Search

The biochemical mechanisms responsible for soil organic C (SOC) accumulation under N enrichment are not well understood. By examining two N types (NH4Cl and NaNO3) and three rates (0, 40, and 120 kg N ha−1 yr−1), we investigated the concentration and distribution of SOC by using soil aggregate separation and physical fractionation of soil organic matter (SOM). The molecular composition of SOM within bulk soil and each physical fraction was determined by pyrolysis–gas chromatography‐mass spectrometry. Nitrogen fertilization led to soil NO3−–N accumulation and intensified soil acidification. Four years of N fertilization did not change total SOC concentrations in the 0‐ to 20‐cm depth but a high rate of NaNO3 addition increased the concentrations of fine particulate organic C and microaggregate‐associated organic C by 133 and 83.4%, respectively. The change in SOC concentration was positively correlated with the change in fine particulate organic C and mineral‐associated organic C concentrations. Four years of N fertilization significantly changed the molecular composition of SOM, with an increase in the relative abundance of furfural and acetic acid and a decrease in pyrrole in the mineral‐associated organic matter (MAOM) fraction. Fertilization increased the furfural/pyrrole and aliphatic/aromatic ratios in the SOM and MAOM fractions, suggesting decreased stability of the passive SOM fraction. The SOC concentration was negatively correlated with the benzene/toluene ratio in both bulk soil and the MAOM fraction. Overall, exogenous N input to this subtropical plantation forest decreased the mineralization and humification of SOM, which could be detrimental to soil C accumulation. [ABSTRACT FROM AUTHOR]

Soil N 2 O responds differently to various soil variables; moreover these driving variables usually have low explanatory capacity of soil N 2 O flux variability. Anthropogenic nitrogen (N) deposition inputs to the subtropical plantation forests markedly increase N 2 O emission from soils, but the relative contribution of deposited NH 4 + and NO 3 − is not well understood. Based on a two-type (NH 4 Cl and NaNO 3 ) and three-level (0, 40, 120 kg N ha −1 yr −1 ) N addition experiment, we simultaneously determined soil N 2 O flux and related soil variables once a week in 2015. Boundary line analysis was used to describe the maximum responses of soil N 2 O fluxes to different soil variables. Four years of N addition led to significant accumulations of soil NH 4 + -N and NO 3 − -N, thereby accelerating soil acidification. Nitrogen addition significantly increased soil N 2 O fluxes by 4.7–19.3 folds relative to control; moreover, the promotions to soil N 2 O emission and soil acidification were greater with ammonium-N addition than with nitrate-N addition. The relationships between the maximum soil N 2 O fluxes and individual soil variables were well fitted to the Gaussian equations, and the optimum values of soil temperature, moisture, pH, and NO 3 − -N content indicate that nitrification dominated soil N 2 O production. Overall, our results suggest that exogenous NH 4 + input to the subtropical plantation appears to have greater negative effects on soil acidification and N 2 O emission compared with NO 3 − input. Boundary line analysis well depicts the shape and magnitude of response functions of soil N 2 O fluxes against soil variables, which should be incorporated into terrestrial ecosystem process models. [ABSTRACT FROM AUTHOR]

Responses of soil organic carbon (SOC) cycling and C budget in forest ecosystems to elevated nitrogen (N) deposition are divergent. Little is known about the N critical loads for the shift between gain and loss of SOC storage in the old-growth temperate forest of Northeast China. The objective of this study was to investigate the nonlinear responses of SOC concentration and composition to multiple rates of N addition, as well as the microbial mechanisms responsible for SOC alteration under N enrichment. Nine rates of urea addition (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha year) with 4 replicates for each treatment were conducted. Soil samples in the 0-10 cm mineral layer were taken after 3 years of N fertilization. Soil aggregate size distribution and SOC physical fractionation were performed to examine SOC dynamics. Phospholipid fatty acid (PLFA) technique was used to measure the abundance and structure of microbial community. Three years of N addition led to significant increases in the concentrations of soil particulate organic C and aggregate-associated organic C fractions only. The responses of total N and each labile SOC fraction to the rates of N addition followed Gaussian equations, with the N critical loads being estimated to be between 80 and 100 kg N ha year. The change in SOC concentration (ΔSOC) was positively correlated with the changes in aggregate associated OC (r > 0.80) and POC concentrations (r > 0.50). Significant correlations among the concentrations of labile SOC fractions, the percentages of soil aggregates, and the abundances of microbial PLFAs were observed, which implies a close linkage between microbial community structure and SOC accumulation and stability. Our results suggest that increase in soil moisture and shift of microbial community structure could control the critical N load for the switch between C accumulation and loss. The current N deposition rate (~ 11 kg N ha year) to the northeast China's forests is favorable for soil C accumulation over the short term. [ABSTRACT FROM AUTHOR]

