosystem. Based on these insights, we present a revised STM and propose future work. [ABSTRACT FROM AUTHOR]

Abstract: Question: Climate change interacts with land use and introduces new pressures that trigger growing concerns about increasing vulnerability of the Eurasian steppes. However, it is not well known how increasing precipitation and atmospheric N deposition interact with the land use legacy to affect nutrient availability, plant species composition and therefore vegetation dynamics of the Mongolian Steppe. Location: Steppe in Xilin River Basin, Inner Mongolia, China. The mean annual precipitation is 343 mm with 60%–80% of it occurring during the plant growing season. Methods: We conducted a 6‐years (2005–2010) field experiment to manipulate N and water availability on sites experiencing two different historical stocking rates. Species composition, above‐ground biomass and plant N concentration were determined at both individual and community levels. Soil cumulative inorganic N and N mineralization rates were determined by laboratory incubation. Results: (1) Supplementary irrigation increased soil cumulative inorganic N and N mineralization rate, plant community N uptake, and the abundance of perennial species for the site with high historical stocking rate. In contrast, long‐term water addition decreased soil cumulative inorganic N and N mineralization rate, and did not change the plant community N uptake, but increased the abundance of Cleistogenes squarrosa as a species indicative of degradation for sites with moderate historic stocking rate. (2) Nitrogen addition increased soil cumulative inorganic N and N mineralization rate irrespective of grazing history under ambient precipitation, and resulted in a burst of annuals in moist years at both sites. Under supplementary irrigation, N addition increased soil cumulative inorganic N and N mineralization rate at the site with moderate historic stocking rate, but not at the site with high historical stocking rate, and increased the abundance of taller perennial species at both sites. Conclusions: Legacy effects of grazing greatly affect plant composition responses to increasing water and N availability. The effectiveness of N application in restoring heavily degraded sites in the Mongolian steppe depends on precipitation availability. In the face of increasing precipitation and atmospheric N deposition, resource managers should be prepared to cope with the different vegetation succession and recovery trajectories as a result of historical land‐use difference. [ABSTRACT FROM AUTHOR]

The article discusses a study published within the issue which developed a model for root nitrogen (N) uptake that accounts for processes of N supply and N demand. Topics covered include the representation of roots in Terrestrial Ecosystem Model (TEMs), prediction of the model on N uptake rates from a unit soil volume with a given root length density, and processes that may be important for inclusion in the root N uptake model when applied at the ecosystem scale.

Abstract: Assessing the status of soil nutrients with their corresponding microbial communities provides important information about degraded soils during the restoration of coastal wet pine forests. Net nitrogen mineralization, nitrogen‐oxidizing bacteria (NOB), and soil microbial biomass were compared with patch‐derived volume along a 110‐year longleaf pine (Pinus palustris Mill.) chronosequence for identifying a trajectory and ecological benchmark during forest restoration. Net nitrogen mineralization rates decreased significantly in the maturing‐aged, pine patches, driven by a larger drop in net nitrification. Net nitrification and abundance of NOB were higher in young pine patches compared to soils from the maturing (86–110 years) pine patches. Gross nitrate fluxes followed the nonfungal portion of the soil microbial biomass along the chronosequence, declining in 64‐year‐old pine patches. Microbial biomass peaked in patches 17–34 years of age, but significantly declined in the older patches. Fungal biomass leveled off without decline. Ammonium was the major source of nitrogen within the maturing pine patches as well as the wetland patches, indicating that ammonium maintains longleaf pine during growth‐limiting conditions. Nitrate dominated during rapid tree growth, optimally in mesic conditions. The relative amounts of available ammonium to nitrate can be used to model nitrogen cycling in facultative‐wetland pine forests of the coastal United States as soils alternate between wet and mesic conditions. A key restoration benchmark occurred after 86 years of pine development when pine patch growth rates slowed, with lower numbers of NOB, when the nonfungal biomass leveled off, and net nitrification rates are at a minimum, during pine maturation. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is the primary factor limiting soil organic matter (SOM) decomposition and carbon (C) storage in the tropical P-deficient plantations. However, the mineralization of SOM derived from N-fixing and non-N-fixing plantations may respond differently to P addition within the laboratoryor field-based incubation experiment. In this study, the effects of P addition (+P) combined with glucose (+Glu) on soil N mineralization within Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) plantations were evaluated using a 60-d laboratory aerobic incubation assay in subtropical China. The results showed that Glu addition significantly affected the net N mineralization and nitrification in both EU and AA plantations. In the 7th day incubation, +Glu treatment enhanced the N mineralization, whereas +Glu+P dramatically accelerated the N immobilization in EU plantation. However, +Glu+P treatment increased N mineralization while +Glu enhanced N immobilization in AA plantation, suggesting that P addition greatly affected the direction of N transformation within different plantations under the labile C input in the early stage of incubation. However, in the 15th day incubation, +Glu increased while +Glu+P greatly reduced the N immobilization in EU plantation. On the contrary, +Glu promoted while +Glu+P further accelerated the N immobilization in AA plantations, indicating that P addition had the contrasting effects on N immobilization within EU and AA plantations. After the 15th day incubation, Glu addition further facilitated the N immobilization in two plantations. Although +Glu increased the fungi and bacteria PLFA biomass, +Glu+P significantly increased F/B ratios, which may contribute to the direction and magnitude of N transformations in two plantations. However, in these two plantation forests with low soil P availability, single P addition would not significantly alter the N mineralization mediated by microbe in the laboratory incubation experiment without the labile C input. Therefore, the significant effects of P addition on the N mineralization or immobilization varied with the incubation stage and plantation types. We recommend that the P availability has profound implications on SOM decomposition and C sequestration in subtropical plantations. [ABSTRACT FROM AUTHOR]

