Elevated temperature shifts soil N cycling from microbial immobilization to enhanced mineralization,nitrification and denitrification across global terrestrial ecosystems

We assessed the response of soil microbial nitrogen (N) cycling and associated functional genes to elevated temperature at the global scale. A meta‐analysis of 1,270 observations from 134 publications indicated that elevated temperature decreased soil microbial biomass N and increased N mineralization rates, both in the presence and absence of plants. These findings infer that elevated temperature drives microbially mediated N cycling processes from dominance by anabolic to catabolic reaction processes. Elevated temperature increased soil nitrification and denitrification rates, leading to an increase in N₂O emissions of up to 227%, whether plants were present or not. Rates of N mineralization, denitrification and N₂O emission demonstrated significant positive relationships with rates of CO₂ emissions under elevated temperatures, suggesting that microbial N cycling processes were associated with enhanced microbial carbon (C) metabolism due to soil warming. The response in the abundance of relevant genes to elevated temperature was not always consistent with changes in N cycling processes. While elevated temperature increased the abundances of the nirS gene with plants and nosZ genes without plants, there was no effect on the abundances of the ammonia‐oxidizing archaea amoA gene, ammonia‐oxidizing bacteria amoA and nirK genes. This study provides the first global‐scale assessment demonstrating that elevated temperature shifts N cycling from microbial immobilization to enhanced mineralization, nitrification and denitrification in terrestrial ecosystems. These findings infer that elevated temperatures have a profound impact on global N cycling processes with implications of a positive feedback to global climate and emphasize the close linkage between soil microbial C and N cycling.     

Can differences in microbial abundances help explain enhanced N2O emissions in a permanent grassland under elevated atmospheric CO2?

Long‐term effects of elevated atmospheric CO₂ on the ammonia‐oxidizing and denitrifying bacteria in a grassland soil were investigated to test whether a shift in abundance of these N‐cycling microorganisms was responsible for enhanced N₂O emissions under elevated atmospheric CO₂. Soil samples (7.5 cm increments to 45 cm depth) were collected in 2008 from the University of Giessen Free Air Carbon dioxide Enrichment (GiFACE), a permanent grassland exposed to moderately elevated atmospheric CO₂ (+20%) since 1998. GiFACE plots lay on a soil moisture gradient because of gradually changing depth to the underlying water table and labeled as the DRY block (furthest from water table), MED block (intermediate to water table), and WET block (nearest to water table). Mean N₂O emissions measured since 1998 have been significantly higher under elevated CO₂. This study sought to identify microbial and biochemical parameters that might explain higher N₂O emissions under elevated CO₂. Soil biochemical parameters [extractable organic carbon (EOC), dissolved organic nitrogen (DON), NH₄⁺, NO₃^?], and abundances of genes encoding the key enzymes involved in ammonia oxidation (amoA) and denitrification (nirK, nirS, nosZ) depended more on soil depth and block (underlying soil moisture gradient) than on elevated CO₂. Ammonia oxidation and denitrification gene abundances, relative abundances (ratios) of nirS to nirK, of nosZ to both nirS and to nirK, and of the measured soil biochemical properties DON and NO₃^? tended to be lower in elevated CO₂ plots as compared with ambient plots in the MED and WET blocks while the DRY block exhibited an opposite trend. High N₂O emissions under elevated CO₂ in the MED and WET blocks correlated with lower nosZ to nirK ratios, suggesting that increased N₂O emissions under elevated CO₂ might be caused by a higher proportion of N₂O‐producing rather than N₂O consuming (N₂ producing) denitrifiers.     

氮磷添加对草甸草原土壤氮磷转化功能基因丰度的影响

氮添加是提高退化草地生产力的主要养分管理措施,而过量的氮输入会导致土壤酸化、增加硝酸盐淋溶损失和温室气体排放。旨在明确草原割草利用下土壤氮、磷转化功能基因丰度对氮磷添加的响应规律,为定向调控打草场土壤氮、磷转化过程,提高养分利用效率,减少温室气体N₂O排放提供科学依据。2018—2020年在呼伦贝尔草甸草原打草场设置了5个施氮水平(0、1.55、4.65、13.95、27.9 g N m^-2 a^-1)和3个磷水平(0、5.24、10.48 g P m^-2 a^-1),裂区试验设计,在植物不同生长时期测定土壤氨氧化(amoA-AOA和amoA-AOB)、反硝化(narG、nirK、nirS和nosZ)和磷转化(phoD)基因丰度。结果表明,土壤氮转化基因丰度受到氮、磷添加的调控,而氮、磷添加对土壤磷转化功能基因丰度无显著影响(P>0.05)。氮添加可提高amoA-AOB基因丰度,增加氨氧化细菌调控土壤总硝化速率的相对重要性,因此能增加硝酸盐淋溶损失潜势。高氮处理下添加磷可降低...     

