Carbon Addition as a Countermeasure Against Biological Invasion by Plants

Increased nitrogen availability is known to favor invasion by non-native plants into natural grasslands. This suggests that decreasing nitrogen availability might serve as a countermeasure against invasion. One way to at least temporarily decrease nitrogen availability to plants is to increase microbial nitrogen uptake by adding carbon to the soil, and sawdust is a carbon source whose low cost could make it a practical conservation tool. To test whether adding sawdust to soil can counter the tendency of nitrogen enrichment to promote invasions by non-native plants, we hand-tilled 1.5kg sawdust/m² into the upper soil of the bare, nitrogen-rich patches left by dead shrubs of the nitrogen-fixing shrub Lupinus arboreus in two nearby areas with contrasting levels of invasion in a coastal grassland in northern California. After two years, in both areas, patches with sawdust had 40% less biomass of non-native plants than patches without sawdust, whereas biomass of native plants was not affected by sawdust addition. The more negative effect of sawdust on non-native species was specifically due to an effect on non-native grasses; adding sawdust increased the frequency of both native and non-native forbs. Results suggest that adding carbon as sawdust to soil can help counter invasion of grassland by non-native plants when invasion is being promoted by increased nitrogen availability, especially when the major invasive species are grasses.     

Temporal variation in microbial and plant biomass during summer in a Mediterranean high-mountain dry grassland

Aims

We assessed the temporal changes on microbial biomass in relation to changes in soil moisture, dissolved organic carbon and plant biomass during the summer season in a Mediterranean high-mountain grassland.

Methods

Temporal variations were tested by two-way ANOVA. The relationships among microbial biomass, plant biomass, soil water content, soil organic carbon, dissolved organic carbon and total soil nitrogen during the summer season were assessed by means of structural equation modeling.

Results

Microbial biomass did not show variation, while dissolved organic carbon and root biomass decreased throughout the summer. Aboveground plant biomass peaked in the middle of the summer, when soil water content was at its minimum. Soil water content directly and negatively affected soil microbial biomass, and positively affected dissolved organic carbon. Moreover soil microbial biomass and dissolved organic carbon were negatively related. Plant biomass effects on soil microbial biomass were driven by root biomass, which indirectly affected soil microbial biomass through effects on soil organic carbon and soil nitrogen.

Conclusions

The temporal dynamic of microbial biomass during the summer season appeared to differ from previous observations in temperate alpine communities, and indicated the drought resistance of the microbial community during the summer in Mediterranean high-mountain grasslands. During the dry period, microbial biomass may play an alternative role in soil carbon conservation.     

Root traits and soil properties in harvested perennial grassland,annual wheat,and never-tilled annual wheat

Background and aims

Root functional traits are determinants of soil carbon storage; plant productivity; and ecosystem properties. However, few studies look at both annual and perennial roots, soil properties, and productivity in the context of field scale agricultural systems.

Methods

In Long Term and Conversion studies in North Central Kansas, USA; root biomass and length, soil carbon and nitrogen, microbial biomass, nematode and micro-arthropod communities were measured to a depth of one meter in paired perennial grassland and cropland wheat sites as well as a grassland site that had been converted to cropland using no tillage five years prior.

Results

In the Long Term Study root biomass was three to seven times greater (9.4 Mg ha^?1 and 2.5 Mg ha^?1 in May), and root length two times greater (52.5 km m^?2 and 24.0 km m^?2 in May) in perennial grassland than in cropland. Soil organic carbon and microbial biomass carbon were larger, numbers of Orbatid mites greater (2084 vs 730 mites m^?2), and nematode communities more structured (Structure Index 67 vs 59) in perennial grassland versus annual cropland. Improved soil physical and biological properties in perennial grasslands were significantly correlated with larger, deeper root systems. In the Conversion Study root length and biomass, microbial biomass carbon, mite abundance and nematode community structure differed at some but not all dates and depths. Isotope analysis showed that five years after no-till conversion old perennial roots remained in soils of annual wheat fields and that all soil fractions except coarse particulate organic matter were derived from C₄ plants.

Conclusions

Significant correlation between larger, longer roots in grasslands compared to annual croplands and improved soil biological, physical and chemical properties suggest that perennial roots are an important factor allowing perennial grasslands to maintain productivity and soil quality with few inputs. Perennial roots may persist and continue to influence soil properties long after conversion to annual systems.     

