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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.

Aims Biomass allocation to different organs is a fundamental plant ecophysiological process to better respond to changing environments; yet, it remains poorly understood how patterns of biomass allocation respond to nitrogen (N) additions across terrestrial ecosystems worldwide. Methods We conducted a meta-analysis using 5474 pairwise observations from 333 articles to assess how N addition affected plant biomass and biomass allocation among different organs. We also tested the ‘ratio-based optimal partitioning’ vs. the ‘isometric allocation’ hypotheses to explain potential N addition effects on biomass allocation. Important Findings We found that (i) N addition significantly increased whole plant biomass and the biomass of different organs, but decreased root:shoot ratio (RS) and root mass fraction (RMF) while no effects of N addition on leaf mass fraction and stem mass fraction at the global scale; (ii) the effects of N addition on ratio-based biomass allocation were mediated by individual or interactive effects of moderator variables such as experimental conditions, plant functional types, latitudes and rates of N addition and (iii) N addition did not affect allometric relationships among different organs, suggesting that decreases in RS and RMF may result from isometric allocation patterns following increases in whole plant biomass. Despite alteration of ratio-based biomass allocation between root and shoot by N addition, the unaffected allometric scaling relationships among different organs (including root vs. shoot) suggest that plant biomass allocation patterns are more appropriately explained by the isometric allocation hypothesis rather than the optimal partitioning hypothesis. Our findings contribute to better understand N-induced effects on allometric relationships of terrestrial plants, and suggest that these ecophysiological responses should be incorporated into models that aim to predict how terrestrial ecosystems may respond to enhanced N deposition under future global change scenarios.

Plant litter decomposition is one of the most important ecosystem processes influencing elemental biogeochemistry across terrestrial and aquatic environments. Calcium (Ca), magnesium (Mg), and manganese (Mn) are essential nutrients, which play a key role in plant energy metabolism, photosynthesis, and membrane transport of plants, and the major source of these nutrients is litter decomposition. Although litter decomposition has been subject to extensive research over the last several decades, the dynamics of Ca, Mg, and Mn in decomposing litter has been poorly assessed. Also, very few decomposition studies have been simultaneously carried out across aquatic and terrestrial ecosystems. Here, we conducted a two-year litterbag experiment across alpine forest aquatic (stream and riparian zone) and terrestrial (forest floor) ecosystems to address the dynamics of Ca, Mg, and Mn during litter decomposition. We found that (1) in general concentrations of Ca, Mg, and Mn all increased (that is, high immobilization) during the period of decomposition; (2) Ca and Mn immobilized in decomposing litter regardless of ecosystem type or litter species identity, while Mg showed release (mineralization) or immobilization depending on litter species identity; (3) a common hierarchy of drivers for Ca, Mg, or Mn release rate was found across all ecosystem types; and (4) Ca, Mg, and Mn concentrations all showed positive relationships with litter decomposition. Our results provide evidence of general patterns of Ca, Mg, and Mn dynamics in decomposing litter and significant positive effects of these nutrients on litter decomposition across terrestrial and aquatic environments associated with a forest ecosystem. We suggest that addressing nutrient dynamics across contrasting environments is crucial for predicting how biogeochemical cycling of different nutrients is modulated by litter mass decomposition.