不同土地利用方式土壤氨氧化微生物和反硝化微生物时空分布特征

研究不同土地利用方式下氮循环相关微生物在不同土壤剖面的分布,可为认识和理解土壤氮转化过程提供科学依据。土壤氨氧化微生物和反硝化微生物在调节氮肥利用率、硝态氮淋溶和氧化亚氮(N₂O)排放等方面有着重要作用。以北京郊区农田和林地两种土地利用方式为研究对象,分析土壤氨氧化潜势和亚硝酸盐氧化潜势在0—100 cm土壤剖面上的季节分布(春季和秋季),并通过实时荧光定量PCR方法表征土壤氨氧化和反硝化微生物的时空分布特征。结果表明,农田土壤氨氧化潜势、亚硝酸盐氧化潜势、氨氧化微生物和反硝化微生物丰度均显著高于林地土壤,且随土壤深度增加而显著降低。除氨氧化古菌amoA基因丰度在不同季节间无显著差异外,春季土壤氨氧化细菌(amoA基因)、反硝化微生物nirS、nirK和典型nosZ I基因的丰度均显著高于秋季。土壤有机质、总氮、NH~+₄-N、NO~-₃-N含量与氨氧化微生物和反硝化微生物的功能基因丰度显著相关。综上,不同土地利用方式下土壤氮循环相关微生物的丰度与土壤氮素的可利用性和转化过程紧密相关,研究结果对土壤氮素利用和养分管理提供...     

Elevated Atmospheric CO2 Impacts Abundance and Diversity of Nitrogen Cycling Functional Genes in Soil

The concentration of CO₂ in the Earth's atmosphere has increased over the last century. Although this increase is unlikely to have direct effects on soil microbial communities, increased atmospheric CO₂ may impact soil ecosystems indirectly through plant responses. This study tested the hypothesis that exposure of plants to elevated CO₂ would impact soil microorganisms responsible for key nitrogen cycling processes, specifically denitrification and nitrification. We grew trembling aspen (Populus tremuloides) trees in outdoor chambers under ambient (360 ppm) or elevated (720 ppm) levels of CO₂ for 5 years and analyzed the microbial communities in the soils below the trees using quantitative polymerase chain reaction and clone library sequencing targeting the nitrite reductase (nirK) and ammonia monooxygenase (amoA) genes. We observed a more than twofold increase in copy numbers of nirK and a decrease in nirK diversity with CO₂ enrichment, with an increased predominance of Bradyrhizobia-like nirK sequences. We suggest that this dramatic increase in nirK-containing bacteria may have contributed to the significant loss of soil N in the CO₂-treated chambers. Elevated CO₂ also resulted in a significant decrease in copy numbers of bacterial amoA, but no change in archaeal amoA copy numbers. The decrease in abundance of bacterial amoA was likely a result of the loss of soil N in the CO₂-treated chambers, while the lack of response for archaeal amoA supports the hypothesis that physiological differences in these two groups of ammonia oxidizers may enable them to occupy distinct ecological niches and respond differently to environmental change.     

