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Microbial and faunal decomposers regulate the flux of carbon and nitrogen belowground, thus controlling the storage/release of carbon and nitrogen in soil systems. Warming is anticipated to alter decomposer biomass, and accelerate organismal metabolism and soil carbon release. We parameterized six soil food webs using empirical data for 18 trophic nodes at two boreal peatland sites under three climate scenarios (control, +2 °C, +4 °C), and model carbon and nitrogen flux, loss and retention using an energetic ecostoichiometric food web model. Differences in microbial biomass between sites dictated flux under warming. The community biomass of the fungal-dominated site was more impacted by warming, but fluxes were more responsive to warming at the bacterial-dominated site. Decreased metabolic efficiency of the soil food web at both sites in response to warming led to greater per capita carbon losses, indicating the long-term carbon storage potential of both systems is diminished. • Six empirical soil food webs were created for two boreal peatland sites. • We modeled carbon flux under three climate scenarios. • Differences in microbial biomass between sites dictated flux under warming. • Fungal biomass was more impacted by warming but bacteria dictate flux response. • Decreased metabolic efficiency under warming diminishes carbon storage potential. [ABSTRACT FROM AUTHOR]

Acid deposition has declined substantially over the last thirty years in the developed world. In forested watersheds previously impacted by acid deposition, evidence suggests that soils are slowly beginning to recover their alkalinity and base cation fertility. Research in these recovering ecosystems suggests that these changes in soil chemistry may result in decreased ecosystem retention and greater hydrologic export of nitrogen (N), although the drivers behind this enhanced export remain poorly understood. A whole-watershed acid rain mitigation experiment at Hubbard Brook Experimental Forest (New Hampshire, USA) offers a unique opportunity to examine how forest N cycling might change as soil recovery progresses over the coming decades. In this experiment, researchers added 1168 kg ha−1 of wollastonite (CaSiO 3) to an 11.8 ha watershed in 1999, leading to sustained increases in soil pH and increasing Ca fertility. By 2008, the experimental watershed began to export significant quantities of nitrogen, becoming a net nitrogen source for the first time since measurements began in 1963. We sought to understand whether shifts in soil N cycling could explain this watershed phenomenon. Across repeated soil sampling campaigns in 2015 and 2016, we found that soil inorganic nitrogen pools in the treated watershed were between 1.8 and 2.6 times higher (p < 0.001) than in a nearby reference watershed. Gross N mineralization rates and immobilization rates were ∼40% higher in the litter layer (Oie horizon) of the treated watershed(p < 0.001). We conclude that an accelerated N cycle in this litter layer, with faster turnover between organic and inorganic N pools, likely resulted in faster replenishment of inorganic N pools that were susceptible to hydrologic loss, resulting in higher N export. Nitrogen cycling rates were uncorrelated with soil acid-base properties, suggesting that direct geochemical controls are not the primary driver of the altered N cycle. • Watershed acid rain mitigation accelerated forest soil gross nitrogen (N) cycling. • Altered soil geochemistry was not responsible for the faster N cycling observed. • Enhanced leaf litter N cycling explains greatly increased watershed N export. [ABSTRACT FROM AUTHOR]

Microbial activity in Arctic tundra ecosystems continues through the winter and is an important component of the annual C budget. This activity is sensitive to climatic variation, particularly snow depth because that regulates soil temperature. The influence of winter conditions on soil N cycling is poorly understood. In this study, we used intact core incubations sampled periodically through the winter and following growing season to measure net N mineralization and nitrification in dry heath and in moist tussock tundra under ambient and experimentally increased snow depths (by use of a snowfence). In dry heath, we sampled soils under Dryas octopetela or Arctostaphylos alpine, while in tussock tundra, we sampled Eriophorum vaginatum tussocks and Sphagnum dominated areas between tussocks. Our objectives were to: (1) examine how different winter snow regimes influenced year-round N dynamics in the two tundra types, and (2) evaluate how these responses are affected by dominant species present in each system. In tussock tundra, soils with increased winter snow cover had high net N mineralization rates during the fall and winter, followed by immobilization during thaw. In contrast, N mineralization only occurred during the autumn in soils with ambient snow cover. During the growing season when N immobilization dominated in areas with ambient snow cover, soils with increased winter snow cover had positive net mineralization and nitrification rates. In dry heath tundra, soils with increased snow depth had high late winter net N mineralization rates, but these rates were: (a) comparable to early winter rates in soils under Arctostaphylos plants with ambient snow cover; (b) greater in soils under Arctostaphylos plants than in soils under Dryas plants; and (c) less than the rates found in tussock tundra. Our findings suggest under ambient snow conditions, low soil temperatures limit soil N mineralization, but that deeper snow conditions with the associated warmer winter soil temperatures dramatically increase over-winter N mineralization and thereby alter the amount and timing of plant-available N in tundra ecosystems. (C) 2003 Elsevier Ltd. All rights reserved.

Abstract Proteins comprise the largest soil N reservoir but cannot be taken up directly by microorganisms and plants due to size constraints and stabilization of proteins in organo-mineral associations. Therefore the cleavage of this high molecular weight organic N to smaller soluble compounds as amino acids is a key step in the terrestrial N cycle. In the last years two isotope pool dilution approaches have been successfully established to measure gross rates of protein depolymerization and microbial amino acid uptake in soils. However, both require laborious sample preparation and analyses, which limits sample throughput. Therefore, we here present a novel isotope pool dilution approach based on the addition of 15N-labeled amino acids to soils and subsequent concentration and 15N analysis by the oxidation of α-amino groups to NO 2 − and further reduction to N 2 O, followed by purge-and-trap isotope ratio mass spectrometry (PT-IRMS). We applied this method in mesocosm experiments with forest and meadow soils as well as with a cropland soil amended with either organic C (cellulose) or organic N (bovine serum albumin). To measure direct organic N mineralization to NH 4 +, the latter was captured in acid traps and analyzed by an elemental analyzer coupled to an isotope ratio mass spectrometer (EA-IRMS). Our results demonstrate that the proposed method provides fast and precise measurements of at%15N even at low amino acid concentrations, allows high sample throughput and enables parallel estimations of instantaneous organic N mineralization rates. Highlights • Novel high-throughput approach for gross protein depolymerization rates in soils tested. • High precision of at%15N measurements even at low soil free amino acid concentrations. • Parallel estimations of instantaneous organic N mineralization rates. [ABSTRACT FROM AUTHOR]

Increasing plant diversity in agroecosystems is proposed to enhance multiple ecosystem services. Adding selected forbs such as caraway ( Carum carvi L.) and plantain ( Plantago lanceolata L.) to productive grass-clover mixtures can further enhance forage yields, root biomass, uptake of mineral nutrients and improve animal performance. Yet, it remains unclear whether and how adding these forbs to grass-clover mixtures can influence soil microbial communities and associated soil carbon (C) and nitrogen (N) cycling. Based on a three-year grassland experiment varying in species diversity and composition with and without fertilizer application, we determined soil microbial community composition and functions related to C and N cycling under laboratory incubations. Results showed that inclusion of caraway modified soil microbial community composition by enhancing fungal-to-bacterial phospholipid fatty acids of the ryegrass-red clover mixture. Adding plantain to the ryegrass-red clover mixture increased the relative decomposition rate of the labile C pool, but not of the recalcitrant C pool. Yet, β-glucosidase activity and net N mineralization were unchanged due to inclusion of either forb. Moreover, fertilization with cattle slurry generally weakened these forb-induced changes in soil microbial properties. These findings demonstrate that adding selected forbs to unfertilized grass-clover leys can modify soil microbial community composition and associated C and N cycling, implying a potential for promoting long-term soil C sequestration through enhanced fungi-to-bacteria ratio, but a limited role in improving soil N fertility. [ABSTRACT FROM AUTHOR]

Extracellular enzymatic depolymerization of polymeric organic nitrogen (PON) is a rate-limiting step in N mineralization. However, enzymatic accessibility to PON might be regulated by physical occlusion of the PON resulting from the architectural packing of soil minerals during aggregate formation. To examine the extent to which enzymatic accessibility to PON is regulated by soil aggregation, we put forward a new approach involving the comparison of relationships between potential N depolymerase activity (protease and β-glucosaminidase; as an estimate of the potential to produce depolymerized products) and net N mineralization (as a bioassay for actual low molecular weight dissolved ON production) in aggregated and corresponding disaggregated soil. Soils were sampled from grassland (GL) and arable land (AL), separated by dry sieving into fractions (4.75–2, 2–0.25 and 0.25–0.063 mm) and fractions mixed (4:4:1 by mass, respectively) to obtain constructed aggregated soils. Corresponding disaggregated soils were prepared using a mortar and pestle. This procedure mainly disrupted the 4.75–2 mm (large macro-aggregate) fraction. Disaggregation significantly promoted (p < 0.05) net N mineralization rates by 1.3 times and 1.5 times in GL and AL soil, respectively. When net N mineralization - potential N depolymerase relationships for GL were examined, a greater slope parameter for disaggregated compared to aggregated soil (p = 0.001; ANCOVA) quantified the extent to which this promoted N mineralization could be attributed to disruption of macroaggregate-increased enzymatic accessibility to PON. For AL, which had low protease and β-glucosaminidase activity, promoted N mineralization rate could not be attributed to increased protease + β-glucosaminidase accessibility to PON reflecting a possible role for other N depolymerases and/or osmolyte/lysate effects. By proposing how differences between mineralization-depolymerase relationships for soils differing in aggregation status might, with assumptions, be interpreted to identify the role of physical occlusion in protection of PON, we give new insight on the regulation of enzymatic depolymerization by physical protection through macro-aggregation for soils from contrasting land use. [ABSTRACT FROM AUTHOR]