There is growing international interest in better managing soils to increase soil organic carbon (SOC) content to contribute to climate change mitigation, to enhance resilience to climate change and to underpin food security, through initiatives such as international ‘4p1000’ initiative and the FAO's Global assessment of SOC sequestration potential (GSOCseq) programme. Since SOC content of soils cannot be easily measured, a key barrier to implementing programmes to increase SOC at large scale, is the need for credible and reliable measurement/monitoring, reporting and verification (MRV) platforms, both for national reporting and for emissions trading. Without such platforms, investments could be considered risky. In this paper, we review methods and challenges of measuring SOC change directly in soils, before examining some recent novel developments that show promise for quantifying SOC. We describe how repeat soil surveys are used to estimate changes in SOC over time, and how long‐term experiments and space‐for‐time substitution sites can serve as sources of knowledge and can be used to test models, and as potential benchmark sites in global frameworks to estimate SOC change. We briefly consider models that can be used to simulate and project change in SOC and examine the MRV platforms for SOC change already in use in various countries/regions. In the final section, we bring together the various components described in this review, to describe a new vision for a global framework for MRV of SOC change, to support national and international initiatives seeking to effect change in the way we manage our soils., Since soil organic carbon (SOC) changes are difficult to measure, a key barrier to implementing programmes to increase SOC is the need for credible and reliable measurement/monitoring, reporting and verification (MRV) platforms. We review methods for measuring SOC change directly in soils, examine novel developments for quantifying SOC change and describe how surveys, long‐term experiments and chronosequences can be used for testing models and as benchmark sites in global frameworks to estimate SOC change. We review MRV platforms for soil organic carbon change already in use and describe a new vision for a global framework for MRV of SOC change.

The role of soils in the global carbon cycle and in reducing GHG emissions from agriculture has been increasingly acknowledged. The '4 per 1000' (4p1000) initiative has become a prominent action plan for climate change mitigation and achieve food security through an annual increase in soil organic carbon (SOC) stocks by 0.4%, (i.e. 4 parts per thousand per year). However, the feasibility of the 4p1000 scenario and, more generally, the capacity of individual countries to implement soil carbon sequestration (SCS) measures remain highly uncertain. Here, we evaluated country-specific SCS potentials of agricultural land for 24 countries in Europe. Based on a detailed survey of available literature, we estimate that between 0.1% and 27% of the agricultural greenhouse gas (GHG) emissions can potentially be compensated by SCS annually within the next decades. Measures varied widely across countries, indicating differences in country-specific environmental conditions and agricultural practices. None of the countries' SCS potential reached the aspirational goal of the 4p1000 initiative, suggesting that in order to achieve this goal, a wider range of measures and implementation pathways need to be explored. Yet, SCS potentials exceeded those from previous pan-European modelling scenarios, underpinning the general need to include national/regional knowledge and expertise to improve estimates of SCS potentials. The complexity of the chosen SCS measurement approaches between countries ranked from tier 1 to tier 3 and included the effect of different controlling factors, suggesting that methodological improvements and standardization of SCS accounting are urgently required. Standardization should include the assessment of key controlling factors such as realistic areas, technical and practical feasibility, trade-offs with other GHG and climate change. Our analysis suggests that country-specific knowledge and SCS estimates together with improved data sharing and harmonization are crucial to better quantify the role of soils in offsetting anthropogenic GHG emissions at global level.

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.

Aim We sought to understand how the individual and combined effects of multiple environmental change drivers differentially influence terrestrial nitrogen (N) concentrations and N pools and whether the interactive effects of these drivers are mainly antagonistic, synergistic or additive. Location Worldwide. Time period Contemporary. Major taxa studied Plants, soil, and soil microbes in terrestrial ecosystems. Methods We synthesized data from manipulative field studies from 758 published articles to estimate the individual, combined and interactive effects of key environmental change drivers (elevated CO2, warming, N addition, phosphorus addition, increased rainfall and drought) on plant, soil, and soil microbe N concentrations and pools using meta‐analyses. We assessed the influences of moderator variables on these effects through structural equation modelling. Results We found that (a) N concentrations and N pools were significantly affected by the individual and combined effects of multiple drivers, with N addition (either alone or in combination with another driver) showing the strongest positive effects; (b) the individual and combined effects of these drivers differed significantly between N concentrations and N pools in plants, but seldom in soils and microbes; (c) additive effects of driver pairs on N concentrations and pools were much more common than synergistic or antagonistic effects across plants, soils and microbes; and (d) environmental and experimental factors were important moderators of the individual, combined and interactive effects of these drivers on terrestrial N. Main conclusions Our results indicate that terrestrial N concentrations and N pools, especially those of plants, can be significantly affected by the individual and combined effects of environmental change drivers, with the interactive effects of these drivers being mostly additive. Our findings are important because they contribute to the development of models to better predict how altered N availability affects ecosystem carbon cycling under future environmental changes. ispartof: GLOBAL ECOLOGY AND BIOGEOGRAPHY vol:28 issue:5 pages:1-35 status: Published online