生物炭对褐土旱地玉米季氮转化功能基因、丛枝菌根真菌及N2O释放的影响

以豫西旱地玉米农田为研究对象,设置不同生物炭施用量处理（T0：不施用生物炭;T1：施用生物炭20 t/hm²;T2：施用生物炭40 t/hm²）,采用密闭式静态箱法测定N₂O排放通量和荧光定量PCR法分析丛枝菌根（arbuscular mycorrhizal,AM）真菌、氨单加氧酶（amoA）、亚硝酸盐还原酶（nirS、nirK）以及氧化亚氮还原酶（nosZ）的基因丰度,同时测定土壤理化性状的变化。研究结果表明,随着生物炭施用量的增加,土壤pH和含水量呈增加趋势,土壤有机碳、全氮和铵态氮含量显著提高,土壤容重和硝态氮含量显著降低。T1和T2处理土壤有机碳含量分别较T0显著提高38.44%和71.01%;T1和T2处理土壤铵态氮含量分别较T0显著增加15.89%和30.46%;T2处理土壤全氮含量较T0处理显著提高14.87%;T1和T2处理土壤硝态氮含量分别较T0减少10.57%和21.40%。随着生物炭施用量的增加,AM真菌侵染率显著增加,T1和T2处理分别较T0处理提高71.88%和115.88%;AOA、AOB、nirK和nirS基因丰度显著降低;nosZ基因丰度增加。施加生物炭处理的N₂O排放通量和累积排放量均低于不施生物炭处理,具体表现为：T0 > T1 > T2。相关分析表明,生物炭施用量与AM真菌基因丰度呈显著正相关;与nosZ基因丰度呈正相关;与AOA、AOB、nirK、nirS基因丰度呈极显著负相关。N₂O排放通量与AOA、nirK、nirS基因丰度呈极显著正相关;与土壤含水量和土壤硝态氮含量呈显著正相关;与AM真菌、nosZ基因丰度、易提取球囊霉素含量、铵态氮含量呈极显著负相关。集成推进树（ABT）分析表明,AOA对N₂O排放的影响最大,其次是AM真菌和nirS。总之,生物炭处理改善土壤理化性质、提高土壤AM真菌侵染率、调节硝化、反硝化相关功能基因的丰度,减少N₂O气体排放,为旱地农田合理施用生物炭减少N₂O气体排放提供理论依据。     

滨海湿地生态系统微生物驱动的氮循环研究进展

滨海湿地生态系统介于陆地生态系统和海洋生态系统之间,其类型多种多样,环境差异极大,微生物种类丰富。近年来,随着人为氮源的大量输入,造成滨海湿地生态系统富营养化污染问题日趋严重。本文主要总结了滨海湿地生态系统微生物驱动的固氮、硝化、反硝化、厌氧氨氧化、NO_3~-还原成铵等主要氮循环过程,并综述了通过功能基因(如nifH、amoA、hzo、nirS、nirK、nrfA)检测微生物群落多样性及其环境影响因素的相关研究,旨在更好理解微生物驱动氮循环过程以去除氮,以期为减轻富营养化和危害性藻类爆发提供科学依据。     

Terrestrial N2O emissions and related functional genes under climate change: A global meta‐analysis

Nitrous oxide (N₂O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N₂O production across terrestrial ecosystems remain poorly understood. Here, we synthesized 46 published studies of N₂O fluxes and relevant soil functional genes (SFGs, that is, archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the data set, temperature increased N₂O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N₂O emissions (most effectively induced by open‐top chambers). Whole‐day or whole‐year warming treatment significantly enhanced N₂O emissions, but daytime, nighttime or short‐season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N₂O emission by an average of 55%, while decreased precipitation suppressed N₂O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U‐shape relationship with soil moisture; further insight into biotic mechanisms underlying N₂O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N₂O emission and highlights the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change.     

Dominance of bacterial ammonium oxidizers and fungal denitrifiers in the complex nitrogen cycle pathways related to nitrous oxide emission

Organic compounds and mineral nitrogen (N) usually increase nitrous oxide (N₂O) emissions. Vinasse, a by‐product of bio‐ethanol production that is rich in carbon, nitrogen, and potassium, is recycled in sugarcane fields as a bio‐fertilizer. Vinasse can contribute significantly to N₂O emissions when applied with N in sugarcane plantations, a common practice. However, the biological processes involved in N₂O emissions under this management practice are unknown. This study investigated the roles of nitrification and denitrification in N₂O emissions from straw‐covered soils amended with different vinasses (CV: concentrated and V: nonconcentrated) before or at the same time as mineral fertilizers at different time points of the sugarcane cycle in two seasons. N₂O emissions were evaluated for 90 days, the period that occurs most of the N₂O emission from fertilizers; the microbial genes encoding enzymes involved in N₂O production (archaeal and bacterial amoA, fungal and bacterial nirK, and bacterial nirS and nosZ), total bacteria, and total fungi were quantified by real‐time PCR. The application of CV and V in conjunction with mineral N resulted in higher N₂O emissions than the application of N fertilizer alone. The strategy of vinasse application 30 days before mineral N reduced N₂O emissions by 65% for CV, but not for V. Independent of rainy or dry season, the microbial processes were nitrification by ammonia‐oxidizing bacteria (AOB) and archaea and denitrification by bacteria and fungi. The contributions of each process differed and depended on soil moisture, soil pH, and N sources. We concluded that amoA‐AOB was the most important gene related to N₂O emissions, which indicates that nitrification by AOB is the main microbial‐driven process linked to N₂O emissions in tropical soil. Interestingly, fungal nirK was also significantly correlated with N₂O emissions, suggesting that denitrification by fungi contributes to N₂O emission in soils receiving straw and vinasse application.     