Improved assessment and prediction of net nitrogen mineralization (NNM) from soil organic matter in field conditions are essential for a better management of nitrogen (N) in arable cropping systems and an accurate simulation of its seasonal dynamics. The first objective of this study was to quantify accurately NNM rates by combining measurements of soil water and mineral N contents in soil profile, daily weather data and the LIXIM calculation model (Mary et al., 1999). The second objective was to develop a predictive model of in situ NNM rates based on basic soil properties and to evaluate if additional factors related to cropping history, organic matter fractions or microbial biomass could improve the model performance. We selected a set of 65 field experiments representative of arable cropping systems in France which were carried out in bare fallow soils without recent addition of crop residue (minimal delay of 4 months) or organic amendment. In these experiments, soil water and mineral N contents were monitored over the soil profile (0–90 to 0–150 cm depth, 3 to 5 layers) during 100–555 days (median value of 272 days). LIXIM was able to satisfactorily simulate water ( EF = 0.58) and mineral N contents ( EF = 0.80) in the several layers, allowing to disentangle the two main processes involved in mineral N changes: NNM (occurring in the upper soil layer) and nitrate transfer (through the whole soil profile). The actual NNM rates varied widely between sites from 0.13 to 1.10 kg N ha −1 day −1 . The potential NNM rates ( Vp ), calculated under standard conditions of water and temperature, varied between 0.17 and 1.67 kg N ha −1 nday −1 (nday = “normalized” day at 15 °C and field capacity). A novel model with a multiplicative structure and a sequential approach was developed by including functions explaining the relationship between mineralization rate Vp (predicted variable) and explanatory variables. Soil organic N (SON) was the most correlated variable with Vp ( r = 0.51, p < 0.001) and the first variable introduced in the model as a linear function. Non-linear functions of soil clay content, pH, C/N ratio and CaCO 3 content were successively introduced, leading to a “soil model” based on five basic soil parameters that allowed explaining 61% of the variance in Vp . Two functions relative to the frequency of rapeseed and legumes in rotations were found to further improve the predictive quality of the soil model, explaining finally 72% of Vp variance (“soil-history model”). Organic matter fractions (particulate or extractible organic matter) did not explain significantly more variance of Vp due to their strong correlations with SON. Microbial biomass explained only 2% additional variance in Vp compared to the “soil-history model”. The predictive error of prediction ( RMSEP ) varied between 0.22, 0.19 and 0.17 kg N ha −1 nday −1 for the “soil” ( n = 65), “soil-history” ( n = 65) and “soil-history-biological” ( n = 47) models. This multiplicative, non-linear approach performed better than a multiple linear regression approach which explained 10–15% less variance of Vp when using the same explanatory variables. The models based on soil properties and cropping history could be further implemented in multidisciplinary soil-crop models and decision support systems in order to enhance the prediction of N dynamics in soils and improve fertilizer recommendations. [ABSTRACT FROM AUTHOR]

Microbial transformations of nitrogen (N) largely determine whether N is retained in ecosystems via net primary productivity or lost via gaseous emissions and leaching. The controls on soil N cycling are often studied at single locales, making it difficult to predict N cycling at regional to global scales. We hypothesized that contemporary soil properties exhibit consistent relationships with instantaneous gross N cycling rates across diverse biomes that create a continuum in these properties. We measured ex situ gross N cycling rates and soil properties at 33 study sites representing five biome classifications in California including deserts, grasslands, shrublands, forest, and wetlands. Desert soils had significantly lower total N, organic carbon (C), microbial biomass N, and soil moisture as well as higher pH than all other biomes, whereas forests and wetlands had significantly lower soil nitrate (NO 3 − ) concentrations (P < 0.001 for all). Gross mineralization rates were best predicted by the combination of soil moisture and soil C:N ratios (R 2 = 0.46), which exerted positive and negative controls, respectively. Grasslands exhibited marginally higher gross mineralization than all other biomes, whereas deserts had the lowest rates due to low soil moisture (P = 0.09). Gross nitrification rates were positively correlated to soil NO 3 − concentrations (R 2 = 0.34) and negatively correlated to soil C:N ratios (R 2 = 0.31). The negative relationship between gross nitrification and soil C:N ratios was driven by forest soils, which had significantly higher C:N ratios and lower gross nitrification than all other biomes (P < 0.05). Dissimilatory NO 3 − reduction to NH 4 + (DNRA) occurred in soils from all biomes. The strong positive correlation between DNRA rates and soil NO 3 − (R 2 = 0.41) suggests NO 3 − limitation of DNRA. Predictable patterns in gross N cycling across biomes in California suggest that contemporary soil properties are important drivers of instantaneous soil N cycling rates that integrate over differences in vegetation type, atmospheric N deposition rates, and local climate. [ABSTRACT FROM AUTHOR]

Activities of burrowing herbivores, including movement of soil and litter and deposition of waste material, can alter the distribution of labile carbon (C) and nitrogen (N) in soil, affecting spatial patterning of nutrient dynamics in ecosystems where they are abundant. Their role in ecosystem processes in surface soil has been studied extensively, but effects of burrowing species on processes in subsurface soil remain poorly known. We investigated the effects of burrowing and grazing by plains vizcachas ( Lagostomus maximus , Chinchilidae), a large colonial burrowing rodent native to South America, on the distribution and dynamics of C and N in soil of a semi-arid scrub ecosystem in central Argentina. In situ N mineralization (N min ), potential N min and CO 2 emissions were measured in surface soil (0–10 cm) and soil at the mean depth of burrows (65 ± 10 cm; mean ± 1 SD) in five colonial burrow systems and adjacent grazed and ungrazed zones. Decomposition and N dynamics of vizcacha feces on the soil surface and in burrow soil was assessed using litterbags. Total C and N in soil in burrows were 1.6 and 5.5 times greater than in undisturbed soil at similar depths, and similar to amounts in surface soil. Inorganic N, particularly NO 3 − , was consistently highest in burrows, intermediate in surface soil on burrow systems, and relatively low in all other zones. Despite high C and N content in all burrows, in situ net N min was highly variable in burrow soil. Feces decomposed and released N more rapidly in burrow soil. Laboratory incubations indicated that soil moisture limited N min under conditions that typically characterize burrow microclimate, and that rates increased dramatically at soil moisture contents >25% field capacity, which likely occurs during pulsed rainfall events. Thus, the high and seasonally stable NO 3 − content in burrow soil likely originated from the accumulation of pulsed mineralization events over time. Burrowing and waste deposition by vizcachas produced “resource islands” at the landscape scale. At a measured density of 0.3 burrow systems per hectare, colonial burrow soil contained an amount of inorganic N equal to 21% and 30% of plant-available N in surface soil and subsurface soil, respectively, in an area that represented only 0.35% of the landscape. Our study indicates that burrowing and deposition of waste results in a highly active subsurface layer in which C and N dynamics function much like surface soil when soil moisture is not limiting. [ABSTRACT FROM AUTHOR]

Nitrogen transfer between litter components is often presented as a key mechanism responsible for the synergistic effect of litter mixtures on decomposition rates. The litter cover is a heterogeneous environment stemming from the input of chemically distinct materials and the transfer of nutrients in this patchy environment is likely to fulfil the specific needs of microbial communities in each micro-environment. Our study aimed to gain insight into the factors controlling N dynamics within and between leaves and woody components in the litter cover. We used 15 N labelling to discriminate endogenous and exogenous N and to measure N transfers between three types of litter components, viz. leaves and twigs (L + T), bark and branches, in 162 litter bags for more than 2 years in two Congolese Eucalyptus forests with contrasting N status (low-N vs high-N litter). Large quantities of N were released from N-rich L + T at the end of the study while early release of leachable N was only observed for the high-N L + T. Exogenous N was only incorporated in N-poor litter components (bark and branches) and a net increase in N compared to the initial quantities only occurred in the low-N bark. The bi-directional N transfers observed between litter components were most likely microbially-mediated rather than driven by abiotic leaching. Nitrogen transfers were controlled by the N status of both source and sink litter components, contrary to the diffusion theory based on concentration gradient. For a given source, more N was transferred to N-rich than to N-poor sink components. Our results suggested that the microbial community might control both the quantity of N available to be transferred to other microsites and the quantity that is actually transferred, presumably because the potential for N immobilization may be limited in N-poor litter components. Interactions among micro-environments can favor chemical convergence from distinct litter components to humified organic matter along the decay continuum. [ABSTRACT FROM AUTHOR]