The need to meet rapidly increasing demands for synthetic nitrogen (N) while reducing dependence on fossil fuels has been driving widespread attention to the recuperation and reuse of nutrients present in digestate and animal manure. The N release and mineralization potential of animal manure ( AM), digestate ( DIG), liquid fraction of digestate ( LFDIG) and mineral concentrate ( MC) were assessed in comparison with N availability from calcium ammonium nitrate ( CAN) as a reference. The release was highly dependent on the product [ABSTRACT FROM AUTHOR]

Herbivores modify their environment by consuming plant biomass and redistributing materials across the landscape. While small mammalian herbivores, such as rodents, are typically inconspicuous, their impacts on plant community structure and chemistry can be large. We used a small mammal exclosure experiment to explore whether rodents in a southeastern old field directly altered the aboveground plant species composition and chemistry, and indirectly altered the belowground soil community composition and activity. In general, when rodents were excluded, C3 graminoids increased in cover and biomass, contributing toward a shift in plant species composition relative to plots where rodents were present. The plant community chemistry also shifted; plant fiber concentration and carbon : nitrogen were higher, whereas plant nitrogen concentration was lower in exclosure plots relative to access plots. While microbial community enzyme activity increased when rodents were excluded, no significant changes in the fungal : bacterial or potential nitrogen mineralization occurred between treatments. Our results show that rodents can rapidly influence aboveground plant community composition and chemistry, but their influence on belowground processes may require plant inputs to the soil to accumulate over longer periods of time. [ABSTRACT FROM AUTHOR]

The supply of nitrogen (N) constrains primary productivity in many ecosystems, raising the question 'what controls the availability and cycling of N'? As a step toward answering this question, we evaluated N cycling processes and aspects of their regulation on a climate gradient on Kohala Volcano, Hawaii, USA. The gradient extends from sites receiving <300 mm/yr of rain to those receiving >3,000 mm/yr, and the pedology and dynamics of rock-derived nutrients in soils on the gradient are well understood. In particular, there is a soil process domain at intermediate rainfall within which ongoing weathering and biological uplift have enriched total and available pools of rock-derived nutrients substantially; sites at higher rainfall than this domain are acid and infertile as a consequence of depletion of rock-derived nutrients, while sites at lower rainfall are unproductive and subject to wind erosion. We found elevated rates of potential net N mineralization in the domain where rock-derived nutrients are enriched. Higher-rainfall sites have low rates of potential net N mineralization and high rates of microbial N immobilization, despite relatively high rates of gross N mineralization. Lower-rainfall sites have moderately low potential net N mineralization, relatively low rates of gross N mineralization, and rates of microbial N immobilization sufficient to sequester almost all the mineral N produced. Bulk soil δ15N also varied along the gradient, from +4‰ at high rainfall sites to +14‰ at low rainfall sites, indicating differences in the sources and dynamics of soil N. Our analysis shows that there is a strong association between N cycling and soil process domains that are defined using soil characteristics independent of N along this gradient, and that short-term controls of N cycling can be understood in terms of the supply of and demand for N. [ABSTRACT FROM AUTHOR]

