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Soil fauna plays a key role in regulating biogeochemical cycles, but how multiple global change factors (GCFs) may affect faunal communities remains poorly studied. We conducted a meta-analysis using 1154 observations to evaluate the individual and combined effects of elevated CO

Grazing by ungulate herbivores can greatly alter nitrogen ([N]) and phosphorus ([P]) concentrations in plants and soils. It is not clear, however, how grazing might affect N:P co‐limitation in grasslands depending on soil N and P availability. Here we selected 173 peer‐reviewed studies, which measured 12 key variables associated with changes in [N], [P] and N:P ratios (i.e. N:P stoichiometry) in soils and plants in the presence or absence of herbivore grazing. Subsequently, we addressed the magnitude and direction of grazing effects on these variables using a meta‐analysis approach. Grazing increased leaf [N] and [P] but decreased total and available soil [N] and [P]. Grazing also increased leaf N:P ratios while decreasing root and total soil N:P ratios. The response ratio (RR) of leaf N:P was negatively correlated with RR of plant‐available soil [P] and positively correlated with RR of available soil N:P ratio (rather than with RR of total soil N:P). Intensive grazing (e.g. heavy grazing or long‐term grazing) had in general more positive effects on plant N:P stoichiometry and negative effects on soil N:P stoichiometry than light grazing. Responses of plant‐soil N:P stoichiometry to grazing greatly varied depending on plant functional group identity, plant organizational level (i.e. species and community) and grassland type. Synthesis and applications. Our study suggests that understanding changes in available soil N:P stoichiometry (rather than total soil N:P) in response to grazing is crucial to predict nutrient co‐limitation in grassland biomes. Our findings show that P is more important for plant growth than generally thought due to greater reduction of plant‐available soil [P] under grazing. A better mechanistic understanding of the relationships between plant and available soil N:P stoichiometry under grazing will greatly help improve the sustainability of natural and semi‐natural grassland ecosystems.

Our knowledge of root mass decomposition processes has greatly benefited from studies carried out in natural and semi‐natural ecosystems. It is less known, however, how root mass loss might occur in intensively managed ecosystems, such as agricultural grasslands, which receive chronic additions of nitrogen (N) fertilizers. Key questions remain about (1) how repeated N applications either in organic or inorganic forms might influence fine root mass decomposition, and (2) what implications root mass loss might have for soil carbon (C) storage. Here, we use a long‐term grassland experiment established in 1970 at Hillsborough, Northern Ireland (UK), to address how repeated additions of organic (i.e., liquid manures) and inorganic (i.e., NPK) fertilizers over >45 yr might influence fine root mass decomposition and how. We found that after a 10‐month incubation period under field conditions, highest root mass loss (~43%) was associated with highest additions of organic slurries, whereas lowest root mass loss (~29%) occurred in inorganically N‐fertilized and unfertilized soils. Root decomposition was significantly and negatively related to root C:N ratios and fungal‐to‐bacterial (F:B) ratios. Root decomposition, however, was not related to changes in soil C stocks or rates of soil C accumulation. N applications either maintained or increased (up to three times) soil C accumulation compared to unfertilized soils after five decades of intensive management. Our findings suggest that in the short‐term organic fertilization can enhance root detritus decomposition (thus promoting N release in soils for plant uptake) while contributing in the long‐term to soil C accumulation either through extra C inputs from manures or through N‐induced effects on microbial activity or both. Further studies need to address how this dual effect of organic fertilization could contribute making grassland intensification more sustainable by reducing the use of artificial NPK fertilizers.

Summary Recent studies have revealed many potential benefits of increasing plant diversity in natural ecosystems, as well as in agroecosystems and production forests. Plant diversity potentially provides a partial to complete substitute for many costly agricultural inputs, such as fertilizers, pesticides, imported pollinators and irrigation. Diversification strategies include enhancing crop genetic diversity, mixed plantings, rotating crops, agroforestry and diversifying landscapes surrounding croplands. Here we briefly review studies considering how increasing plant diversity influences the production of crops, forage, and wood, yield stability, and several regulating and supporting agroecosystem services. We also discuss challenges and recommendations for diversifying agroecosystems. There is consistently strong evidence that strategically increasing plant diversity increases crop and forage yield, wood production, yield stability, pollinators, weed suppression and pest suppression, whereas effects of diversification on soil nutrients and carbon remain poorly understood. Synthesis. The benefits of diversifying agroecosystems are expected to be greatest where the aims are to sustainably intensify production while reducing conventional inputs or to optimize both yields and ecosystem services. Over the next few decades, as monoculture yields continue to decelerate or decline for many crops, and as demand for ecosystem services continues to rise, diversification could become an essential tool for sustaining production and ecosystem services in croplands, rangelands and production forests.