Agricultural grasslands comprise over 50% of the total land area of the UK and provide important ecosystem services that include livestock and forage production. These services are rarely measured against the effects that key management practices might have on the long-term ability of grassland soils to cycle and store carbon (C). The common management practice of re-seeding (i.e. the ploughing and seeding of grasslands with more productive grass cultivars) can cause significant soil disturbance; yet the net long-term effects of re-seeding on soil C gains and losses in permanent grasslands are poorly understood. Here, we selected a chronosequence of 45 permanent grasslands across Northern Ireland with a well-documented history of single re-seeding events over the last 50 years. Second, we asked whether and how soil C cycling and storage might differ between recently re-seeded ‘young’ grasslands and increasingly ‘older’ (or never re-seeded) grasslands. We measured (1) soil CO2 fluxes, (2) soil C stocks, (3) the C content of different soil aggregate fractions, and (4) root C stocks. We found that soil CO2 fluxes were significantly higher in recently re-seeded, ‘young’, grasslands. However, total soil C stocks (0–20 cm depth) did not increase in ‘older’ grasslands despite these grasslands showing greater root C stocks. Instead, soil C stocks significantly decreased with increases in soil bulk density. Higher soil bulk density was also associated with lower C content in smaller organo-mineral aggregate sizes (i.e. more recalcitrant C pools) regardless of grassland age (time since re-seeding). Overall, our results suggest that management-induced effects on key soil physical properties, i.e. bulk density, may have significantly greater implications for C sequestration in permanent grassland soils than high disturbance, but infrequent, re-seeding events.

Plant elemental composition can indicate resource limitation, and changes in key elemental ratios (e.g. plant C:N ratios) can influence rates including herbivory, nutrient recycling, and pathogen infection. Although plant stoichiometry can influence ecosystem-level processes, very few studies have addressed whether and how plant C:N stoichiometry changes with plant diversity and composition. Here, using two long-term experimental manipulations of plant diversity (Jena and Cedar Creek), we test whether plant richness (species and functional groups) or composition (functional group proportions) affects temporal trends and variability of community-wide C:N stoichiometry. Site fertility determined the initial community-scale C:N ratio. Communities growing on N-poor soil (Cedar Creek) began with higher C:N ratios than communities growing on N-rich soil (Jena). However, site-level plant C:N ratios converged through time, most rapidly in high diversity plots. In Jena, plant community C:N ratios increased. This temporal trend was stronger with increasing richness. However, temporal variability of C:N decreased as plant richness increased. In contrast, C:N decreased over time at Cedar Creek, most strongly at high species and functional richness, whereas the temporal variability of C:N increased with both measures of diversity at this site. Thus, temporal trends in the mean and variability of C:N were underlain by concordant changes among sites in functional group proportions. In particular, the convergence of community-scale C:N over time at these very different sites was mainly due to increasing proportions of forbs at both sites, replacing high mean C:N (C4 grasses, Cedar Creek) or low C:N (legumes, Jena) species. Diversity amplified this convergence; although temporal trends differed in sign between the sites, these trends increased in magnitude with increasing species richness. Our results suggest a predictive mechanistic link between trends in plant diversity and functional group composition and trends in the many ecosystem rates that depend on aboveground community C:N. Synthesis We compared the effect of plant diversity on the temporal dynamics of community stoichiometry in two long-term grassland diversity experiments: the Cedar Creek and Jena Experiments. Changes in community C:N ratios were accelerated by increasing diversity at both sites, but in opposite directions depending on soil fertility. Stoichiometry changes were driven by shifts of functional group composition differing in their elemental compositions, the identity of the functional groups depending on the site. Thus, we highlighted that community turnover constrained the effect of diversity on plant stoichiometry at both sites.