Agricultural crop rotations have been shown to increase soil carbon (C), nitrogen (N), and microbial biomass. The mechanisms behind these increases remain unclear, but may be linked to the diversity of crop residue inputs to soil organic matter (SOM). We used a residue mixture incubation to examine how variation in long-term diversity of plant communities in agroecosystems influences decomposition of residue mixtures, thus providing a comparison of the effects of plant diversification on decomposition in the long term (via crop rotation) and short term (via residue mixtures). Three crop residue mixtures, ranging in diversity from two to four species, were incubated for 360 d with soils from five crop rotations, ranging from monoculture corn (mC) to a complex five-crop rotation. In response, we measured fundamental soil pools and processes underlying C and N cycling. These included soil respiration, inorganic N, microbial biomass, and extracellular enzymes. We hypothesized that soils with more diverse cropping histories would show greater synergistic mixture effects than mC. For most variables (except extracellular enzymes), crop rotation history, or the long-term history of plant diversity in the field, had a stronger effect on soil processes than mixture composition. In contrast to our hypothesis, the mC soil had nearly three and seven times greater synergistic mixture effects for respiration and microbial biomass N, respectively, compared with soils from crop rotations. This was due to the low response of the mC soils to poor quality residues (corn and wheat), likely resulting from a lack of available C and nutrients to cometabolize these residues. These results indicate that diversifying crop rotations in agricultural systems alter the decomposition dynamics of new residue inputs, which may be linked to the benefits of increasing crop rotation diversity on soil nutrient cycling, SOM dynamics, and yields. [ABSTRACT FROM AUTHOR]

Biochar management has been proposed as a possible tool to mitigate anthropogenic CO2 emissions, and thus far its impacts in forested environments remain poorly understood. We conducted a large-scale, replicated field experiment using 0.05-ha plots in the boreal region in northern Sweden to evaluate how soil and vegetation properties and processes responded to biochar application and the disturbance associated with burying biochar in the soil. We employed a randomized block design, where biochar and soil mixing treatments were established in factorial combination (i.e., control, soil mixing only, biochar only, and biochar and soil mixing; n = 6 plots of each). After two growing seasons, we found that biochar application enhanced net soil N mineralization rates and soil [ABSTRACT FROM AUTHOR]

Soil test indicators are needed to predict the contribution of soil organic N to crop N requirements. Labile organic matter (OM) fractions containing C and N are readily metabolized by soil microorganisms, which leads to N mineralization and contributes to the soil N supply to crops. The objective of this study was to identify labile OM fractions that could be indicators of the soil N supply by evaluating the relationship between the soil N supply, the C and N concentrations, and C/N ratios of water extractable OM, hot-water extractable OM, particulate OM, microbial biomass, and salt extractable OM. Labile OM fractions were measured before planting spring wheat ( Triticum aestivum L.) in fertilized soils and the soil N supply was determined from the wheat N uptake and soil mineral N concentration after 6 weeks. Prior to the study, fertilized sandy loam and silty clay soils received three annual applications of 90 kg available N (ha · y)−1 from mineral fertilizer, liquid dairy cattle manure, liquid swine manure or solid poultry litter, and there was a zero-N control. Water extractable organic N was the only labile OM fraction to be affected by fertilization in both soil types ( P < 0.01). Across both test soils, the soil N supply was significantly correlated with the particulate OM N ( r = 0.87, P < 0.001), the particulate OM C ( r = 0.83, P < 0.001), and hot-water extractable organic N ( r = 0.81, P < 0.001). We conclude that pre-planting concentrations of particulate OM and hot-water extractable organic N could be early season indicators of the soil N supply in fertilized soils of the Saint Lawrence River Lowlands in Quebec, Canada. The suitability of these pre-planting indicators to predict the soil N supply under field conditions and in fertilized soils from other regions remains to be determined. [ABSTRACT FROM AUTHOR]

Understanding the changes in the plant community and soil nitrogen (N) availability during natural recovery of a forest following clear-cutting is critical to develop post-disturbance forest management strategies. Therefore, post-clearcut changes in vegetation, net N mineralization rates and available N concentrations in soil were studied in a montane subtropical broadleaved forest during the early years of recovery following a chronosequence approach. Two replicate stands were selected for each treatment of old-growth undisturbed forest and recovering forest stands aged 1, 5 and 10 years. Following disturbance, tree species diversity and basal area declined but shrub and herb diversity increased. High percentage dissimilarity among the stands in tree, shrub and herb species composition indicated that species colonization varied with recovery age. NH-N and NO-N concentrations increased significantly ( P < 0.001) following clear-cutting and the increasing trend continued until 5 years post-recovery, after which it declined. While ammonification rate remained unchanged with recovery age, nitrification and net N mineralization rates in 10-year-old and undisturbed forest were significantly greater ( P < 0.001) than in 1- and 5-year-old forest, indicating that mineralization rates increased with recovery age. Despite high mineralization rates, low availability of soil N in the later years of recovery was attributed to high N uptake by growing vegetation and leaching because of high rainfall in Cherrapunjee. To ensure N availability during the later years of recovery, we suggest that forest management intervention should encourage aided regeneration of native N-fixing trees, e.g., Myrica esculenta, during the initial years of recovery. [ABSTRACT FROM AUTHOR]

