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Understanding the controls on the amount and persistence of soil organic carbon (C) is essential for predicting its sensitivity to global change. The response may depend on whether C is unprotected, isolated within aggregates, or protected from decomposition by mineral associations. Here, we present a global synthesis of the relative influence of environmental factors on soil organic C partitioning among pools, abundance in each pool (mg C g-1 soil), and persistence (as approximated by radiocarbon abundance) in relatively unprotected particulate and protected mineral-bound pools. We show that C within particulate and mineral-associated pools consistently differed from one another in degree of persistence and relationship to environmental factors. Soil depth was the best predictor of C abundance and persistence, though it accounted for more variance in persistence. Persistence of all C pools decreased with increasing mean annual temperature (MAT) throughout the soil profile whereas persistence increased with increasing wetness index (MAP/PET) in subsurface soils (30-176 cm). The relationship of C abundance (mg C g-1 soil) to climate varied among pools and with depth. Mineral-associated C in surface soils (

Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon (SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant-derived, particulate organic C (POC), and largely microbially derived, mineral-associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non-acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long-term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca-stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss.

Nitrogen enrichment due to atmospheric nitrogen deposition has affected plant growth and microbial activity globally, leading to an increase in soil organic carbon in many ecosystems. Drylands cover ~45% of the global land area and constitute ~32% of the global carbon stocks, but the response of dryland carbon storage to atmospheric nitrogen deposition remains unclear and understudied relative to mesic systems. Observations from mesic systems suggest that nitrogen enrichment can increase the efficiency by which microbes incorporate carbon into mineral-associated forms if pH stays constant. Under acidification, a common response to nitrogen deposition, microbial biomass and enzymatic organic matter decay often decrease, leading to a build-up in plant-derived particulate organic carbon. However, in drylands, where organic carbon often associates with mineral surfaces via Ca-bridging, acidification may also abiotically decrease mineral-associated organic carbon if Ca is leached. In this study we tested how experimental nitrogen deposition affects different soil organic carbon fractions in drylands through microbial and abiotic effects.We used four long-running nitrogen deposition experiments in Mediterranean shrub- and grassland ecosystems in Southern California, where two of the sites showed strong nitrogen-induced acidification (pH drop by ~1.5 units). We studied changes in soil organic carbon fractions, soil extracellular enzyme activity, microbial carbon stabilization efficiency and exchangeable Ca. Experimental nitrogen deposition had relatively small effects on soil organic carbon storage, which appeared to be mostly driven by soil physicochemical changes. Particulate organic carbon did not increase despite previously reported increases in plant biomass and decreases in microbial biomass and extracellular enzyme activity in acidified sites. Furthermore, microbial carbon stabilization efficiency was unaffected by N fertilization in non-acidified sites and decreased in short-term but not long-term incubations in acidified sites. Importantly, mineral-associated organic carbon decreased significantly by 20% in response to N fertilization at one of the acidified sites, likely as result of pH-induced loss of Ca, which dropped by 48%. Our measurements suggest that long-term effects of nitrogen fertilization on dryland carbon storage might be primarily abiotic in nature, such that drylands, which may undergo acidification and where Ca-stabilization of soil organic carbon is prevalent, may be most at risk for loss of mineral-associated organic carbon.

Wetting of dry soil triggers a pulse of microbial respiration that has been attributed to two broad mechanisms: (1) recycling of microbial cellular carbon (C), and (2) consumption of extracellular organic C made available to microbes by wetting. We evaluated these two mechanisms by measuring cumulative CO2 release, changes in the size and chemical composition of microbial biomass, and water-extractable organic carbon (WEOC) concentrations following artificial wetting of soil sampled from two depths at each of seven sites across California spanning a range of geologic parent materials. In samples collected from surface soil (0–10 cm depth), we found that cumulative CO2 release after wetting in the laboratory was most strongly correlated with microbial biomass. In these samples, the relative abundance of trehalose—a putative microbial osmolyte—decreased from 25% (SD = 12) to 16% (SD = 7) of the chloroform-labile fraction of the microbial biomass after wetting. This suggested a role for osmolyte consumption in generating the respiration pulse. In subsoil (40–50 cm depth, or sampled at contact with rock), however, the cumulative CO2 release after wetting was unrelated to microbial biomass and more strongly related to WEOC. The concentrations of selected microbial biomass constituents (e.g. trehalose and amino acids) in WEOC were negligible (

Most of the carbon (C) that enters the soil is broken down by the microbial community and either respired or stored in soil depending on the microbial allocation strategy. Changes in how the microbial community uses C can significantly affect soil C pool sizes, so new models have begun to explicitly represent microbial allocation. Most models use a parameter called carbon use efficiency (CUE) to represent microbial allocation, which partitions consumed C between respiration and growth. Here we compare a “Typical Microbial Model” with this representation of microbial allocation to two other models. One is the “Microbial Allocation Model” that represents CUE as an emergent property of the microbial community, explicitly modeling multiple processes involved in CUE. The second is the “Two-Pool Biomass Model” that similarly accounts for CUE as an emergent property but also represents the biomass using two C pools with different turnover times. We assessed the models’ relative ability to track a14C-glucose tracer pulse over three weeks through the extractable C pool, the microbial biomass, and respiration. We also used the 14C data to test how estimates of microbial CUE change during the incubation. Our results suggest that CUE estimates in soil are sensitive to incubation timing and are at no point stable. Isotopic data can best parameterize models when a time course of measurements is used. Our model comparison showed that the Two-Pool Biomass Model best fit our data. Using the Two-Pool Biomass model to represent microbial allocation is more biologically realistic and better matches the dynamics observed in our microbial C partitioning data. Mechanistic biogeochemical models should consider using a dynamic allocation scheme to represent CUE, rather than assuming a static value.

Our understanding of terrestrial nitrogen (N) cycling is changing as new processes are uncovered, including the sources, turnover and losses of N from ecosystems. We integrate recent insights into an updated N‐cycling framework and discuss how a new understanding integrates eco‐evolutionary dynamics with nutrient cycling. These insights include (a) the significance of rock weathering as a biologically meaningful N source to plants and microbes; (b) the lack of consistent N limitation of organic matter decomposition by soil microbes; (c) species‐specific variation in plant N limitation; and (d) how fire effects on soil N shift with ecosystem properties. Using an eco‐evolutionary framework and revised knowledge of N cycling, we describe how (a) rock N weathering could have contributed more strongly to gradients in soil N availability than previously recognized, (b) evolution and co‐evolution of plant and soil microbial resource‐use traits underlie whether decomposition and production are N‐limited, and (c) the effects of fire on soil N pools are mediated by composition of plant species and time‐scale. Our revised framework of N cycling provides a way forward for improving biogeochemical models to more accurately estimate rates of plant production and decomposition, and total soil N. A free Plain Language Summary can be found within the Supporting Information of this article.