During the initial stages of leaf and needle litter decomposition, microorganisms face nitrogen (N) and phosphorus (P) scarcity since plant litter is very N- and P-poor compared to microbial biomass. The processes that microorganisms use to cope with the unfavorable stoichiometry, such as transport of nutrients into decomposing litter, are still not fully understood. The aim of the study was to explore the import and release of nutrients (N, P, K, Mn, Ca, and Mg) into and from decomposing Norway spruce (Picea abies Karst) and Scots pine (Pinus silvestris L.) needle litter. For this purpose, we conducted a paired-stand litterbag study at eight temperate and boreal forest sites in Sweden that each have a spruce and a pine stand, over a period of five years. The mass of N in decomposing spruce and pine needle litter increased during the first 172 and 356 days, on average by 19% and 30%, respectively, compared to the initial masses of the element in the litter. The mass of P in pine litter increased during the first 526 days of decomposition, on average by 48%. Net release of N from spruce litter, relative to the initial N amount, only began after 895 days of decomposition. Net release of N and P from pine litter, relative to the initial amounts of the elements, started only after 1097 days. In contrast, K, Mn, Ca, and Mg were released right from the beginning of the decomposition process. The results show that N and P import into decomposing plant litter is a quantitatively important process in temperate and boreal coniferous forests during the first stage of litter decomposition when N and P concentrations are low. Nutrient import alleviates stoichiometric imbalance between the microbial biomass and the litter and likely contributes to microbial nutrient acquisition. • Mass of N in spruce litter increased by 19% during the first 172 days of decomposition. • Mass of N in pine litter increased by 30% during the first 356 days of decomposition. • Mass of P in pine litter increased by 48% during the first 526 days of decomposition. • Net release of N and P from litter only began after 895–1097 days. • N and P import into decomposing needle litter is a quantitatively important process. [ABSTRACT FROM AUTHOR]

While many ecosystem processes depend on biodiversity, the relationships between agricultural plant diversity and soil carbon (C) and nitrogen (N) dynamics remains controversial. Our objective was to examine how temporal plant diversity (i.e. crop rotation) influences residue decomposition, a key ecosystem function that regulates nutrient cycling, greenhouse gas emissions, and soil organic matter formation. We incubated soils from five long-term crop rotations, located at W.K. Kellogg Biological Station LTER in southwestern Michigan, USA, with and without four chemically diverse crop residues. Increasing crop biodiversity increased soil potentially mineralizable C by 125%, increased hydrolytic enzyme activity by 46%, but decreased oxidative enzyme activity by 20% in soils before residue was added. After residue additions, soils from more diverse cropping systems decomposed all residues more rapidly (0.2-8.3% greater mass loss) compared to monoculture corn. The fast-cycling, 'Active C' pool and microbial biomass N increased with higher cropping diversity, but the differences among rotations in Active C pools was higher for the most recalcitrant residues. Further, the ratio of the cellulose degrading enzyme (β-glucosidase) to the lignin degrading enzyme (phenol oxidase) was highest in the two most diverse crop rotations regardless of residue additions, providing additional evidence of enhanced microbial activity and substrate acquisition in more diverse rotations. Our study shows that crop diversity over time influences the processing of newly-added residues, microbial dynamics, and nutrient cycling. Diversifying crop rotations has the potential to enhance soil ecosystem functions and is critical to maintaining soil services in agricultural systems. [ABSTRACT FROM AUTHOR]

Amino acids (AAs) have been the focus of an increasing amount of research in relation to nitrogen fluxes in soils. We use five temperate grassland sites across an altitudinal gradient to establish relationships between soil properties and the size of individual AA pools and their mineralisation rate. Soil and soil solution chemistry, vegetation and microbial community structure, and standing concentrations of free- and peptide-AAs, were quantified. Mineralisation of AAs was universally rapid (t1/2 < 5 h). We found no relationship between standing AA pool chemistry and rate of AA mineralisation. Instead, soil pH and total microbial biomass, and vegetation community structure were most strongly related to AA turnover rate, highlighting the regulatory role of pH on soil microbial function. [ABSTRACT FROM AUTHOR]

Saprophagous macroarthropods are usually described as litter transformers that have low assimilation efficiencies and little direct effect on carbon mineralization. They are considered to enhance decomposition indirectly, by fragmenting leaf litter and increasing the surface area available for microbial colonization, thus stimulating microbial activity in their faeces. A review of experimental studies on the direct and indirect effects of macroarthropods on leaf litter decomposition does not confirm these views. (1) Laboratory estimates of assimilation efficiency are highly variable across studies, ranging from less than 5% to over 50%; this suggests that under field conditions that offer a variety of food choices, direct impacts of macroarthropods on carbon mineralization may be stronger than generally assumed. (2) Macroarthropod faeces are poor in easily assimilable organic compounds and rich in lignin; microbial respiration is only transiently stimulated in this material and, in the long term, there is no evidence of increased mass loss in faeces compared with intact leaf litter; faecal pellets are more akin to partially stabilized organic matter than to hotspots of microbial activity. (3) The overall impact of macroarthropods on microbial respiration in litter or soil-litter microcosms can be positive or negative; the results vary depending on animal abundance and litter type, but macroarthropod-microorganism interactions in unconsumed leaf litter may also be involved; recent studies have shown that macroarthropods grazing on fungi have complex, species-specific effects on fungal-mediated decomposition, which may partly explain the variability of microbial responses in microcosm experiments. (4) The most consistent effect of macroarthropods in decomposing leaf litter is an increased rate of nitrogen mineralization, which results predominantly from interactions with microorganisms and not from excretion; fresh macroarthropod faeces probably stimulate microfaunal activity, thereby increasing nitrogen release, although the actual mechanism remains unclear. It is concluded that soil macroarthropods play important roles in nutrient cycling, while their impact on carbon mineralization is much less clear. Significant alterations of carbon and nutrient dynamics may result from their interactions with fungi and more research is required in this area. [ABSTRACT FROM AUTHOR]

In temperate forest ecosystems, accelerated freeze-thaw cycles caused by winter climate change are expected to affect nitrogen (N) cycling in soils. Net N mineralization and nitrification rates were investigated via incubations of sieved soils transplanted from ten temperate forest ecosystems to two northern Japan sites with natural snowfall gradients. This was done to address: 1) how freeze-thaw cycles affect N mineralization and nitrification in temperate forest soils; 2) whether freeze-thaw cycles change the soil N transformation rates in the following growing season; and 3) which soil characteristics affect the response of the N transformation rates to freeze-thaw cycles. The effect of freeze-thaw cycles on inorganic N and dissolved organic carbon productions differed among soils, that is, some soils produced more inorganic N and dissolved organic carbon in the conditions imposed by freeze thaw cycles than in the non-frozen treatment but the others did not. The response to the freeze-thaw cycles was explained by soil microbial activity (gross N mineralization and nitrification rate) and soil fertility (inorganic N pools in the early spring and water soluble ions). Freeze-thaw cycles significantly increased N transformation rates in the following growing season, suggesting that winter climate change might also affect nutrient availability for vegetation and soil microbes in the growing season. The magnitude and frequency of freeze-thaw cycles were considered to be important indicators of N transformation rates during the growing season, suggesting that the higher intensity of freeze-thaw cycles in the original locations of soils changed the microbial communities and functions with high tolerance to freeze-thaw cycles; this resulted in greater N transformation rates in the following growing season. Microbial activity, soil fertility and climate patterns in the original locations of soils are believed to have an effect on the response to winter climate change and to cause large variability of soil response of N transformation rates to freeze-thaw cycles in both the dormant and growing seasons. [ABSTRACT FROM AUTHOR]

NH4+, NO3−, and amino acids are generally thought to be either mobile in the soil solution or adsorbed to ion exchange sites on soil surfaces. Here we examine a third possibility: some labile N compounds may be spatially inaccessible to plants and microbes due to isolation in soil microsites with low water potential. We tested this hypothesis in Arctic tussock tundra soils by comparing soil NH4+, NO3−, and amino acid concentrations using tension lysimetry, water extractions, and salt extractions. We used lysimeters to measure labile N that is in the mobile soil solution, and water extracts and salt extracts as combined measurements of mobile, adsorbed, and spatially inaccessible labile N. Averaged through the 2011 growing season, 37 ± 28% of labile N was adsorbed to ion exchange sites, 10 ± 4% was in the mobile soil solution, and the remaining 52 ± 26% was neither mobile nor on exchange sites. Labile N concentrations in water extractions were, on average, ~5 times higher than those in lysimeters. These results suggest that a large quantity of the water-extractable N is not available in the mobile soil solution. The estimated quantity of spatially inaccessible labile N declined over the course of the season, suggesting that these pools can be gradually drawn down by plant or microbial uptake. To place these results in context, we reviewed published labile N concentrations in lysimeters and extractions in other ecosystems and these published values were consistent with our results, showing much higher concentrations of labile N in extractions than in lysimeters. Overall, our study supports the hypothesis that soils can contain a substantial spatially inaccessible pool of extractable labile N that is not immediately available to plants and soil microbes. [ABSTRACT FROM AUTHOR]

