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The interaction of organic carbon (OC) with clay and metals stabilizes soil carbon (C), but the influence of specific clay-metal-OC assemblages (flocs) needs further evaluation. This study aimed to investigate the stability of flocs in soil as affected by external C inputs. Flocs representing OC-mineral soil fractions were synthesized using dissolved organic C (DOC) combined with kaolinite (1:1 layer structure) or montmorillonite (2:1 layer structure) clays in the absence or presence of two levels of Fe (III) (named low or high Fe). Flocs were mixed with soil (classified as Luvisol) and incubated with or without 13C labelled plant residue (i.e., ryegrass) for 30 days. The CO2 emissions and DOC concentrations as well as their 13C signatures from all treatments were examined. Total C mineralization from flocs was approximately 70% lower than non-flocced DOC. The flocs made with montmorillonite had 16–43% lower C mineralization rate than those made with kaolinite with no Fe or low Fe. However, when flocs were made with high Fe, clay mineralogy did not significantly affect total C mineralization. A positive priming effect (PE) of flocs on native soil OC was observed in all treatments, with a stronger PE found in lower Fe treatments. The high-Fe clay flocs inhibited ryegrass decomposition, while the flocs made without clay had no impact on it. Interestingly, flocs significantly decreased the PE of ryegrass on native soil OC decomposition. These results indicate that the adsorption of DOC onto clay minerals in the presence of Fe (III) stabilizes it against decomposition processes and its stability increases as Fe in flocs increases. Flocs also protect soil OC from the PE of external degradable plant C input. This study showed that Fe level and clay mineralogy play an important role in controlling soil C stability.

An adequate supply of bioavailable nitrogen (N) is critical to soil microbial communities and plants. Over the last decades, research efforts have rarely considered the importance of reactive iron (Fe) minerals in the processes that produce or consume bioavailable N in soils, compared to other factors such as soil texture, pH, and organic matter (OM). However, Fe is involved in both enzymatic and non-enzymatic reactions that influence the N cycle. More broadly, reactive Fe minerals restrict soil organic matter (SOM) cycling through sorption processes, but also promote SOM decomposition and denitrification in anoxic conditions. By synthesizing available research, we show that Fe plays diverse roles in N bioavailability. Fe affects N bioavailability directly by acting as a sorbent, catalyst, and electron transfer agent, or indirectly by promoting certain soil features, such as aggregate formation and stability, which affect N turnover processes. These roles can lead to different outcomes on N bioavailability, depending on environmental conditions such as soil redox shifts during wet-dry cycles. We provide examples of Fe-N interactions and discuss the possible underlying mechanisms, which can be abiotic or microbially meditated. We also discuss methodological constraints that hinder the development of mechanistic understanding of Fe in controlling N bioavailability and highlight the areas of needed research.

This research aimed to estimate methane (CH4) and nitrous oxide (N2O) fluxes and subsequent global warming potential (GWP) for a Douglas fir (Pseudotsuga menziesii) bark-based substrate production ...

Soil nitrous oxide (N2O) is an important source of greenhouse gas contributing to climate change. Many processes produce N2O in soils and the production rate of each process is affected variably by climatic-edaphic factors, making the soil-to-atmosphere N2O flux extremely dynamic. Experimental approaches, including natural and enriched isotopic methods, have been developed to separate and quantify the N2O production from different processes. However, these methods are often costly or difficult to conduct, hampering their widely applications. This study aimed to develop a mechanistic model quantifying the soil N2O production from nitrifier nitrification (NN), nitrifier denitrification (ND), and heterotrophic denitrification (HD), which are considered as the most important biological contributors, and to investigate how climatic-edaphic factors affect individual processes as well as total N2O production rates. The developed model demonstrated its robustness and capability by reliably reproducing N2O productions from the three individual processes of NN, ND, and HD under different moisture contents and oxygen concentrations. The model simulations unraveled how environmental conditions and soil properties controlled the total N2O production rate by regulating individual rates variably. Therefore, the mechanistic model is able to potentially elucidate the large spatiotemporal variances of in-situ soil N2O flux and improve the assessment of soil N2O emission at regional and global scales.

Comammox, one nitrifying microorganism carries out the complete oxidation of ammonia to nitrate, have been recently discovered, and are found in a wide range of environments, including soil. However, conditions under which they actually contribute to nitrification in soil have not yet been demonstrated. By 13CO2-based DNA stable isotope probing with real-time quantitative PCR and gene sequence, we reported two uncultured strains, which are closely related to comammox Nitrospira clade B, autotrophically grew in both forest and paddy soils only in the absence of ammonium amendment. Furthermore, all clade B amoA sequences amplified from isotopically enriched genomic DNA in both soils were derived from one or two phylotypes, indicating a low diversity of active comammox strains in soils.