Grasslands cover more than a third of the European agricultural area and are often intensively managed to support the livestock-farming sector. Despite grassland intensification can greatly influence soil carbon (C) cycling, changes in soil C stocks both across years and at increasing soil depths remain difficult to quantify simultaneously. Here we measured spatial and temporal changes in soil C stocks to assess C stocks’ response to intensive grassland management. We measured soil C stocks (a) in a long-term nutrient fertilization experiment on permanent grassland (established in 1970), and (b) in 126 grassland fields distributed across 11 lowland farms in Northern Ireland (UK), which are associated with different frequencies of soil tillage. Using 45 years of data from the plot-scale grassland experiment we found evidence that significant changes in soil C stocks mainly occurred in the soil top 20 cm (not in deeper soils) and only between ‘extreme’ nutrient treatments (i.e. unfertilized vs. highly fertilized soils). Soil physical fractionation, radiocarbon and stable nitrogen isotope analyses all suggest that new C has accumulated in these soils but perhaps not fast enough to affect C stocks in deeper soil layers. Results at the field-scale from intensively managed grasslands show how the frequency of soil tillage neither affected soil C stocks between 0–20, 20–40 or 40–60 cm depth layers nor the C pool of different soil physical fractions at increasing soil depths. Our findings demonstrate how the response of soil C stocks to grassland intensification (i.e. C stocks response to different rates of nutrient fertilization or frequency of soil tillage) can be very slow both in time and space under cool and humid climate conditions. We suggest how the persistence of these soil C stocks under intensive grassland management offers the unique opportunity to improve nutrient use efficiency and cycling thus promoting the delivery of multiple ecosystem processes.

The functioning of human-managed grassland ecosystems strongly depends on how common management practices will affect grassland ‘belowground compartment’ including soil biogeochemistry and plant roots. Key questions remain about how animal grazing, liming (e.g. the addition of CaCO3 to soils), and nutrient fertilization might affect, in the long-term, soil nutrient cycling and multiple root traits. Here we focus on a mesotrophic grassland located in Berkshire, UK, where contrasting levels of rabbit grazing, liming and different inorganic fertilizers have been applied since 1991. We ask how (1) soil nitrogen (N) availability and cycling, (2) total root mass, (3) root mass decomposition, and (4) arbuscular mycorrhizal fungal (AMF) root colonisation might respond to 22 years of very different management. We found that liming strongly affected total root mass, root decomposition, root AMF colonisation as well as soil N availability and cycling and that these effects were mainly driven by liming-induced increases in soil pH. Increases in soil pH were associated with significant (1) decreases in root mass, (2) increases in root mass decomposability and in the mineralization of N in decomposing root detritus, and (3) increases in AMF infection. Soil pH was also significantly related to greater N availability (i.e. soil NO3 levels) and to lower δ15N natural abundance, which suggests more efficient N uptake by plants in limed soils as we found in our study. The application of multiple nutrients (NPKMg) also reduced total root mass, while N-only fertilization was associated with greater AMF infection. Surprisingly the long-term impact of grazing was generally weak and not significant on most plant and soil parameters. Despite soil pH affecting most belowground variables, changes in soil pH were not associated with any change in soil C and N stocks. Because liming can improve nutrient cycling (and benefits soil pH and grass yields) without negatively affecting soil C sequestration, we suggest that regular liming applications may provide management solutions for increasing the long-term sustainability of permanent grassland. This article is protected by copyright. All rights reserved.

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.