Recent research has shown that microbial-feeding invertebrate soil fauna species can significantly contribute to N2O emissions. However, in soil food webs microbial-feeding soil fauna interact with each other and with their predators, which affects microbial activity. To date we lack empirical tests of whether or not these interactions play a significant role in N2O emissions from soil. Therefore we studied how interactions between soil microbes, two groups of microbial-feeding soil fauna (enchytraeids and fungivorous mites) and their predators (predatory mites) affect soil N2O emissions. We hypothesized that: 1) the presence of two microbial-feeding fauna groups (enchytraeids and fungivorous mites) together increase N2O emissions more than when only a single group is present; and 2) the addition of predatory mites further enhances N2O emissions. We assembled soil food webs consisting of soil microbes, enchytraeids, fungivorous and predatory mites in microcosms with sandy loamy soil and sterilised hay as a substrate for the soil microbes. N2O emissions were measured during 56 days. We found no support for our first yet support for our second hypothesis. Addition of predatory mites to microcosms with enchytraeids and fungivorous mites increased N2O emissions significantly from 135.3 to 482.1 mg N m−2, which was also significantly higher than the control without fauna (83 mg N m−2) (P < 0.001). In presence of enchytraeids, fungivorous and predatory mites, we found much higher nitrate availability at the time of the N2O peak on Day 35 (10.9 versus 5.5 mg N per kg soil without soil fauna), indicating that the major increase in N2O emissions in this treatment may be due to increased nitrification. Increased nitrification may be attributed to higher availability of N from the dead tissues of fungivorous mites and increased activity of the enchytraeids that might also have affected soil structure and contributed to increased N2O emissions. This study demonstrates the importance of interactions between microbial-feeding invertebrate soil fauna and their predators in understanding N2O emissions. [ABSTRACT FROM AUTHOR]

Labile organic nitrogen (N) fractions are actively involved in short-term N mineralization, but the extent to which each fraction contributes to N mineralization is not fully understood. The objective of this study was to examine the flow of 15N-labelled faba bean (Vicia faba L.) and wheat (Triticum aestivum L.) residues through the soil microbial biomass N (MBN), water-extractable organic N (WEON), light fraction organic matter N (LFOMN), particulate organic matter N (POMN) and mineral N pools in sandy-loam and clay soils under controlled conditions. After 3 d, 17-30% of the residue 15N was recovered in the POMN fraction, with a greater proportion of the wheat than faba bean residue recovered as POM15N. This POM15N probably included undecomposed residues and LFOM15N. Net N mineralization was greater in faba bean- than wheat-amended soils and greater in the sandy-loam than the clay soil. The LFOM15N concentrations decreased throughout the study, while POM15N concentrations increased or remained constant for 28 d in the sandy-loam and until 56 d in the clay soil. This suggests possible encrustation of LFOMN with soil mineral particles causing increased densification and recovery in the POMN fraction. The subsequent decrease in POM15N concentrations corresponded with mineral 15N accumulation in the soils. Mineral 15N concentration after 112 d was positively related to the initial POM15N concentration (r = 0.78, P < 0.001) but not to the initial LFOM15N concentration (r = −0.48, P > 0.05). The WEON and MBN appeared as transient, intermediary pools. The results of this study suggest that mineralization of POMN is a major pathway through which mineral N is supplied in agricultural soils, with C/N ratio of crop residues and to a lesser extent soil properties, influencing the mineralization rate. [ABSTRACT FROM AUTHOR]

Abstract: Turbic Cryosols (permafrost soils characterized by cryoturbation, i.e., by mixing of soil layers due to freezing and thawing) are widespread across the Arctic, and contain large amounts of poorly decomposed organic material buried in the subsoil. This cryoturbated organic matter exhibits retarded decomposition compared to organic material in the topsoil. Since soil organic matter (SOM) decomposition is known to be tightly linked to N availability, we investigated N transformation rates in different soil horizons of three tundra sites in north-eastern Siberia and Greenland. We measured gross rates of protein depolymerization, N mineralization (ammonification) and nitrification, as well as microbial uptake of amino acids and NH4 + using an array of 15N pool dilution approaches. We found that all sites and horizons were characterized by low N availability, as indicated by low N mineralization compared to protein depolymerization rates (with gross N mineralization accounting on average for 14% of gross protein depolymerization). The proportion of organic N mineralized was significantly higher at the Greenland than at the Siberian sites, suggesting differences in N limitation. The proportion of organic N mineralized, however, did not differ significantly between soil horizons, pointing to a similar N demand of the microbial community of each horizon. In contrast, absolute N transformation rates were significantly lower in cryoturbated than in organic horizons, with cryoturbated horizons reaching not more than 32% of the transformation rates in organic horizons. Our results thus indicate a deceleration of the entire N cycle in cryoturbated soil horizons, especially strongly reduced rates of protein depolymerization (16% of organic horizons) which is considered the rate-limiting step in soil N cycling. [Copyright &y& Elsevier]

Abstract: The controlling effects of soil moisture on soil nitrogen (N) supply to plants have been well established from lab experiments, and are often incorporated in soil organic matter (SOM) models, but rarely tested in the field along natural gradients of soil moisture. With a large dataset of in-situ incubation experiments across a wide range of natural ecosystems (mostly with shallow groundwater) in the Netherlands and Belgium, we tested whether and how soil moisture influences N mineralization rates (N min) in natural soils, and what type of hydrological coupling (in terms of estimation of soil moisture and formulation of soil moisture effect) is suited to predict across-site variation of N min with a SOM model, CENTURY. Our results show that soil moisture was related to a small (17%), but a larger than other site factors, proportion of the difference between measured and reference (i.e. simulated with no moisture effects) N min, indicating that soil moisture has a subtle yet important influence on N min of natural soils. Coupling CENTURY with a simple tipping-bucket hydrological module, a common way to estimate soil moisture in existing models, did not improve model performance on N min compared to a model without soil moisture effects, irrespective of the formulation of moisture effect applied. Coupling of CENTURY model with a more detailed hydrological module improved the model performance, but only when an appropriate moisture function was applied. Our results also indicated that model performance largely improved if indirect moisture effects through N losses via denitrification were accounted for. With our best model, inclusion of soil moisture information improved the normalized residual variance from 28% to 18% of the observed range of N min. We conclude that appropriate estimates of soil moisture and proper representation of its effects on mineralization are crucial to properly reflect between-site variation in soil N supply in natural ecosystems. [Copyright &y& Elsevier]

Abstract: Soil organic matter (SOM) decomposition has the potential to radically affect carbon dioxide concentrations in the atmosphere. Priming, the increased decomposition of SOM after the addition of a labile carbon (C) source, may be an important regulator of SOM dynamics, yet little is known about the mechanisms of the priming effect. Two hypotheses generated in the last decade have suggested that priming is caused by either the nutrient conditions in soil or the response of the microbial community to labile C addition. We used a three million year substrate age gradient, with associated changes in nutrient availability and microbial communities, to test these two hypotheses. We added 13C labeled glucose to soil in quantities similar to increases in root exudation that can be expected in a high carbon dioxide environment, and traced the effect of C addition on soil C, the microbial community, and soil nutrient pools and fluxes. We observed positive priming, negative priming, and no net priming depending on substrate age. Priming was most positive at the youngest sites with the smallest nitrogen (N) pools, and most negative at the site with the most available N. In contrast, we found no significant relationships between priming and phosphorus availability. Though components of the microbial community size and structure (measured by phospholipid fatty acid analysis) changed as a result of C addition, soil N availability was a better explanatory mechanism of priming effects than microbial community dynamics. Our results suggest close linkages between C and N cycles regulate the magnitude and direction of priming. If general, the stability of SOM in temperate ecosystems is likely to be governed by soil N status. [Copyright &y& Elsevier]

Abstract: The availability of inorganic N has been shown to be one of the major factors limiting primary productivity in high latitude ecosystems. The factors regulating the rate of transformation of organic N to nitrate and ammonium, however, remain poorly understood. The aim of this study was to investigate the nature of the soluble N pool in forest soils and to determine the relative rate of inorganic N production from high and low molecular weight (MW) dissolved organic nitrogen (DON) compounds in black spruce forest soils. DON was found to be the dominant N form in soil solution, however, most of this DON was of high MW of which >75% remained unidentified. Free amino acids constituted less than 5% of the total DON pool. The concentration of NO3 − and NH4 + was low in all soils but significantly greater than the concentration of free amino acids. Incubations of low MW DON with soil indicated a rapid processing of amino acids, di- and tri-peptides to NH4 + followed by a slower transformation of the NH4 + pool to NO3 −. The rate of protein transformation to NH4 + was slower than for amino acids and peptides suggesting that the block in N mineralization in taiga forest soils is the transformation of high MW DON to low MW DON and not low MW DON to NH4 + or NH4 + to NO3 −. Calculated turnover rates of amino acid-derived C and N immobilized in the soil microbial biomass were similar with a half-life of approximately 30 d indicating congruent C and N mineralization. [Copyright &y& Elsevier]