A Mechanistic Study of Plant and Microbial Controls over R* for Nitrogen in an Annual Grassland

Differences in species'' abilities to capture resources can drive competitive hierarchies, successional dynamics, community diversity, and invasions. To investigate mechanisms of resource competition within a nitrogen (N) limited California grassland community, we established a manipulative experiment using an R* framework. R* theory holds that better competitors within a N limited community should better depress available N in monoculture plots and obtain higher abundance in mixture plots. We asked whether (1) plant uptake or (2) plant species influences on microbial dynamics were the primary drivers of available soil N levels in this system where N structures plant communities. To disentangle the relative roles of plant uptake and microbially-mediated processes in resource competition, we quantified soil N dynamics as well as N pools in plant and microbial biomass in monoculture plots of 11 native or exotic annual grassland plants over one growing season. We found a negative correlation between plant N content and soil dissolved inorganic nitrogen (DIN, our measure of R*), suggesting that plant uptake drives R*. In contrast, we found no relationship between microbial biomass N or potential net N mineralization and DIN. We conclude that while plant-microbial interactions may have altered the overall quantity of N that plants take up, the relationship between species'' abundance and available N in monoculture was largely driven by plant N uptake in this first year of growth.     

Prediction of the non-fertilizer N supply of mineral grassland soils

Different methods for estimating the non-fertilizer N supply (NFNS) of mineral grassland soils were compared. NFNS was defined as the N uptake on unfertilized plots. The potential mineralization rate (0–12 weeks), macroorganic matter and active microbial biomass (determined by the substrate-induced respiration method; SIR) were correlated positively with NFNS. The difference between the actual soil organic N or microbial N content (determined by the fumigation incubation method) and their contents under equilibrium conditions ( org. N and MB-N), however, gave the best estimations of NFNS. For field conditions the best estimation for NFNS was: NFNS (kg N ha^–1 yr^–1)=132.3+42.1× org. N (g kg^–1 soil; r=0.80). This method is based on the observation that, under old grassland swards, close relationships exist between soil texture and the amounts of soil organic N and microbial N. These relationships are assumed to represent equilibrium conditions as under old swards under constant management, the gain in soil organic N and microbial N equals the losses. Soils under young grassland and recently reclaimed soils contained less soil organic N and microbial N. In such soils the amounts of organic N and microbial N increase with time, which is reflected in a lower NFNS. The annual accumulation of organic and microbial N gradually becomes smaller until organic N, microbial N and NFNS reach equilibrium. The main advantage of the difference method in comparison with the other methods is its speed and simplicity.FAX no: +31 50337291     

Nitrogen fertilization affects bacteria utilizing plant-derived carbon in the rhizosphere of beech seedlings

Background and aims

Variation in fire intensity within an ecosystem is likely to moderate fire effects on plant and soil properties. We tested the effect of fire intensity on grassland biomass, soil microbial biomass, and soil nutrients. Additional tests determined plant-microbe, plant-nutrient, and microbe-nutrient associations.

Methods

A replicated field experiment produced a fire intensity gradient. We measured plant and soil microbial biomasses at peak plant productivity the first growing season after fire. We concurrently measured flux in 11 soil nutrients and soil moisture.

Results

Fire intensity positively affected soil nitrogen, phosphorus (P), and zinc but did not appreciably affect plant biomass, microbial biomass, and other soil nutrients. Plant biomass was seemingly (co-)limited by boron, manganese, and P. Microbial biomass was (co-)limited mainly by P and also iron.

Conclusions

In the Northern Great Plains, plant and soil microbial biomasses were limited mainly by P and some micronutrients. Fire intensity affected soil nutrients, however, pulsed P (due to fire) did not result in appreciable fire intensity effects on plant and microbial biomasses. Variable responses in plant productivity to fire are common and indicate the complexity of factors that regulate plant production after fire.