Over the last few decades there has been an increasing number of controlled-manipulative experiments to investigate how plants and soils might respond to global change. These experiments typically examined the effects of each of three global change drivers (i.e. nitrogen (N) deposition, warming, and elevated CO2) on primary productivity and on the biogeochemistry of carbon (C), N and phosphorus (P) across different terrestrial ecosystems. Here we capitalize on this large amount of information by performing a comprehensive meta-analysis (>2000 case studies worldwide) to address how C:N:P stoichiometry of plants, soils and soil microbial biomass might respond to individual vs. combined effects of the three global change drivers. Our results show that (1) individual effects of N addition and elevated CO2 on C:N:P stoichiometry are stronger than warming, (2) combined effects of pairs of global change drivers (e.g., N addition + elevated CO2, warming + elevated CO2) on C:N:P stoichiometry were generally weaker than the individual effects of each of these drivers, (3) additive interactions (i.e. when combined effects are equal to or no significantly different from the sum of individual effects) were more common than synergistic or antagonistic interactions, (4) C:N:P stoichiometry of soil and soil microbial biomass shows high homeostasis under global change manipulations, and (5) C:N:P responses to global change are strongly affected by ecosystem type, local climate and experimental conditions. Our study is one of the first to compare individual vs. combined effects of the three global change drivers on terrestrial C:N:P ratios using a large set of data. To further improve our understanding of how ecosystems might respond to future global change, long-term ecosystem-scale studies testing multifactor effects on plants and soils are urgently required across different world regions. This article is protected by copyright. All rights reserved.

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.

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.

Two primary approaches to perennial biofuel crop production studied so far are fertilized grassmonocultures and low-input high-diversity grasslands. While high-yielding perennial grass varieties arebeing developed in fertilized monocultures, breeding for yield in low-input high-diversity systemswould be difficult. Before initiating breeding for low-input systems, it is therefore important to know theminimum number of functional groups and species required for maximum biomass harvest from lowinputgrasslands. We controlled the number of perennial grassland species in 168 plots in Minnesota,USA. Species were selected at random from a pool of 18, and 1, 2, 4, 8, or 16 were planted in each plot.Aboveground biomass was measured annually, and the plots were burned each spring. We found astrongly positive log-linear relationship between average annual aboveground biomass and plantedspecies number, but a large proportion of plot-to-plot variability remained unexplained. Weperformed aconditional analysis of the aboveground biomass data to determine whether considering species identitywould reduce the minimum number of species necessary in order to achieve yields similar to the highestdiversity treatments. A model that accounted for the presence of legumes in general, and for the presenceof the legume species Lupinus perennis in particular, showed no increase in biomass yield with increasedspecies number. Over 11 years, average yields of L. perennis/C4 grass bicultures were similar to those of16-species (maximum diversity) plots, and both were >200% greater than the average of monocultures.Thus, under low-input conditions, the choice of the appropriate few perennial plant species for eachlocation might result in systems with biomass yields similar to those from high-diversity systems.Because breeding biofuel crops in diverse mixtures would introduce complexity that is unwarranted interms of maximum biomass yield, the first biofuel crop breeding programs for low-input systems arelikely to accelerate progress by focusing on grass–legume bicultures.

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.

We studied the composition of a savanna woody plant community across a natural herbivory gradient maintained by both browsing and grazing ungulates in an arid part of the Kruger National Park, South Africa. We focused on (1) short-term browsing effects on reproductive and morphological traits of a dominant-palatable woody species, Acacia nigrescens, Miller, (2) the relationship between browsing–grazing intensity and soil parameters, (3) the effects of herbivore–soil interactions on woody species richness and composition, and (4) browser-induced effects on the representation of woody plant functional traits. We show that the number of pods carried by A. nigrescens trees as well as the internode length of external tree branches both decreased significantly at high browsing intensity. Moreover, we found that total soil nitrogen (N) and soil cation (Ca, Na, Mg, and K) concentrations varied significantly according to grazing rather than browsing intensity with soil nutrients decreasing at heavily grazed sites. Although herbivory and soil