Abstract: A three-month mesocosm experiment was performed to unravel interactions between pesticides (difenoconazole: fungicide, deltamethrin: insecticide, ethofumesate: herbicide) and fertilizers (NPK synthetic fertilizer, compost) regarding the potential non-target effects of pesticides on soil microbial communities. To this aim, pesticides and fertilizers were applied to soil at a rate of 5 mg active ingredient kg−1 DW soil and 185 mg N kg−1 DW soil, respectively. Soil sampling was done after 0, 7, 30, 60 and 90 days of incubation in order to determine pesticide degradation rates and microbial properties: enzyme activities, basal respiration, substrate-induced respiration, potentially mineralizable N, nitrification rate and denitrification potential. By the end of the incubation, difenoconazole, deltamethrin and ethofumesate in non-fertilized soils were degraded by 52, 85 and 93%, with half-lives of 86, 36 and 29 days, respectively. Compost application had a stimulatory effect on difenoconazole and deltamethrin degradation. NPK fertilization led to a 26% increase in ethofumesate half-life in soil. Difenoconazole and deltamethrin caused a short-term inhibitory effect on microbial activity in non-fertilized soils, but not in fertilized soils. A short-term antagonistic effect between NPK fertilization and deltamethrin or ethofumesate presence was found regarding their inhibitory effect on potentially mineralizable N. In compost-fertilized soils, pesticides (especially, ethofumesate) counteracted the stimulatory effect of compost on denitrification potential. Pesticides caused a slight negative effect on the capacity of soils to recycle nutrients that was counteracted at day 90 by the addition of compost, as reflected by the values of the treated-soil quality index. We concluded that fertilizers can modify both pesticide degradation rates and their non-target effects on soil microbial communities. [Copyright &y& Elsevier]

Abstract: Changes in plant species diversity can result in synergistic increases in decomposition rates, while elevated atmospheric CO2 can slow the decomposition rates; yet it remains unclear how diversity and changes in atmospheric CO2 may interact to alter root decomposition. To investigate how elevated CO2 interacts with changes in root-litter diversity to alter decomposition rates, we conducted a 120-day laboratory incubation. Roots from three species (Trifolium repens, Lespedeza cuneata, and Festuca pratense) grown under ambient or elevated CO2 were incubated individually or in combination in soils that were exposed to ambient or elevated CO2 for five years. Our experiment resulted in two main findings: (1) Roots from T. repens and L. cuneata, both nitrogen (N) fixers, grown under elevated CO2 treatments had significantly slower decomposition rates than similar roots grown under ambient CO2 treatments; but the decomposition rate of F. pratense roots (a non-N-fixing species) was similar regardless of CO2 treatment. (2) Roots of the three species grown under ambient CO2 and decomposed in combination with each other had faster decomposition rates than when they were decomposed as single species. However, roots of the three species grown under elevated CO2 had similar decomposition rates when they were incubated alone or in combination with other species. These data suggest that if elevated CO2 reduces the root decomposition rate of even a few species in the community, it may slow root decomposition of the entire plant community. [Copyright &y& Elsevier]

Abstract: Elevated CO2 and defoliation effects on nitrogen (N) cycling in rangeland soils remain poorly understood. Here we tested whether effects of elevated CO2 (720 μl L−1) and defoliation (clipping to 2.5 cm height) on N cycling depended on soil N availability (addition of 1 vs. 11 g N m−2) in intact mesocosms extracted from a semiarid grassland. Mesocosms were kept inside growth chambers for one growing season, and the experiment was repeated the next year. We added 15N (1 g m−2) to all mesocosms at the start of the growing season. We measured total N and 15N in plant, soil inorganic, microbial and soil organic pools at different times of the growing season. We combined the plant, soil inorganic, and microbial N pools into one pool (PIM-N pool) to separate biotic + inorganic from abiotic N residing in soil organic matter (SOM). With the 15N measurements we were then able to calculate transfer rates of N from the active PIM-N pool into SOM (soil N immobilization) and vice versa (soil N mobilization) throughout the growing season. We observed significant interactive effects of elevated CO2 with N addition and defoliation with N addition on soil N mobilization and immobilization. However, no interactive effects were observed for net transfer rates. Net N transfer from the PIM-N pool into SOM increased under elevated CO2, but was unaffected by defoliation. Elevated CO2 and defoliation effects on the net transfer of N into SOM may not depend on soil N availability in semiarid grasslands, but may depend on the balance of root litter production affecting soil N immobilization and root exudation affecting soil N mobilization. We observed no interactive effects of elevated CO2 with defoliation. We conclude that elevated CO2, but not defoliation, may limit plant productivity in the long-term through increased soil N immobilization. [Copyright &y& Elsevier]

Abstract: We examined the impact of long-term cattle grazing on soil processes and microbial activity in a temperate salt marsh. Soil conditions, microbial biomass and respiration, mineralization and denitrification rates were measured in upper salt marsh that had been ungrazed or cattle grazed for several decades. Increased microbial biomass and soil respiration were observed in grazed marsh, most likely stimulated by enhanced rates of root turnover and root exudation. We found a significant positive effect of grazing on potential N mineralization rates measured in the laboratory, but this difference did not translate to in situ net mineralization measured monthly from May to September. Rates of denitrification were lowest in the grazed marsh and appeared to be limited by nitrate availability, possibly due to more anoxic conditions and lower rates of nitrification. The major effect of grazing on N cycling therefore appeared to be in limiting losses of N through denitrification, which may lead to enhanced nutrient availability to saltmarsh plants, but a reduced ability of the marsh to act as a buffer for land-derived nutrients to adjacent coastal areas. Additionally, we investigated if grazing influences the rates of turnover of labile and refractory C in saltmarsh soils by adding 14C-labelled leaf litter or root exudates to soil samples and monitoring the evolution of 14CO2. Grazing had little effect on the rates of mineralization of 14C used as a respiratory substrate, but a larger proportion of 14C was partitioned into microbial biomass and immobilized in long- and medium-term storage pools in the grazed treatment. Grazing slowed down the turnover of the microbial biomass, which resulted in longer turnover times for both leaf litter and root exudates. Grazing may therefore affect the longevity of C in the soil and alter C storage and utilization pathways in the microbial community. [ABSTRACT FROM AUTHOR]

Abstract: Bt (Bacillus thuringiensis) corn is reported to produce lignin-rich residues, compared to non-Bt (NBt) corn, suggesting it is more resistant to decomposition. As the Bt gene is expressed selectively in stem and leaf tissue, it could affect lignin distribution in corn, which naturally has greater lignin content in roots than in stems and leaves. Our objective was to evaluate the effects of corn plant components, the Bt gene and elevated-lignin inputs on decomposition. Roots, stems and leaves from Bt corn and NBt corn isolines enriched with 13C and 15N were finely ground and mixed separately with soil, then incubated at 20 °C for 36 weeks. The effect of elevated lignin on decomposition was tested by adding a commercial lignin source (indulin lignin) to half of the samples. In addition to weekly CO2 analysis and regular measurement of N mineralization, the degree of lignin degradation was evaluated at 1 and 36 weeks from the acid to aldehyde ratio (Ad/Al) of vanillyl and syringyl lignin-derived phenols. The CO2 production and N mineralization was lower in root-amended soils than stem- and leaf-amended soils. The Bt genetic modification increased CO2 production from stem-amended soils (P < 0.05) and decreased N mineralization in root-amended soils. The 13C and 15N results also showed more residue-C and -N retained in soils mixed with NBt stem residues. After 36 weeks leaf- and stem-amended soils with indulin lignin had a lower Ad/Al ratio and were less degraded than soils without exogenous lignin. In conclusion, plant lignin and nitrogen contents were good predictors of CO2 production and N mineralization potential. Corn roots decomposed more slowly than aboveground components emphasizing the importance of recalcitrant root residues in sustaining the organic matter content of soil. [ABSTRACT FROM AUTHOR]

Abstract: Projected changes in precipitation patterns in the northeastern U.S. may alter soil moisture dynamics and cause a shift in the structure and function of soil microbial communities. We studied the potential for such changes by manipulating annual precipitation amount in an oak–pine forest of the New Jersey Pinelands. During a two-year field study we tested the effects of a complete rain exclusion, as well as a doubling of rainfall, on soil microbial biomass, community composition (phospholipid fatty acid analysis) and the production of plant-available nitrogen (nitrogen mineralization + amino-acid production). We found that neither microbial biomass nor community composition was affected by the experimental manipulations. Despite having studied the organic horizon, the relatively high sand content appeared to influence this response by limiting the extent to which soil moisture increased in response to elevated rainfall. Furthermore, a strong correlation between soil moisture and the physiological status of Gram-negative bacteria suggested that soil microbes in the New Jersey Pinelands are well adapted to soil drying. We observed a sustained accumulation of ammonium in drought plots that was more than four times the value of all other treatments after one year. The relationship between soil moisture and nitrogen mineralization changed with season, suggesting that the effect of changing rainfall patterns on nitrogen cycling will depend upon microbial physiological demand and substrate diffusion. Based on available estimates of foliar N concentration in the New Jersey Pinelands, we conclude that neither the accumulation of ammonium in drought plots, nor the changes in nitrogen mineralization rates in response to high and low soil moisture will affect plant nitrogen demand. However, if the ammonium pool in dry soil had been mobilized by precipitation, a shift towards a higher bacteria:fungi ratio – and therefore higher nitrogen mineralization rates – may have occurred. [Copyright &y& Elsevier]