In addition to warming temperatures, Arctic ecosystems are responding to climate change with earlier snowmelt and soil thaw. Earlier snowmelt has been examined infrequently in field experiments, and we lack a comprehensive look at belowground responses of the soil biogeochemical system that includes plant roots, decomposers, and soil nutrients. We experimentally advanced the timing of snowmelt in factorial combination with an open‐top chamber warming treatment over a 3‐year period and evaluated the responses of decomposers and nutrient cycling processes. We tested two alternative hypotheses: (a) Early snowmelt and warming advance the timing of root growth and nutrient uptake, altering the timing of microbial and invertebrate activity and key nutrient cycling events; and (b) loss of insulating snow cover damages plants, leading to reductions in root growth and altered biological activity. During the 3 years of our study (2010–2012), we advanced snowmelt by 4, 15, and 10 days, respectively. Despite advancing aboveground plant phenology, particularly in the year with the warmest early‐season temperatures (2012), belowground effects were primarily seen only on the first sampling date of the season or restricted to particular years or soil type. Overall, consistent and substantial responses to early snowmelt were not observed, counter to both of our hypotheses. The data on soil physical conditions, as well interannual comparisons of our results, suggest that this limited response was because of the earlier date of snowmelt that did not coincide with substantially warmer air and soil temperatures as they might in response to a natural climate event. We conclude that the interaction of snowmelt timing with soil temperatures is important to how the ecosystem will respond, but that 1‐ to 2‐week changes in timing of snowmelt alone are not enough to drive season‐long changes in soil microbial and nutrient cycling processes.

Throughout Earth's history, drought has been a common crisis in terrestrial ecosystems; in human societies, it can cause famine, one of the Four Horsemen of the apocalypse. As the global hydrological cycle intensifies with global warming, deeper droughts and rewetting will alter, and possibly transform, ecosystems. Soil communities, however, seem more tolerant than plants or animals are to water stress—the main effects, in fact, on soil processes appear to be limited diffusion and the limited supply of resources to soil organisms. Thus, the rains that end a drought not only release soil microbes from stress but also create a resource pulse that fuels soil microbial activity. It remains unclear whether the effects of drought on soil processes result from drying or rewetting. It is also unclear whether the flush of activity on rewetting is driven by microbial growth or by the physical/chemical processes that mobilize organic matter. In this review, I discuss how soil water, and the lack of it, regulates microbial life and biogeochemical processes. I first focus on organismal-level responses and then consider how these influence whole-soil organic matter dynamics. A final focus is on how to incorporate these effects into Earth System models that can effectively capture dry–wet cycling.

Soils contain more carbon than plants or the atmosphere, and sensitivities of soil organic carbon (SOC) stocks to changing climate and plant productivity are a major uncertainty in global carbon cycle projections. Despite a consensus that microbial degradation and mineral stabilization processes control SOC cycling, no systematic synthesis of long-term warming and litter addition experiments has been used to test process-based microbe-mineral SOC models. We explored SOC responses to warming and increased carbon inputs using a synthesis of 147 field manipulation experiments and five SOC models with different representations of microbial and mineral processes. Model projections diverged but encompassed a similar range of variability as the experimental results. Experimental measurements were insufficient to eliminate or validate individual model outcomes. While all models projected that CO2 efflux would increase and SOC stocks would decline under warming, nearly one-third of experiments observed decreases in CO2 flux and nearly half of experiments observed increases in SOC stocks under warming. Long-term measurements of C inputs to soil and their changes under warming are needed to reconcile modeled and observed patterns. Measurements separating the responses of mineral-protected and unprotected SOC fractions in manipulation experiments are needed to address key uncertainties in microbial degradation and mineral stabilization mechanisms. Integrating models with experimental design will allow targeting of these uncertainties and help to reconcile divergence among models to produce more confident projections of SOC responses to global changes.

Areas with Mediterranean climate regimes, such as California, have cool wet winter growing seasons and hot dry summers. Summer is a time of stress for plants, yet soil microbes survive and biochemical processes continue. One mechanism soil microorganisms might use to survive drought is to produce extracellular polysaccharides (EPSac). We hypothesized that in dry soils, pools of microbial EPSac would therefore increase, but that this increase would depend on having carbon available from fresh plant inputs. We manipulated plant cover and dry season length and measured soil saccharides in a seasonally dry California grassland soil; we evaluated total sugars as well as the mix of sugars present in the soil. Soil cores were collected monthly from July 2014 to February 2015. Sugar residues were analyzed using Gas Chromatography—Mass Spectroscopy (GC-MS). Drier soils showed larger pools of sugar residues; these residues decreased as moisture increased across sample dates and treatments. Plant removal only slightly reduced soil saccharide levels. However, the pools of individual saccharides varied only modestly across all treatments and dates, and correlated with total microbial biomass, suggesting that extracellular polysaccharides may be a constitutive response to survival in soil, rather than an inducible response to dry conditions.

High latitude ecosystems are characterized by cold soils and long winters, with much of their biogeochemistry directly or indirectly controlled by temperature. Climate warming has led to an expansion of shrubby plant communities across tussock tundra, but whether these clear aboveground shifts correspond to changes in the microbial community belowground remains less certain. Using bromodeoxyuridine to label growing cells, we evaluated how total and actively growing bacterial communities varied throughout a year and following 22 years of passive summer warming. We found that changes in total and actively growing bacterial community structures were correlated with edaphic factors and time point sampled, but were unaffected by warming. The aboveground plant community had become more shrub-dominated with warming at this site, and so our results indicate that belowground bacterial communities did not track changes in the aboveground plant community. As such, studies that have used space-for-time methods to predict how increased shrub cover has altered bacterial communities may not be representative of how the microbial community will be affected by in situ changes in the plant community as the Arctic continues to warm.

The microbial metabolic quotient (MMQ), microbial respiration per unit of biomass, is a fundamental factor controlling heterotrophic respiration, the largest carbon flux in soils. The magnitude and controls of MMQ at regional scale remain uncertain. We compiled a comprehensive data set of MMQ to investigate the global patterns and controls of MMQ in top 30cm soils. Published MMQ values, generally measured in laboratory microcosms, were adjusted on ambient soil temperature using long-term (30yr) average site soil temperature and a Q(10)=2. The area-weighted global average of MMQ_Soil is estimated as 1.8 (1.5-2.2) (95% confidence interval) molCh(-1)mmol(-1) microbial biomass carbon (MBC) with substantial variations across biomes and between cropland and natural ecosystems. Variation was most closely associated with biological factors, followed by edaphic and meteorological parameters. MMQ_Soil was greatest in sandy clay and sandy clay loam and showed a pH maximum of 6.7 +/- 0.1 (mean +/- se). At large scale, MMQ_Soil varied with latitude and mean annual temperature (MAT), and was negatively correlated with microbial N:P ratio, supporting growth rate theory. These trends led to large differences in MMQ_Soil between natural ecosystems and cropland. When MMQ was adjusted to 11 degrees C (MMQ_Ref), the global MAT in the top 30cm of soils, the area-weighted global averages of MMQ_Ref was 1.5 (1.3-1.8) molCmmolMBC(-1)h(-1). The values, trends, and controls of MMQ_Soil add to our understanding of soil microbial influences on soil carbon cycling and could be used to represent microbial activity in global carbon models.