Response of the Abundance of Key Soil Microbial Nitrogen-Cycling Genes to Multi-Factorial Global Changes

Multiple co-occurring environmental changes are affecting soil nitrogen cycling processes, which are mainly mediated by microbes. While it is likely that various nitrogen-cycling functional groups will respond differently to such environmental changes, very little is known about their relative responsiveness. Here we conducted four long-term experiments in a steppe ecosystem by removing plant functional groups, mowing, adding nitrogen, adding phosphorus, watering, warming, and manipulating some of their combinations. We quantified the abundance of seven nitrogen-cycling genes, including those for fixation (nifH), mineralization (chiA), nitrification (amoA of ammonia-oxidizing bacteria (AOB) or archaea (AOA)), and denitrification (nirS, nirK and nosZ). First, for each gene, we compared its sensitivities to different environmental changes and found that the abundances of various genes were sensitive to distinct and different factors. Overall, the abundances of nearly all genes were sensitive to nitrogen enrichment. In addition, the abundances of the chiA and nosZ genes were sensitive to plant functional group removal, the AOB-amoA gene abundance to phosphorus enrichment when nitrogen was added simultaneously, and the nirS and nirK gene abundances responded to watering. Second, for each single- or multi-factorial environmental change, we compared the sensitivities of the abundances of different genes and found that different environmental changes primarily affected different gene abundances. Overall, AOB-amoA gene abundance was most responsive, followed by the two denitrifying genes nosZ and nirS, while the other genes were less sensitive. These results provide, for the first time, systematic insights into how the abundance of each type of nitrogen-cycling gene and the equilibrium state of all these nitrogen-cycling gene abundances would shift under each single- or multi-factorial global change.     

Reduced N cycling in response to elevated CO2, warming,and drought in a Danish heathland: Synthesizing results of the CLIMAITE project after two years of treatments

Field‐scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a seminatural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO₂), warming, and extended summer drought in all combinations. Here, we report on the short‐term responses of the nitrogen (N) cycle after 2 years of treatments. Elevated CO₂ significantly affected aboveground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co‐dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Belowground, elevated CO₂ had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH₄⁺ consumption, gross mineralization, potential nitrification, denitrification and N₂O emissions. Drought reduced belowground gross N mineralization and decreased fauna N mass and fauna N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue in response to elevated CO₂, as well as drought‐induced decreases in litter N production and fauna N mineralization prevailed in the full treatment combination. Overall, the simulated future climate scenario therefore lead to reduced N turnover, which could act to reduce the potential growth response of plants to elevated atmospheric CO₂ concentration.     

Elevated atmospheric CO2 in open top chambers increases net nitrification and potential denitrification

The control of soil nitrogen (N) availability under elevated atmospheric CO₂ is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO₂ on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO₂ (ambient + 400 µmol mol^?1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N₂O emissions were significantly higher in soils from the elevated CO₂ treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO₃^– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO₂ treatment in the field when incubated in the laboratory under elevated CO₂ (increase of 100%), but there was no effect when incubated under ambient CO₂. Net nitrate production of the soil originating from the ambient CO₂ treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO₂ on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO₂ through higher soil CO₂ concentrations. Alternatively, lower microbial NH₄ assimilation under elevated CO₂ might explain the higher net nitrification. We conclude that elevated atmospheric CO₂ has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO₂ concentrations compared to atmospheric concentrations.     

Who contributes more to N2O emission during sludge bio-drying with two different aeration strategies,nitrifiers or denitrifiers?