Soil redox is a critical environmental factor shaping the microbial community structure and ultimately alters the nutrient cycling. However, the response of soil microbial community structure to prolonged or repeated redox fluctuations is not yet clear. To study the dynamic effects of prolonged redox disturbances to the soil microbial community structure, soil samples experiencing 8, 5 and 0 alternating oxic-anoxic cycles within approximately 6 months each year were collected and the microbial community structure were evaluated using phospholipid fatty acid analysis (PLFA) profiles. Prolonged redox disturbances had significant effects on soil physiochemical properties and soil microbial community structure. The relative abundance of straight chain saturated PLFAs, cyclopropyl, and terminal- and mid-branched chain saturated PLFAs increased due to prolonged redox disturbances, but there was a consistent decrease in linear monounsaturated PLFAs and polyunsaturated PLFAs in the fluctuating zone. Prolonged redox disturbances had a negative impact on the total PLFA content (a proxy for biomass). Both the fluctuating zone (8-cycle and 5-cycle plots) and the never flooded zone (0-cycle plots) were dominated by Gram-positive bacteria and a low content of fungi, actinomycetes and protozoa. The fungi and protozoa abundance decreased significantly with an increase in the occurrence of alternating flooding-dry events, suggesting that the prolonged redox disturbance leads to high stress on the fungi and protozoa populations. Moreover, total organic matter (TOC) and C:N ratio, environmental factors that can be influenced by recurring redox fluctuations, also influenced the microbial community structure.

Microbial biomass (MB) plays a critical role in residue decomposition and soil organic matter (SOM) turnover. We investigated the effects of the initial size of soil MB pool and nitrogen (N) availability on the incorporation of ryegrass residue carbon (C) into microbial biomass C (MBC) and dissolved organic C (DOC). Soils from a row crop system (CS) and an adjacent grass sod (GS) were preincubated with (i.e., increased MB) and without (i.e., unchanged MB) glucose to produce soils with two levels of initial MB size before residue additions. Ammonium sulfate was added to test the effect of N availability. Residue addition increased DOC production in both soils, regardless of initial MB size and N availability, contributing 9 to ∼22% to the total pool. Residue quality was observed to affect the incorporation of residue C into DOC, which depended on soil type and initial MB size. However, the assimilation of residue C into MBC was not affected by the quality of ryegrass residue. Compared with the control (which did not have residue and N additions), a significant increase in SOM-derived MBC by residue addition was observed in CS without glucose preincubation but not in the glucose preincubated CS and in GS. This indicated that the primed MBC by residue addition depended on initial MB size and soil type. The assimilation of residue C into MB was marginally inhibited by the preincubation with glucose in GS but was promoted in CS. With the addition of extra N, higher ryegrass-derived MBC was observed in CS with glucose preincubation compared with the corresponding treatments without preincubation, which was not found in GS. These results suggested that residue-derived MBC was not only regulated by initial MB size and N availability but also was affected by soil management history. However, N addition reduced ryegrass-derived DOC production in GS without glucose preincubation. Apparently, both soil MB size and N availability largely affected the assimilation and incorporation of residue C in soil labile pools (i.e., DOC and MBC), and the extent of this relationship varied between two agricultural soils.

Soil pH is often changed by anthropogenic activities such as agronomic management, land use or acidifying pollution, and is widely considered to be a dominant factor affecting soil nitrogen cycling. In this study, three purple soils originating from similar parent materials but varying in pH (5.7, 7.3, and 8.0), representing acidic, neutral and alkaline soils, were selected to determine the effect of background pH on net nitrification rate (NNR) and ammonia oxidizers (AOA and AOB). The background pH of each soil was modified to the pH of the other soils to investigate the effect of adjusted pH on NNR and ammonia oxidizers. Net nitrification rates varied significantly with adjusted pH in neutral soils but did not change in acidic and alkaline soils, suggesting that soil at neutral pH is more sensitive to changes in nitrification. The AOB abundance increased in neutral soils adjusted to high pH, whereas AOA decreased with increased pH in acidic and neutral soils, which indicated that the activity and abundance of AOA and AOB is the more important factor affecting nitrification in neutral soils. The ratios of AOA to AOB in the unmodified acidic, neutral and alkaline soils were 120, 1.55 and 0.07, respectively. The highest AOA and AOB abundances occurred in unmodified pH neutral soil. However, the highest NNR was found in alkaline soils (7.04 mg N kg−1 dry soil d−1), which was significantly higher than that in neutral and acidic soils (2.31 and − 0.23 mg N kg−1 dry soil d−1, respectively). These results indicate that substrate competition between AOA and AOB exists in neutral soils, which can provide insight and improve our understanding of microbial regulation of N cycling in terrestrial ecosystems.