Soil can be analogized as a “democracy” in which there are three “political parties” (microbes, minerals, and organic molecules) that sometimes compete and sometimes cooperate in controlling biogeochemical process in the soil system. These interactions are in turn regulated by the media that regulate the flow of material and information through the system; in soil the key medium is moisture, which regulates substrate solubility and transport. Modern soil science is increasingly sophisticated in our tools to analyze these phenomena with high resolution at fine scales— ‘omics identifies thousands of microbial taxa, tomography maps the 3-D pore structure of a single aggregate, and chromatography-mass spectrometry systems identify molecules in soil. Yet, we are still challenged to assimilate the masses of data these methods produce to explain the overall functioning of the soil system. For example, why do molecules that microbes can metabolize in minutes still persist for centuries or millennia? How we assimilate these new data into the models (conceptual and mathematical) that we use to understand and characterize the soil system is fundamental to developing integrated perspectives on soil and its roles in biogeochemical cycles. In this chapter, I discuss ideas and approaches for integrating data to better bridge from the micron-scale processes that occur on mineral surfaces or in individual soil pores to meter- and kilometer-scale phenomena within soil profiles and landscapes and which drive the functioning of terrestrial ecosystems.

Soil organic nitrogen (N) is a critical resource for plants and microbes, but the processes that govern its cycle are not well-described. To promote a holistic understanding of soil N dynamics, we need an integrated model that links soil organic matter (SOM) cycling to bioavailable N in both unmanaged and managed landscapes, including agroecosystems. We present a framework that unifies recent conceptual advances in our understanding of three critical steps in bioavailable N cycling: organic N (ON) depolymerization and solubilization; bioavailable N sorption and desorption on mineral surfaces; and microbial ON turnover including assimilation, mineralization, and the recycling of microbial products. Consideration of the balance between these processes provides insight into the sources, sinks, and flux rates of bioavailable N. By accounting for interactions among the biological, physical, and chemical controls over ON and its availability to plants and microbes, our conceptual model unifies complex mechanisms of ON transformation in a concrete conceptual framework that is amenable to experimental testing and translates into ideas for new management practices. This framework will allow researchers and practitioners to use common measurements of particulate organic matter (POM) and mineral-associated organic matter (MAOM) to design strategic organic N-cycle interventions that optimize ecosystem productivity and minimize environmental N loss. Supplementary Information The online version contains supplementary material available at 10.1007/s10533-021-00793-9.

Organic nitrogen (N) is abundant in soils, but early conceptual frameworks considered it nonessential for plant growth. It is now well recognised that plants have the potential to take up organic N. However, it is still unclear whether plants supplement their N requirements by taking up organic N in situ: at what rate is organic N diffusing towards roots and are plants taking it up? We combined microdialysis with live-root uptake experiments to measure amino acid speciation and diffusion rates towards roots of Eriophorum vaginatum. Amino acid diffusion rates (321 ng N cm-2 h-1 ) were c. 3× higher than those for inorganic N. Positively charged amino acids made up 68% of the N diffusing through soils compared with neutral and negatively charged amino acids. Live-root uptake experiments confirmed that amino acids are taken up by plants (up to 1 µg N g-1 min-1 potential net uptake). Amino acids must be considered when forecasting plant-available N, especially when they dominate the N supply, and when acidity favours proteolysis over net N mineralisation. Determining amino acid production pathways and supply rates will become increasingly important in projecting the extent and consequences of shrub expansion, especially considering the higher C : N ratio of plants relative to soil.

Soil organic carbon management has the potential to aid climate change mitigation through drawdown of atmospheric carbon dioxide. To be effective, such management must account for processes influencing carbon storage and re-emission at different space and time scales. Achieving this requires a conceptual advance in our understanding to link carbon dynamics from the scales at which processes occur to the scales at which decisions are made. Here, we propose that soil carbon persistence can be understood through the lens of decomposers as a result of functional complexity derived from the interplay between spatial and temporal variation of molecular diversity and composition. For example, co-location alone can determine whether a molecule is decomposed, with rapid changes in moisture leading to transport of organic matter and constraining the fitness of the microbial community, while greater molecular diversity may increase the metabolic demand of, and thus potentially limit, decomposition. This conceptual shift accounts for emergent behaviour of the microbial community and would enable soil carbon changes to be predicted without invoking recalcitrant carbon forms that have not been observed experimentally. Functional complexity as a driver of soil carbon persistence suggests soil management should be based on constant care rather than one-time action to lock away carbon in soils. Dynamic interactions between chemical and biological controls govern the stability of soil organic carbon and drive complex, emergent patterns in soil carbon persistence.

Soil drying and wetting cycles promote carbon (C) release through large heterotrophic respiration pulses at rewetting, known as Birch effect. Empirical evidence shows that drier conditions before rewetting and larger changes in soil moisture at rewetting cause larger respiration pulses. Because soil moisture varies in response to rainfall, also these respiration pulses depend on the random timing and intensity of precipitation. In addition to rewetting pulses, heterotrophic respiration continues during soil drying, eventually ceasing when soils are too dry to sustain microbial activity. The importance of respiration pulses in contributing to the overall soil respiration flux has been demonstrated empirically, but no theoretical investigation has so far evaluated how the relative contribution of these pulses may change along climatic gradients or as precipitation regimes shift in a given location. To fill this gap, we start by assuming that rewetting pulses and respiration rates during soil drying can be treated as random variables dependent on soil moisture fluctuations, and develop a stochastic model for soil heterotrophic respiration rates that analytically links the statistical properties of respiration to those of precipitation. Model results show that both the mean rewetting pulse respiration and the mean respiration during drying increase with increasing mean precipitation. However, the contribution of respiration pulses to the total heterotrophic respiration increases with decreasing precipitation frequency and to a lesser degree with decreasing precipitation depth, leading to an overall higher contribution of respiration pulses under future more intermittent and intense precipitation. Moreover, the variability of both components of soil respiration is also predicted to increase under these conditions. Therefore, our results suggest that with future more intermittent precipitation, respiration pulses and the associated nutrient release will intensify and become more variable, contributing more to soil biogeochemical cycling.