The US Department of Energy has mandated the production of 16 billion gallons (60.6 billion liters) of renewable biofuel from cellulosic feedstocks by 2022. The perennial grass, Miscanthus × giganteus, is a potential candidate for cellulosic biofuel production because of high productivity with minimal inputs. This study determined the effect of three different spring fertilizer treatments (0, 60, and 120 kg N ha−1 yr−1 as urea) on biomass production, soil organic matter ( SOM), and inorganic N leaching in Illinois, Kentucky, Nebraska, New Jersey, and Virginia, along with N2O and CO2 emissions at the IL site. There were no significant yield responses to fertilizer treatments, except at the IL site in 2012 (yields in 2012, year 4, varied from 10 to 23.7 Mg ha−1 across all sites). Potentially mineralizable N increased across all fertilizer treatments and sites in the 0-10 cm soil depth. An increase in permanganate oxidizable carbon ( POX-C, labile C) in surface soils occurred at the IL and NJ sites, which were regularly tilled before planting. Decreases in POX-C were observed in the 0 - 10 cm soil depth at the KY and NE sites where highly managed turfgrass was grown prior to planting. Growing M. × giganteus altered SOM composition in only 4 years of production by increasing the amount of potentially mineralizable N at every site, regardless of fertilization amount. Nitrogen applications increased N leaching and N2O emission without increasing biomass production. This suggests that for the initial period (4 years) of M. × giganteus production, N application has a detrimental environmental impact without any yield benefits and thus should not be recommended. Further research is needed to define a time when N application to M. × giganteus results in increased biomass production. [ABSTRACT FROM AUTHOR]

Understanding the role of soil microbial communities in coupled carbon and nitrogen cycles has become an area of great interest as we strive to understand how global change will influence ecosystem function. In this endeavor, microbially explicit decomposition models are being adopted because they include microbial stoichiometry- and biomass-mediated mechanisms that may be important in shaping ecosystem response to environmental change. Yet there has been a dearth of empirical tests to verify the predictions of these models and hence identify potential improvements. We measured the response of soil microbes to multiple rates of carbon and nitrogen amendment in experimental microcosms, and used the respiration and nitrogen mineralization responses to assess a well-established, single-pool, microbial decomposition model. The model generally predicted the empirical trends in carbon and nitrogen fluxes, but failed to predict the empirical trends in microbial biomass. Further examination of this discontinuity indicated that the model successfully predicted carbon and nitrogen cycling because stoichiometry overrode microbial biomass as a regulator of cycling rates. Stoichiometric control meant that the addition of carbon generally increased respiration and decreased nitrogen mineralization, whereas nitrogen had the opposite effects. Biomass only assumed importance as a control on cycling rates when stoichiometric ratios of resource inputs were a close match to those of the microbial biomass. Our work highlights the need to advance our understanding of the stoichiometric demands of microbial biomass in order to better understand biogeochemical cycles in the face of changing organic- and inorganic-matter inputs to terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Climatic change is altering ecosystem structure and function, especially in the southwestern United States where trees are near their physiological water stress threshold. In piñon-juniper ( Pinus edulis- Juniperus monosperma; PJ) woodlands, increased drought is causing differential mortality of piñon resulting in an ecosystem that is becoming juniper dominated., Using a precipitation manipulation, we assessed how both increased and decreased precipitation altered ecosystem function beneath piñon and juniper. We predicted that changes in precipitation would alter nitrogen ( N) availability and mineralization at the site. Further, we predicted that these responses would differ beneath piñon and juniper crowns due to plant-level differences in transpiration and N uptake in response to drought., We found minimal interactions between tree species and the precipitation treatments on N cycling. However, across all years measured, soil nitrate decreased with increasing soil volumetric water content; a pattern that is likely due to reduced turnover in dry plots. In contrast, potential soil net-nitrogen mineralization was reduced in water removal plots relative to water addition plots indicating that nitrogen cycling rates were slower under drought. Tree type also influenced nitrogen dynamics in this woodland. Across all 4 years, soil N availability and potential soil net-mineralization rates were higher in soils beneath piñon relative to juniper across all treatments. Interestingly, the observed shifts in N cycling were not reflected in the abundance of N in microbial biomass or in ammonia-oxidizing bacteria, which are responsible for nitrification. The observed patterns may be due to increased N leaching from the soil during periods of increased rainfall or due to decreased microbial activity or plant N uptake when conditions are dry., The effect of precipitation change on N cycling may have long-term consequences on the plant community in this semi-arid ecosystem. Nitrogen concentrations are highest in the soil when water availability is low, thus when N concentrations are high, plants and microbes are relatively inactive and unable to use this resource. [ABSTRACT FROM AUTHOR]