The interaction of organic carbon (OC) with clay and metals stabilizes soil carbon (C), but the influence of specific clay-metal-OC assemblages (flocs) needs further evaluation. Clay-metal-OC assemblages representing OC-mineral soil fractions were synthesized using dissolved organic C (DOC) combined with kaolinite (1:1 layer structure) or montmorillonite (2:1 layer structure) clays in the absence or presence of two levels of Fe (III). Flocs were mixed with soil and incubated for 30 days. Total C mineralization from flocs was approximately 70% lower than non-flocced DOC. The flocs made with montmorillonite had 16–43% lower C mineralization rate than those made with kaolinite with no Fe or low Fe. However, when flocs were made with high Fe, clay mineralogy did not significantly affect total C mineralization. A positive priming effect (PE) of flocs on native soil OC was observed in all treatments, with a stronger PE found in lower Fe treatments. The high-Fe clay flocs inhibited ryegrass decomposition, while the flocs made without clay had no impact on it. Interestingly, flocs significantly decreased the PE of ryegrass on native soil OC decomposition. These results indicate that the adsorption of DOC onto clay minerals in the presence of Fe (III) stabilizes it against decomposition processes and its stability increases as Fe in flocs increases. Flocs also protect soil OC from the PE of external degradable plant C input. This study showed that Fe level and clay mineralogy play an important role in controlling soil C stability.

Ammonia (NH3 ) emissions from fertilized soils to the atmosphere and the subsequent deposition to land surface exert adverse effects on biogeochemical nitrogen (N) cycling. The region- and crop-specific emission factors (EFs) of N fertilizer for NH3 are poorly developed and therefore the global estimate of soil NH3 emissions from agricultural N fertilizer application is constrained. Here we quantified the region- and crop-specific NH3 EFs of N fertilizer by compiling data from 324 worldwide manipulative studies and focused to map the global soil NH3 emissions from agricultural N fertilizer application. Globally, the NH3 EFs averaged 12.56% and 14.12% for synthetic N fertilizer and manure, respectively. Regionally, south-eastern Asia had the highest NH3 EFs of synthetic N fertilizer (19.48%) and Europe had the lowest (6%), which might have been associated with the regional discrepancy in the form and rate of N fertilizer use and management practices in agricultural production. Global agricultural NH3 emissions from the use of synthetic N fertilizer and manure in 2014 were estimated to be 12.32 and 3.79 Tg N/year, respectively. China (4.20 Tg N/year) followed by India (2.37 Tg N/year) and America (1.05 Tg N/year) together contributed to over 60% of the total global agricultural NH3 emissions from the use of synthetic N fertilizer. For crop-specific emissions, the NH3 EFs averaged 11.13%-13.95% for the three main staple crops (i.e., maize, wheat, and rice), together accounting for 72% of synthetic N fertilizer-induced NH3 emissions from croplands in the world and 70% in China. The region- and crop-specific NH3 EFs of N fertilizer established in this study offer references to update the default EF in the IPCC Tier 1 guideline. This work also provides an insight into the spatial variation of soil-derived NH3 emissions from the use of synthetic N fertilizer in agriculture at the global and regional scales.

Increasing evidence suggests that autotrophic ammonia oxidation in acidic soils is largely performed by archaea. However, ammonia oxidizing bacteria (AOB) are also found in acidic soils, and mechanisms have been demonstrated that enable the growth of AOB in low pH soils. This work used DNA stable isotope probing to examine the response of autotrophic ammonia oxidizers in an acidic forest soil (pH 5.0) following urea amendment. After application, increases in nitrate concentrations correlated only with increases in AOB and not ammonia oxidizing archaeal (AOA) amoA gene abundance, indicating that AOB may be responsible for urea-derived ammonia oxidation. To identify growing autotrophic populations associated with ammonia oxidation, qPCR and high throughput sequencing analysis of 16S rRNA gene amplicons demonstrated that 13CO2 was assimilated by Nitrosospira AOB rather than archaeal ammonia oxidizing populations, and Nitrospira-associated nitrite-oxidizing bacteria (NOB). Phylogenetic analysis of amoA genes demonstrated that the active ammonia oxidizers were closely related to neutrophilic Nitrosospira multiformis within Nitrosospira cluster 3a.2. These results demonstrate that AOB can dominate ammonia oxidation in acidic soils under specific conditions.