The responses of soil-atmosphere carbon (C) exchange fluxes to growing atmospheric nitrogen (N) deposition are controversial, leading to large uncertainty in the estimated C sink of global forest ecosystems experiencing substantial N inputs. However, it is challenging to quantify critical load of N input for the alteration of the soil C fluxes, and what factors controlled the changes in soil CO 2 and CH 4 fluxes under N enrichment. Nine levels of urea addition experiment (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha −1 yr −1 ) were conducted in the needle-broadleaved mixed forest in Changbai Mountain, Northeast China. Soil CO 2 and CH 4 fluxes were monitored weekly using the static chamber and gas chromatograph technique. Environmental variables (soil temperature and moisture in the 0–10 cm depth) and dissolved N (NH 4 + -N, NO 3 − -N, total dissolved N (TDN), and dissolved organic N (DON)) in the organic layer and the 0–10 cm mineral soil layer were simultaneously measured. High rates of N addition (≥60 kg N ha −1 yr −1 ) significantly increased soil NO 3 − -N contents in the organic layer and the mineral layer by 120%-180% and 56.4%-84.6%, respectively. However, N application did not lead to a significant accumulation of soil NH 4 + -N contents in the two soil layers except for a few treatments. N addition at a low rate of 10 kg N ha −1 yr −1 significantly stimulated, whereas high rate of N addition (140 kg N ha −1 yr −1 ) significantly inhibited soil CO 2 emission and CH 4 uptake. Significant negative relationships were observed between changes in soil CO 2 emission and CH 4 uptake and changes in soil NO 3 − -N and moisture contents under N enrichment. These results suggest that soil nitrification and NO 3 − -N accumulation could be important regulators of soil CO 2 emission and CH 4 uptake in the temperate needle-broadleaved mixed forest. The nonlinear responses to exogenous N inputs and the critical level of N in terms of soil C fluxes should be considered in the ecological process models and ecosystem management. [ABSTRACT FROM AUTHOR]

Interactions between soil organic matter (SOM) composition and microbial communities determine soil basal respiration (BR, SOM-derived CO 2). However, few studies have investigated the relative importance of SOM chemical structure, microbial community composition, and soil environmental factors to the spatial variability of soil BR on a large scale. Here, organic and mineral layer soils were collected along a 650-km boreal forest transect in Northeast China. Combining 13C-nuclear magnetic resonance spectroscopy and high-throughput sequencing techniques, SOM chemical structure, microbial community composition, and soil physicochemical properties were determined to investigate the biotic and abiotic drivers of soil BR. Soil BR, microbial communities, and soil physicochemical properties exhibited significant spatial heterogeneity across sites, but the variations were independent of latitude regardless of soil layer. Soil BR rates were significantly higher in the organic layer than in the mineral layer (0.82 vs. 0.31 CO 2 -C kg−1soil−1 h−1, P < 0.001). Multi-model averaging and partial regression showed that SOM chemical structure exhibited the strongest control on the spatial variation of soil BR (contributing 23.0 %), followed by the organic layer soil C/N ratio (contributing 17 %). However, only soil C/N ratio was the most important predictor of soil BR variation in the mineral layer. Copiotrophic bacteria had a higher impact on soil BR than oligotrophic bacteria. In addition, soil physicochemical properties (i.e. pH, mechanical composition) indirectly affected soil BR through regulating soil microbial composition, especially bacteria. Overall, these results highlighted that SOM chemical structure directly or indirectly affected the microbial decomposition of SOM, and contributed more relative to SOM quality. These findings should be incorporated into ecosystem process models to better predict the response of boreal forest soil C emissions to climate change. [Display omitted] • Spatial variation of soil BR, SOM, and microbial community are independent of latitude. • Labile and recalcitrant SOM components are differentially associated with soil BR. • Copiotrophic bacteria have a greater impact on soil BR than ologotrophic bacteria. • SOM chemical structure indirectly affects soil BR through altering bacterial communities. [ABSTRACT FROM AUTHOR]