Engineered nanomaterials (ENMs) can enter agroecosystems because of their widespread use and disposal. Within soil, ENMs may affect legumes and their dinitrogen (N2) fixation, which are critical fo...

Soil organic matter (SOM) turnover increasingly is conceptualized as a tension between accessibility to microorganisms and protection from decomposition via physical and chemical association with minerals in emerging soil biogeochemical theory. Yet, these components are missing from the original mathematical models of belowground carbon dynamics and remain underrepresented in more recent compartmental models that separate SOM into discrete pools with differing turnover times. Thus, a gap currently exists between the emergent understanding of SOM dynamics and our ability to improve terrestrial biogeochemical projections that rely on the existing models. In this opinion paper, we portray the SOM paradigm as a triangle composed of three nodes: conceptual theory, analytical measurement, and numerical models. In successful approaches, we contend that the nodes are connected—models capture the essential features of dominant theories while measurement tools generate data adequate to parameterize and evaluate the models—and balanced—models can inspire new theories via emergent behaviors, pushing empiricists to devise new measurements. Many exciting advances recently pushed the boundaries on one or more nodes. However, newly integrated triangles have yet to coalesce. We conclude that our ability to incorporate mechanisms of microbial decomposition and physicochemical protection into predictions of SOM change is limited by current disconnections and imbalances among theory, measurement, and modeling. Opportunities to reintegrate the three components of the SOM paradigm exist by carefully considering their linkages and feedbacks at specific scales of observation.

Despite decades of research progress, ecologists are still debating which pools and fluxes provide nitrogen (N) to plants and soil microbes across different ecosystems. Depolymerization of soil organic N is recognized as the rate-limiting step in the production of bioavailable N, and it is generally assumed that detrital N is the main source. However, in many mineral soils, detrital polymers constitute a minor fraction of total soil organic N. The majority of organic N is associated with clay-sized particles where physicochemical interactions may limit the accessibility of N-containing compounds. Although mineral-associated organic matter (MAOM) has historically been considered a critical, but relatively passive, reservoir of soil N, a growing body of research now points to the dynamic nature of mineral-organic associations and their potential for destabilization. Here we synthesize evidence from biogeoscience and soil ecology to demonstrate how MAOM is an important, yet overlooked, mediator of bioavailable N, especially in the rhizosphere. We highlight several biochemical strategies that enable plants and microbes to disrupt mineral-organic interactions and access MAOM. In particular, root-deposited low-molecular-weight exudates may enhance the mobilization and solubilization of MAOM, increasing its bioavailability. However, the competitive balance between the possible fates of N monomers—bound to mineral surfaces versus dissolved and available for assimilation—will depend on the specific interaction between mineral properties, soil solution, mineral-bound organic matter, and microbes. Building off our emerging understanding of MAOM as a source of bioavailable N, we propose a revision of the Schimel and Bennett (Ecology 85:591–602, 2004) model (which emphasizes N depolymerization), by incorporating MAOM as a potential proximal mediator of bioavailable N.

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.

Earthworms and arbuscular mycorrhizal fungi (AMF) interact to regulate plant nitrogen (N) supply, but the mechanisms through which they affect plant N uptake are unclear. We hypothesized that earthworms, plants and the associated AMF exhibit different preferences for different forms of inorganic N (NH4+ and NO3−), which could regulate the effect of earthworms and AMF interaction on plant N acquisition. We outlined three independent but complementary experiments to test this hypothesis in the context of exotic earthworm, Pontoscolex corethrurus. The earthworm is dominating the plantation forests in subtropical and tropical regions of China, which have their understory dominated by the fern Dicranopteris dichotoma. By employing an excised root 15N incubation experiment and a field in situ 15N experiment, we found that the fern prefers to use NH4+ rather than NO3−. Then we did a 2 × 2 factorial microcosm experiment using AMF (Rhizophagus intraradices) and earthworms (P. corethrurus). The exotic earthworm increased soil NH4+ concentration but did not affect soil NO3− concentration, while the AMF decreased soil NH4+ concentration but had no effect on soil NO3− concentration. The increase in soil NH4+ induced by the earthworms was efficiently utilized by the AMF, and significantly increased the total N uptake by the fern. In contrast, the AMF alone increased the N concentration of leaves and coarse roots, but not the total plant N uptake, primarily due to the lower levels of available NH4+ compared with the earthworm treatments. The uninoculated fern did not benefit from the earthworm-induced increase in soil NH4+, suggesting that the root of the fern cannot access the ‘NH4+ hotspots’ created by the earthworms. Our work suggests that successful cooperation of earthworms and AMF on plant N uptake depends on the correct match in N-form.

Drought is common in soil, yet its intensity and immediate effects vary with the timing of precipitation and the depth in the soil profile. We set out to analyze the patterns of soil C and N dynamics through the dry summer in a California annual grassland, and to analyze the processes that control those dynamics. During the dry season, inorganic N, soluble organic matter, and microbial biomass accumulated in soil. Concentrations were generally greater in the surface soil layer (0–10 cm) but increased in both surface and deeper soil (10–20 cm) over the course of the summer. There was a positive relationship between drought length, microbial biomass C and N, extractable C and N, and microbial activity upon rewetting. Upon rewetting of dry soils there was a pulse in nitrate availability (under field conditions) and, in the top 10 cm, a positive correlation between the initial CO2 pulse released upon rewetting and the drought length. In contrast, below 10 cm there was no correlation between the rewetting CO2 pulse and drought length. Soils from the top 10 cm and below 10 cm dried under controlled laboratory conditions showed the same CO2 pulse response to the length of drying as those in the field. These results suggest that the substrates that accumulate during the dry season are relatively labile, and that, in surface soils, biomass/substrate amounts and water availability control their persistence, while in deeper layers, some other factor is more important.

