Different nitrogen saturation thresholds for above-, below-, and total net primary productivity in a temperate steppe

Identifying the thresholds for the positive responses of total net primary productivity (NPP) to nitrogen (N) enrichment is an essential prerequisite for predicting the benefits of N deposition on ecosystem carbon sequestration. However, the responses of below-ground NPP (BNPP) to N enrichment are unknown in many ecosystems, which limits our ability to understand the carbon cycling under the scenario of increasing N availability. We examined the changes in above-ground NPP (ANPP), BNPP, and NPP of a temperate meadow steppe across a wide-ranging N addition gradient (0, 2, 5, 10, 20, and 50 g N m⁻² year⁻¹) during 5 years. Both ANPP and NPP increased nonlinearly with N addition rates. The N saturation threshold for ANPP (T_A) and NPP (T_N) was at the rate of 13.11 and 6.70 g N m⁻² year⁻¹, respectively. BNPP decreased with increasing N addition when N addition rates ˃5 g N m⁻² year⁻¹, resulting in much lower T_N than T_A. Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, and consequently on the earlier occurrence of N saturation threshold for NPP. Our results highlight the negative effects of soil N enrichment on NPP in natural grasslands super-saturated with N. Furthermore, by considering ANPP and BNPP simultaneously, our results indicate that previous findings from above-ground might have over-estimated the positive effects of N deposition on primary productivity.     

降水时间对内蒙古温带草原地上净初级生产力的影响

全球气候变化下降水时间的改变将深刻影响草原生态系统地上净初级生产力(ANPP),而草原生态系统ANPP是区域碳循环的重要过程.利用1998-2007年的SPOT-VEG NDVI数据并结合111个样点的ANPP地面样方调查数据,获得了内蒙古温带草原1998-2007年的ANPP区域数据,依此分析了中国内蒙古温带草原以及区域内的3种植被类型(荒漠草原、典型草原、草甸草原)降水时间对ANPP的影响.研究结果表明,对于整个内蒙古温带草原来说,一个水分年内(从上一年9月份到当年地上生物量达最大值时的8月份)影响ANPP较为重要的降水月份为2-7月份,其中,5-7月份降水尤为重要.具体到每个月降水的影响,研究发现,7月份降水最重要,而仍处于生长季的8月份降水相对于其他生长季降水作用最小;影响不同草地类型最重要的降水时期存在一定差异,对荒漠草原和典型草原地区来说,ANPP达最大值前3个月(5-7月份)的生长季降水最重要,而8月份降水影响较小,而草甸草原地区8月份和非生长季的3、4月份降水最重要,但各个降水时期降水对ANPP的影响都较荒漠草原和典型草原小,大部分地区降水对ANPP的影响不显著.     

Interactive effects of soil nitrogen and water availability on leaf mass loss in a temperate steppe

Although the link between leaf mass loss and assessment of ecosystem nutrient use efficiency and plant nutrient resorption efficiency has received considerable attention in various ecosystems, there has been relatively little effort to assess plant leaf mass loss during senescence, especially for herbaceous species. We conducted experimental studies to assess leaf mass loss during senescence in five dominant herbaceous species and examined the effects of increasing nitrogen (N) and water availability on leaf mass loss of four species in a temperate steppe in northern China. We nondestructively estimated mature leaf mass based on leaf length and width. Leaf mass loss varied substantially among species, ranging from 20–50%. On average across all species, N and water addition increased leaf mass loss by 30% and 19%, respectively. N and water addition interacted to affect leaf mass loss, as water addition had a significant positive effect on leaf mass loss under enriched N conditions but showed no effect under ambient N levels. We conclude that leaf mass loss of herbaceous plants was considerable and can potentially be more pronounced with increasing N and water availability. It is notable that the responses of plant species to N and water addition were variable. We suggest that leaf mass loss during senescence should be given full consideration in assessing nutrient use and resorption efficiency in semi-arid areas.     