This study was conducted to estimate nitrogen (N) mineralization of anaerobically digested centrifuge cake from the Stickney Water Reclamation Plant (SWRP) and Calumet Water Reclamation Plant (CWRP), lagoon-aged air-dried biosolids from the CWRP, and Milorganite at three rates of application (0, 12.5 and 25 Mg ha-1). The N mineralized varied among biosolids as follows: Milorganite (44%) > SWRP centrifuge cake (35%) > CWRP centrifuge cake (31%) > aged air-dried (13%). The N mineralized in the SWRP cake (32%) and CWRP aged air-dried biosolids (12%) determined from the 15N study were in agreement with the first study. The N mineralization value for centrifuge cake biosolids observed in our study is higher than the value given in the Part 503 rule and Illinois Part 391 guidelines. These results will be used to fine-tune biosolids application rate to match crop N demand without compromising yield while minimizing any adverse effect on the environment. [ABSTRACT FROM AUTHOR]

Few studies have quantified pathogen impacts to ecosystem processes, despite the fact that pathogens cause or contribute to regional-scale tree mortality., We measured litterfall mass, litterfall chemistry, and soil nitrogen ( N) cycling associated with multiple hosts along a gradient of mortality caused by Phytophthora ramorum, the cause of sudden oak death., In redwood forests, the epidemiological and ecological characteristics of the major overstory species determine disease patterns and the magnitude and nature of ecosystem change. Bay laurel ( Umbellularia californica) has high litterfall N (0.992%), greater soil extractable NO3- N, and transmits infection without suffering mortality. Tanoak ( Notholithocarpus densiflorus) has moderate litterfall N (0.723%) and transmits infection while suffering extensive mortality that leads to higher extractable soil NO3- N. Redwood ( Sequoia sempervirens) has relatively low litterfall N (0.519%), does not suffer mortality or transmit the pathogen, but dominates forest biomass., The strongest impact of pathogen-caused mortality was the potential shift in species composition, which will alter litterfall chemistry, patterns and dynamics of litterfall mass, and increase soil NO3- N availability. Patterns of P. ramorum spread and consequent mortality are closely associated with bay laurel abundances, suggesting this species will drive both disease emergence and subsequent ecosystem function. [ABSTRACT FROM AUTHOR]

Soil organic nitrogen mineralization rates and possible predictors thereof were investigated for vegetable-growing soils in Belgium. Soil organic matter ( SOM) was fractionated into sand (> 53 μm) and silt+clay (< 53 μm) fractions. The latter fraction was further separated into 6%Na OCl-oxidation labile (6%Na OCl-ox) and resistant N and C and subsequently into 10% HF-extractable (mineral bound) and resistant (recalcitrant) N and C. The N mineralization turnover rate (% of soil N/year) correlated with several of the investigated N or C fractions and stepwise linear regression confirmed that the 6%Na OCl-ox N was the best predictor. However, the small R² (0.42) of the regression model suggests that soil parameters other than the soil fractions isolated here would be required to explain the significant residual variation in N mineralization rate. A next step could be to look for alternative SOM fractionations capable of isolating bioavailable N. However, it would appear that the observed relationships between N fractions and N mineralization may not be causal but indirect. The number of vegetable crops per rotation did not influence N mineralization, but it did influence 6%Na OCl-ox N, probably as an effect of differences in crop residues returned and organic manure supply. However, the nature of this relation between management, SOM quality and N mineralization is not clear. Explanation of correlations between N mineralization and presumed bioavailable N fractions, like the 6%Na OCl-ox N, requires further mechanistic elucidation of the N mineralization process. [ABSTRACT FROM AUTHOR]

Questions How is the composition of wet meadow plant communities influenced by water regime? What controls soil nitrogen ( N) availability along a gentle soil water regime gradient? Is vegetation response to subtle changes in soil water regime mediated by N availability? Location Cricklade, Wiltshire, southwest England. Methods A field survey of community composition and soil N availability was conducted along a gentle gradient of water table depth (mean water table depth between 25 and 70 cm) at a species-rich hay meadow in southwest England. This study was complemented by a controlled mescosm study undertaken at water table depths between 5 and 45 cm, and a controlled laboratory N mineralization study with water tension varying between 0 and 100 cm (0-10 kPa), the latter augmented with a microbial community study. Results The field, mesocosm and laboratory studies showed that the soil N mineralization rate and N availability responded to water regime. In particular, the N mineralization rate strongly declined when the soil air-filled pore space dropped below 10% of total soil volume. In the field, plant cover distribution (individual species as well as functional groups) correlated with increase in soil water table depth ( r = −0.86 and r = 0.89 for graminoids and forbs, respectively) and soil N availability ( r = 0.77 and r = −0.85, respectively). A switch in dominance between forbs and graminoids occurred at a mean water table depth of 55 cm and mean soil inorganic N of 20 mg·kg−1). Study of the soil microbial community composition in the mesocosms, using phospholipid fatty acid ( PLFA) signatures, showed that microbial community composition was also significantly influenced by soil water status. Conclusions We show that soil moisture content and soil aeration control the availability of N, a limiting nutrient, in species-rich wet meadows. The mechanism is linked to changes in suitability of the soil water regime for microbially mediated N mineralization. Maintaining the water regime is therefore critical because adjustment of the hydrological niche also conditions species' responses along the N availability gradient. This could subsequently prove a useful tool for conservation managers trying to sustain a target vegetation type. [ABSTRACT FROM AUTHOR]