Drying and rewetting of soils triggers a cascade of physical, chemical, and biological processes; understanding these responses to varying moisture levels becomes increasingly important in the context of changing precipitation patterns. When soils dry and water content decreases, diffusion is limited and substrates can accumulate. Upon rewetting, these substrates are mobilized and can energize hot moments of intense biogeochemical cycling, leading to pulses of trace gas emissions. Until recently, it was difficult to follow the rewetting dynamics of nutrient cycling in the field without physically disturbing the soil. Here we present a study that combines real-time trace gas measurements with high-resolution measurements of diffusive nutrient fluxes in intact soils. Our goal was to distinguish the contribution of different inorganic and organic nitrogen (N) forms to the rewetting substrate flush and the production of nitric oxide (NO) and nitrous oxide (N 2 O). Diffusive flux of N-bearing substrates (NO 2 − , NO 3 − , NH 4 + and amino acids) was determined in situ in hourly resolution using a microdialysis approach. We conducted an irrigation experiment in a semi-arid California grassland at the end of the dry season, and followed soil N flux and N trace gas emissions over the course of 30 h post-wetting. Upon rewetting, both inorganic and organic N diffused through the soil, with inorganic N contributing most to the rewetting N flush. Emissions of NO and N 2 O rapidly increased and remained elevated for the duration of our measurements, whereas diffusive soil N flux was characterized by large temporal variation. Immediately after rewetting, NO 3 − contributed 80% to the total diffusive N flux but was consumed rapidly, possibly due to fast microbial uptake or denitrification. Ammonium flux contributed only ∼10% to the initial diffusive N flux, but it dominated total N diffusion 27 h post-wetting, coinciding with peak N-gas emissions. This suggests nitrification may control most of the N trace gases produced during the late stages of a rewetting pulse. Nitrite contributed only 1% to total N diffusion and did not show a clear temporal pattern. Amino acids contributed roughly as much as NH 4 + to the initial diffusive N flux, but the organic N pulse was short-lived, indicating that organic N did not contribute substantially to N-gas formation shortly after rewetting at our study site. Our results support the hypothesis that in semi-arid environments N-bearing substrates concentrate during dry periods and, upon rewetting, can lead to pulses of NO and N 2 O when they react chemically or are transformed by microorganisms.

Extracellular enzymes in soil are central to the decomposition of plant and microbial detritus and they are increasingly incorporated into soil biogeochemical models as drivers of detritus breakdown. In enzyme-driven models, a critical parameter is the functional lifespan of the enzymes, yet this is poorly constrained by experimental data. We evaluated how long soil enzymes remain active in five soils spanning from arctic tussock tundra to a tropical forest/grassland soil. We incubated soils under continuous fumigation with CHCl 3 vapor to kill microbes and prevent the synthesis of new enzymes. We monitored the activities of six hydrolytic and two oxidative enzymes over a 12-week incubation. Initial activities of the various soil enzymes varied substantially across the ecosystems; they were generally highest (per gram soil) in the tussock tundra, followed by temperate hardwood forest, grassland, tropical forest/grassland, and then chaparral. In tussock tundra organic soil, activities of all enzymes decreased rapidly following first-order decay curves; the half-lives of enzyme activity were typically several weeks. In the mineral soils, the time course of loss of hydrolytic enzyme activities could always be described by a first-order decay curve, but in many cases, activity could equally be described by a zero-order, linear, decay function because the rate of loss was slow. Although α-glucosidase lost activity rapidly, for other enzymes substantial activity remained even after 12 weeks of incubation. This likely resulted from stabilization by mineral surfaces, stabilization that might constrain activity against native polymeric substrates. Measured turnover rate constants fell within the broad range that model have used, but that range remains exceedingly broad.

Wetting dry soil triggers emission of nitric oxide (NO), which can harm air quality and influence ecosystem N balance. NO released after wetting may be derived from two primary mechanisms: (1) abiotic decomposition of nitrite (NO2−) present in the dry soil and (2) biologically-mediated production of NO. The relative contributions of these mechanisms to NO emission are challenging to partition. To differentiate the two mechanisms, we made continuous observations of NO emission from a range of soils that varied in texture, pH, and exchangeable cations. We coupled NO gas exchange measurements with automated, non-destructive high-resolution sampling of inorganic N species via microdialysis. We identified two distinct phases of NO production (1) an initial phase that reached its maximum less than 30 min after rewetting; and (2) a second phase lasting up to 36 h. The magnitude of the initial phase of NO production was correlated with the initial nitrous acid concentration (calculated from NO2− concentrations and soil pH) and soil exchangeable calcium. The second phase of NO emission coincided with a peak in NO2− that preceded NO3− accumulation. Together, these results suggest that NO emissions following wetting are defined by an initial pulse of abiotic NO formation mediated by soil acidity at mineral surfaces, followed by a prolonged pulse of NO emission driven by a lag between the biological processes of ammonia oxidation and nitrite oxidation. These results can inform mechanistic models of NO emissions from soils to improve predictions of the terrestrial N cycle and atmospheric chemistry.

Rapid arctic vegetation change as a result of global warming includes an increase in the cover and biomass of deciduous shrubs. Increases in shrub abundance will result in a proportional increase of shrub litter in the litter community, potentially affecting carbon turnover rates in arctic ecosystems. We investigated the effects of leaf and root litter of a deciduous shrub, Betula nana, on decomposition, by examining species-specific decomposition patterns, as well as effects of Betula litter on the decomposition of other species. We conducted a 2-yr decomposition experiment in moist acidic tundra in northern Alaska, where we decomposed three tundra species (Vaccinium vitis-idaea, Rhododendron palustre, and Eriophorum vaginatum) alone and in combination with Betula litter. Decomposition patterns for leaf and root litter were determined using three different measures of decomposition (mass loss, respiration, extracellular enzyme activity). We report faster decomposition of Betula leaf litter compared to other species, with support for species differences coming from all three measures of decomposition. Mixing effects were less consistent among the measures, with negative mixing effects shown only for mass loss. In contrast, there were few species differences or mixing effects for root decomposition. Overall, we attribute longer-term litter mass loss patterns to patterns created by early decomposition processes in the first winter. We note numerous differences for species patterns between leaf and root decomposition, indicating that conclusions from leaf litter experiments should not be extrapolated to below-ground decomposition. The high decomposition rates of Betula leaf litter aboveground, and relatively similar decomposition rates of multiple species below, suggest a potential for increases in turnover in the fast-decomposing carbon pool of leaves and fine roots as the dominance of deciduous shrubs in the Arctic increases, but this outcome may be tempered by negative litter mixing effects during the early stages of encroachment.

In the arctic tundra, N controls the productivity and composition of tundra plant communities. In the Alaskan tundra, microbes only ever appear to be N-limited in Eriophorum vaginatum -dominated tussock tundra, and that only during the summer growing season; during winter microbes continue to respire but show no evidence of N-limitation. What drives the shifts in microbial N-limitation and how are they developed by the metabolic pathways involved in processing available soil organic compounds? To answer these questions, we collected soils from tussock and shrub tundra during four seasons and incubated them with 13 C-labeled glucose, glutamate, and protein; then we measured 13 C in CO 2 , microbial biomass, and specific phospholipid fatty acids (PLFAs). By analyzing 13 C-PLFAs, we were able to assess whether different groups of microbes processed the substrates differently, and to assess changes in N-use brought on by the Arctic winter. Monomers were metabolized during both seasons. In tussock tundra, glutamate-C was assimilated into PLFAs more extensively in winter than summer, suggesting glutamate was used as a C-source during winter but as a N-source during summer. In shrub soils, the flow of C from glutamate to PLFAs tracked with glucose year-round suggesting that the communities were primarily using glutamate as a C source. These results parallel biogeochemical evidence showing shifts in N availability and limitation. Protein metabolism was negligible in winter in soils other than tussocks. In summer, protein was broken down and all communities incorporated C; however, fungi did not assimilate protein-C at all while Gram-positive bacteria appeared to be proteolytic specialists. The different patterns of metabolism of C vs. N containing compounds across microbial groups regulate the dynamics of both soil communities and of soil carbon and nitrogen in tundra soils. As such, the flow of 13 C into different biomarker PLFAs provided a lens to evaluate the shifting dynamics of microbial communities and of the resource environment in which they find themselves.