The interactive effects of multiple global change drivers on terrestrial carbon (C) storage remain poorly understood. Here, we synthesise data from 633 published studies to show how the interactive effects of multiple drivers are generally additive (i.e. not differing from the sum of their individual effects) rather than synergistic or antagonistic. We further show that (1) elevated CO2 , warming, N addition, P addition and increased rainfall, all exerted positive individual effects on plant C pools at both single-plant and plant-community levels; (2) plant C pool responses to individual or combined effects of multiple drivers are seldom scale-dependent (i.e. not differing from single-plant to plant-community levels) and (3) soil and microbial biomass C pools are significantly less sensitive than plant C pools to individual or combined effects. We provide a quantitative basis for integrating additive effects of multiple global change drivers into future assessments of the C storage ability of terrestrial ecosystems.

Summary The productivity of species-diverse plant assemblages strongly depends on the temporal dynamics of nutrient uptake by competing neighbouring plants. Our understanding, however, of how rates of nitrogen (N), phosphorous (P) and potassium (K) uptake might change through time between neighbouring plant species under field conditions is still very limited. Here, we specifically measure the temporal trajectories of N, P and K uptake by staple food plants such as wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and maize (Zea mays L.) when growing either in monocultures or in intercropping systems. We ask how (i) plant species combinations, (ii) N fertilization and (iii) film mulching might affect key indexes of N, P and K uptake over time. We fit logistic models to characterize the nutrient uptake trajectories. Maximum cumulative N, P and K uptake (kg ha−1) by wheat and barley were significantly greater in wheat–maize or barley–maize intercropping systems than in wheat or barley monocultures. Cumulative nutrient uptake by intercropped maize (either with wheat or with barley) was reduced by interspecific competition at early growth stages, but it increased rapidly after wheat and barley were harvested. Maximum cumulative N and P (but not K) uptake by intercropped maize were significantly higher than the uptake by monoculture maize, particularly when N fertilizer or film mulching was applied. Intercropping induced a significant temporal niche differentiation in maximum daily nutrient uptake rates (kg ha−1 day−1) between intercropped species. Fertilization had much stronger effects on maximum cumulative nutrient uptake of maize than that of wheat and barley. Mulching significantly increased the maximum cumulative nutrient uptake of maize and advanced the time to reach its maximum daily P and K uptake rates. Our study provides evidence of an important temporal niche differentiation mechanism (‘temporal complementarity’) in nutrient uptake rates between neighbouring plant species. A better understanding of temporal trajectories of interspecific nutrient uptake rates remains crucial if we want to maximize the nutrient-use efficiency and sustain overyielding (i.e. high food production) in plant species-diverse systems such as intercropping.

The relationship between plant diversity and productivity in grasslands could depend, partly, on how diversity affects vertical distributions of root biomass in soil; yet, no prior study has evaluated the links among diversity, root depth distributions, and productivity in a long-term experiment. We use data from a 12-year experiment to ask how plant species richness and composition influenced both observed and expected root depth distributions of plant communities. Expected root depth distributions were based on the abundance of species in each community and two traits of species that were measured in monocultures: root depth distributions and root to shoot ratios. The observed proportion of deep root biomass increased more than expected with species richness and was positively correlated with aboveground productivity. Indeed, the proportion of deep root biomass explained variation in productivity even after accounting for legume presence/abundance, and greater nitrogen availability in diverse plots. Diverse plots had root depth distributions that were twice as deep as expected from their species composition and corresponding monoculture traits, partly due to interactions between C4 grasses and legumes. These results suggest the productivity of diverse plant communities was partly dependent on belowground plant interactions that caused roots to be distributed more deeply in soil.