The effects of global environmental changes on soil nitrogen ( N) pools and fluxes have consequences for ecosystem functions such as plant productivity and N retention. In a 13-year grassland experiment, we evaluated how elevated atmospheric carbon dioxide ( CO2), N fertilization, and plant species richness alter soil N cycling. We focused on soil inorganic N pools, including ammonium and nitrate, and two N fluxes, net N mineralization and net nitrification. In contrast with existing hypotheses, such as progressive N limitation, and with observations from other, often shorter, studies, elevated CO2 had relatively static and small, or insignificant, effects on soil inorganic N pools and fluxes. Nitrogen fertilization had inconsistent effects on soil N transformations, but increased soil nitrate and ammonium concentrations. Plant species richness had increasingly positive effects on soil N transformations over time, likely because in diverse subplots the concentrations of N in roots increased over time. Species richness also had increasingly positive effects on concentrations of ammonium in soil, perhaps because more carbon accumulated in soils of diverse subplots, providing exchange sites for ammonium. By contrast, subplots planted with 16 species had lower soil nitrate concentrations than less diverse subplots, especially when fertilized, probably due to greater N uptake capacity of subplots with 16 species. Monocultures of different plant functional types had distinct effects on N transformations and nitrate concentrations, such that not all monocultures differed from diverse subplots in the same manner. The first few years of data would not have adequately forecast the effects of N fertilization and diversity on soil N cycling in later years; therefore, the dearth of long-term manipulations of plant species richness and N inputs is a hindrance to forecasting the state of the soil N cycle and ecosystem functions in extant plant communities. [ABSTRACT FROM AUTHOR]

Decomposition is a large term in the global carbon budget, but models of the earth system that simulate carbon cycle-climate feedbacks are largely untested with respect to litter decomposition. We tested the litter decomposition parameterization of the community land model version 4 ( CLM4), the terrestrial component of the community earth system model, with data from the long-term intersite decomposition experiment team ( LIDET). The LIDET dataset is a 10-year study of litter decomposition at multiple sites across North America and Central America. We performed 10-year litter decomposition simulations comparable with LIDET for 9 litter types and 20 sites in tundra, grassland, and boreal, conifer, deciduous, and tropical forest biomes using the LIDET-provided climatic decomposition index to constrain temperature and moisture effects on decomposition. We performed additional simulations with DAYCENT, a version of the CENTURY model, to ask how well an established ecosystem model matches the observations. The results show large discrepancy between the laboratory microcosm studies used to parameterize the CLM4 litter decomposition and the LIDET field study. Simulated carbon loss is more rapid than the observations across all sites, and nitrogen immobilization is biased high. Closer agreement with the observations requires much lower decomposition rates, obtained with the assumption that soil mineral nitrogen severely limits decomposition. DAYCENT better replicates the observations, for both carbon mass remaining and nitrogen, independent of nitrogen limitation. CLM4 has low soil carbon in global earth system simulations. These results suggest that this bias arises, in part, from too rapid litter decomposition. More broadly, the terrestrial biogeochemistry of earth system models must be critically tested with observations, and the consequences of particular model choices must be documented. Long-term litter decomposition experiments such as LIDET provide a real-world process-oriented benchmark to evaluate models. [ABSTRACT FROM AUTHOR]

Thermophilic bacteria were isolated from a soil of an olive grove in Alentejo (Portugal) and characterized morphologically, physiologically, and at the molecular level by partial sequencing of the 16S ribosomal RNA genes, followed by subsequent phylogenetic analysis. The isolates were shown to be gram-positive rods, motile, and to belong to the Firmicutes Phylum. They were able to produce ammonium and sulfate during growth, the levels of which vary among the bacterial isolates. This ability suggests that these bacteria may have an important role in the mobilization of organic S and N in the soil and therefore in providing essential nutrients for plant growth. Tests using two isolates indicated a positive effect on initial seedling development suggesting their potential use in soil nutrient supplementation. The presence of these thermophiles in arable temperate soils might be increasingly important, particularly when the predicted global climate warming is considered, and their features are discussed in this context and considering actual strategies to improve soil fertilization. [ABSTRACT FROM AUTHOR]