Abstract: Diverse plant litter mixtures frequently decompose differently than expected compared to the average of the component species decomposing alone, and it remains unclear why decomposition may respond non-additively to diversity. Here, we hypothesized that litter chemical composition and chemical diversity would be important determinants of the strength and direction (synergistic versus antagonistic) of non-additive soil carbon (C) and nitrogen (N) cycling responses to litter mixtures. To test this, we performed a soil incubation experiment using litter mixtures comprised of up to four plant species, and we measured three components of decomposition: respiration, net N mineralization, and microbial biomass N accumulation. We used nine chemical traits to calculate the chemical composition and diversity of the litter mixtures. First, we found that respiration responded as the average of the individual species in the mixture (i.e. additively), rather than non-additively as initially predicted. Second, litter mixtures stimulated significantly more net N immobilization than expected in 64% of cases, and non-additive responses were highly dependent on mixture chemical composition, and were influenced to a lesser degree by chemical diversity. Specifically, concentrations of tannins and certain low molecular weight phenolics in the mixtures were positively correlated with greater N immobilization than expected. Non-additive N mineralization responses were poorly correlated with traditional measures of litter chemistry like N concentration, C:N, lignin:N, and phenolic:N. Our results also show that non-additive N mineralization responses were affected by loss of some species significantly more than others, and the effects of species loss could depend on 1) whether a species contains compounds with strong effects on non-additive responses; and 2) whether those compounds are also found in other species. Finally, litter mixtures stimulated more microbial biomass N than expected in 45% of cases, but non-additive responses were only weakly dependent on the litter chemistry variables that we measured. [Copyright &y& Elsevier]

Abstract: Although the rates of gross protein depolymerization are central in litter decomposition, they cannot be measured directly by any existing method. Here we report the development of a novel assay to quantify gross protein depolymerization (i.e. gross amino acid production) based on a 15N isotope pool dilution technique. The assay is based on the concurrent labeling of the pool of 18 proteinogenic amino acids, which are present in a free form in litter, and the measurement of 15N:14N ratios in the individual amino acids by GC–MS over time. The method proved to be highly linear, precise and sufficiently sensitive. We tested the applicability of the novel method on a litter decomposition experiment, in which we could demonstrate that gross protein depolymerization exceeded gross N mineralization by >8 fold indicating that only a small fraction of amino acids released by extracellular enzymes was actually mineralized to ammonium. Moreover, the results provide evidence that protein depolymerization was limited by protein availability or accessibility, not by the size of protease pool. [Copyright &y& Elsevier]

Abstract: Wetlands are subject to changes in soil moisture as a result of both short-term seasonal climate variations and long-term changes in regional water resource management, both of which can modify the dynamics of ground and surface water inputs. In the New Jersey Pinelands, forested wetlands that differ in both plant communities and soil structure occur along a topographic and hydrological gradient associated with an unconfined aquifer. Proposed groundwater withdrawals may affect water content of soils along this gradient. We hypothesized that prolonged changes in soil moisture would alter net nitrogen mineralization and nitrification rates in proportion to the amount of moisture change, and that these changes would be similar for the different soils along the drainage catena. Soils from two catenary sequences of wetlands, including pine-dominated (driest landscape position), hardwood-dominated, and Atlantic white-cedar-dominated (wettest landscape position) communities were used in long-term laboratory incubations (36 weeks). Production of NH4 +–N, NO3 −–N, and dissolved organic N were measured under two sets of conditions: constant moisture levels of 100%, 60% and 30% water-holding capacity (WHC), and fluctuating moisture levels (alternating 2 week periods at 100% and 30% WHC). In soils from most of the wetlands, we observed increases in net mineralization and nitrification when constant low-moisture conditions were established, but not under fluctuating conditions. Contrary to expectations, responses to the drying treatments varied between wetland types and between replicate wetlands of each type. Under constant-moisture conditions, nitrification increased more in cedar swamps than in either type of pine wetland. Under all conditions, soils from all the wetlands within one of the catenas produced more inorganic and organic soluble N than did the wetlands from the other catena, suggesting that area-wide effects are as important as wetland type in regulating production of soluble N. Within both catenas, pine-hardwood wetlands generated more soluble N under all moisture conditions than did either pine-dominated or cedar wetlands. Our results suggest that changes in soil moisture due to management of water resources will affect N cycling in wetland soils, but that the magnitude of the effects, and the potential for large releases of nitrate, will depend on the specific soil properties of affected wetlands. [Copyright &y& Elsevier]

Abstract: Soil microbial organisms are central to carbon (C) and nitrogen (N) transformations in soils, yet not much is known about the stable isotope composition of these essential regulators of element cycles. We investigated the relationship between C and N availability and stable C and N isotope composition of soil microbial biomass across a three million year old semiarid substrate age gradient in northern Arizona. The δ 15N of soil microbial biomass was on average 7.2‰ higher than that of soil total N for all substrate ages and 1.6‰ higher than that of extractable N, but not significantly different for the youngest and oldest sites. Microbial 15N enrichment relative to soil extractable and total N was low at the youngest site, increased to a maximum after 55,000 years, and then decreased slightly with age. The degree of 15N enrichment of microbial biomass correlated negatively with the C:N mass ratio of the soil extractable pool. The δ 13C signature of soil microbial biomass was 1.4‰ and 4.6‰ enriched relative to that of soil total and extractable pools respectively and showed significant differences between sites. However, microbial 13C enrichment was unrelated to measures of C and N availability. Our results confirm that 15N, but not 13C enrichment of soil microbial biomass reflects changes in C and N availability and N processing during long-term ecosystem development. [Copyright &y& Elsevier]

Abstract: The purpose of this research was to compare soil chemistry, microbially mediated carbon (C) and nitrogen (N) transformations and microbial biomass in forest floors under European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Lieblein), Norway spruce (Picea abies (L.) Karst) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) at four study sites. We measured soil chemical characteristics, net N mineralization, potential and relative nitrification, basal respiration, microbial and metabolic quotient and microbial biomass C and N under monoculture stands at all sites (one mixed stand). Tree species affected soil chemistry, microbial activities and biomass, but these effects varied between sites. Our results indicated that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Differences in potential nitrification and relative nitrification might be related to the presence of ground vegetation through its influence on soil NH4 and labile C availability. Our findings highlight the need to study the effects of tree species on microbial activities at several sites to elucidate complex N cycle interactions between tree species, ground vegetation, soil characteristics and microbial processes. [Copyright &y& Elsevier]

Abstract: Elevated atmospheric CO2 tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO2 concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO2. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO2 plots at 0–10cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO2 stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO2 increased net N mineralization in the 10–30cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO2. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO2 studies. Increased mineralization of stable soil organic carbon by a CO2-induced priming effect may diminish the terrestrial carbon sink globally. [Copyright &y& Elsevier]

Abstract: This study was designed to address how earthworm activity influences soil mineral nitrogen (N), plant N uptake and forage yield in grass-based hayfields. Earthworm populations were reduced by applying carbaryl pesticide to the experimental field plots every 2-weeks, effectively eliminating the earthworms for up to 12-weeks from May to August. Grass yields and tissue N concentrations were measured every 2 weeks, and the soil mineral N concentration determined at the final harvest. Reducing earthworm populations for up to 12-weeks did not affect grass yield or N uptake. However, regression analysis showed that plots with undisturbed earthworm populations had higher soil N by 0.8kgNha−1 per week, representing mineralization of about 10kgNha−1 during the 12-week study. This was a fraction of the fertilizer N recommendation (75kgNha−1) for grass-based hayfields in this region. Therefore, the increase in soil mineral N from earthworm activity was small, relative to the N requirements of the hayfield. [Copyright &y& Elsevier]

Abstract: A study was conducted at two experimental tree plantations in the Pacific Northwest to assess the roles of bacteria and fungi in nitrogen (N) cycling. Soils from red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) plots in low- (H.J. Andrews) and high- (Cascade Head) productivity stands were sampled in 2005 and 2006. Fungal:bacterial ratios were determined using phospholipid fatty acid (PLFA) profiles and quantitative (Q)-PCR. Ratios from these two molecular methods were highly correlated and showed that microbial biomass varied significantly between the two experimental sites and to a lesser extent between tree types with fungal:bacterial biomass ratios lower in more N-rich plots. 15N isotope dilution experiments, with ammonium (NH4 +) and nitrate (NO3 −), were paired with antibiotics that blocked bacterial (bronopol) and fungal (cycloheximide) protein synthesis. This modified isotope dilution technique was used to determine the relative contribution of bacteria and fungi to net N mineralization and gross rates of ammonification and nitrification. When bacterial protein synthesis was blocked NH4 + consumption and nitrification rates decreased in all treatments except for NH4 + consumption in the Douglas-fir plots at H.J. Andrews, suggesting that prokaryotic nitrifiers are a major sink for mineral NH4 + in forest soils with higher N availability. Cycloheximide consistently increased NH4 + consumption, however the trend was not statistically significant. Both antibiotics additions also significantly increased gross ammonification, which may have been due to continued activity of extra- and intracellular enzymes involved in producing NH4 + combined with the inhibition of NH4 + assimilation into proteins. The implication of this result is that microorganisms are likely a major sink for soil dissolved organic N (DON) in soils. [Copyright &y& Elsevier]