Elevated nitrogen (N) deposition variously affects the soil carbon (C) cycle. It is not clear how deposited NH 4 + and NO 3 − divergently affect the amount and stability of soil organic C (SOC) in the N-limiting forests. A multi-form N addition experiment was conducted in a boreal forest in the Great Khingan mountain in 2010. Three fertilizers, NH 4 Cl, KNO 3 and NH 4 NO 3 , were applied at four rates of 0, 10, 20, and 40 kg N ha − 1 yr − 1 . Solid-state 13 C nuclear magnetic resonance spectroscopy and elemental analysis were used to determine the chemical structure and C contents in bulk soils and/or two density fractions (< 1.70 g cm − 3 light fraction and > 1.70 g cm − 3 heavy fraction). NH 4 Cl addition significantly decreased the SOC contents in the organic layer by 28.35% to 54.23%, but KNO 3 addition significantly increased the SOC contents in the organic layer and mineral layer by 26.53% and 87.44%, respectively. Nitrogen addition rates significantly influenced the degradability (alkyl-C/O-alkyl-C) and hydrophobicity ((alkyl C + aromatic C)/(O-alkyl C + carboxyl C)) of light and heavy fractions, whereas N addition forms only impacted their aromaticity (aromatic C/(aromatic C + alkyl C + O-alkyl C)). The movement between residue-C and stable humin fraction, as well as the chemical stability of SOM could profoundly affect the storage of SOC under N enrichment. Overall, added NH 4 + and NO 3 − differently affect C sequestration in the N-limiting forest soils. This should be differentiated in the biogeochemical models of C N cycle coupling. [ABSTRACT FROM AUTHOR]

The responses of nitrous oxide (N 2 O) emission from forest soils to increasing atmospheric nitrogen (N) deposition are controversial. In this study, our objectives were to explore the response curves of soil N 2 O flux to multi-level N inputs, as well as to examine the key factors dominating the changes in soil N 2 O emission caused by N enrichment in the temperate needle-broadleaved mixed forest, Northeast China. The study consists of nine levels of urea addition (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha − 1 yr − 1 ) with 4 replicates for each treatment. Soil N 2 O fluxes were monitored weekly using the static chamber and gas chromatograph technique. NH 4 + -N, NO 3 − -N, total dissolved N (TDN), dissolved organic N (DON), and auxiliary variables (soil temperate and moisture in 0–10 cm depth) were measured at the same frequency to examine the regulation of soil N 2 O flux. The results showed that high rates of urea inputs (> 60 kg N ha − 1 yr − 1 ) significantly increased soil NO 3 − -N concentrations in litter layer and mineral layer (0–10 cm depth) by 120–180% and 56.4–84.6%, respectively. Soil N 2 O flux increased exponentially with increase in the rates of urea addition by 194% to 334% for the 60 to 140 kg N ha − 1 yr − 1 treatments relative to the control. The critical level of N input for the significant alternation of soil N accumulation and N 2 O emission was approximately 70 kg N ha − 1 yr − 1 . The changes in soil N 2 O flux elicited by N addition were positively related to those of soil NO 3 − -N and soil moisture contents. These results suggest that exogenous N input at the rate below the critical load will not significantly promote soil N 2 O emission over the short term, which is favorable to carbon sequestration of the temperate needle-broadleaved mixed forest, Northeast China. [ABSTRACT FROM AUTHOR]

Ammonia-oxidizers play an essential role in nitrogen (N) transformation and nitrous oxide (N 2 O) emission in forest soils. It remains unclear if ammonia-oxidizers affect interaction between methane (CH 4 ) uptake and N 2 O emission. Our specific goal was to test the impacts of changes in ammonia-oxidizing communities elicited by N enrichment on soil CH 4 uptake and N 2 O emission. Based on a field experiment, two-forms (NH 4 Cl and NaNO 3 ) and two levels (40 and 120 kg N ha −1 yr −1 ) of N were applied in the subtropical plantation forest of southern China. Soil CH 4 and N 2 O fluxes, the abundance and structure of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities were measured using static chamber-gas chromatography, quantitative PCR (qPCR), and terminal-restriction fragment length polymorphism (T-RFLP). Nitrogen addition tended to inhibit soil CH 4 uptake, but significantly promoted soil N 2 O emission; moreover, these impacts were more significant with NH 4 + − N than with NO 3 − − N addition. NH 4 Cl addition significantly changed ammonia-oxidizer abundance with an increase in AOA and a decrease in AOB. Nitrogen additions significantly decreased the relative abundance of 329 bp and 421 bp of archaeal amoA gene. Negative relationships occurred between soil CH 4 uptake and AOA abundance and between soil CH 4 uptake and AOA/AOB ratio; however, a positive relationship was found between soil N 2 O emission and AOA abundance. These results indicate that a shift in abundance and composition of ammonia-oxidizing communities is closely linked to changes in soil CH 4 uptake and N 2 O emission under N enrichment. Furthermore, AOA communities play a contrasting role from AOB communities for regulating the fluctuation between soil CH 4 and N 2 O fluxes. [ABSTRACT FROM AUTHOR]