长期氮添加对典型草原植物多样性与初级生产力的影响及途径

氮(N)沉降对陆地生态系统的结构和功能已产生了重要的影响, N也是中国北方草原植物生长和初级生产力的主要限制性元素。物种多样性和功能多样性是揭示生物多样性对生态系统功能维持机制的关键指标, 然而, 关于长期N添加下草原物种多样性与功能多样性的关系, 及其对初级生产力的影响途径及机制, 尚不十分清楚。为此, 该研究依托在内蒙古典型草原建立的长期N添加实验平台, 实验处理包括1个完全对照(不添加任何肥料)和6个N添加水平(0、1.75、5.25、10.50、17.50和28.00 g·m^-2·a^-1), 研究了长期N添加对典型草原物种多样性、功能多样性和初级生产力的影响大小及途径。结果表明: 1) N添加显著降低了典型草原的物种丰富度和Shannon-Wiener指数, 但对功能多样性(包括功能性状多样性指数和群落加权性状值)无显著的影响。2)结构方程模型分析表明, 功能多样性主要受物种丰富度的影响, 但是物种多样性减少并没有导致功能多样性降低, 其原因主要是功能群组成发生了改变, 即群落内多年生根茎禾草所占比例显著增加, 以致群落加权性状值变化不大。3) N通过影响物种丰富度和功能群组成, 间接影响群落加权性状值, 进而影响群落净初级生产力。其中, 群落加权性状值是最重要的影响因子, 可解释48%的初级生产力变化, 表明初级生产力主要是由群落内优势物种的生物量及功能性状所决定, 因此该研究的结果很好地支持了质量比假说。     

Increasing water and nitrogen availability enhanced net ecosystem CO2 assimilation of a temperate semiarid steppe

Boron (B) is an essential micronutrient, whose deficiency is common in boreal forests. Our aim was to investigate the effects of the B supply on the retranslocation of micro- and macro nutrients in seedlings of Betula pendula Roth. One-year-old seedlings were grown under three different levels of B: 0%, 30% and 100% of the standard level for complete nutrient solution. Half of the seedlings were harvested after summer period and another half when leaves abscised. Boron was not resorbed in significant amounts from senescing birch leaves prior to abscission. The only micronutrients resorbed were Zn and Ni. Three macronutrients, N, P, and S, were resorbed efficiently from senescing leaves and accumulated into the stems. The resorption of nutrients was the mostly pronounced in B0 seedlings and minimal in B30 seedlings, which, however, showed the highest accumulation of nutrients during autumn period at least partly independently from the resorption from senescing leaves. Boron was shown to be an immobile element in silver birch seedlings that was not withdrawn from senescing leaves prior to abscission. This may increase the B availability for other tree species but also increase the potential for its leaching.     

氮水添加对放牧背景下荒漠草原生产力的影响

水分与氮素作为干旱和半干旱草原生产力的共同限制性因子在退化草原的生态快速修复过程中备受关注。以不同放牧强度背景下的短花针茅荒漠草原为研究对象,开展围封模拟放牧利用实验,同时添加氮素和水分。通过分析历史放牧强度与年份对生产力的影响,以及添加氮素和水分对不同功能群植物生物量的作用,探讨放牧强度对短花针茅草原生产力的内在作用机制,以及如何实现荒漠草原资源合理开发和可持续利用。研究结果显示,降雨量与放牧强度决定着短花针茅草原的植物群落结构。氮素和水分添加可分别提升11%-29%和12%-32%的群落地上生物量,且二者存在显著的交互作用。不同功能群植物的地上生物量对氮素与水分添加的响应存在差异,多年生丛生禾草对氮素和水分添加响应最敏感。氮素与水分添加可显著提高多年生丛生禾草的地上生物量,但与自然降水量相关。氮素添加对地上生物量的影响在正常降雨和稍旱年份作用显著,而水分添加在干旱年份作用显著。在正常降雨年份,以半灌木植物为优势种的轻度放牧背景以添加水分对提升生产力最宜,以多年生丛生禾草和半灌木为共优种的中度放牧背景和以多年生丛生禾草为优势种的重度放牧以同时添加水分和氮素对提升生产力最为宜;在干旱年份不同放牧强度背景下均以同时添加水分和氮素对提升生产力最为宜。我们的结果表明了养分与资源的改善有利于退化短花针茅草原的快速恢复和可持续生产。     

Differential responses of auto‐ and heterotrophic soil respiration to water and nitrogen addition in a semiarid temperate steppe