     

Invasive plants decrease microbial capacity to nitrify and denitrify compared to native California grassland communities

Exotic plant invasions are a major driver of global environmental change that can significantly alter the availability of limiting nutrients such as nitrogen (N). Beginning with European colonization of California, native grasslands were replaced almost entirely by annual exotic grasses, many of which are now so ubiquitous that they are considered part of the regional flora (“naturalized”). A new wave of invasive plants, such as Aegilops triuncialis (Barb goatgrass) and Elymus caput-medusae (Medusahead), continue to spread throughout the state today. To determine whether these new-wave invasive plants alter soil N dynamics, we measured inorganic N pools, nitrification and denitrification potentials, and possible mediating factors such as microbial biomass and soil pH in experimental grasslands comprised of A. triuncialis and E. caput-medusae. We compared these measurements with those from experimental grasslands containing: (1) native annuals and perennials and (2) naturalized exotic annuals. We found that A. triuncialis and E. caput-medusae significantly reduced ion-exchange resin estimates of nitrate (NO₃ ^?) availability as well as nitrification and denitrification potentials compared to native communities. Active microbial biomass was also lower in invaded soils. In contrast, potential measurements of nitrification and denitrification were similar between invaded and naturalized communities. These results suggest that invasion by A. triuncialis and E. caput-medusae may significantly alter the capacity for soil microbial communities to nitrify or denitrify, and by extension alter soil N availability and rates of N transformations during invasion of remnant native-dominated sites.     

Carbon and Nitrogen Decoupling Under an 11-Year Drought in the Shortgrass Steppe

The frequency and magnitude of drought is expected to increase in the US Great Plains under future climate regimes. Although semiarid systems are considered highly resistant to water limitation, novel drought events could alter linkages among biogeochemical processes, and result in new feedbacks that influence the timescale of ecosystem recovery. We examined changes in carbon and nitrogen cycling in the last 2 years of an 11-year drought manipulation in the shortgrass steppe, and under the first 2 years of recovery from drought. We measured plant production, plant tissue chemistry, soil trace gas flux, and soil inorganic nitrogen dynamics to test the extent that this magnitude of drought altered carbon and nitrogen fluxes and how these changes affected post-drought dynamics. We found that soil inorganic nitrogen was up to five times higher under severe drought than under control conditions, but that this nitrogen may not have been accessible to plants and microbial communities during drought due to diffusion limitations. Drought plots had higher N₂O flux when they received equal rainfall pulses, showing that this accumulated N may be vulnerable to loss. In addition, plants in drought plots had higher tissue nitrogen for 2 years following drought. These results show that decadal-length droughts that may occur under future precipitation regimes are likely to alter ecosystem properties through interactions among precipitation, vegetation, and N cycling. Shifts in plant N, vulnerability of nitrogen to loss, and rainfall use efficiency that we observed are likely to affect the recovery time of semiarid systems subject to droughts of this magnitude.     

Temporal variation overshadows the response of leaf litter microbial communities to simulated global change

Bacteria and fungi drive the decomposition of dead plant biomass (litter), an important step in the terrestrial carbon cycle. Here we investigate the sensitivity of litter microbial communities to simulated global change (drought and nitrogen addition) in a California annual grassland. Using 16S and 28S rDNA amplicon pyrosequencing, we quantify the response of the bacterial and fungal communities to the treatments and compare these results to background, temporal (seasonal and interannual) variability of the communities. We found that the drought and nitrogen treatments both had significant effects on microbial community composition, explaining 2–6% of total compositional variation. However, microbial composition was even more strongly influenced by seasonal and annual variation (explaining 14–39%). The response of microbial composition to drought varied by season, while the effect of the nitrogen addition treatment was constant through time. These compositional responses were similar in magnitude to those seen in microbial enzyme activities and the surrounding plant community, but did not correspond to a consistent effect on leaf litter decomposition rate. Overall, these patterns indicate that, in this ecosystem, temporal variability in the composition of leaf litter microorganisms largely surpasses that expected in a short-term global change experiment. Thus, as for plant communities, future microbial communities will likely be determined by the interplay between rapid, local background variability and slower, global changes.     