Background and aims Deposited NH 4 + and NO 3 − differently affect soil carbon (C) and nitrogen (N) cycles due to their contrasting actions in terrestrial ecosystems. However, little information on the effects of exogenous NH 4 + and NO 3 − inputs on the exchange of greenhouse gases (GHGs) from the subtropical plantation soils as well as their contribution to global warming is available to date. Methods Based on a field experiment, two-form (NH 4 Cl and NaNO 3 ) and two-level (40 and 120 kg N ha −1 yr −1 ) of N addition, in a slash pine plantation of southern China, we investigated soil CO 2 , CH 4 and N 2 O fluxes and related auxiliary variables (soil temperature and moisture) twice a week using static chamber–gas chromatography. The total global warming potential (GWP) of soil GHG fluxes and N 2 O emission factor (EF) were calculated. Results Low level of NaNO 3 addition significantly increased cumulative annual soil CO 2 emission by 33.7%. N addition significantly promoted annual soil N 2 O emission by 2.4–6.9 folds; moreover, ammonium-N addition had a greater promotion to soil N 2 O emission than nitrate-N addition. However, short-term N addition did not change soil CH 4 uptake. Also, soil CO 2 and N 2 O fluxes were positively correlated with soil temperature and moisture, while soil CH 4 uptake was only driven by soil moisture. Overall, elevated N addition increased the total GWP, and changed the temperature sensitivity (Q 10 ) of soil CO 2 and N 2 O fluxes. Conclusions These results suggest that chronic atmospheric N deposition changes soil-atmospheric GHG fluxes in the subtropical plantation of southern China depending on the levels and forms of N input, and would exacerbate global warming. [ABSTRACT FROM AUTHOR]

Increased nitrogen (N) deposition has been found controversial affecting soil CO emission in terrestrial ecosystems, which leads to serious debate on the efficiency of estimated C sequestration induced by N enrichment. The forms of input N might be responsible for this controversy. This study aims to explore the effects of NH (reduced N) and NO (oxidized N) on soil CO flux and the underlying microbial mechanisms. An N addition experiment, two N fertilizers (NHCl and NaNO) and two rates (40 and 120 kg N ha year), was carried out in a slash pine plantation of southern China. Soil-atmospheric CO exchange, soil microbial biomass, and community composition were measured using static chamber-gas chromatography and phospholipid fatty acid (PLFA) analyses in the active growing and nonactive growing seasons, respectively. Low level of NaNO addition significantly increased soil CO flux in the active growing season, whereas other N treatments did not change soil CO flux. High level of NHCl addition significantly reduced soil fungal biomass (fungal PLFA) and changed microbial community composition (ratio of fungal to bacterial (F/B) PLFAs). The positive relationships between the change in soil CO flux and the change in fungal biomass, as well as between the change in soil CO flux and the change in community composition, were observed in the nonactive growing season. The N forms as NO or NH are important factors affecting C cycles in the subtropical coniferous plantation. These results suggested that the variations of soil CO emission and microbial biomass and community composition in the subtropical plantation depended on the seasons and the levels and forms of N addition. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO) enrichment and increased nitrogen (N) deposition can change microbial activity and dissolved organic carbon (DOC) turnover, consequently affecting carbon sequestration in soils. However, we do not have much available information on the relationship between soil DOC and microbial activity under CO enrichment and N addition in semi-arid agroecosystems. Using free air CO enrichment (FACE), soybean and winter wheat were grown in the field under ambient CO (350 μmol mol) and elevated CO (550 μmol mol) conditions subjected to two N fertilizer regimes (132 and 306 kg N ha year). Rhizosphere soils and bulk soils at three depths, 0-10, 10-20 and 20-40 cm, were collected to determine water extractable organic matter (WEOM) characteristics with fluorescence spectroscopy and parallel factor analyses of excitation/emission matrix, as well as five extracellular enzymes activities. All significant effects were observed in the topsoil (0-10 cm): elevated CO decreased water extractable organic carbon concentration of the rhizosphere soils and bulk soils by 8.5 and 10.1 %, respectively. Furthermore, elevated CO changed the composition and structure of soil WEOM by increasing the plant- and microbial-derived components in the rhizosphere and solubilizing soil organic matter (SOM). The activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase were stimulated by elevated CO in the rhizosphere soils and bulk soils. Our findings suggest that the stimulation of microbial activity elicited by elevated CO increased the turnover of labile WEOM and the solubilization of SOM in the topsoils, which could be adverse to the accumulation and stability of soil carbon in the semi-arid agroecosystems in northern China. [ABSTRACT FROM AUTHOR]