With increasing use, manufactured nanomaterials (MNMs) may enter soils and impact agriculture. Herein, soybean (Glycine max) was grown in soil amended with either nano-CeO2 (0.1, 0.5, or 1.0gkg-1 soil) or nano-ZnO (0.05, 0.1, or 0.5gkg-1 soil). Leaf chlorosis, necrosis, and photosystem II (PSII) quantum efficiency were monitored during plant growth. Seed protein and protein carbonyl, plus leaf chlorophyll, reactive oxygen species (ROS), lipid peroxidation, and genotoxicity were measured for plants at harvest. Neither PSII quantum efficiency, seed protein, nor protein carbonyl indicated negative MNM effects. However, increased ROS, lipid peroxidation, and visible damage, along with decreased total chlorophyll concentrations, were observed for soybean leaves in the nano-CeO2 treatments. These effects correlated to aboveground leaf, pod, and stem production, and to root nodule N2 fixation potential. Soybeans grown in soil amended with nano-ZnO maintained growth, yield, and N2 fixation potential similarly to the controls, without increased leaf ROS or lipid peroxidation. Leaf damage was observed for the nano-ZnO treatments, and genotoxicity appeared for the highest nano-ZnO treatment, but only for one plant. Total chlorophyll concentrations decreased with increasing leaf Zn concentration, which was attributable to zinc complexes-not nano-ZnO-in the leaves. Overall, nano-ZnO and nano-CeO2 amended to soils differentially triggered aboveground soybean leaf stress and damage. However, the consequences of leaf stress and damage to N2 fixation, plant growth, and yield were only observed for nano-CeO2.

Climate change models predict that interannual rainfall variability will increase in California over the next several decades; these changes will likely influence how frequently California ecosystems burn and how they respond to fire. Fires uncouple N mobilization from uptake by destroying plant biomass and increasing nitrification. Following fire, autumn and winter rains can leach N into streams from slopes that have been denuded. The amount of N exported depends on how rapidly soil microbes metabolize it into mobile forms such as NO3− , and the rate that recovering plants take up available N. However, the long-term effects of a changing climate on postfire N dynamics remain unknown. We used the ecohydrologic model RHESSys (regional hydro-ecologic simulation system) to evaluate how interannual climate variability may affect the magnitude of N mineralization, nitrification, N export, and plant recovery following fire. N export was highest when fire was followed by drought; even though there was less water moving through the system, dry conditions prolonged the period during which N mobilization was decoupled from plant uptake. We also found that the effects of drought on N export were magnified in stands dominated by obligate seeders, which initially recovered more slowly than resprouters. These findings suggest that climate may regulate N balance most powerfully by influencing how quickly plants “turn on” and begin to immobilize N.

Mediterranean-type ecosystems are structured by fire. In California chaparral, fires uncouple inorganic nitrogen (N) production and consumption by enhancing nitrification and reducing plant uptake. Nitrate (NO3−) that accumulates after fire is vulnerable to leaching. However, the controls over N metabolism can change as chaparral ecosystems recover from fire, and these mechanisms are not well understood. To evaluate how ammonium (NH4+) availability, pH, and char influence N cycling, we measured inorganic N concentration and microbial biomass in chaparral soils that burned 1.5, 4, 23, and 49 years prior to sampling. We then experimentally adjusted NH4+ concentration, pH, and char content for all soils in a factorial design, and incubated them for 8 weeks. Each week, we measured respiration, exchangeable NH4+ and NO3− content, nitrification potential, microbial biomass, and pH. Within each watershed, NO3− was higher in soils collected from recently burned sites, than in soils from neighboring mature sites. Also, NO3− concentrations increased over the course of incubation in soils from all sites, especially with the addition of NH4+. When NH4+ was sufficiently high, pH determined the relative proportion of inorganic N that was nitrified. Char did not have a strong impact on N cycling, but in soils from 23- and 49-year-old sites, char treatments reduced the decline of microbial biomass C and N relative to control and fertilized treatments, perhaps by supplying C to soil microbes. These findings suggest that under some circumstances char may decelerate N cycling if it promotes immobilization and limits NH4+ supply to nitrifiers.

Mechanistic descriptions of microbial processes are difficult to embed in ecosystem models because they require complex mathematical formulations. The interactions between microbes, soil carbon (C), and water availability are particularly complex, as they involve coupled physical (advection and diffusion in unsaturated media) and biochemical processes (enzymatic reactions, C uptake by microbes). Here we propose an approximated equation based on a quasi-equilibrium assumption that describes microbial uptake of soil C as a function of soil moisture and organic matter content during soil drying. The equation predicts that uptake depends on two terms, one dependent on soil organic C concentration and enzyme availability (analogous to a Michaelis–Menten equation) and one dependent on soil moisture via its effects on enzyme and solute mass transfer, and microbial uptake kinetics. Assuming that uptake is proportional to microbial respiration, model results are compared to measured respiration–water potential curves. Using independently estimated parameter values (except for the calibrated microbial uptake efficiency), the theoretical model captures well the respiration decline during drying and provides an explanation of respiration pulses at rewetting. Thus, this simple formulation could be employed in ecosystem models as an alternative to empirical respiration–moisture response functions.

Fire is a fundamental reorganizing force in chaparral and other Mediterranean-type ecosystems. Postfire nutrient redistribution and cycling are frequently invoked as drivers of ecosystem recovery. The extent to which N is transported from slopes to streams following fire is a function of the balance between the rate at which soil microbes retain and metabolize N into forms that readily dissolve or leach, and how rapidly recovering plants sequester this mobilized N. To better understand how fire impacts this balance, we sampled soil and plant N dynamics in 17 plots distributed across two burned, chaparral-dominated watersheds in Santa Barbara County, California. We measured a variety of ecosystem properties in both burned and unburned plots on a periodic basis for 2 years, including soil water content, pH, soil and plant carbon and nitrogen, extractable inorganic nitrogen, dissolved organic nitrogen, and microbial biomass. In burned plots, nitrification was significantly enhanced relative to rates measured in unburned plots. Ephemeral herbs established quickly following the first postfire rain events. Aboveground plant biomass assimilated N commensurate with soil net mineralization, implying tight N cycling during the early stages of recovery. Microbial biomass N, on the other hand, remained low throughout the study. These findings highlight the importance of herbaceous species in conserving ecosystem nutrients as shrubs gradually recover.