荒漠草地土壤微生物生物量和微生物熵对沙漠化的响应

采用空间序列代替时间演替的方法,分析宁夏中北部盐池县荒漠草地不同沙漠化阶段(荒漠草地、固定沙地、半固定沙地和流动沙地)土壤微生物生物量(SMB)和微生物熵(qMB)的变化特征及其影响因子.结果表明:从荒漠草地到流动沙地,土壤微生物生物量碳、氮、磷分别降低46.1%、80.8%和30.0%.随着荒漠草地沙漠化程度的加剧,土壤微生物熵碳(qMBC)、土壤微生物熵氮(qMBN)、土壤微生物熵磷(qMBP)均表现为荒漠草地>固定沙地>半固定沙地>流动沙地,而土壤-微生物化学计量不平衡性(C∶Nimb、C∶Pimb、N∶Pimb)基本呈增加趋势.土壤微生物生物量氮与C∶Nimb呈显著正相关,与N∶Pimb呈显著负相关;土壤微生物生物量磷与C∶Pimb呈显著正相关.冗余分析(RDA)显示,土壤生态化学计量(C∶N、C∶P)对微生物熵碳的负效应最明显.荒漠草地沙漠化显著影响土壤微生物生物量和微生物熵.     

Soil Biological and Chemical Properties in Restored Perennial Grassland in California

Restoration of California native perennial grassland is often initiated with cultivation to reduce the density and cover of non‐native annual grasses before seeding with native perennials. Tillage is known to adversely impact agriculturally cultivated land; thus changes in soil biological functions, as indicated by carbon (C) turnover and C retention, may also be negatively affected by these restoration techniques. We investigated a restored perennial grassland in the fourth year after planting Nassella pulchra, Elymus glaucus, and Hordeum brachyantherum ssp. californicum for total soil C and nitrogen (N), microbial biomass C, microbial respiration, CO₂ concentrations in the soil atmosphere, surface efflux of CO₂, and root distribution (0‐ to 15‐, 15‐ to 30‐, 30‐ to 60‐, and 60‐ to 80‐cm depths). A comparison was made between untreated annual grassland and plots without plant cover still maintained by tillage and herbicide. In the uppermost layer (0‐ to 15‐cm depth), total C, microbial biomass C, and respiration were lower in the tilled, bare soil than in the grassland soils, as was CO₂ efflux from the soil surface. Root length near perennial bunchgrasses was lower at the surface and greater at lower depths than in the annual grass–dominated areas; a similar but less pronounced trend was observed for root biomass. Few differences in soil biological or chemical properties occurred below 15‐cm depth, except that at lower depths, the CO₂ concentration in the soil atmosphere was lower in the plots without vegetation, possibly from reduced production of CO₂ due to the lack of root respiration. Similar microbiological properties in soil layers below 15‐cm depth suggest that deeper microbiota rely on more recalcitrant C sources and are less affected by plant removal than in the surface layer, even after 6 years. Without primary production, restoration procedures with extended periods of tillage and herbicide applications led to net losses of C during the plant‐free periods. However, at 4 years after planting native grasses, soil microbial biomass and activity were nearly the same as the former conditions represented by annual grassland, suggesting high resilience to the temporary disturbance caused by tillage.     

Response of soil microbial activity to grazing, nitrogen deposition, and exotic cover in a serpentine grassland

Background and aims

Exotic species, nitrogen (N) deposition, and grazing are major drivers of change in grasslands. However little is known about the interactive effects of these factors on below-ground microbial communities.

Methods

We simulated realistic N deposition increases with low-level fertilization and manipulated grazing with fencing in a split-plot experiment in California’s largest serpentine grassland. We also monitored grazing intensity using camera traps and measured total available N to assess grazing and nutrient enrichment effects on microbial extracellular enzyme activity (EEA), microbial N mineralization, and respiration rates in soil.

Results

Continuous measures of grazing intensity and N availability showed that increased grazing and N were correlated with increased microbial activity and were stronger predictors than the categorical grazing and fertilization measures. Exotic cover was also generally correlated with increased microbial activity resulting from exotic-driven nutrient cycling alterations. Seasonal effects, on abiotic factors and plant phenology, were also an important factor in EEA with lower activity occurring at peak plant biomass.

Conclusions

In combination with previous studies from this serpentine grassland, our results suggest that grazing intensity and soil N availability may affect the soil microbial community indirectly via effects on exotic cover and associated changes in nutrient cycling while grazing directly impacts soil community function.     