Human-induced increases in nitrogen (N) deposition are common across many terrestrial ecosystems worldwide. Greater N availability not only reduces biological diversity, but also affects the biogeochemical coupling of carbon (C) and N cycles in soil ecosystems. Soils are the largest active terrestrial C pool and N deposition effects on soil C sequestration or release could have global importance. Here, we show that 27 years of chronic N additions to prairie grasslands increased C sequestration in mineral soils and that a potential mechanism responsible for this C accrual was an N-induced increase in root mass. Greater soil C sequestration followed a dramatic shift in plant community composition from native-species-rich C4 grasslands to naturalized-species-rich C3 grasslands, which, despite lower soil C gains per unit of N added, still acted as soil C sinks. Since both high plant diversity and elevated N deposition may increase soil C sequestration, but N deposition also decreases plant diversity, more research is needed to address the long-term implications for soil C storage of these two factors. Finally, because exotic C3 grasses often come to dominate N-enriched grasslands, it is important to determine if such N-dependent soil C sequestration occurs across C3 grasslands in other regions worldwide.

Despite the topic of soil nitrogen (N) mineralization being well-studied, very few studies have addressed the relative contribution of different plant and soil variables in influencing soil N mineralization rates, and thus the supply of inorganic N to plants. Here, we used data from a well-studied N-limited grassland to address the relative effects of six plant and soil variables on net and on gross rates of soil N mineralization. We also addressed whether plant effects on soil N mineralization were mediated by changes in C and N concentrations of multiple soil organic matter (SOM) fractions. Regression analyses show that key plant traits (i.e., plant C:N ratios and total root mass) were more important than total C and N concentrations of bulk soil in influencing N mineralization. This was mainly because plant traits influenced the C and N concentration (and C:N ratios) of different SOM fractions, which in turn were significantly associated with changes in net and gross N mineralization. In particular, C:N ratios of a labile soil fraction were negatively related to net soil N mineralization rates, whereas total soil C and N concentrations of more recalcitrant fractions were positively related to gross N mineralization. Our study suggests that changes in belowground N-cycling can be better predicted by simultaneously addressing how plant C:N ratios and root mass affect the composition and distribution of different SOM pools in N-limited grassland systems.

The application of calcium- and magnesium-rich materials to soil, known as liming, has long been a foundation of many agro-ecosystems worldwide because of its role in counteracting soil acidity. Although liming contributes to increased rates of respiration from soil thereby potentially reducing soils ability to act as a CO2 sink, the long-term effects of liming on soil organic carbon (Corg) sequestration are largely unknown. Here, using data spanning 129 years of the Park Grass Experiment at Rothamsted (UK), we show net Corg sequestration measured in the 0–23cm layer at different time intervals since 1876 was 2–20 times greater in limed than in unlimed soils. The main cause of this large Corg accrual was greater biological activity in limed soils, which despite increasing soil respiration rates, led to plant C inputs being processed and incorporated into resistant soil organo-mineral pools. Limed organo-mineral soils showed: (1) greater Corg content for similar plant productivity levels (i.e. hay yields); (2) higher 14C incorporation after 1950s atomic bomb testing and (3) lower C:N ratios than unlimed organo-mineral soils, which also indicate higher microbial processing of plant C. Our results show that greater Corg sequestration in limed soils strongly reduced the global warming potential of long-term liming to permanent grassland suggesting the net contribution of agricultural liming to global warming could be lower than previously estimated. Our study demonstrates that liming might prove to be an effective mitigation strategy, especially because liming applications can be associated with a reduced use of nitrogen fertilizer which is a key cause for increased greenhouse gas emissions from agro-ecosystems.

In a 13-year grassland biodiversity experiment in Minnesota, USA, we addressed two main questions: What set of ecological mechanisms caused aboveground productivity to become approximately 340% greater in highly diverse plant mixtures than in the average monoculture? Why did the effect of diversity on productivity become so much stronger through time? Because our grassland system is N limited, we simultaneously measured critical variables associated with the storage and cycling of this element, such as plant and soil N pools, soil N availability, soil N mineralization rates, and plant N-use efficiency, as well as the initial soil N concentration of each diversity plot when the experiment was established in 1994. We used linear and multiple regression analyses to test for potential effects of these variables on aboveground productivity and to address whether and how such variables were in turn affected by plant species diversity and functional composition across years and also at different time intervals within the same year. We found that seven variables simultaneously controlled productivity: (1) initial total soil nitrogen (N) of each plot, (2) diversity-dependent increases in total soil N through time, (3) soil N mineralization rates, (4) soil nitrate (NO3-) utilization, (5) increases in plant N-use efficiency at greater plant diversity, (6) legume presence, and (7) higher species numbers. The surprising continued significance of higher plant diversity may occur because of its effects on seasonal capture of soil NO3- and moisture and on the accumulation of root-N pools, all of which may have also increased productivity through time at higher species numbers.