Microbial biomass plays a critical role in soil organic carbon (SOC) mineralization. However, the effects of microbial biomass size on SOC mineralization are poorly understood. We investigated how the priming effect (PE) of plant residue inputs on native SOC mineralization responds to changes in microbial biomass size and nitrogen (N) availability in the same soil with a 23 y history of crops and grass cover with contrasting SOC contents. The size of the soil microbial biomass was changed by pre-incubating soils with glucose. The pre-incubated soils were then treated with 13C-labeled ryegrass residue combined with or without N to determine the PE. In all soils, the addition of ryegrass residue significantly increased cumulative carbon dioxide (CO2) production, whereas the addition of N decreased it compared to the control (no ryegrass or N addition). After a 42-day incubation, only 9–16% of the ryegrass carbon (C) was mineralized to CO2, which contributed approximately 55 and 34% to the total CO2 production in crop and grass soils, respectively. The addition of N decreased CO2 production during the ryegrass decomposition by 9–45%, while the change in soil microbial biomass size had no impact. In addition, a positive PE was generally found in the soils amended with ryegrass alone, while the application of ryegrass residue combined with N decreased the PE. Moreover, the priming effect was independent of the size of the microbial biomass in crop soil. However, we observed a significant interaction of microbial biomass size and N availability on the priming effect in a grass soil but not in a crop soil. Our results indicate that soil microbial biomass size and soil C influence the magnitude and direction of the priming effect from C inputs depending on N availability.

Soil organic carbon (SOC) content and its 13C natural abundance along soil profile are sensitive to the changes of land use since these changes alter external carbon (C) inputs and belowground C turnover processes. A study was conducted to investigate how the conversion of cropland to forest, grassland or bare land (no vegetation) affect the distribution of SOC and its 13C values in 200-cm soil profiles. After 28 years of land conversion, the cropland that was remained as cropland (treated as control) had a significantly higher δ13C value than the bare land, grassland or forest at the top 10 cm soil depth. Regardless of the land use type, the mean soil 13C values in the subsurface soil layers depleted and δ13C signature decreased by 1%−5% compared with the top 10 cm soil layer, while the corresponded SOC content decreased by 3.5% to 91.2% as soil depth went down deeper. The SOC stock in the 0–200 cm profile was 16% and 12% higher in the grassland and forest, respectively, than in the control. The conservation of cropland to grassland and forest largely increased the SOC stock in the 20–40 cm soil layer compared with the control. The newly formed SOC was higher in the grassland than in the forest, and accounted for 15.4% and 9.7% of the SOC stock in the soil depth of 0–60 cm in the grassland and forest, respectively. Soil δ13C values and SOC contents were negatively correlated with pH and clay content, while they were positively correlated with nitrogen and C/N ratio along the soil profile. We infer that the conversion of cropland to forest or grassland is a beneficial practice for SOC storage, especially in the 0–60 cm depth. Such conversion has little impact on the SOC turnovers in deep soil layers during the study period.

Soil ecosystem represents the largest contributor to global nitrous oxide (N2 O) production, which is regulated by a wide variety of microbial communities in multiple biological pathways. A mechanistic understanding of these N2 O production biological pathways in complex soil environment is essential for improving model performance and developing innovative mitigation strategies. Here, combined approaches of the 15 N-18 O labelling technique, transcriptome analysis, and Illumina MiSeq sequencing were used to identify the relative contributions of four N2 O pathways including nitrification, nitrifier-induced denitrification (nitrifier denitrification and nitrification-coupled denitrification) and heterotrophic denitrification in six soils (alkaline vs. acid soils). In alkaline soils, nitrification and nitrifier-induced denitrification were the dominant pathways of N2 O production, and application of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) significantly reduced the N2 O production from these pathways; this is probably due to the observed reduction in the expression of the amoA gene in ammonia-oxidizing bacteria (AOB) in the DMPP-amended treatments. In acid soils, however, heterotrophic denitrification was the main source for N2 O production, and was not impacted by the application of DMPP. Our results provide robust evidence that the nitrification inhibitor DMPP can inhibit the N2 O production from nitrifier-induced denitrification, a potential significant source of N2 O production in agricultural soils.

Temperature sensitivity of organic carbon (C) mineralization is affected by C inputs, but predicting the magnitude of the response remains a challenge. We investigated how temperature and exogenous C inputs affected the apparent organic C mineralization in soils with a range of soil organic C (SOC) contents. Soils with 2.9, 4.0, and 6.8% SOC content were incubated with or without C sources (either plant residues or glucose), at two temperatures (15 or 25 °C) for 120 days. Apparent organic C mineralization was significantly affected by SOC content and C inputs. Cumulative CO 2 production in soils with a higher SOC content was less sensitive to C inputs. Glucose was always more effective in stimulating CO 2 production than plant residue. Without exogenous C inputs, the temperature sensitivity of organic C mineralization (described by Q 10 value) was higher in the 4.0% SOC soil (2.31) than in the 2.9% (1.12) and 6.8% SOC soils (1.30). The addition of exogenous C decreased Q 10 values by up to 131% in the 4.0% SOC soil, which was not observed in other two soils. In general, C inputs increased the Q 10 of the estimated active ( C a ) and stable C ( C s ) pools in all the tested soils, especially the C s pool in the 4.0 and 6.8% SOC soils. Results indicated that the effect of C inputs on apparent organic C mineralization (either stimulatory or inhibitory) is influenced by SOC content and C source, and that temperature sensitivity of organic C mineralization in the presence of exogenous C is highly sensitive to SOC content.