Global warming effects have drawn more and more attention to studying all sources and sinks of nitrous oxide (N₂O). Sludge bio-drying, as an effective sludge treatment technology, is being adopted worldwide. In this study, two aeration strategies (piles I and II) were compared to investigate the primary contributors to N₂O emission during sludge bio-drying through studying the evolution of functional genes involved in nitrification (amoA, hao, and nxrA) and denitrification (narG, nirS, nirK, norB, and nosZ) by quantitative PCR (qPCR). Results showed that the profile of N₂O emission can be divided into three stages, traditional denitrification contributed largely to N₂O emission at stage I (days 1–5), but N₂O emission mainly happened at stage II (days 5–14) due to nitrifier denitrification and NH₂OH accumulation by ammonia-oxidizing bacteria (AOB), accounting for 51.4% and 58.2% of total N₂O emission for piles I and II, respectively. At stage III (days 14–21), nitrifier denitrification was inhibited because sludge bio-drying proceeded mainly by the physical aeration, thus N₂O emission decreased and changed little. The improved aeration strategy availed pile I to reduce N₂O emission much especially at stages II and III, respectively. These results indicated that nitrifier denitrification by AOB and biological NH₂OH oxidation due to AOB made more contribution to N₂O emission, and aeration strategy was crucial to mitigate N₂O emission during sludge bio-drying.

     

Genetic and Environmental Controls on Nitrous Oxide Accumulation in Lakes

We studied potential links between environmental factors, nitrous oxide (N₂O) accumulation, and genetic indicators of nitrite and N₂O reducing bacteria in 12 boreal lakes. Denitrifying bacteria were investigated by quantifying genes encoding nitrite and N₂O reductases (nirS/nirK and nosZ, respectively, including the two phylogenetically distinct clades nosZ _I and nosZ _II) in lake sediments. Summertime N₂O accumulation and hypolimnetic nitrate concentrations were positively correlated both at the inter-lake scale and within a depth transect of an individual lake (Lake Vanajavesi). The variability in the individual nirS, nirK, nosZ _I, and nosZ _II gene abundances was high (up to tenfold) among the lakes, which allowed us to study the expected links between the ecosystem’s nir-vs-nos gene inventories and N₂O accumulation. Inter-lake variation in N₂O accumulation was indeed connected to the relative abundance of nitrite versus N₂O reductase genes, i.e. the (nirS+nirK)/nosZ _I gene ratio. In addition, the ratios of (nirS+nirK)/nosZ _I at the inter-lake scale and (nirS+nirK)/nosZ _I+II within Lake Vanajavesi correlated positively with nitrate availability. The results suggest that ambient nitrate concentration can be an important modulator of the N₂O accumulation in lake ecosystems, either directly by increasing the overall rate of denitrification or indirectly by controlling the balance of nitrite versus N₂O reductase carrying organisms.     

Ectomycorrhizal fungi inoculation alleviates simulated acid rain effects on soil ammonia oxidizers and denitrifiers in Masson pine forest

Acid rain can cause severe effects on soil biota and nutrient biogeochemical cycles in the forest ecosystem, but how plant-symbiotic ectomycorrhizal fungi will modulate the effects remains unknown. Here, we conducted a full factorial field experiment in a Masson pine forest by simultaneously controlling the acidity of the simulated rain (pH 5.6 vs. pH 3.5) and the ectomycorrhizal fungi Pisolithus tinctorius inoculation (non-inoculation vs. inoculation), to investigate the effects on ammonia oxidizers and denitrifiers. After 10 months, compared with the control (rain pH 5.6, and non-inoculation), simulated acid rain (pH 3.5) reduced soil nutrient content, decreased archaeal amoA gene abundance and inhibited denitrification enzyme activity. Also, simulated acid rain altered the community compositions of all the examined functional genes (archaeal amoA, bacterial amoA, nirK, nirS and nosZ). However, inoculation with ectomycorrhizal fungi under acid rain stress recovered soil nutrient content, archaeal amoA gene abundance and denitrification enzyme activity to levels comparable to the control, suggesting that ectomycorrhizal fungi inoculation ameliorates simulated acid rain effects. Taken together, ectomycorrhizal fungi inoculation – potentially through improving soil substrate availability – could alleviate the deleterious effects of acid rain on nitrogen cycling microbes in forest soils.     