Summary 1. Plant functional composition may indirectly affect fine root processes both qualitatively (e.g. by influencing root chemistry) and quantitatively (e.g. by influencing root biomass and thus soil carbon (C) inputs and the soil environment). Despite the potential implications for ecosystem nitrogen (N) cycling, few studies have addressed the linkages between plant functional composition, root decay, root detritus N dynamics and soil N mineralization rates. 2. Here, using data from a large grassland biodiversity experiment, we first show that plant functional composition affected fine root mass loss, root detritus N dynamics and net soil N mineralization rates through its effects on root chemistry rather than on the environment of decomposition. In particular, the presence of legumes and non-leguminous forbs contributed to greater fine root decomposition which in turn enhanced root N release and net soil N mineralization rates compared with C3 and C4 grasses. 3. Second, we show that all fine roots released N immediately during decomposition and showed very little N immobilization regardless of plant composition. As a consequence, there was no evidence of increased root or soil N immobilization rates with increased below-ground plant biomass (i.e. increased soil C inputs) even though root biomass negatively affected root decay. 4. Our results suggest that fine roots represent an active soil N pool that may sustain plant uptake while other soil N forms are being immobilized in microbial biomass and/or sequestered into soil organic matter. However, fine roots may also represent a source of recalcitrant plant detritus that is returned to the soil (i.e. fine roots of C4 and C3 grasses) and that can contribute to an increase in the soil organic matter pool. 5. Synthesis . An important implication of our study is that the simultaneous presence of different plant functional groups (in plant mixtures) with opposite effects on root mass loss, root N release and soil N mineralization rates may be crucial for sustaining multiple ecosystem services such as productivity and soil C and N sequestration in many N-limited grassland systems.

Summary 1. The mechanisms controlling soil carbon (C) and nitrogen (N) accumulation are crucial for explaining why soils are major terrestrial C sinks. Such mechanisms have been mainly addressed by imposing short-term, step-changes in CO 2 , temperature and N fertilization rates on either monocultures or low-diversity plant assemblages. No studies have addressed the long-term effects of plant functional diversity (i.e. plant functional composition) on rates of soil C accumulation in N-limited grasslands where fixation is the main source of N for plants. 2. Here we measure net soil C and N accumulation to 1 m soil-depth during a 12-year-long grassland biodiversity experiment established on agriculturally degraded soils at Cedar Creek, Minnesota, USA. 3. We show that high-diversity mixtures of perennial grassland plant species stored 500% and 600% more soil C and N than, on average, did monoculture plots of the same species. Moreover, the presence of C4 grasses and legumes increased soil C accumulation by 193% and 522%, respectively. Higher soil C and N accrual resulted both from increased C and N inputs via (i) higher root biomass, and (ii) from greater root biomass accumulation to 60 cm soil depth resulting from the presence of highly complementary functional groups (i.e. C4 grasses and legumes). 4. Our results suggest that the joint presence of C4 grass and legume species is a key cause of greater soil C and N accumulation in both higher and lower diversity plant assemblages. This is because legumes have unique access to N, and C4 grasses take up and use N efficiently, increasing below-ground biomass and thus soil C and N inputs. 5. Synthesis. We demonstrate that plant functional complementarity is a key reason why higher plant diversity leads to greater soil C and N accumulation on agriculturally degraded soils. We suggest the combination of key C4 grass‐legume species may greatly increase ecosystem services such as soil C accumulation and biomass (biofuel) production in both high- and low-diversity N-limited grassland systems.

We measured browsing-induced responses of Acacia trees to investigate "browsing lawns" as an analogy to grazing lawns in a semiarid eutrophic African savanna. During the two-year field study, we measured plant tolerance, resistance, and phenological traits, while comparing variation in leaf nitrogen and specific leaf area (SLA) across stands of Acacia nigrescens, Miller, that had experienced markedly different histories of attack from large herbivores. Trees in heavily browsed stands developed (1) tolerance traits such as high regrowth abilities in shoots and leaves, high annual branch growth rates, extensive tree branching and evidence of internal N translocation, and (2) resistance traits such as close thorn spacing. However, phenological "escape" responses were weak even in heavily browsed stands. Overall, browsing strongly affected plant morpho-functional traits and decreased both the number of trees carrying pods and the number of pods per tree in heavily browsed stands. Hence, there is experimental evidence that tolerance and resistance traits may occur simultaneously at heavily browsed sites, but this comes at the expense of reproductive success. Such tolerance and resistance traits may coexist if browsers trigger and maintain a positive feedback loop in which trees are continually investing in regrowth (tolerance), and if the plant's physical defenses (resistance) are not nutritionally costly and are long-lived. Our results confirm that chronic browsing by ungulates can maintain A. nigrescens trees in a hedged state that is analogous to a grazing lawn. Further research is needed to fully understand the long-term effects of chronic browsing on reproduction within such tree populations, as well as the overall effects on nutrient cycling at the ecosystem level.