Soil microbial communities play an essential role in soil carbon (C) emission and C sequestration in forest ecosystems. However, little information is available regarding the relationship between soil C dynamics and microbial substrate utilization at large scales. Along the North-South Transect of Eastern China (NSTEC), seven forests representative of boreal, temperate and tropical biomes were examined. Soil heterotrophic respiration (R), soil dissolved organic C (DOC), microbial biomass C (MBC), and microbial community-level physiological profiles (CLPPs) were investigated using biochemical measurements, static chamber-gas chromatography analysis, and Biolog-Eco microplates, respectively. We found that soil R rates were significantly higher in subtropical and boreal forests than in temperate forests. Conversely, the concentrations of soil DOC and MBC, as well as microbial metabolic activity and functional diversity, were consistently higher in temperate forests than in subtropical forests. There were considerably different substrate utilization profiles among the boreal, temperate, and subtropical forests. Soil microorganisms from the temperate and boreal forests mainly metabolized high-energy substrates, while those from the subtropical forests used all substrates equally. In addition, soil R rates were significantly and negatively related to soil labile C concentrations, total metabolic activity, and the intensity of individual substrate utilization, indicating that soil microbes assimilated more soil substrates, thereby reducing CO emissions. Overall, our study suggests that climate factors, as well as substrate availability, dominate the activities and functions of soil microbes at large scales. [ABSTRACT FROM AUTHOR]

Highlights: [•] N dose rather than N form significantly changed quantity and quality of soil DOC. [•] The responses of soil DOC amount and composition to N addition depend on soil layer. [•] Changes in microbial decomposition and humification processes dominate DOC dynamics. [•] Increasing atmospheric N deposition could decrease soil C storage in the alpine meadow. [Copyright &y& Elsevier]

To date, few studies are conducted to quantify the effects of reduced ammonium (NH4+) and oxidized nitrate (NO3−) on soil CH4 uptake and N2O emission in the subtropical forests. In this study, NH4Cl and NaNO3 fertilizers were applied at three rates: 0, 40 and 120 kg N ha−1 yr−1. Soil CH4 and N2O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH4 and N2O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH4+-N concentrations, but high levels of applied NH4Cl and NaNO3 fertilizers significantly increased soil NO3−-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH4 uptake, but significantly promoted soil N2O emission by 403% to 762%. Furthermore, NH4+-N fertilizer application had a stronger inhibition to soil CH4 uptake and a stronger promotion to soil N2O emission than NO3−-N application. Also, both soil CH4 and N2O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH4 uptake and N2O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH4 uptake and N2O emission. [ABSTRACT FROM AUTHOR]

Abstract: It is crucial to understand the effects of enhanced nitrogen (N) deposition on soil methane (CH4) uptake to develop a better comprehension of carbon (C) dynamics in terrestrial ecosystems. A two-year field study was conducted to assess the effects of various forms of N (NH4 + and NO3 −) and associated N deposition rates (0, 10, 20 and 40kgNha−1 yr−1) on alpine meadow soil CH4 fluxes on the Qinghai–Tibetan Plateau, China. Soil CH4 fluxes, soil temperature, and soil moisture were monitored weekly using the static chamber technique and gas chromatography. Soil inorganic N pools, soil pH and aboveground biomass were measured monthly to examine the key controlling factors of soil CH4 flux. Our results showed that N addition significantly promoted plant growth and changed soil water-filled pore space (WFPS), but did not alter soil inorganic N storages over the short term. Low rates of N addition significantly decreased the seasonal amount of CH4 uptake by 8.6% compared with the control. Soil CH4 fluxes were mainly determined by soil WFPS, followed by inorganic N availability. N addition increased the contribution of soil WFPS, pH and soil NO3 − storage. The observed reduction in CH4 uptake caused by N addition may be largely due to a decrease in physical diffusion, as the biochemical inhibition effects on methanotrophic bacteria are minor. These results suggest that soil inorganic N is a regulatory factor of soil CH4 uptake, and its promotion or inhibition to soil CH4 uptake depends on the N status in terrestrial ecosystems. [Copyright &y& Elsevier]

Aims: To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai-Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO) exchange to N addition. Methods: Based on a multi-form, low-level N addition experiment, soil CO effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO efflux. Results: The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH-N pool. N applications significantly increased aboveground biomass and soil CO efflux; moreover, this effect was more significant from NH-N than from NO-N fertilizer. In addition, the soil CO efflux was mainly driven by soil temperature, followed by aboveground biomass and NH-N pool. Conclusions: These results suggest that chronic atmospheric N deposition will stimulate soil CO efflux in the alpine meadow on the Qinghai-Tibetan Plateau by increasing available N content and promoting plant growth. [ABSTRACT FROM AUTHOR]