Microbial communities and their responses to simulated global change fluctuate greatly over multiple years

We used microbial lipid analysis to analyze microbial biomass and community structure during 6 years of experimental treatment at the Jasper Ridge Global Change Experiment (JRGCE), a long‐term multi‐factor global change experiment in a California annual grassland. The microbial community fingerprint and specific biomarkers varied substantially from year to year, in both control and experimental treatment plots. Possible drivers of the variability included plant growth, soil moisture, and ambient temperature. Surprisingly, background variation in the microbial community was of a larger magnitude than even very significant treatment effects, and this variation appeared to constrain responses to treatment. Microbial communities were mostly not responsive or not consistently responsive to the experimental treatments. Both arbuscular mycorrhizal fungi biomarker abundance (16 : 1 ω5c) and the fungal to bacterial ratio were lower under nitrogen addition in most years. Bacterial lipid biomarker abundances (15 : 0 iso and 16 : 1 ω7c) were higher under nitrogen addition in 2002, the year of largest microbial biomass, suggesting that bacteria could respond more to nitrogen addition in years of better growth conditions. Nitrogen addition and warming led to an interactive effect on the Gram‐positive bacterial biomarker and the fungal to bacterial ratio. These patterns indicate that in California grassland ecosystems, microbial communities may not respond substantially to future changes in climate and that nitrogen deposition may be a determinant of the soil response to global change. Further, year‐to‐year variation in microbial growth or community composition may be important determinants of ecosystem response to global change.     

退化草地恢复过程中土壤氮素状况以及与植被地上绿色生物量形成关系的研究

 研究了在不同放牧率下形成的不同退化阶段的草地各形态氮素（全氮、硝态氮、铵态氮、无机氮和微生物氮）的变化情况,同时也研究了植被地上绿色生物量与各形态氮素季节变化的同步性关系。土壤全氮含量相对稳定,随草地植被状况和植物生长时期变化不大,说明土壤总氮库有相当的弹性。土壤硝态氮（NO－3-N）、铵态氮（NH+4-N）、无机氮（IN）和微生物氮（Micro-N）季节变化明显。土壤Micro-N和NO-3-N含量随植物生长逐渐降低,到植物枯黄期含量又回复到较高的水平;土壤NH+4-N含量随植物生长有逐渐升高的趋势;IN则随着植物的生长出现低-高-低-高的特点,且与植被地上绿色生物量呈显著负相关（R=-0.247, p<0.01）。在放牧条件下草原植物优先利用NO－3-N,NO－3-N与植被地上绿色生物量有显著的负相关性,是形成草原植被地上绿色生物量的有效性氮素。Micro-N能解释土壤IN 22.3%的变异（R2=0.223, p<0.01）,Micro-N是土壤无机氮的重要来源。土壤NH+4-N与Micro-N呈显著负相关（R=-0.222, p<0.01）,说明土壤微生物对土壤NH+4-N有偏好吸收。总体上,不同形态的氮素在各土壤层次间差异显著,随土壤层次的加深含量逐步降低。连续放牧11年恢复两年后,各氮素组分对放牧压力消除的响应并不一致。土壤全氮含量与停止放牧前相比变化差异不显著;而Micro-N对放牧压力消失的响应在不同处理下整个生长季的结果比较一致,即以前过度和中度放牧处理的Micro-N含量较高,无牧和轻牧含量较低;IN、NH+4-N和NO－3-N变化比较复杂,在不同放牧恢复处理上结果并不一致。总的来看,以前中度和过度放牧的IN、NH+4-N和NO－3-N含量较高,存在潜在损失的可能。经过两年的恢复,植被地上绿色生物量（8月）过牧处理与无牧处理差异不显著。     

Climate Change Affects Carbon Allocation to the Soil in Shrublands

Climate change may affect ecosystem functioning through increased temperatures or changes in precipitation patterns. Temperature and water availability are important drivers for ecosystem processes such as photosynthesis, carbon translocation, and organic matter decomposition. These climate changes may affect the supply of carbon and energy to the soil microbial population and subsequently alter decomposition and mineralization, important ecosystem processes in carbon and nutrient cycling. In this study, carried out within the cross-European research project CLIMOOR, the effect of climate change, resulting from imposed manipulations, on carbon dynamics in shrubland ecosystems was examined. We performed a ¹⁴C-labeling experiment to probe changes in net carbon uptake and allocation to the roots and soil compartments as affected by a higher temperature during the year and a drought period in the growing season. Differences in climate, soil, and plant characteristics resulted in a gradient in the severity of the drought effects on net carbon uptake by plants with the impact being most severe in Spain, followed by Denmark, with the UK showing few negative effects at significance levels of p 0.10. Drought clearly reduced carbon flow from the roots to the soil compartments. The fraction of the ¹⁴C fixed by the plants and allocated into the soluble carbon fraction in the soil and to soil microbial biomass in Denmark and the UK decreased by more than 60%. The effects of warming were not significant, but, as with the drought treatment, a negative effect on carbon allocation to soil microbial biomass was found. The changes in carbon allocation to soil microbial biomass at the northern sites in this study indicate that soil microbial biomass is a sensitive, early indicator of drought- or temperature-initiated changes in these shrubland ecosystems. The reduced supply of substrate to the soil and the response of the soil microbial biomass may help to explain the observed acclimation of CO₂ exchange in other ecosystems.     