The rhizosphere priming effect is caused by root carbon (C) exudation into the rhizosphere; the role of this effect in nutrient cycling and ecosystem recovery of natural grasslands as affected by different grazing intensities is still unknown. The objective of the present study was to investigate the relationships among root C exudation, rhizospheric microbial activity, and their influences on plant nutrient uptake during grazing and recovery periods. Field experiments were conducted in the Hongyuan Alpine Meadow to measure root exudation rate and nutrient cycling processes of the dominant species Elymus nutans. Three grazing intensities (an ungrazed control, moderate grazing and heavy grazing) were introduced for two months, following which all treatments received a recovery practice (no grazing for 21 days). Heavy grazing significantly decreased root exudation rate, soil nitrogen (N) mineralization rate, β-1,4-glucosidase (BG) activity, and foliar C concentration, while moderate grazing had no influence on these parameters compared to the control. After the 21 days of recovery, all these parameters, except N mineralization rate and foliar C concentrations in the heavy grazing treatment, returned to similar levels as in the control, whereas root exudation rate and BG activity rose to even higher levels. Meanwhile, moderate grazing significantly promoted root exudation rate, soil inorganic N concentration, net soil N mineralization rate, and β-N-acetylglucosaminidase (NAG) activity during the recovery stage as compared to the control. Foliar quality was also improved by the recovery practice, indicating that the high availability of N and P is a consequence of the positive root–microbe feedback and will ultimately benefit grazers. The flush of labile C released to the rhizosphere by grazed plants stimulated extracellular enzyme activities, enhanced soil N mineralization, and increased plant nutrient uptake. These results imply that reasonable (i.e. moderate) grazing followed by a recovery practice can effectively restore and strengthen grassland vegetation, and contribute to the sustainable use of alpine meadows such as Hongyuan.

As a result of global climate change, frequent and intensive extreme climate events are observed and predicted. These extreme climate events can change ecosystem functions through driving plant growth and mortality, as well as ecological and evolutionary processes into different directions. We simulated a 1-in-100-year extreme precipitation event in different plant growth seasons (i.e. spring, summer, or autumn) to investigate the functional responses of a mesophytic alpine grassland ecosystem to climate change in the Tibetan plateau. The results demonstrated that the seasonal distribution of extreme precipitation is a critical factor in determining soil microbial- and plant-productivity. The response of vegetation net primary productivity (NPP) to extreme precipitation depends on the growth seasons and plant types. Total NPP in the treatments experienced extreme precipitation in the early (spring), mid- (summer) and late- (autumn) plant growth seasons were significant lower, higher, and no difference, respectively, compared with the control. Soil temperature and moisture were the key abiotic factors that affected ecosystem functions. For example, in the early plant growth season, no changes of soil moisture and the decreased temperature in response to the extreme precipitation resulted in a substantial decline in NPP. By contrast, in the mid-plant growth season, higher temperature and soil moisture elicited positive synergistic effects on plant growth and soil microbial processes. The increased sensitivity of above-ground NPP and the shift of dominant species from sedges to less palatable forbs might inevitably exaggerate the degradation of this grassland. Nevertheless, a high resilience index in the related ecological processes could potentially contribute to the grassland acclimation and stability, as most parameters returned to the similar levels as in the control after the extreme events ceased. Therefore, several synergistic or antagonistic mechanisms are hypothesized to operate in parallel or at different levels of organization and timescales. Further studies involving a range of different potential scenarios and longer periods are needed to predict the future climate change impacts on mesophytic grassland.

We investigated the extent water soluble and insoluble (hexane soluble) plant residue fractions in influence the priming of soil organic carbon (SOC). Carbon-13-labeled water-soluble and insoluble plant materials were added to an organic or mineral soil at three moisture levels and incubated for 57 days. Plant material decomposition and resulting priming effect (PE) was quantified. Water-soluble additions were not always mineralized at higher rates than insoluble material and their mineralization rates varied greatly across soil type and moisture content. Regardless of solubility, less than 33 % of the added carbon was mineralized to CO2 by the end of the experiment, but a positive PE was observed in both soils. In general, water-soluble material caused a greater PE than insoluble organics, though the difference was not always significant throughout the experiment. Both additions induced greater PE in mineral soil than in an organic soil on a soil organic carbon basis. Moreover, the PE of soluble substrates generally increased when soil moisture increased, which was not observed for the PE of insoluble substrates. Solubility alone is insufficient in protecting organic substrates against microbial decomposition, with soluble substrates causing only a short and pulsed response to priming of SOC, and may therefore have a less pronounced impact on soil processes and final C balance than insoluble substrates in the long-term.