• High rate of urea addition significantly enhances gross nitrification and reduced gross N immobilization. • Acid-tolerant AOA and fungi are responsible for the changes in autotrophic and heterotrophic nitrification, respectively. • Elevated organic N deposition tends to cause decoupling of microbial N cycle and increase the risk of N losses. • The threshold rate causing the shift of soil N transformations is estimated to be 60 to 120 kg N ha−1 yr−1. External inorganic and organic nitrogen (N) inputs can contrastingly affect the transformation and availability of N in forest soils. Studies have mainly focused on the effects of inorganic N enrichment, whereas little is known about the effects of organic N input on soil gross N transformation and the underlying microbial mechanisms. Here we conducted a laboratory 15N tracing study in a temperate needle-broadleaved mixed forest with a fertilization rate of 0, 20, 60, and 120 kg urea-N ha−1 yr−1 over three years. We investigated the key drivers of soil N transformation processes using a 15N tracing model in the context of selected soil chemical properties and microbial characteristics. Urea addition did not change soil gross N mineralization rates, while stimulating mineralization of labile organic N (M Nlab). Urea addition at a rate of 120 kg N ha−1 yr−1 significantly increased autotrophic nitrification and gross nitrification rates by 88% and 96%, respectively. In contrast, all the three levels of urea addition significantly reduced gross microbial N immobilization by 28% to 52%, leading to an increase in the accumulation of soil NO 3 −-N in the top 10 cm soil layer by 38% to 88%. The changes in autotrophic nitrification were primarily driven by acid-tolerant ammonia-oxidizing archaea (AOA). Fungi were responsible for the change in heterotrophic nitrification under organic N enrichment. Gross N transformation rates were predominately regulated by AOA and fungal abundances as well as soil NO 3 −-N content under high level of organic N addition. The response of soil N transformation to exogenous organic N input depended on N addition level with the threshold rate being estimated to be 60–120 kg N ha−1 yr−1. All lines of evidence showed that the temperate needle-broadleaved mixed forest is moving towards an opener microbial N cycle under elevated organic N deposition. Our finding suggests that the effect of organic N input on soil gross N transformation is different from that of inorganic N input, which should be considered in ecosystem process models. [ABSTRACT FROM AUTHOR]

Exogenous nitrogen (N) inputs greatly change the emission and uptake of carbon dioxide (CO2) and methane (CH4) from temperate grassland soils, thereby affecting the carbon (C) budget of regional terrestrial ecosystems. Relevant research focused on natural grassland, but the effects of N fertilization on C exchange fluxes from different forage soils and the driving mechanisms were poorly understood. Here, a three-year N addition experiment was conducted on cultivated grassland planted with alfalfa (Medicago sativa) and bromegrass (Bromus inermis) in Inner Mongolia. The fluxes of soil-atmospheric CO2 and CH4; the content of the total dissolved N (TDN); the dissolved organic N (DON); the dissolved organic C (DOC); NH4+–N and NO3−–N in soil; enzyme activity; and auxiliary variables (soil temperature and moisture) were simultaneously measured. The results showed that N fertilization (>75 kg N ha−1 year−1) caused more serious soil acidification for alfalfa planting than for brome planting. N fertilization stimulated P-acquiring hydrolase (AP) in soil for growing Bromus inermis but did not affect C- and N-acquiring hydrolases (AG, BG, CBH, BX, LAP, and NAG). The oxidase activities (PHO and PER) of soil for planting Bromus inermis were higher than soil for planting Medicago sativa, regardless of N, whether fertilization was applied or not. Forage species and N fertilization did not affect soil CO2 flux, whereas a high rate of N fertilization (150 kg N ha−1 year−1) significantly inhibited CH4 uptake in soil for planting Medicago sativa. A synergistic effect between CO2 emission and CH4 uptake in soil was found over the short term. Our findings highlight that forage species affect soil enzyme activity in response to N fertilization. Soil enzyme activity may be an important regulatory factor for C exchange from temperate artificial grassland soil in response to N fertilization. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen (N) deposition markedly increases nitrous oxide (N 2 O) emissions from subtropical plantation soils. However, the contrasting effects of deposited NH 4 + and NO 3 − on soil N 2 O emissions and the underlying mechanisms are not well understood. Based on the long-term N fertilization manipulative experiment with two types (NH 4 Cl and NaNO 3) and three levels (0, 40, and 120 kg N ha−1 yr−1), we investigated the response of soil N 2 O emissions to N addition, as well as the environmental and microbial control on soil N 2 O emissions using flux observations and molecular biology assays. Nitrogen addition significantly decreased soil pH by 0.30–0.67 units and increased soil N 2 O emissions by 6- to 21-fold relative to the control; furthermore, the effect of NH 4 Cl addition was significantly stronger than that of NaNO 3 addition. Nitrogen addition significantly increased the abundance of ammonia-oxidizing archaea (AOA), whereas high levels of NH 4 Cl addition (120 kg N ha−1 yr−1) significantly decreased the abundances of nirS , nirK , and nosZ. In addition, NH 4 Cl addition decreased the Shannon diversity indices of AOA, nirK , and nosZ communities. Changes in soil N 2 O flux were positively correlated with changes in soil NO 3 −-N concentration and AOA abundance, whereas negatively correlated with changes in soil pH. Soil acidification and enhanced AOA-dominated nitrification led to higher soil N 2 O emissions under NH 4 Cl addition relative to NaNO 3 addition. Overall, the contrasting effects of reduced NH 4 + and oxidized NO 3 − should be incorporated into ecosystem process models to accurately assess the response of N 2 O emissions from subtropical plantation soils to exogenous N input. [Display omitted] [ABSTRACT FROM AUTHOR]