Herbivore influence on soil microbial biomass and nitrogen mineralization in a northern grassland ecosystem: Yellowstone National Park

Microorganisms are largely responsible for soil nutrient cycling and energy flow in terrestrial ecosystems. Although soil microorganisms are affected by topography and grazing, little is known about how these two variables may interact to influence microbial processes. Even less is known about how these variables influence microorganisms in systems that contain large populations of free-roaming ungulates. In this study, we compared microbial biomass size and activity, as measured by in situ net N mineralization, inside and outside 35- to 40-year exclosures across a topographic gradient in northern Yellowstone National Park. The objective was to determine the relative effect of topography and large grazers on microbial biomass and nitrogen mineralization. Microbial C and N varied by almost an order of magnitude across sites. Topographic depressions that contained high plant biomass and fine-textured soils supported the greatest microbial biomass. We found that plant biomass accurately predicted microbial biomass across our sites suggesting that carbon inputs from plants constrained microbial biomass. Chronic grazing neither depleted soil C nor reduced microbial biomass. We hypothesize that microbial populations in grazed grasslands are sustained mainly by inputs of labile C from dung deposition and increased root turnover or root exudation beneath grazed plants. Mineral N fluxes were affected more by grazing than topography. Net N mineralization rates were highest in grazed grassland and increased from dry, unproductive to mesic, highly productive communities. Overall, our results indicate that topography mainly influences microbial biomass size, while mineral N fluxes (microbial activity) are affected more by grazing in this grassland ecosystem. Received: 4 June 1997 / Accepted: 16 December 1997     

Chemistry and Microbial Functional Diversity Differences in Biofuel Crop and Grassland Soils in Multiple Geographies

We obtained soil samples from geographically diverse switchgrass (Panicum virgatum L.) and sorghum (Sorghum bicolor L.) crop sites and from nearby reference grasslands and compared their edaphic properties, microbial gene diversity and abundance, and active microbial biomass content. We hypothesized that soils under switchgrass, a perennial, would be more similar to reference grassland soils than sorghum, an annual crop. Sorghum crop soils had significantly higher NO₃ ^? -N, NH₄ ⁺ -N, SO₄ ^2? -S, and Cu levels than grassland soils. In contrast, few significant differences in soil chemistry were observed between switchgrass crop and grassland soils. Active bacterial biomass was significantly lower in sorghum soils than switchgrass soils. Using GeoChip 4.0 functional gene arrays, we observed that microbial gene diversity was significantly lower in sorghum soils than grassland soils. Gene diversity at sorghum locations was negatively correlated with NO₃ ^? -N, NH₄ ⁺ -N, and SO₄ ^2? -S in C and N cycling microbial gene categories. Microbial gene diversity at switchgrass sites varied among geographic locations, but crop and grassland sites tended to be similar. Microbial gene abundance did not differ between sorghum crop and grassland soils, but was generally lower in switchgrass crop soils compared to grassland soils. Our results suggest that switchgrass has fewer adverse impacts on microbial soil ecosystem services than cultivation of an annual biofuel crop such as sorghum. Multi-year, multi-disciplinary regional studies comparing these and additional annual and perennial biofuel crop and grassland soils are recommended to help define sustainable crop production and soil ecosystem service practices.     