Evaluating how autotrophic (SR_A), heterotrophic (SR_H) and total soil respiration (SR_TOT) respond differently to changes of environmental factors is critical to get an understanding of ecosystem carbon (C) cycling and its feedback processes to climate change. A field experiment was conducted to examine the responses of SR_A and SR_H to water and nitrogen (N) addition in a temperate steppe in northern China during two hydrologically contrasting growing seasons. Water addition stimulated SR_A and SR_H in both years, and their increases were significantly greater in a dry year (2007) than in a wet year (2006). N addition increased SR_A in 2006 but not in 2007, while it decreased SR_H in both years, leading to a positive response of SR_TOT in 2006 but a negative one in 2007. The different responses of SR_A and SR_H indicate that it will be uncertain to predict soil C storage if SR_TOT is used instead of SR_H to estimate variations in soil C storage. Overall, N addition is likely to enhance soil C storage, while the impacts of water addition are determined by its relative effects on carbon input (plant growth) and SR_H. Antecedent water conditions played an important role in controlling responses of SR_A, SR_H and the consequent SR_TOT to water and N addition. Our findings highlight the predominance of hydrological conditions in regulating the responses of C cycling to global change in the semiarid temperate steppe of northern China.     

水氮添加对内蒙古温带典型草原植被氮含量季节和年际动态的影响

作为对全球变化响应最敏感的生态系统类型之一,草原生态系统植被氮含量的季节、年际变化及其对气候变化(氮沉降、降水格局改变)的响应研究相对匮乏。基于内蒙古温带典型草原5年的氮添加(10 g N m^-2 a^-1)和水添加(添加量80 mm,分2 mm×40次、5 mm×16次、10 mm×8次、20 mm×4次、40 mm×2次5种处理)控制试验分析了水氮添加后植被氮含量在生态系统和物种两个水平的季节和年际变化。结果表明,高强度水添加处理有降低(10 mm/次和40 mm/次)生态系统氮含量的趋势,但不显著,小强度水添加处理(2 mm/次、5 mm/次)在不同年份之间无一致的升高或降低趋势,但所有水添加处理有降低两种优势物种整个生长季氮含量的趋势。氮添加促进生态系统和两种优势物种整个生长季的氮含量,但该促进作用可被水添加抵消,且这种抵消作用随水氮添加年限的延长而加剧。水氮添加均增加了生态系统氮含量的年际变异,但对特定物种季节内变异的影响在湿润和干旱年份存在一定差异。本研究将为预测草原生态系统对未来氮沉降增加和降水格局改变的响应及模型改进提供科学依据...     

Hierarchical responses of plant stoichiometry to nitrogen deposition and mowing in a temperate steppe

Background and aims

Stoichiometric relations drive powerful constraints on many ecosystem processes. However, our understanding of the hierarchical responses of plant C:N:P stoichiometry at different levels of biological organization to global change factors remains limited.

Methods

we examined the plant C:N:P stoichiometric responses to N deposition and mowing (hay making) at both species- and community-level by carrying out a 4-year field experiment in the temperate steppe of northern China.

Results

Our results showed that N addition and mowing resulted in higher plant N concentrations, lower C:N, and higher N:P at both species- and community-level. Mowing had a limited negative influence on the effects of N addition. We observed divergent responses of both plant P concentrations and C:P to N addition at species-level and community-level: N addition led to higher plant P and lower C:P at species-level, but this effect was not observed at the community-level.

Conclusions

Our results indicate that stoichiometric responses at community-level to N addition and mowing diverge from more traditionally examined species-specific responses. Our results suggest that the hierarchical responses of plant stoichiometry to anthropogenic disturbance deserves more attention when we model the interactions of terrestrial ecosystem C, N, and P cycling under scenarios of increasing N availability concomitantly occurring with active land management.     

Nutrient co-limitation of primary producer communities

Synergistic interactions between multiple limiting resources are common, highlighting the importance of co-limitation as a constraint on primary production. Our concept of resource limitation has shifted over the past two decades from an earlier paradigm of single-resource limitation towards concepts of co-limitation by multiple resources, which are predicted by various theories. Herein, we summarise multiple-resource limitation responses in plant communities using a dataset of 641 studies that applied factorial addition of nitrogen (N) and phosphorus (P) in freshwater, marine and terrestrial systems. We found that more than half of the studies displayed some type of synergistic response to N and P addition. We found support for strict definitions of co-limitation in 28% of the studies: i.e. community biomass responded to only combined N and P addition, or to both N and P when added separately. Our results highlight the importance of interactions between N and P in regulating primary producer community biomass and point to the need for future studies that address the multiple mechanisms that could lead to different types of co-limitation.     