• FTCs increase DOC release and change DOM chemical structure in boreal forest soils. • DOM activation is detrimental to DOC accumulation. • FTCs complicate the interactions between bacteria taxa and DOM component. Soil dissolved organic matter (DOM) is mainly derived from the microbial degradation of organic matter. Intense freeze–thaw cycles (FTCs) elicited by warming alter the production and consumption of DOM in boreal forest soils, but little is known about the interactions between soil microbial community and DOM chemical composition. In this study, a FTCs incubation experiment was conducted to investigate the association between DOM dynamics and microbial community. DOM fingerprints were characterized using pyrolysis gas chromatography-mass spectrometry (Py-GC/MS) technology, and bacterial and fungal community composition was analyzed using high-throughput sequencing and bioinformatics. Both two and six freeze–thaw cycles (2FTCs and 6FTCs) significantly increased soil dissolved organic carbon (DOC) and NH 4 +-N contents at 0–10 cm and 50–60 cm depths. FTCs significantly decreased the relative abundance of aromatic components in DOM, whereas increasing the relative abundance of polysaccharides and fatty acid methyl esters (FAMEs) in DOM. The degradation of lignin-derived aromatic compounds was accelerated and microbial-derived components accumulated. Also, bacterial community responded more sensitively to FTCs than fungal community. Significant changes in bacterial community network was observed during FTCs with increased linkages, shortened average path lengths, reduced modularity and relative abundance of dominant phyla, and decreased α diversity. However, FTCs only increased the average path length of soil fungal community. The higher connectivity indicated that FTCs enhanced the interactions between microbial community and DOM chemical components. FTCs led to a shift in bacterial keystone taxa from being present in specific DOM molecules to competing for common substrates. Overall, FTCs tended to activate DOM and increase availability, which could promote DOM turnover and increase soil C stabilization. These findings provide new insights into the molecular mechanisms of DOM dynamics in the boreal forest soils under increased FTCs. [ABSTRACT FROM AUTHOR]

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of N (δN) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, NO emission and inorganic N leaching as well as N concentration and δN of leaves, litters and soils. We found that foliar N concentration and δN were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly increased foliar δN ( p < 0.05). Also, N addition decreased the δN of top soil in the N-poor pine and mixed forests and significantly increased that of organic and mineral soils in N-rich broadleaf forests ( p < 0.05). In addition, the soil NO emission flux and inorganic N leaching rate increased with increasing N addition and were positively correlated with the N enrichment factor ( ε) of forest ecosystems. Our study indicates that δN of leaf, litter and soil integrates various information on plant species, forest stand age, exogenous N input and soil N transformation and loss, which can be used to monitor N availability and N dynamics in forest ecosystems caused by increasing N deposition in the future. [ABSTRACT FROM AUTHOR]

• Ammonium and nitrate fertilization had opposite effects on SOC contents. • Nitrate fertilization sequestrated more SOC through increasing plant inputs. • Ammonium treatment shifted microbial community structure from bacteria to fungi. • Py-GC/MS is a powerful technique to investigate the chemical structure of SOM. Elevated nitrogen (N) deposition plays a critical role in soil organic matter (SOM) transformation, but the responses of the quantity and quality of SOM to ammonium and nitrate fertilization remain unclear. In this work, a 6-year field experiment amended with ammonium chloride (NH 4 Cl) and potassium nitrate (KNO 3) fertilizers at four rates of 0, 10, 20, and 40 kg N ha−1 yr−1 was conducted in a boreal forest. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analysis and elemental analysis were used to investigate the chemical composition and soil organic carbon (SOC) contents in bulk soils and two density fractions (light fraction < 1.70 g cm−3 and heavy fraction > 1.70 g cm−3). Soil microbial community composition was determined based on phospholipid fatty acids (PLFAs) analysis. Results showed that the SOC contents in the organic and mineral layers under nitrate addition were significantly increased by 27–87%. By contrast, ammonium addition significantly decreased the SOC contents in the organic layer by 28–54%. Nitrate addition increased relative proportions of acetic acid in the light fraction and toluene in bulk soil, but decreased relative abundance of phenol in bulk soil, indicating that more plant-derived input contributed to the increase of SOC. Contrastingly, ammonium addition increased the proportions of benzene in bulk soil and pyrrole as well as the mineralization index (pyrrole/phenol) in the heavy fraction, suggesting more advanced mineralization degree of SOM. Moreover, ammonium addition altered microbial community structure with a higher ratio of fungi/bacteria (F/B) while nitrate addition had little effect on soil microbial community. Overall, our findings highlight the contrasting roles of ammonium and nitrate in SOM dynamics, which should be distinguished to accurately predict the responses of SOC sequestration to N deposition especially in N-limited boreal forests. [ABSTRACT FROM AUTHOR]