Direct land application as an alternative to green waste (GW) disposal in landfills or composting requires an understanding of its impacts on greenhouse gas (GHG) and volatile organic compound (VOC) emissions. We investigated the effects of two approaches of GW direct land application, surface application and soil incorporation, on carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4), and VOC emissions for a 12month period. Five treatments were applied in fall 2013 on fallow land under a Mediterranean climate in California: 30cm height GW on surface; 15cm height GW on surface; 15cm height GW tilled into soil; control+till; control+no till. In addition, a laboratory experiment was conducted to develop a mechanistic understanding of the influence of GW application on soil O2 consumption and GHG emission. The annual cumulative N2O, CO2 and VOC emissions ranged from 1.6 to 5.5kgN2O-Nha(-1), 5.3 to 40.6MgCO2-Cha(-1) and 0.6 to 9.9kgVOCha(-1), respectively, and were greatly reduced by GW soil incorporation compared to surface application. Application of GW quickly consumed soil O2 within one day in the lab incubation. These results indicate that to reduce GHG and VOC emissions of GW direct land application, GW incorporation into soil is recommended.

Nitrogen (N) application is the main agricultural management that increases nitrous oxide (N2O) concentration in the atmosphere. Freezing conditions are common phenomenon in the northern China that significantly affect soil N2O emissions through alterations in nutrients availability and microbial population. To develop a comprehensive understanding of how N fertilizer managements affect soil N2O emissions during the freezing process, a lab incubation was conducted in three typical cultivated soils (black soil, fluvo-aquic soil, or loess soil) by adding different N fertilizer sources, including ammonium chloride, sodium nitrate, or urea at different N levels (0, 80, 200, or 500 mg N/kg) at the start of freezing. The N2O emissions in the fluvo-aquic soil were significantly higher than in other soils. The application of nitrate in the fluvo-aquic soil promoted N2O emissions by five- and seven-fold higher compared to ammonium chloride and urea, whereas N2O emissions in black soil were enhanced by appl...

Sympatric differentiation of ammonia-oxidizing archaea and bacteria may occur due to differences in ammonia availability during agricultural activities such as fertilization, which shift key biological drivers of soil nitrification processes. Restoring native habitat from intensive agricultural activities (e.g. converting cropland into fallow land) reduces fertilizer inputs, which has the potential to change active ammonia-oxidizers and thus nitrification rates. We incubated soils from fields under 30 years fallow and under continuous cropping to understand shifts in ammonia-oxidizers and nitrification processes. Thirty years of land fallow practices sharply decreased the abundance and activity of soil ammonia-oxidizing bacteria compared to continuous cropping. Nitrification in cropland was driven mainly by ammonia-oxidizing bacteria while ammonia-oxidizing archaea were the main drivers of nitrification in fallow soil. Conversion of cropland into fallow land shifted the dominant active ammonia-oxidizers from ammonia-oxidizing bacteria to archaea, and the low nitrification rates in fallow soil were caused by the lack of active ammonia-oxidizing bacteria rather than N substrate availability.

The biology of soil is critical to its health; the available carbon and nitrogen from plant litter and mineral fertilizers in the soil determines the structure of microbial communities and thus the functioning of soil. We used amino sugars (ASs) and phospholipid fatty acids (PLFAs) as microbial biomarkers to assist in identifying the effect of practices of agricultural management on the microbial community. We determined the contents and compositions of ASs and PLFAs in the soils of five long-term (12 years) practices of land management: bare land (BL), grassland (GL), and land with continuous wheat (WC), soybean (SC), and maize (MC) cropping systems. BL and GL were not fertilized, but WC, SC, and MC were supplied with either mineral fertilizer (WCF, SCF, and MCF, respectively) or no fertilizer as controls (WC, SC, and MC, respectively). The contents of ASs and PLFAs in WC, SC, and MC were somewhere in the middle of those between GL and BL. Soils amended with maize material accumulated more ASs than soils amended with soybean or wheat material. The AS contents were 7.2 and 10.3% lower in WCF and SCF than WC and SC, respectively, but were 6.4% higher in MCF than MC. The contribution of fungi to the pools of soil organic carbon were significantly lower when mineral fertilizer was added, indicated by structural equation modeling under a continuous supply of N. The new carbon accumulated by microbes was positively contributed more by residues derived bacteria than from fungi. These results indicated that long-term practices of land management significantly altered the soil microbial communities and their contribution to SOC accumulation.