Effects of drought and N-fertilization on N cycling in two grassland soils

Changes in frequency and intensity of drought events are anticipated in many areas of the world. In pasture, drought effects on soil nitrogen (N) cycling are spatially and temporally heterogeneous due to N redistribution by grazers. We studied soil N cycling responses to simulated summer drought and N deposition by grazers in a 3-year field experiment replicated in two grasslands differing in climate and management. Cattle urine and NH₄NO₃ application increased soil NH₄ ⁺ and NO₃ ^? concentrations, and more so under drought due to reduced plant uptake and reduced nitrification and denitrification. Drought effects were, however, reflected to a minor extent only in potential nitrification, denitrifying enzyme activity (DEA), and the abundance of functional genes characteristic of nitrifying (bacterial and archaeal amoA) and denitrifying (narG, nirS, nirK, nosZ) micro-organisms. N₂O emissions, however, were much reduced under drought, suggesting that this effect was driven by environmental limitations rather than by changes in the activity potential or the size of the respective microbial communities. Cattle urine stimulated nitrification and, to a lesser extent, also DEA, but more so in the absence of drought. In contrast, NH₄NO₃ reduced the activity of nitrifiers and denitrifiers due to top-soil acidification. In summary, our data demonstrate that complex interactions between drought, mineral N availability, soil acidification, and plant nutrient uptake control soil N cycling and associated N₂O emissions. These interactive effects differed between processes of the soil N cycle, suggesting that the spatial heterogeneity in pastures needs to be taken into account when predicting changes in N cycling and associated N₂O emissions in a changing climate.     

Interactions between plant growth and soil nutrient cycling under elevated CO2: a meta-analysis

free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO₂ on nutrient cycling in terrestrial ecosystems. Using meta‐analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N₂ fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO₂ alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO₂ stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO₂ by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr^?1. Namely, elevated CO₂ stimulated overall above‐ and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO₂ respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr^?1) and above‐ and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO₂ in experiments receiving the high N treatments. Under low N availability, above‐ and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO₂ only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO₂ in the long‐term. Therefore, increased soil C input and soil C sequestration under elevated CO₂ can only be sustained in the long‐term when additional nutrients are supplied.     

Effects of elevated atmospheric CO2 on soil microbiota in calcareous grassland

Microbial responses to three years of CO₂ enrichment (600 μL L^–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO₂. Microbial C:N ratios significantly decreased under elevated CO₂ (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO₂ (qCO₂ = basal respiration/microbial C) did not exhibit any significant differences between CO₂ treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO₂ and phosphorus (P) fertilizer (1 g P m^–2 y^–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO₂. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short-term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO₂ on microbial activity or biomass in such nutrient-poor natural ecosystems is in marked contrast to previously published large and short-term microbial responses to CO₂ enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in disturbed or fertilized systems and that therefore microbial responses to elevated CO₂ must be studied in natural, undisturbed systems.     

Intergenomic Comparisons Highlight Modularity of the Denitrification Pathway and Underpin the Importance of Community Structure for N2O Emissions

Nitrous oxide (N₂O) is a potent greenhouse gas and the predominant ozone depleting substance. The only enzyme known to reduce N₂O is the nitrous oxide reductase, encoded by the nosZ gene, which is present among bacteria and archaea capable of either complete denitrification or only N₂O reduction to di-nitrogen gas. To determine whether the occurrence of nosZ, being a proxy for the trait N₂O reduction, differed among taxonomic groups, preferred habitats or organisms having either NirK or NirS nitrite reductases encoded by the nirK and nirS genes, respectively, 652 microbial genomes across 18 phyla were compared. Furthermore, the association of different co-occurrence patterns with enzymes reducing nitric oxide to N₂O encoded by nor genes was examined. We observed that co-occurrence patterns of denitrification genes were not randomly distributed across taxa, as specific patterns were found to be more dominant or absent than expected within different taxonomic groups. The nosZ gene had a significantly higher frequency of co-occurrence with nirS than with nirK and the presence or absence of a nor gene largely explained this pattern, as nirS almost always co-occurred with nor. This suggests that nirS type denitrifiers are more likely to be capable of complete denitrification and thus contribute less to N₂O emissions than nirK type denitrifiers under favorable environmental conditions_. Comparative phylogenetic analysis indicated a greater degree of shared evolutionary history between nosZ and nirS. However 30% of the organisms with nosZ did not possess either nir gene, with several of these also lacking nor, suggesting a potentially important role in N₂O reduction. Co-occurrence patterns were also non-randomly distributed amongst preferred habitat categories, with several habitats showing significant differences in the frequencies of nirS and nirK type denitrifiers. These results demonstrate that the denitrification pathway is highly modular, thus underpinning the importance of community structure for N₂O emissions.