Temporal variability of foliar nutrients: responses to nitrogen deposition and prescribed fire in a temperate steppe

Plant nutrient concentrations and stoichiometry drive fundamental ecosystem processes, with important implications for primary production, diversity, and ecosystem sustainability. While a range of evidence exists regarding how plant nutrients vary across spatial scales, our understanding of their temporal variation remains less well understood. Nevertheless, we know nutrients regulate plant function across time, and that important temporal controls could strongly interact with environmental change. Here, we report results from a 3-year assessment of inter-annual changes of foliar nitrogen (N) and phosphorus (P) concentrations and stoichiometry in three dominant grasses in response to N deposition and prescribed fire in a temperate steppe of northern China. Foliar N and P concentrations and their ratios varied greatly among years, with this temporal variation strongly related to inter-annual variation in precipitation. Nitrogen deposition significantly increased foliar N concentrations and N:P ratios in all species, while fire significantly altered foliar N and P concentrations but had no significant impacts on N:P ratios. Generally, N addition enhanced the temporal stability of foliar N and decreased that of foliar P and of N:P ratios. Our results indicate that plant nutrient status and response to environmental change are temporally dynamic and that there are differential effects on the interactions between environmental change drivers and timing for different nutrients. These responses have important implications for consideration of global change effects on plant community structure and function, management strategies, and the modeling of biogeochemical cycles under global change scenarios.     

Microbial nitrogen and phosphorus co-limitation across permafrost region

The status of plant and microbial nutrient limitation have profound impacts on ecosystem carbon cycle in permafrost areas, which store large amounts of carbon and experience pronounced climatic warming. Despite the long-term standing paradigm assumes that cold ecosystems primarily have nitrogen deficiency, large-scale empirical tests of microbial nutrient limitation are lacking. Here we assessed the potential microbial nutrient limitation across the Tibetan alpine permafrost region, using the combination of enzymatic and elemental stoichiometry, genes abundance and fertilization method. In contrast with the traditional view, the four independent approaches congruently detected widespread microbial nitrogen and phosphorus co-limitation in both the surface soil and deep permafrost deposits, with stronger limitation in the topsoil. Further analysis revealed that soil resources stoichiometry and microbial community composition were the two best predictors of the magnitude of microbial nutrient limitation. High ratio of available soil carbon to nutrient and low fungal/bacterial ratio corresponded to strong microbial nutrient limitation. These findings suggest that warming-induced enhancement in soil nutrient availability could stimulate microbial activity, and probably amplify soil carbon losses from permafrost areas.     

Estimates of bacterial productivity in marine sediments and water from a temperate saltmarsh lagoon

Tritiated thymidine incorporation (TTI) into DNA was used to estimate bacterial productivity in sediment and water samples from two sites in Langebaan Lagoon, South Africa. Routine analysis of isotope dilution showed seasonal variations of approximately threefold in the thymidine precursor pool sizes for bacterial assemblages from each site. Dual label incorporation of [³H]-thymidine and ¹⁴C-leucine into DNA and protein, respectively, showed that pelagic but not sediment assemblages were in a balanced state of growth during TTI. This is the first report of dual label measurements of bacterial production in sediments. Sediments supported bacterial productivities that exceeded those in the water column by factors from five- to 950-fold, whereas bacterial abundance supported by sediments exceeded that in the water column by more than 3 orders of magnitude. Estimates of bacterial productivities in sediments were coincident with levels of organic content in sediments, but not with bacterial abundance. Measurements of TTI activity for 5 different benthic microhabitats at one lagoon site showed highest activity associated with seagrass beds (2.11 ± 0.84 nmol thymidine hours^–1 g-1 dry weight), whereas activities decreased with depth (0.46 ± 0.21 nmol thymidine hours^–1 g^–I dry weight) below sediment surface.Offprint requests to: B. J. Tibbles.     

Increasing nitrogen deposition enhances post-drought recovery of grassland productivity in the Mongolian steppe

Arid regions are prone to drought because annual rainfall accumulation depends on a few rainfall events. Natural plant communities are damaged by drought, but atmospheric nitrogen (N) deposition may enhance the recovery of plant productivity after drought. Here, we investigated the effect of increasing N deposition on post-drought recovery of grassland productivity in the Mongolian steppe, and we examined the influence of grazing in this recovery. We added different amounts of N to a Mongolian grassland during two sequential drought years (2006 and 2007) and the subsequent 3 years of normal rainfall (2008–2010) under grazed and nongrazed conditions. Aboveground biomass and number of shoots were surveyed annually for each species. Nitrogen addition increased grassland productivity after drought irrespective of the grazing regime. The increase in grassland productivity was associated with an increase in the size of an annual, Salsola collina, under grazed conditions, and with an increase in shoot emergence of a perennial, Artemisia adamsii, under nongrazed conditions. The addition of low N content simulating N deposition around the study area by the year 2050 did not significantly increase grassland productivity. Our results suggest that increasing N deposition can enhance grassland recovery after a drought even in arid environments, such as the Mongolian steppe. This enhancement may be accompanied by a loss of grassland quality caused by an increase in the unpalatable species A. adamsii and largely depends on future human activities and the consequent deposition of N in Mongolia.     