Recent empirical and theoretical advances inform us about multiple drivers of soil organic matter ( SOM) decomposition and microbial responses to warming. Absent from our conceptual framework of how soil respiration will respond to warming are adequate links between microbial resource demands, kinetic theory, and substrate stoichiometry. Here, we describe two important concepts either insufficiently explored in current investigations of SOM responses to temperature, or not yet addressed. First, we describe the complete range of responses for how warming may change microbial resource demands, physiology, community structure, and total biomass. Second, we describe how any relationship between SOM activation energy of decay and carbon (C) and nitrogen (N) stoichiometry can alter the relative availability of C and N as temperature changes. Changing availabilities of C and N liberated from their organic precursors can feedback to microbial resource demands, which in turn influence the aggregated respiratory response to temperature we observe. An unsuspecting biogeochemist focused primarily on temperature sensitivity of substrate decay thus cannot make accurate projections of heterotrophic CO2 losses from diverse organic matter reservoirs in a warming world. We establish the linkages between enzyme kinetics, SOM characteristics, and potential for microbial adaptation critical for making such projections. By examining how changing microbial needs interact with inherent SOM structure and composition, and thus reactivity, we demonstrate the means by which increasing temperature could result in increasing, unchanging, or even decreasing respiration rates observed in soils. We use this exercise to highlight ideas for future research that will develop our abilities to predict SOM feedbacks to climate. [ABSTRACT FROM AUTHOR]

The microbial conversion of organic nitrogen (N) to plant available forms is a critical determinant of plant growth and carbon sequestration in forests worldwide. In temperate zones, microbial activity is coupled to variations in temperature, yet at the ecosystem level, microbial N mineralization seems to play a minor role in determining patterns of N loss. Rather, N losses often appear to vary with seasonality in hydrology and plant demand, while exports over longer periods are thought to be associated with increasing rates of anthropogenic N deposition. We analyzed long-term (21-32 years) time series of climate and stream and atmospheric chemistry from two temperate deciduous forest watersheds in the southeastern USA to understand the sensitivity of internal forest N cycles to climate variation and atmospheric deposition. We evaluated the time series with a simple analytical model that incorporates key biotic constraints and mechanisms of N limitation and cycling in plant-soil systems. Through maximum likelihood analysis, we derive biologically realistic estimates of N mineralization and its temperature sensitivity ( Q). We find that seasonality and long-term trends in stream nitrate (NO) concentrations can in large part be explained by the dynamics of internal biological cycling responding to climate rather than external forcing from atmospheric chemistry. In particular, our model analysis suggests that much of the variation in N cycling in these forests results from the response of microbial activity to temperature, causing NO losses to peak in the growing season and to accelerate with recent warming. Extrapolation of current trends in temperature and N deposition suggests that the upturn in temperature may increase future N export by greater than threefold more than from increasing deposition, revealing a potential direct effect of anthropogenic warming on terrestrial N cycles. [ABSTRACT FROM AUTHOR]

Understanding the environmental basis for soil-site quality relationships requires that we connect the environmental factors important to resource availability to the physiological processes influencing tree productivity. The nitrogen productivity concept provides this link by relating nitrogen uptake rate to plant growth, although the concept has been verified almost exclusively by laboratory experiments on tree seedlings. We tested the nitrogen productivity concept in a field setting by relating foliage production to nitrogen mineralization rate in 19 mature ponderosa pine (Pinus ponderosa) stands across a moisture gradient in central Oregon, USA. Models developed following the nitrogen productivity concept predicted annual foliage production precisely and adequately represented the different influences of nitrogen and water stress. Current-year foliage production was proportional to older foliage nitrogen content (R² = 0.82), and a model including a water stress index (stable carbon isotope ratio, δ13C) further explained 95% of the variability. A direct link between soil nitrogen availability and canopy nutrition was less clear. Annual foliage production was positively, but weakly, correlated with soil-estimated N-uptake (estimated in situ), likely because annual nitrogen uptake was small relative to nitrogen retained in the canopy. Foliage nitrogen was highly conserved with a mean retention time of 10.5 yr, which was 2.2 times longer than foliage retention. Annual nitrogen uptake amounted to 0-11% of total canopy N. Multiyear estimates of cumulative N fluxes are needed to adequately assess N availability. Soil nutrient pools were poorly correlated with nutrient uptake and were not useful for predicting stand productivity. [ABSTRACT FROM AUTHOR]