The increasing abundance of the greenhouse gas nitrous oxide (N2O) in the atmosphere is a global concern. Although it is well known that soils are a major source of atmospheric N2O, elucidation of the complexity of the underlying microbial and chemical production processes and the roles of biotic and abiotic factors is still needed. Land management practices, especially nitrogen (N) fertilization in agroecosystems, are important determinants of soil N2O production and its consequent contribution to climate change. Here, we summarize the known N2O production pathways that contribute to soil N2O production and the factors that control these pathways, highlight the importance of land management practices in controlling soil N2O production, and discuss management strategies to reduce soil N2O emissions. We also address the roles of climate change itself and adopted strategies to mitigate climate change on soil N2O emissions. Although management practices have been identified to reduce soil N2O emissions, understanding of how these management practices act in combination and whether they lead to additive or multiplicative reductions in N2O remains limited. Examination of the underlying mechanisms of soil N cycling and N2O production and collection of empirical data across multiple management practices and soil landscapes under conditions that reflect a changing climate will facilitate model development and identification of effective N2O mitigation strategies.

The effect of nitrogen (N) application rate on nitrous oxide (N 2 O) emissions in grain cropping systems has been extensively studied, but little attention has been given to N source as a factor influencing emissions. To expand our understanding of the conditions that affect N 2 O emissions and production pathways, and develop management practices that farmers can use to maintain crop productivity while mitigating N 2 O emissions, we conducted field experiments during two years in wheat ( Triticum aestivum ) cropping systems in the Sacramento Valley, California. Five levels of total N were applied, ranging from 0 to 254 in the 2009–10 season, and from 0 to 266 kg N ha −1 in the 2010–11 season. At planting in November, either ammonium sulfate (AS) or anhydrous ammonia (AA) was applied, and either urea or calcium nitrate was used as a topdress at the stem elongation stage. The cumulative N 2 O emissions ranged from 0.24 (±0.07) to 1.31 (±0.35) kg N 2 O-N ha −1 in 2009–10 and from 0.74 (±0.24) to 2.2 (±0.23) kg N 2 O-N ha −1 in 2010–11. In both seasons, the highest total N 2 O and yield-scaled N 2 O emissions were recorded in the treatments that included 112 kg N ha −1 as knife-injected AA. In 2010–11, the total N 2 O and yield-scaled N 2 O emissions following 168 kg N ha −1 broadcast AS starter were similar as those of the AA treatment. Significantly higher N 2 O fluxes after a topdress application of 98 kg N ha −1 urea compared to a nitrate addition of the same magnitude indicates that ammonia oxidation was likely the source of N 2 O production. In contrast to other studies, N use efficiency was not negatively correlated to N uptake-scaled N 2 O emissions, mainly because N 2 O emissions were a function of fertilizer type and application method.

The continuous rise of atmospheric nitrous oxide (N2O) is an environmental issue of global concern. In biogeochemical studies, N2O production is commonly assumed to arise solely from enzymatic reactions in microbes and fungi. However, iron, manganese and organic compounds readily undergo redox reactions with intermediates in the nitrogen cycle that produce N2O abiotically under relevant environmental conditions at circumneutral pH. Although these abiotic N2O production pathways have been known to occur for close to a century, they are often neglected in modern ecological studies. In this Synthesis and Emerging Ideas paper, we highlight the defining characteristics, environmental controls, and isotopic signatures of abiotic reactions between nitrogen cycle intermediates (hydroxylamine, nitric oxide, and nitrite), redox-active metals (iron and manganese) and organic matter (humic and fulvic acids) that can lead to N2O production. We also discuss the emerging idea that abiotic reactions coupled to biotic processes have widespread ecological relevance and encourage consideration of abiotic production mechanisms in future biogeochemical investigations of N2O cycling.

The increasing use of green waste compost as an agricultural soil amendment requires an understanding of its role in influencing nitrous oxide (N2O) emission, particularly in managing these amendments on different soil types and cropping systems. We conducted a set of experiments to determine the influence of compost application on the pathways and sources of N2O production. 15N-labeled soil nitrogen (N) and fertilizer N were used separately in different soil types to differentiate the sources of N2O from compost, fertilizer and soil N and the different pathways of heterotrophic denitrification and ammonia oxidation. In sandy soils amended with N fertilizer + compost, total N2O emissions and nitrification rates, respectively, ranged from 1.4 to 2.9 and 0.2 to 16 times greater than soils amended solely with N fertilizer. In contrast, no significant difference in N2O emissions was found between compost and non-compost treatments in clay loam soils, likely due to the high capacity of these soils to buffer changes in biochemical properties such as nitrifier activity or pH. Compost application in sandy soils increased N2O emitted from both heterotrophic denitrification and ammonia oxidation by a factor of 1.2 and 2, respectively. In these soils, compost not only acted as a source of N for N2O, but also increased N2O produced from fertilizer (NH4)2SO4. However, N2O derived from soil N was suppressed by compost application, indicating that the overall N2O emissions could be decreased by compost application when applied to soil with high N2O production potential. Therefore, strategies to reduce the environmental impact of green waste compost and mitigate N2O emission from agricultural soil should potentially be aware that green waste compost application to sandy soils could increase N2O production, though further study is needed to predict a generalized response in coarser-textured soils.