Abstract: Organic nitrogen (N) uptake, rather than solely inorganic N (DIN), is considered a significant pathway for plant nutrition, especially in arctic, alpine and boreal ecosystems. Assays of plant-available N in these ecosystems might therefore be improved with measures of dissolved organic N (DON). We examined DON and DIN abundance from an in situ 5-week incubation across plant associations that represent the widest range in site potential in southern boreal forests of British Columbia, Canada. The supply of N from forest floors and mineral soils (20cm depth) was measured separately and then combined (kgha−1) to facilitate comparisons of sites. DON was the predominant form of extractable N, and was increasingly supplemented, rather than replaced, by NH4 + and NO3 − on productive sites. The amount of DIN produced in the soils was very low, perhaps too small to support forest needs, and the correlation of DIN to asymptotic stand height (a measure of site potential) was significant but nonlinear. The combined amount of DON+DIN was considered a more effective index of plant-available N because it was strongly significant as a linear correlation to stand height and more typical of annual forest N uptake. The relative shift in N forms, from a predominance of DON to progressively greater ratios of DIN:DON, was consistent with the current paradigm of N forms across gradients of N availability, although the actual amounts of DON increased, rather than decreased, with site potential. Based on this, we suggest organic N uptake has the potential to contribute to plant nutrition across the entire productivity gradient of soils in southern boreal forests. Although other N indices were effective in characterizing forest productivity, a combined assay of DON+DIN production could provide new insights into functional differences in plant-available N. [Copyright &y& Elsevier]

Abstract: Nitrogen mineralization and denitrification potential in litter were measured during a dry and a wet period in a Bornean Lowland Evergreen Rain Forest (LERF) and two nearby Heath Forests (HF) of contrasting stature. Nitrification was very low or non-existent in all forest types and ammonification was the major constituent of nitrogen (N) mineralization. Rates of net N mineralization in the HFs on infertile sandy soils were lower than in the LERF on a more nutrient-rich clay soil or other LERFs, both during dry and wet conditions. We attribute the differences to the lower litter quality in the HFs compared to LERF. When dissolved organic nitrogen (DON-N) was included, N uptake was the same (15–17μgg−1 d−1) in all three forest types. We conclude that N availability is the same in all three forest types and that N deficiency is not the reason for the reduced stature of Heath Forests compared to LERF. All three-forest types had denitrifiers present in the ectorganic layers but denitrification will only play a minor role in the N-cycle as nitrification rates were very low. [Copyright &y& Elsevier]

Abstract: The aim of this study was to compare microbial activities in the litter (L), fermentation (F) and humified (H) layers of the forest floor under silver birch (Betula pendula Roth), Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.). Soil pH, C-to-N ratio, respiration rates, concentration of NH4-N, net N mineralization and nitrification rates, gross NH4 + production and consumption rates and amounts of C (C mic) and N (N mic) in the microbial biomass were determined from samples taken from the L, F and H layers under silver birch, Norway spruce and Scots pine. The forest floors under birch and spruce were more active than that under pine, having higher respiration and net N mineralization rates, and higher C mic and N mic values than pine forest floor. Differences between tree species were smaller in the H layer than in the L and F layers. The L layer had the highest rates of respiration for all tree species, while rates of net N mineralization were highest in the F layer for birch and spruce. Pine showed negligible net N mineralization in all layers. Concentration of NH4-N was the best predictor of rate of net N mineralization (). In general, C mic and N mic were higher in the L and F layers than in the H layer, as were their relative proportions of total C (C tot) and N (N tot), respectively. C mic correlated positively with soil respiration () and N mic with concentration of NH4-N (). [Copyright &y& Elsevier]

Abstract: A sterilized, but undecomposed, organic by-product of municipal waste processing was incubated in sandy soils to compare C and N mineralization with mature municipal waste compost. Waste products were added to two soils at rates of 17.9, 35.8, 71.6, and dry weight and incubated at for 90d. Every 30d, nitrate and ammonium concentrations were analyzed and C mineralization was measured as total evolved and added total organic C. Carbon mineralization of the undecomposed waste decreased over time, was directly related to application rate and soil nutrient status, and was significantly higher than C mineralization of the compost, in which C evolution was relatively unaffected across time, soils, and application rates. Carbon mineralization, measured as percentage C added by the wastes, also indicated no differences between composted waste treatments. However, mineralization as a percentage of C added in the undecomposed waste treatments was inversely related to application rate in the more productive soil, and no rate differences were observed in the highly degraded soil. Total inorganic N concentrations were much higher in the compost- and un-amended soils than in undecomposed waste treatments. Significant N immobilization occurred in all undecomposed waste treatments. Because C mineralization of the undecomposed waste was dependant on soil nutrient status and led to significant immobilization of N, this material appears to be best suited for highly degraded soils low in organic matter where restoration of vegetation adapted to nutrient poor soils is desired. [Copyright &y& Elsevier]

Abstract: Condensed tannins (CTs) are a major component of litter inputs, but little is known about the effects of tannin structural variations on soil biological processes and organic matter development. Four different CTs extracted from balsam fir, western red cedar, kalmia and black spruce were added to Corsican pine litter and subsequently incubated for 16 weeks in order to investigate the effect of the B ring hydroxylation pattern on C, N and P transformations. While for C mineralization the chain length and stereochemistry of the CTs seemed to be a more important parameter, net N and P mineralization rates were clearly reduced compared with non-amended litter. With regard to the B ring hydroxylation, the prodelphinidin (PD) CTs having predominantly three hydroxy groups at the B ring (balsam fir and western red cedar) exhibited significantly lower mineralization rates than the procyanidin (PC) CTs having two OH groups (kalmia and black spruce). The same was true for net nitrification, but this process was only slightly affected by the CTs. Although based on only four CTs, this study indicates that B ring hydroxylation is an important variable determining net N and P mineralization rates. Our results support previous suggestions that PD tannins bind to or react more strongly with soil organic matter. Therefore, more than PC tannins, they reduce the availability of organic N for mineralization as well as their own detectability by standard methods for soil CT. [Copyright &y& Elsevier]

Abstract: In arid areas of North America, nests of the seed-harvesting ant Pogonomyrmex rugosus tend to be elevated in mineral nitrogen and other soil nutrients relative to other microhabitats. We investigated the roles of decomposition, N mineralization, and plant nutrient uptake in maintaining high standing stocks of nutrients in P. rugosus ant nests. Decomposition rates of standard cellulose substrates placed on the surface of ant nests and other desert microhabitats suggest that conditions found in ant nests and bare areas are conducive to higher rates of decomposition than conditions under shrubs. In laboratory incubations of moist soil, net N mineralization rates were significantly higher in soil from ant nests than from bare areas and under two of three plant species. Net N mineralization rates measured in situ were much lower than those measured in laboratory incubations, but ant nest soil still exhibited higher rates at one of two sites. Litter collected from ant mounds, composed chiefly of seed chaff, was similar in N content to litter collected from underneath the dominant plant species, but had a significantly higher mean δ 15N. Using this distinctive isotope signature as a tracer, we found no evidence that large perennial shrubs tap ant nests as a source of N. An invasive, annual grass species was significantly enriched in 15N, had higher leaf %N, and produced more seeds when growing on the mound than when growing several meters away; however P. rugosus nest surfaces are typically free of such annuals. We conclude that both high rates of nutrient cycling relative to other Mojave Desert microhabitats and low N utilization by the surrounding vegetation contribute to high standing stocks of mineral N in P. rugosus nests. [Copyright &y& Elsevier]

Abstract: To test if native perennial bunchgrasses cultivate the same microbial community composition across a gradient in land-use intensification, soils were sampled in fall, winter and spring in areas under bunchgrasses (‘plant’) and in bare soils (‘removal’) in which plots were cleared of living plants adjacent to native perennial bunchgrasses (Nassella pulchra). The gradient in land-use intensification was represented by a relict perennial grassland, a restored perennial grassland, and a perennial grass agriculture site on the same soil type. An exotic annual grassland site was also included because perennial bunchgrasses often exist within a matrix of annual grasses in California. Differences in soil resource pools between ‘plant’ and ‘removal’ soils were observed mainly in the relict perennial grassland and perennial grass agriculture site. Seasonal responses occurred in all sites. Microbial biomass carbon (C) and dissolved organic C were greater under perennial bunchgrasses in the relict perennial grassland and perennial grass agriculture site when comparing treatment means of ‘plant’ vs. ‘removal’ soil. In general, soil moisture, microbial respiration, and nitrate decreased from fall to spring in ‘plant’ and ‘removal’ soils, while soil ammonium and net mineralizable nitrogen (N) increased only in ‘plant’ soils. A canonical correspondence analysis (CCA) of phospholipid fatty acid (PLFA) profiles from all sites showed that land-use history limits the similarity of microbial community composition as do soil C and N dynamics among sites. When PLFA profiles from individual sites were analyzed by CCA, different microbial PLFA markers were associated with N. pulchra in each site, indicating that the same plant species does not retain a unique microbial fingerprint across the gradient of land-use intensification. [Copyright &y& Elsevier]