In an era of increasingly multidisciplinary science, it is essential to identify the frontiers as well as the core of an inherently holistic discipline: ecosystem ecology. To achieve this, we led a series of town hall events at multiple scientific-society meetings over a two-year period followed by a workshop with a diverse set of ecosystem scientists to review and expand on those outcomes. For the society town hall events ~70 individuals were asked to give short, provocative (the so-called, soapbox) presentations and audience members (~250) filled out tailored surveys. Both presentations and surveys were transcribed and themes were extracted and analyzed before and during the follow-up workshop. Formal ethnographic analysis of the soapbox texts produced three major themes: “frontiers,” “capacity building,” and “barriers to implementation,” including several subthemes. A workshop was held to analyze the ethnographic data where workshop participants further grouped key frontiers as (1) rethinking the drivers of ecosystem change, (2) new insights into ecosystem process and function, (3) evaluating human dimensions of ecosystem ecology, and (4) new angles on problem-solving/applied research. In addition, 13 experts were interviewed to crosscheck interpretations. The survey data, workshop deliberations, and expert interviews suggest that the core of these frontiers defines the current state and provides the foundational knowledge that bounds ecosystem ecology as a discipline. In response to emerging complex environmental issues and ongoing socioecological challenges, the edges of these frontiers expand fundamental ecosystem ecology to engage and intersect with disciplinary realms to create new ways of making sense of complexity, and to develop an even more holistic understanding of ecological systems. In this paper, we present our synthesis of the frontier and core research themes with the goal of inspiring the next wave of studies in ecosystem ecology.

Wetting dry soil triggers a pulse of microbial respiration. Respiration pulses after wetting can be complex, evolving through several stages before subsiding—possibly reflecting contributions from multiple carbon (C) sources. We hypothesized that respiration after wetting combines C from cellular sources (notably trehalose, a cellular osmo-protectant) and soluble extracellular C made available by the physical effects of wetting, with cellular sources contributing early during the pulse and extracellular C dominating later. We tested this hypothesis by making measurements of soil respiration, microbial biomass, trehalose, and water extractable organic C after wetting dry soil in the laboratory. We then devised several alternative minimal models to evaluate the potential contributions of trehalose and extracellular C to the respiration pulse. We found that the chloroform-labile fraction of the microbial biomass was comprised of 50% trehalose in dry soil. Both trehalose and water extractable organic C pools declined rapidly after wetting; trehalose disappeared within 3 h of water addition. Chloroform-labile glucose increased 3 h after water addition and then declined, suggesting rapid hydrolysis of trehalose and consumption of the resulting glucose. Respiration dynamics after wetting could be best explained by models that included two C sources, combining rapid metabolism of trehalose with slower consumption of extracellular soluble C. These results suggest that pulses of respiration after wetting reflect both osmolyte consumption within the microbial biomass and enhanced extracellular C availability; hence, models of microbial drought response must account for both processes to fully resolve soil C dynamics.

Carbon use efficiency (CUE), the proportion of carbon (C) consumed by microbes that is converted into biomass, is an important parameter for soil C models with explicit microbial controls. While often considered as a single parameter, CUE is an emergent property of multiple microbial processes, including assimilation efficiency, biomass-specific respiration, enzyme production, and respiratory costs of enzyme production. These processes occur over variable time scales and imply different fates for C, and the same emergent CUE value can result when C is allocated in fundamentally different ways (e.g. a high investment in enzyme production vs. a high assimilation cost). We developed a model that represents the individual processes underlying emergent CUE to test how shifts in microbial allocation alter equilibrium soil C pool sizes. We found that an increase in emergent CUE that results from a change in assimilation efficiency, biomass specific respiration, or respiration costs from enzyme production causes soil organic C (SOC) to decline, while the same change in emergent CUE resulting from a change in enzyme production causes SOC to increase. We also used the model to test the sensitivity of CUE from isotopic C tracer estimates to changes in microbial allocation processes. We found that these estimates do not account for the same microbial processes represented by emergent CUE in models. We propose that considering microbial processes explicitly rather than representing CUE as a single parameter can improve data-model integration. In addition, modeling microbial processes explicitly will account for a wider range of possible outcomes from shifts in microbial C allocation, such as when increased SOC results from increasing CUE.

Carbonaceous nanomaterials (CNMs) can affect agricultural soil prokaryotic communities, but how the effects vary with the crop growth stage is unknown. To investigate this, soybean plants were cultivated in soils amended with 0, 0.1, 100, or 1000 mg kg–1 of carbon black, multiwalled carbon nanotubes (MWCNTs), or graphene. Soil prokaryotic communities were analyzed by Illumina sequencing at day 0 and at the soybean vegetative and reproductive stages. The sequencing data were functionally annotated using the functional annotation of prokaryotic taxa (FAPROTAX) database. The prokaryotic communities were unaffected at day 0 and were altered at the plant vegetative stage only by 0.1 mg kg–1 MWCNTs. However, at the reproductive stage, when pods were filling, most treatments (except 1000 mg kg–1 MWCNTs) altered the prokaryotic community composition, including functional groups associated with C, N, and S cycling. The lower doses of CNMs, which were previously shown to be less agglomerated and thus more bioavaila...

Soil moisture controls microbial activity and soil carbon cycling. Because microbial activity decreases as soils dry, decomposition of soil organic matter (SOM) is thought to decrease with increasing drought length. Yet, microbial biomass and a pool of water-extractable organic carbon (WEOC) can increase as soils dry, perhaps implying microbes may continue to break down SOM even if drought stressed. Here, we test the hypothesis that WEOC increases as soils dry because exoenzymes continue to break down litter, while their products accumulate because they cannot diffuse to microbes. To test this hypothesis, we manipulated field plots by cutting off litter inputs and by irrigating and excluding precipitation inputs to extend or shorten the length of the dry season. We expected that the longer the soils would remain dry, the more WEOC would accumulate in the presence of litter, whereas shortening the length of the dry season, or cutting off litter inputs, would reduce WEOC accumulation. Lastly, we incubated grass roots in the laboratory and measured the concentration of reducing sugars and potential hydrolytic enzyme activities, strictly to understand the mechanisms whereby exoenzymes break down litter over the dry season. As expected, extending dry season length increased WEOC concentrations by 30% above the 108 μg C/g measured in untreated plots, whereas keeping soils moist prevented WEOC from accumulating. Contrary to our hypothesis, excluding plant litter inputs actually increased WEOC concentrations by 40% above the 105 μg C/g measured in plots with plants. Reducing sugars did not accumulate in dry senesced roots in our laboratory incubation. Potential rates of reducing sugar production by hydrolytic enzymes ranged from 0.7 to 10 μmol·g-1 ·h-1 and far exceeded the rates of reducing sugar accumulation (~0.001 μmol·g-1 ·h-1 ). Our observations do not support the hypothesis that exoenzymes continue to break down litter to produce WEOC in dry soils. Instead, we develop the argument that physical processes are more likely to govern short-term WEOC dynamics via slaking of microaggregates that stabilize SOM and through WEOC redistribution when soils wet up, as well as through less understood effects of drought on the soil mineral matrix.

© Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA. All rights reserved. Soil nitrite (NO2 -) is an important source of nitrous acid to the atmosphere as well an intermediate in nitrification and denitrification. Few studies, however, have directly linked NO2- pools with N emissions because NO2 - is reactive and seldom detectable in soils. Here, we test whether the elusiveness of soil NO2 - is due to its reactivity or to problems associated with conventional 2 M KCl extractions. We extracted acidic, neutral, and alkaline soils (pH 5.4- 8.2) in 2 M KCl, pH-8-adjusted 2 M KCl, and deionized water (DIW). Unbuffered KCl consistently underestimated soil NO2 - compared with DIW; soils with lower pH had lower NO2 - in unbuffered KCl than in DIW water. In acidic soils, unbuffered KCl favored the transformation of NO2 - to nitric oxide. Because KCl lowers the pH of extracts by ~1 unit, this increase in acidity likely favored the transformation of NO2 - to gaseous N products. Although buffered KCl minimizes NO2 - destruction, it can cause colorimetric interferences when done on small soil samples (4 g). Deionized water extractions offer an alternative for measuring NO2 - in small samples, but filtering beyond the traditionally used 2.5-mm filter paper is necessary to remove suspended solids. Despite its widespread use, unbuffered 2 M KCl should not be used for the analysis of soil NO2 -.

The potential effects of carbonaceous nanomaterials (CNMs) on agricultural plants are of concern. However, little research has been performed using plants cultivated to maturity in soils contaminated with various CNMs at different concentrations. Here, we grew soybean for 39 days to seed production in soil amended with 0.1, 100, or 1000 mg kg-1 of either multiwalled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), or carbon black (CB) and studied plant growth, nodulation, and dinitrogen (N2) fixation potential. Plants in all CNM treatments flowered earlier (producing 60% to 372% more flowers when reproduction started) than the unamended controls. The low MWCNT-treated plants were shorter (by 15%) with slower leaf cover expansion (by 26%) and less final leaf area (by 24%) than the controls. Nodulation and N2 fixation potential appeared negatively impacted by CNMs, with stronger effects at lower CNM concentrations. All CNM treatments reduced the whole-plant N2 fixation potential, with the highest reductions (by over 91%) in the low and medium CB and the low MWCNT treatments. CB and GNPs appeared to accumulate inside nodules as observed by transmission electron microscopy. CNM dispersal in aqueous soil extracts was studied to explain the inverse dose-response relationships, showing that CNMs at higher concentrations were more agglomerated (over 90% CNMs settled as agglomerates >3 μm after 12 h) and therefore proportionally less bioavailable. Overall, our findings suggest that lower concentrations of CNMs in soils could be more impactful to leguminous N2 fixation, owing to greater CNM dispersal and therefore increased bioavailability at lower concentrations.

Decomposition rate constants were measured for boles of 155 large dead trees (>10 cm diameter) in central Amazon forests. Mortality data from 21 ha of permanent inventory plots, monitored for 10–15 years, were used to select dead trees for sampling. Measured rate constants varied by over 1.5 orders of magnitude (0.015–0.67 year–1), averaging 0.19 year–1 with predicted error of 0.026 year. Wood density and bole diameter were significantly and inversely correlated with rate constants. A tree of average biomass was predicted to decompose at 0.17 year–1. Based on mortality data, an average of 7.0 trees ha–1 year–1 died producing 3.6 Mg ha–1 year–1 of coarse litter (>10 cm diameter). Mean coarse litter standing-stocks were predicted to be 21 Mg ha–1, with a mean residence time of 5.9 years, and a maximum mean carbon flux to the atmosphere of 1.8 Mg C ha–1 year–1. Total litter is estimated to be partitioned into 16% fine wood, 30% coarse wood, and 54% non-woody litter (e.g., leaves, fruits, flowers). Decomposition rate constants for coarse litter were compiled from 20 globally distributed studies. Rates were highly correlated with mean annual temperature, giving a respiration quotient (Q 10) of 2.4 (10°C–1).

Soil microbes face highly variable moisture conditions that force them to develop adaptations to tolerate or avoid drought. Drought conditions also limit the supply of vital substrates by inhibiting diffusion in dry conditions. How these biological and physical factors affect carbon (C) cycling in soils is addressed here by means of a novel process-based model. The model accounts for different microbial response strategies, including different modes of osmoregulation, drought avoidance through dormancy, and extra-cellular enzyme production. Diffusion limitations induced by low moisture levels for both extracellular enzymes and solutes are also described and coupled to the biological responses. Alternative microbial life-history strategies, each encoded in a set of model parameters, are considered and their effects on C cycling assessed both in the long term (steady state analysis) and in the short term (transient analysis during soil drying and rewetting). Drought resistance achieved by active osmoregulation requiring large C investment is not useful in soils where growth in dry conditions is limited by C supply. In contrast, dormancy followed by rapid reactivation upon rewetting seems to be a better strategy in such conditions. Synthesizing more enzymes may also be advantageous because it causes larger accumulation of depolymerized products during dry periods that can be used upon rewetting. Based on key model parameters, a spectrum of life-history strategies thus emerges, providing a possible classification of microbial responses to drought.

Manufactured nanomaterials (MNMs) are increasingly incorporated into everyday products and thus are entering the environment via manufacturing, product use, and waste disposal. Still, understanding MNM environmental hazards and fates lags MNM industry growth. To catch up, keep pace, and influence future MNM safe design strategies, rapid safety assessments are needed. Bacteria are important ecological nanotoxicology targets to consider when assessing MNM safety: bacteria are exposed to MNMs in water, sewage, soils, and sediments, wherein they influence MNM fates; bacteria can also be impacted-with potential health and ecosystem consequences. Routinely using bacteria for assessing MNMs would promote effective management of the environmental risks of this rapidly growing industry, but appropriate protocols and policies for this assessment need to be instituted.