The functioning of human-managed grassland ecosystems strongly depends on how common management practices (e.g., animal grazing and the chronic addition of fertilizing materials to soils) interact to influence plant and soil element stoichiometry. Here we use data from a 22-yr-long grassland experiment to address whether and how plant element stoichiometry (i.e., carbon [C], nitrogen [N], phosphorus [P] ratios) might respond to (1) animal grazing, (2) agricultural liming (i.e., CaCO3) applications, and (3) nutrient fertilization. We also ask whether plant C:N:P stoichiometry could predict changes in soil N and P availability and in soil C, N, and P stocks. We found that grassland management significantly affected plant C:N:P ratios as predicted by ecological stoichiometry theory. For example, plant aboveground and belowground C:N and C:P ratios decreased under chronic N and P fertilization, respectively. Plant C:N and C:P ratios were significantly greater in unfertilized (control) soils. Also plant C:N ratios were highest under P-only additions, whereas plant C:P ratios were highest under N-only additions. However, unpredictable changes in C:N:P ratios also occurred, suggesting that plant tissue chemistry may not be a simple reflection of soil nutrient availability. Changes in plant C:nutrient ratios well predicted variation in soil nutrient availability, but not in soil C, N, and P stocks. Contrary to expectations, soil C stocks significantly increased with decreasing plant C:N ratios in the nutrient-fertilized grasslands and not with increasing plant C:N ratios in the unfertilized grasslands. We suggest that a better mechanistic understanding of the negative relationship between plant C:N stoichiometry and soil C accrual will greatly help in improving the sustainability of human-managed grasslands.

Human activities have greatly increased the availability of biologically active forms of nutrients [e.g., nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg)] in many soil ecosystems worldwide. Multi-nutrient fertilization strongly increases plant productivity but may also alter the storage of carbon (C) in soil, which represents the largest terrestrial pool of organic C. Despite this issue is important from a global change perspective, key questions remain on how the single addition of N or the combination of N with other nutrients might affect C sequestration in human-managed soils. Here, we use a 19-year old nutrient addition experiment on a permanent grassland to test for nutrient-induced effects on soil C sequestration. We show that combined NPKMg additions to permanent grassland have 'constrained' soil C sequestration to levels similar to unfertilized plots whereas the single addition of N significantly enhanced soil C stocks (N-only fertilized soils store, on average, 11 t C ha(-1) more than unfertilized soils). These results were consistent across grazing and liming treatments suggesting that whilst multi-nutrient additions increase plant productivity, soil C sequestration is increased by N-only additions. The positive N-only effect on soil C content was not related to changes in plant species diversity or to the functional composition of the plant community. N-only fertilized grasslands show, however, increases in total root mass and the accumulation of organic matter detritus in topsoils. Finally, soils receiving any N addition (N only or N in combination with other nutrients) were associated with high N losses. Overall, our results demonstrate that nutrient fertilization remains an important global change driver of ecosystem functioning, which can strongly affect the long-term sustainability of grassland soil ecosystems (e.g., soils ability to deliver multiple ecosystem services).

Biomass allocation to plant aboveground and belowground compartments may change in response to changes in soil nutrient fertility (e.g., nitrogen (N) and phosphorous (P) availability). It is not clear however whether and how changes in soil fertility could influence carbon(C):nutrient stoichiometry of wild plants distributed along environmental gradients. Here we use ecological stoichiometric theory to test two hypotheses: (1) C:N (or C:P) ratios of different plant compartments (i.e., roots, stems and leaves) increase when soil N (or P) availability decreases, (2) the relative availability of N compared to P in soils predictably influences C:nutrient ratios (e.g., high soil N:P ratios are associated with high plant C:P ratios). Data from 72 wild plant species were collected along a gradient of soil development determined by the temporal progression of four primary ecological successions across Europe. We found significant changes in soil N and P availability and content along the four vegetation successio...