Abstract: The effects of maize expressing the Bacillus thuringiensis Cry1Ab protein (Bt maize) on decomposition processes under three different European climatic conditions were assessed in the field. Farming practices using Bt maize were compared with conventional farming practices using near-isogenic non-Bt maize lines under realistic agricultural practices. The litter-bag method was used to study litter decomposition and nitrogen mineralization dynamics of wheat straw. After 4 months incubation in the field, decomposition and mineralization were mainly influenced by climatic conditions with no negative effect of the Bt toxin on decomposition processes. [Copyright &y& Elsevier]

Abstract: Coarse woody debris (CWD) could alter N availability and transformations in the underlying soil and therefore contribute to spatial heterogeneity and influence ecosystem loss of N. We measured soil N concentrations and transformations in soil beneath CWD and beneath a litter layer at a mixed forest in NY State. We found that total and microbial biomass N was lower and that microbial biomass C-to-N ratio was higher in soil beneath CWD. Rates of N2O production and denitrification enzyme activity were reduced beneath CWD. These results suggest that CWD is an important controller of spatial heterogeneity in N dynamics and may influence the magnitude of N loss in temperate forests. [Copyright &y& Elsevier]

Managed pastures have potential for C and N sequestration in addition to providing forage for livestock. Our objectives were to investigate changes in soil organic C (SOC) and soil organic N (SON) concentrations and mineralizable C and N in cattle (Bos indicus) grazed bermudagrass [Cynodon dactylon (L.) Pers.] pastures up to 32 y after establishment. Management included low- and high-grazing intensity, fertilization, and winter overseeding with annual ryegrass (Lolium multiflorum Lam.) and clover (Trifolium sp.). Soil (0–15cm) was sampled 7, 15, 26, and 32 y after establishment of Coastal and common bermudagrass pastures. No significant differences in SOC or SON concentrations were observed between Coastal and common bermudagrass pastures. Grazing strategies played important roles in C and N sequestration, as high-grazing intensity resulted in a lower increase in SOC and SON concentrations over time compared to low-grazing intensity. Increases in SOC were observed up to 26 y, while increases in SON were observed up to 32 y after establishment of bermudagrass pastures. Soil organic C increased 67 and 39% from 7 to 26 y at low-grazing intensity for bermudagrass+ryegrass and bermudagrass+clover pastures, respectively. SOC and SON concentrations did not increase beyond 15 y after bermudagrass establishment at high-grazing intensity. An exception was the Coastal bermudagrass+ryegrass pastures, which exhibited higher SON at 32 y than at 7 y at both grazing intensities. By 32 y, SON increased 83 and 45% in Coastal bermudagrass+ryegrass pastures at low- and high-grazing intensity, respectively, compared to 7 y. The introduction of clover to pastures decreased SOC and SON relative to ryegrass at high- but not at low-grazing intensity. Potentially mineralizable C increased from 7 to 15 y, while mineralizable N increased from 7 to 32 y. Potentially mineralizable N was also greater for bermudagrass+clover than bermudagrass+ryegrass pastures. Long-term increases in SOC and SON concentrations suggest that managed and grazed pastures have strong potential for C and N sequestration. [Copyright &y& Elsevier]

Ecosystem processes such as N transformations have seldom been studied in urban and suburban areas. Here we report the temporal and spatial variations in soil N measured continuously over 16 months in remnant forests dominated by northern red oak (Quercus rubraL.) along a 130 km urban–rural transect in the New York City metropolitan area. Urban, suburban and rural forests all exhibited clear seasonal patterns in soil N concentrations and transformation rates. Concentrations of extractable inorganic N were highest in early spring, while net N mineralization and nitrification rates were highest in summer. Peak N mineralization and nitrification in urban stands tended to occur a month earlier than in rural stands. Daily net N mineralization rates averaged 4.45 mg N kg−1 soil organic matter (SOM) in urban stands, 3.51 in suburban stands, and 2.49 in rural stands. In urban and suburban forests, between 23.2–73.8% of the annual net N mineralized was nitrified, but in rural forests, net nitrification was mostly below the detection limit. Annual net N mineralization rates, expressed on an areal basis (to a depth of 7.5 cm), averaged 11.6 g m−2 in urban forests, 11.3 g m−2 in suburban sites, and 7.3 g m−2 in rural forests. N returns in oak litter fall were 2.15, 1.32, and 1.81 g m−2 in urban, suburban, and rural stands, respectively. The elevated N transformation rates and nitrate production, in combination with possible pollution constraints on tree growth in urban environments, raises concern that these urban and suburban forests may be approaching an N saturated status. [Copyright &y& Elsevier]

More than 200,000 ha of short rotation Eucalyptus globulus plantations have been established in south-western Australia to supply wood for the pulp and paper industries. Sustaining the productivity of these tree crops over successive rotations will depend in part on maintenance of soil fertility, especially soil nitrogen (N) supply. We investigated the impact of four alternative strategies for management of harvest residues on soil N dynamics in recently logged first rotation plantations. The experiments were conducted over 5 years following harvesting at two sites with contrasting soils—a coarse textured grey sand over laterite (Podzol) with low natural fertility and a relatively fertile red earth soil (Ferralsol). At the grey sand site, 31 t ha−1 of residues containing 219 kg N ha−1 were deposited following harvest while at the red earth site the equivalent figures were 51 t ha−1 of residues and 347 kg N ha−1. Experimental treatments applied included residues burned, removed, retained and retained with double the amount of residues. The impact of treatments on soil nitrogen supply was investigated by incubating intact soil cores in the field to determine rates of net N mineralization. Additionally, the effect of treatments on soil moisture and temperature, the resident pool of soil mineral N and the amount of N potentially available for mineralization was assessed. The mulching effect of retained residues resulted in higher soil moisture where residues had been retained and a trend for soil on these treatments to dry out more slowly with the onset of the dry summer season, especially in the first year following harvest. Diurnal variations in soil temperature were moderated and average soil temperatures were reduced during summer where residues were retained. Concentrations of mineral N in soil were high in the 2 years following harvest at both sites and declined as newly established seedlings developed. At the more fertile site, where mineral N occurred predominantly as nitrate, retention of residues resulted in lower pools of soil mineral N following harvest. The effect of residue treatments on soil mineral N pools was less marked at the grey sand site. Concentrations of potentially mineralizable soil N and the amounts of N mineralized annually were greater where residues were retained at both sites. The results indicate that retention of harvest residues will favour the conservation of N following logging. However, accumulation of soil mineral N following harvesting due to reduced plant uptake will result in leaching of N early in the rotation that is largely independent of residue management. Retaining harvest residues will contribute to enhanced N supply for the next tree crop through mineralization in the long term. However, on some sites, additions of nitrogenous fertilizers will still be required to maximise the rate of tree growth. [Copyright &y& Elsevier]

In mid-latitude climatic regions (35–65°) soils may be subjected to freeze–thaw cycles (FTC) which can occur frequently in late winter and early spring. FTC often results in flushes in C and N mineralization and could therefore be an important factor controlling C and N mineralization rates. Laboratory experiments were carried out to characterize the source of organic matter that becomes available upon FTC. Soils, differing in the quantity and quality of organic matter inputs they had received since 1956, were sampled, pre-incubated to reduce amounts of labile organic matter and subsequently exposed to repeated FTC. Each FTC consisted of 6 h at −2 °C, 16 h at −5 °C, 4 h at +2 °C and 22 h at +5 °C, giving a total of 48 h for each FTC. The contribution of microbial biomass C to the C flush upon FTC was determined by labeling the native microbial biomass with a small amount of 14C-labeled glucose and comparing the specific activity of the C flush upon FTC with that upon chloroform fumigation. Temperature corrected amounts of C and N mineralized in soil incubated at constant temperatures acted as control in the calculation of the flush. FTC increased the amounts of C and N mineralized 2–3 fold. The flush was short-lived and highest in the first four FTC, suggesting that easily decomposable material became available upon FTC and that the size of the freeze–thaw susceptible pool was limited. The C flush was linearly related to organic C, water-soluble organic C, microbial biomass C and basal respiration, but only proportional to the latter two. Labeling the native microbial biomass with a small amount of glucose suggested that microbial biomass C contributed ca. 65% to the C flush upon FTC, while representing only about 5% of microbial biomass C. We have no direct evidence for the source of the remaining 35% of the C flush or for the mechanism of its release. In soils subjected to chloroform fumigation prior to being exposed to FTC organic matter released by fumigation became a more important source to the flush than the microbial biomass, suggesting that labile organic matter is highly susceptible to FTC. From our results, effects of FTC have little consequence for annual C and N budgets, but may need to be taken into account when modeling C and N mineralization during the late winter and early spring period in mid-latitude climatic regions. [Copyright &y& Elsevier]