Cultivated grassland can serve as one solution to the degradation of natural grassland. However, less is understood of the response of the microbial communities in cultivated grassland soils to global change factors such as nitrogen (N) deposition and precipitation change, and their linkages with the dynamics of soil organic carbon (SOC). In this study, field experimental plots that simulate different N deposition and precipitation levels were established to investigate the responses of soil microbial communities and their linkages with particulate-sized SOC fractions. Results showed that the activities of hydrolytic enzymes responded significantly to N addition, watering and their combinations, whereas the abundance and composition of microbial communities showed no significant difference among treatments. Addition of N and water generally promoted the activities of N-degrading enzymes such as β-N-acetylglucosaminidase (NAG) and leucine aminopeptidase (LAP), and thus decreased the C- to N-degrading activity (enzyme C/N ratio). This decrease in enzyme C/N ratio was significantly correlated with the accumulation of microbial-accessible particulate organic carbon (POC), indicating that three years of N application did not alleviate microbial N limitation so that microbial communities had invested more in acquisition of N instead of C. In contrast, the content of microbial-inaccessible mineral associated organic carbon (MAOC) was decreased probably as a result of N limitation on the suppressed microbial growth. Collectively, these results highlight that stoichiometric deviation between substrate and microbial demand can be one critical driver for SOC dynamics in cultivated grassland under global change scenarios. • Nitrogen addition and watering significantly affected soil enzymatic activity. • The C- to N-degrading activity was decreased by N addition and watering. • Nitrogen addition and watering generally promotes POC but reduces MAOC. • Changes in POC and MAOC were tightly related to alterations in enzyme C:N ratio. [ABSTRACT FROM AUTHOR]

• The modules reproduced the patterns of N 2 O emission, but some failed to simulate larger emission due to the addition of NH 4 +-N compared to NO 3 –-N. • The analysis highlights the need to improve the representation of N 2 O production and diffusion process in soil. • The use of WFPS in model methodology as a key scheme mediating microbial processes could probably improve the performance of N 2 O model. The contributions of long-lived nitrous oxide (N 2 O) to global climate and environment have received increasing attention. Especially, atmospheric nitrogen (N) deposition has substantially increased in recent decades due to the extensive use of fossil fuels in industry, which strongly stimulates the N 2 O emissions of terrestrial ecosystem. Several models have been developed to simulate the impacts of environmental factors on N 2 O emission from soil, but there are still large differences in the simulations of N 2 O emission and their responses to atmospheric deposition over global or regional scales. Using observations from N addition experiments in a subtropical forest, this study compared five widely-used N 2 O modules or algorithms (i.e. the N 2 O modules of DayCENT, PnET-N-DNDC and DyN, and the algorithm of NOE and NGAS) to investigate their performances for reproducing N 2 O emission, and especially the impacts of two forms of N additions (i.e. NH 4 +-N and NO 3 –-N, respectively) of two levels (low and high) on N 2 O emission. In general, the five modules reproduced the seasonal variations of N 2 O emission. Under the high levels of N addition compared to low ones for both NH 4 +-N and NO 3 –-N, however, not all modules can reproduce larger N 2 O emission. Relatively larger N 2 O emissions in measurements due to NH 4 +-N compared to NO 3 –-N additions were not indicated neither in all the modules. Moreover, there were substantial differences in simulating the ratios of N 2 O emission from nitrification and denitrification processes due to disagreements in the structure of these modules or algorithms. The comparison highlights the need to improve the representation of N 2 O production and diffusion processes. At the same time, it also highlights the application of WFPS in the model methodology as a key scheme that mediates the two microbial processes, i.e. nitrification and denitrification, could probably improve the performances of N 2 O models in future research. [ABSTRACT FROM AUTHOR]