Asymmetry in above‐ and belowground productivity responses to N addition in a semi‐arid temperate steppe

Nitrogen (N) enrichment often increases aboveground net primary productivity (ANPP) of the ecosystem, but it is unclear if belowground net primary productivity (BNPP) track responses of ANPP. Moreover, the frequency of N inputs may affect primary productivity but is rarely studied. To assess the response patterns of above‐ and belowground productivity to rates of N addition under different addition frequencies, we manipulated the rate (0–50 g N m^?2 year^?1) and frequency (twice vs. monthly additions per year) of NH₄NO₃ inputs for six consecutive years in a temperate grassland in northern China and measured ANPP and BNPP from 2012 to 2014. In the low range of N addition rates, BNPP showed the greatest negative response and ANPP showed the greatest positive responses with increases in N addition (<10 g N m^?2 year^?1). As N addition increased beyond 10 g N m^?2 year^?1, increases in ANPP dampened and decreases in BNPP ceased altogether. The response pattern of net primary productivity (combined above‐ and belowground; NPP) corresponded more closely to ANPP than to BNPP. The N effects on BNPP and BNPP/NPP (f_BNPP) were not dependent on N addition frequency in the range of N additions typically associated with N deposition. BNPP was more sensitive to N addition frequency than ANPP, especially at low rates of N addition. Our findings provide new insights into how plants regulate carbon allocation to different organs with increasing N rates and changing addition frequencies. These root response patterns, if incorporated into Earth system models, may improve the predictive power of C dynamics in dryland ecosystems in the face of global atmospheric N deposition.     

植被退化对温带典型草原根系-土壤系统碳氮分配的影响

以3种不同退化程度的温带典型草原(大针茅轻度退化、中度退化和重度退化)为研究对象,研究植被退化对温带典型草原土壤及根系碳氮含量及储量的影响。结果显示:(1)植被退化对地下根系碳含量影响不显著(P0.05),而对地下根系氮含量的影响显著(P0.05),中度退化样地根系氮含量显著高于轻度退化和重度退化样地(P0.05)。(2)植被退化对根系碳氮储量影响显著(P0.05),根系碳氮储量随着土层深度增加而减少,总根系碳氮储量随退化程度加剧而降低。(3)土壤有机碳、总碳和总氮含量及储量均受退化程度和采样深度的影响显著(P0.05),其含量随着土壤深度的增加而显著减少,随退化程度加剧而显著降低(P0.05)。(4)土壤是根系-土壤系统碳氮储存的最主要场所,储量占比90%以上。虽然土壤碳氮储量均存在表层聚集现象,但表层储量所占比例在各样地间差异显著(P0.05)。     

Above and belowground net primary productivity of grassland influenced by supplemental water and nitrogen in Inner Mongolia

While water availability determines grassland productivity in semiarid regions, nutrient availability is the main limiting factor under wet conditions. An experiment was conducted in 2008 at two sites in Inner Mongolia with histories of heavy grazing (HG) and moderate grazing (MG) to study the interactive effects of water and nitrogen on above- and belowground net primary productivity (ANPP and BNPP), biomass partitioning, and plant species composition. The study comprises two water treatments (no irrigation and irrigated when soil water content was below 70% of the field capacity), and two nitrogen (N) levels (0 and 100 kg N ha^?1). Mean values of ANPP at the peak biomass time reached 1,028?±?95 SD g m^?2 at the HG site and 568?±?32 SD g m^?2 at the MG site in irrigated and fertilized treatment. Nitrogen use efficiency (NUE) was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites. Water use efficiency (WUEt) based on total water input and ANPP decreased with irrigation at the HG site. Meanwhile, N application significantly increased WUEt, WUEp (based on precipitation), and WUEi (based on irrigation water) at both sites. BNPP was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites, and it tended to decrease with N addition. However, the fraction of belowground to total biomass (f _BNPP = BNPP/(ANPP+BNPP) decreased with the addition of supplemental resources and exhibited a negative correlation with ANPP. Species diversity remained lower at the HG site compared to the MG site; it decreased with the addition of supplemental resources at the latter site. The annual Salsola collina contributed the most to the total biomass under irrigation. Based on global climate models, more frequent extreme climates are predicted in the future, which can result in changes in resource availabilities. Therefore, our research results have important implications for predicting the production and other properties of grassland ecosystems.     