Soil ecosystem represents the largest contributor to global nitrous oxide (N

Greenhouse gas (GHG) emissions from arid irrigated agricultural soil in California have been predicted to represent 8% of the state’s total GHG emissions. Although specialty crops compose the majority of the state’s crops in both economic value and land area, the portion of GHG emissions contributed by them is still highly uncertain. Current and emerging soil management practices affect the mitigation of those emissions. Herein, we review the scientific literature on the impact of soil management practices in California specialty crop systems on GHG nitrous oxide emissions. As such studies from most major specialty crop systems in California are limited, we focus on two annual and two perennial crops with the most data from the state: tomato, lettuce, wine grapes and almond. Nitrous oxide emission factors were developed and compared to Intergovernmental Panel on Climate Change (IPCC) emission factors, and state-wide emissions for these four crops were calculated for specific soil management practices. Dependent on crop systems and specific management practices, the emission factors developed in this study were either higher, lower or comparable to IPCC emission factors. Uncertainties caused by low gas sampling frequency in these studies were identified and discussed. These uncertainties can be remediated by robust and standardized estimates of nitrous oxide emissions from changes in soil management practices in California specialty crop systems. Promising practices to reduce nitrous oxide emissions and meet crop production goals, pertinent gaps in knowledge on this topic and limitations of this approach are discussed.

Soil-plant-atmosphere interactions regulate the impact of climate on forest ecosystems., Many forest ecosystems have experienced recent declines in productivity; however, in some alpine regions, tree growth and forest expansion are increasing at marked rates. Dendrochronological analyses at the upper limit of alpine forests in the Tibetan Plateau show a steady increase in tree growth since the early 1900s, which intensified during the 1930s and 1960s, and have reached unprecedented levels since 1760. This recent growth acceleration was observed in small/young and large/old trees and coincided with the establishment of trees outside the forest range, reflecting a connection between the physiological performance of dominant species and shifts in forest distribution. Measurements of stable isotopes (carbon, oxygen, and nitrogen) in tree rings indicate that tree growth has been stimulated by the synergistic effect of rising atmospheric CO2 and a warming-induced increase in water and nutrient availability from thawing permafrost. These findings illustrate the importance of considering soil-plant-atmosphere interactions to understand current and anticipate future changes in productivity and distribution of forest ecosystems.

The process of composting is a source of greenhouse gases (GHG) that contribute to climate change. We monitored three field-scale green waste compost windrows over a one-year period to measure the seasonal variance of the GHG fluxes. The compost pile that experienced the wettest and coolest weather had the highest average CH4 emission of 254 ± 76 g C day−1 dry weight (DW) Mg−1 and lowest average N2O emission of 152 ± 21 mg N day−1 DW Mg−1compared to the other seasonal piles. The highest N2O emissions (342 ± 41 mg N day−1 DW Mg−1) came from the pile that underwent the driest and hottest weather. The compost windrow oxygen (O2) concentration and moisture content were the most consistent factors predicting N2O and CH4 emissions from all seasonal compost piles. Compared to N2O, CH4 was a higher contributor to the overall global warming potential (GWP) expressed as CO2 equivalents (CO2 eq.). Therefore, CH4 mitigation practices, such as increasing O2 concentration in the compost windrows through moisture control, feedstock changes to increase porosity, and windrow turning, may reduce the overall GWP of composting. Based on the results of the present study, statewide total GHG emissions of green waste composting were estimated at 789,000 Mg of CO2 eq., representing 2.1% of total annual GHG emissions of the California agricultural sector and 0.18% of the total state emissions.

Iron (Fe) affects soil nitrogen (N) cycling processes both in anoxic and oxic environments. The role of Fe in soil N transformations including nitrification, mineralization, and immobilization, is influenced by redox activity, which is regulated by soil pH. The effect of Fe minerals, particularly oxides, on soil N transformation processes depends on soil pH, with Fe oxide often stimulating nitrification activity in the soil with low pH. We conducted lab incubations to investigate the effect of Fe oxide on N transformation rates in two subtropical agricultural soils with low pH (pH 5.1) and high pH (pH 7.8). 15N-labeled ammonium and nitrate were used separately to determine N transformation rates combined with Fe oxide (ferrihydrite) addition. Iron oxide stimulated net nitrification in low-pH soil (pH 5.1), while the opposite occurred in high-pH soil (pH 7.8). Compared to the control, Fe oxide decreased microbial immobilization of inorganic N by 50 % in low-pH soil but increased it by 45 % in high-pH soil. A likely explanation for the effects at low pH is that Fe oxide increased NH3-N availability by stimulating N mineralization and inhibiting N immobilization. These results indicate that Fe oxide plays an important role in soil N transformation processes and the magnitude of the effect of Fe oxide is dependent significantly on soil pH.