Historical photographs document that during the last century, forests have expanded upwards by 60-80 m into former tundra of the pristine Ural mountains. We assessed how the shift of the high-altitude treeline ecotone might affect soil organic matter (SOM) dynamics. On the gentle slopes of Mali Iremel in the Southern Urals, we (1) determined the differences in SOM stocks and properties from the tundra at 1360 m above sea level (a.s.l.) to the subalpine forest at 1260 m a.s.l., and (2) measured carbon (C) and nitrogen (N) mineralization from tundra and forest soils at 7 and 20 °C in a 6-month incubation experiment. C stocks of organic layers were 3.6±0.3 kg C m-2 in the tundra and 1.9±0.2 kg C m-2 in the forest. Mineral soils down to the bedrock stored significantly more C in the forest, and thus, total soil C stocks were slightly but insignificantly greater in the forest (+3 kg C m-2). Assuming a space for time approach based on tree ages suggests that the soil C sink due to the forest expansion during the last century was at most 30 g C m-2 yr-1. Diffuse reflective infrared spectroscopy and scanning calorimetry revealed that SOM under forest was less humified in both organic and mineral horizons and, therefore, contained more available substrate. Consistent with this result, C mineralization rates of organic layers and A horizons of the forest were two to four times greater than those of tundra soils. This difference was similar in magnitude to the effect of increasing the incubation temperature from 7 to 20 °C. Hence, indirect climate change effects through an upward expansion of forests can be much larger than direct warming effects (Δ0.3 K across the treeline). Net N mineralization was 2.5 to six times greater in forest than in tundra soils, suggesting that an advancing treeline likely increases N availability. This may provide a nutritional basis for the fivefold increase in plant biomass and a tripling in productivity from the tundra to the forest. In summary, our results suggest that an upward expansion of forest has small net effects on C storage in soils but leads to changes in SOM quality, accelerates C cycling and increases net N mineralization, which in turn might stimulate plant growth and thus C sequestration in tree biomass. [ABSTRACT FROM AUTHOR]

1. While rising atmospheric CO2 has the potential to enhance plant growth and biomass accumulation, rates of these processes may be constrained by soil nitrogen (N) availability. Despite much effort, it is still uncertain how elevated CO2 affects long-term soil N dynamics. 2. We used open-top chambers to examine the effect of 5 years of elevated atmospheric CO2 concentration (720 vs. 368 p.p.m.) on N dynamics in a semi-arid grassland ecosystem in north-eastern Colorado, USA. In the first year 0·5 g m−2 of ammonium nitrate-N, 99·9 atom%15N, was added to each plot. We examined the effect of elevated CO2 on N mineralization and plant N uptake by tracking the labelled and total N in plant and soil over the following 5 years. 3. Plant growth and plant N uptake remained significantly higher under elevated than under ambient CO2. The fraction of labelled N (expressed per unit of total N) in above-ground biomass declined over time, and this decline was greater under elevated CO2. The amount and fraction of labelled N in the soil did not change with time and was unaffected by elevated CO2. These results suggest that with time, N released from mineralization in the soil diluted the labelled N in above-ground biomass and that this dilution effect caused by N mineralization was greater under elevated CO2. More of the mineralized N ended up in the above-ground biomass of Stipa comata and forbs (C3) than in Bouteloua gracilis (C4) under elevated CO2. 4. Increased soil moisture under elevated CO2 likely supported higher rates of N mineralization, thereby reducing N constraints on plant growth. Therefore, in semi-arid systems, plant growth and species composition responses to elevated CO2 may be more persistent than in mesic systems where N mineralization is less constrained by soil moisture. [ABSTRACT FROM AUTHOR]

Restoring native plant communities on sites formerly occupied by invasive nitrogen-fixing species poses unique problems due to elevated soil nitrogen availability. Mitigation practices that reduce available nitrogen may ameliorate this problem. We evaluated the effects of tree removal followed by soil preparation or mulching on native plant growth and soil nitrogen transformations in a pine–oak system formerly occupied by exotic nitrogen-fixing Black locust ( Robinia pseudoacacia) trees. Greenhouse growth experiments with native grasses, Andropogon gerardii and Sorghastrum nutans, showed elevated relative growth rates in soils from Black locust compared with pine–oak stands. Field soil nutrient concentrations and rates of net nitrification and total net N-mineralization were compared 2 and 4 years since Black locust removal and in control sites. Although soil nitrogen concentrations and total net N-mineralization rates in the restored sites were reduced to levels that were similar to paired pine–oak stands after only 2 years, net nitrification rates remained 3–34 times higher in the restored sites. Other nutrient ion concentrations (Ca, Mg) and organic matter content were reduced, whereas phosphorus levels remained elevated in restored sites. Thus, 2–4 years following Black locust tree removal and soil horizon mixing achieved through site preparation, the concentrations of many soil nutrients returned to preinvasion levels. However, net nitrification rates remained elevated; cover cropping or carbon addition during restoration of sites invaded by nitrogen fixers could increase nitrogen immobilization and/or reduce nitrate availability, making sites more amenable to native plant establishment. [ABSTRACT FROM AUTHOR]

The article presents a study on the effects of water, nitrogen cycling and forest thinning and burning on soil nitrogen pools and fluxes in ponderosa pine forests near Flagstaff, Arizona. The study used a nitrogen-15 depleted fertilizer and documented the distribution of nitrogen in soil N pools. It was found that thinning and burning in ponderosa pine forests will not increase resistance to changes in soil N dynamics resulting from increased atmospheric N deposition or increased precipitation due to climatic change.