Challenging the paradigm of nitrogen cycling: no evidence of in situ resource partitioning by coexisting plant species in grasslands of contrasting fertility

In monoculture, certain plant species are able to preferentially utilize different nitrogen (N) forms, both inorganic and organic, including amino acids and peptides, thus forming fundamental niches based on the chemical form of N. Results from field studies, however, are inconsistent: Some showing that coexisting plant species predominantly utilize inorganic N, while others reveal distinct interspecies preferences for different N forms. As a result, the extent to which hypothetical niches are realized in nature remains unclear. Here, we used in situ stable isotope tracer techniques to test the idea, in temperate grassland, that niche partitioning of N based on chemical form is related to plant productivity and the relative availability of organic and inorganic N. We also tested in situ whether grassland plants vary in their ability to compete for, and utilize peptides, which have recently been shown to act as an N source for plants in strongly N-limited ecosystems. We hypothesized that plants would preferentially use NO₃⁻-N and NH₄⁺-N over dissolved organic N in high-productivity grassland where inorganic N availability is high. On the other hand, in low-productivity grasslands, where the availability of dissolved inorganic N is low, and soil availability of dissolved organic N is greater, we predicted that plants would preferentially use N from amino acids and peptides, prior to microbial mineralization. Turves from two well-characterized grasslands of contrasting productivity and soil N availability were injected, in situ, with mixtures of ¹⁵N-labeled inorganic N (NO₃⁻ and NH₄⁺) and ¹³C¹⁵N labeled amino acid (l-alanine) and peptide (l-tri-alanine). In order to measure rapid assimilation of these N forms by soil microbes and plants, the uptake of these substrates was traced within 2.5 hours into the shoots of the most abundant plant species, as well as roots and the soil microbial biomass. We found that, contrary to our hypothesis, the majority of plant species across both grasslands took up most N in the form of NH₄⁺, suggesting that inorganic N is their predominant N source. However, we did find that organic N was a source of N which could be utilized by plant species at both sites, and in the low-productivity grassland, plants were able to capture some tri-alanine-N directly. Although our findings did not support the hypothesis that differences in the availability of inorganic and organic N facilitate resource partitioning in grassland, they do support the emerging view that peptides represent a significant, but until now neglected, component of the terrestrial N cycle.     

Herbivory and Stoichiometric Feedbacks to Primary Production

Established theory addresses the idea that herbivory can have positive feedbacks on nutrient flow to plants. Positive feedbacks likely emerge from a greater availability of organic carbon that primes the soil by supporting nutrient turnover through consumer and especially microbially-mediated metabolism in the detrital pool. We developed an entirely novel stoichiometric model that demonstrates the mechanism of a positive feedback. In particular, we show that sloppy or partial feeding by herbivores increases detrital carbon and nitrogen allowing for greater nitrogen mineralization and nutritive feedback to plants. The model consists of differential equations coupling flows among pools of: plants, herbivores, detrital carbon and nitrogen, and inorganic nitrogen. We test the effects of different levels of herbivore grazing completion and of the stoichiometric quality (carbon to nitrogen ratio, C:N) of the host plant. Our model analyses show that partial feeding and plant C:N interact because when herbivores are sloppy and plant biomass is diverted to the detrital pool, more mineral nitrogen is available to plants because of the stoichiometric difference between the organisms in the detrital pool and the herbivore. This model helps to identify how herbivory may feedback positively on primary production, and it mechanistically connects direct and indirect feedbacks from soil to plant production.     

Nitrogen deposition weakens plant–microbe interactions in grassland ecosystems

Soil carbon (C) and nitrogen (N) stoichiometry is a main driver of ecosystem functioning. Global N enrichment has greatly changed soil C : N ratios, but how altered resource stoichiometry influences the complexity of direct and indirect interactions among plants, soils, and microbial communities has rarely been explored. Here, we investigated the responses of the plant‐soil‐microbe system to multi‐level N additions and the role of dissolved organic carbon (DOC) and inorganic N stoichiometry in regulating microbial biomass in semiarid grassland in northern China. We documented a significant positive correlation between DOC and inorganic N across the N addition gradient, which contradicts the negative nonlinear correlation between nitrate accrual and DOC availability commonly observed in natural ecosystems. Using hierarchical structural equation modeling, we found that soil acidification resulting from N addition, rather than changes in the plant community, was most closely related to shifts in soil microbial community composition and decline of microbial respiration. These findings indicate a down‐regulating effect of high N availability on plant–microbe interactions. That is, with the limiting factor for microbial biomass shifting from resource stoichiometry to soil acidity, N enrichment weakens the bottom‐up control of soil microorganisms by plant‐derived C sources. These results highlight the importance of integratively studying the plant‐soil‐microbe system in improving our understanding of ecosystem functioning under conditions of global N enrichment.