Nitrogen addition decreases seed germination in a temperate steppe

Seed germination and seedling establishment play an important role in driving the responses of plant community structure and function to global change. Nitrogen (N) deposition is one of the driving factors of global change, which often leads to a loss in species richness in grassland ecosystems. However, how seed germination responds to N addition remains unclear. A pot incubation test was conducted in a semi‐arid grassland in the Mongolian Plateau, Northern China, to investigate the effect of N addition (0, 5, 10, 20, 40, and 80 g N/m²) on seed germination from May to October 2016. Twenty species germinated under all treatments; however, the responses of the 20 species to N addition were different. The densities of Stipa krylovii, Leymus chinensis, and Artemisia frigida, which are the dominant species in this temperate steppe, decreased significantly as the amount of N addition. Moreover, N addition significantly suppressed seedling densities of the community, perennial forbs, perennial grasses, and annuals and biennials. Furthermore, species richness of the community, perennial forbs, and annuals and biennials decreased sharply with increasing N addition level, but perennial grass species richness did not change. The Shannon–Wiener diversity index also decreased as the amount of N addition increased. Our results suggest that N enrichment plays an important role in the seed germination stage and decreases supplements of seedlings to adult plants. These findings may help explain the causes of species loss by atmospheric N deposition in grassland ecosystems.     

Effects of increased nitrogen deposition and precipitation on seed and seedling production of Potentilla tanacetifolia in a temperate steppe ecosystem

Background

The responses of plant seeds and seedlings to changing atmospheric nitrogen (N) deposition and precipitation regimes determine plant population dynamics and community composition under global change.

Methodology/Principal Findings

In a temperate steppe in northern China, seeds of P. tanacetifolia were collected from a field-based experiment with N addition and increased precipitation to measure changes in their traits (production, mass, germination). Seedlings germinated from those seeds were grown in a greenhouse to examine the effects of improved N and water availability in maternal and offspring environments on seedling growth. Maternal N-addition stimulated seed production, but it suppressed seed mass, germination rate and seedling biomass of P. tanacetifolia. Maternal N-addition also enhanced responses of seedlings to N and water addition in the offspring environment. Maternal increased-precipitation stimulated seed production, but it had no effect on seed mass and germination rate. Maternal increased-precipitation enhanced seedling growth when grown under similar conditions, whereas seedling responses to offspring N- and water-addition were suppressed by maternal increased-precipitation. Both offspring N-addition and increased-precipitation stimulated growth of seedlings germinated from seeds collected from the maternal control environment without either N or water addition. Our observations indicate that both maternal and offspring environments can influence seedling growth of P. tanacetifolia with consequent impacts on the future population dynamics of this species in the study area.

Conclusion/Significance

The findings highlight the importance of the maternal effects on seed and seedling production as well as responses of offspring to changing environmental drivers in mechanistic understanding and projecting of plant population dynamics under global change.     

Interactive effects of water and nitrogen addition on soil microbial communities in a semiarid steppe

Aims Better understanding of microbial compositional and physiological acclimation mechanisms is critical for predicting terrestrial ecosystem responses to global change. The aim is to assess variations in soil microbial communities under future scenarios of changing precipitation and N deposition in a semiarid grassland of northern China.Methods In order to explicitly estimate microbial responses, a field experiment with water and N addition was established in April 2005 and continuously conducted for 4 years. Specifically, soil microbial community composition and microbial C utilization potential were determined by phospholipid fatty acid (PLFA) and community-level physiological profiles, respectively.Important findings Water addition had no effects on the PLFA concentrations of gram-positive (GP) and negative bacteria (GN), total bacteria and fungi. However, N addition caused significant reductions in the PLFA concentrations of GP, GN, total bacteria and fungi and thus decreased total PLFA of microbial communities. Moreover, there were interactive effects of water and N addition on GN/GP and the ratio of fungal to bacterial PLFA (F/B). In addition, synergistic effects were found between water and nitrogen in affecting microbial C utilization potentials, which implies that microbial C utilization potentials tend to be enhanced when both N and water availability are sufficient. Overall, the microbial responses to water and N addition support our hypothesis that water and N addition may be combined together to affect microbial communities in the semiarid grassland.