Suppressed growth forms of woody species are common where fire and herbivory are major ecosystem drivers, such as in African savannas. Nevertheless, despite their importance in maintaining plant population viability, woody plants in suppressed growth forms have received little attention in ecological studies. We measured a set of morpho-functional traits and investigated plant density variation in suppressed growth forms (hereafter we refer to them as saplings) of two common species, Acacia nigrescens and Acacia tortilis, across sites that had undergone very different histories of attack from large herbivores while fire had been absent for at least 13 y. We show that heavily browsed saplings have higher regrowth abilities, twice the number of stems produced by the main root crown and twice the root diameter at 5 cm soil depth, than lightly browsed saplings. This suggests that Acacia saplings are resilient to chronic herbivory and show high morphological plasticity. However, we show that mammalian herbivores can strongly limit sapling recruitment to mature size classes and possibly affect variation in sapling density between heavily and lightly browsed sites. Further studies should investigate whether the persistence of the sapling bank can be ascribed to the ``storage effect'' by which a plant's reproductive potential is ``stored'' in a suppressed growth form until a window of opportunity allows rapid maturity.

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 investigated the linkages between leaf litter quality and decomposability in a savanna plant community dominated by palatable-spinescent tree species. We measured: (1) leaf litter decomposability across five woody species that differ in leaf chemistry; (2) mass decomposition, nitrogen (N); and carbon (C) dynamics in leaf litter of a staple browse species (Acacia nigrescens) as well as (3) variation in litter composition across six sites that experienced very different histories of attack from large herbivores. All decomposition trials included litter bags filled with chopped straw to control for variation in site effects. We found a positive relationship between litter quality and decomposability, but we also found that Acacia and straw litter mass remaining did not significantly vary between heavily and lightly browsed sites. This is despite the fact that both the quality and composition of litter returned to the soil were significantly different across sites. We observed greater N resorption from senescing Acacia leaves at heavily browsed sites, which in turn contributed to increase the C:N ratio of leaf litter and caused greater litter N immobilization over time. This, together with the significantly lower tree- and herb-leaf litter mass beneath heavily browsed trees, should negatively affect decomposition rates. However, estimated dung and urine N deposition from both browsers and grazers was significantly greater at high- than at low-herbivory sites. We hypothesize that N inputs from dung and urine boost litter N mineralization and decomposition (especially following seasonal rainfall events), and thereby offset the effects of poor leaf litter quality at chronically browsed sites.

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.

Many tropical pioneer species depend on the presence of high seed densities in the soil for successful recruitment following canopy disturbance (Cheke et al. 1979, Dalling & Hubbell 2002, Guevara Sada & Gómez Pompa 1972, Whitmore 1983). However determinants of variation in the composition and abundance of soil seed banks remain poorly understood. Seed bank densities can be affected by rates of seed predation and pathogen infection on the surface and in the soil, by intrinsic rates of loss in viability following dispersal, and by variation in the timing and duration of fruit production (Dalling et al. 1997, Garwood 1983, Murray & Garcia 2002). Here we compare seasonal fluctuations in seed bank density in five Panamanian forests varying in elevation and seasonality of precipitation (Table 1). We predict that lowland forests should show stronger intra-annual fluctuation in seed bank densities than montane forests because seed production and loss rates should be higher under conditions of greater resource availability, and where consistent high temperatures support greater abundance or activity of seed predators and pathogens (Brühl et al. 1999). Secondly, among lowland sites, we predict greater fluctuations in seed bank densities at drier, more seasonal sites where seasonally favourable conditions for seedling recruitment may select for interspecific synchrony in fruit production (Daubenmire 1972, Garwood 1983).

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...

Spatiotemporal redistribution of incident rainfall in vegetated ecosystems results from the partitioning by plants into intercepted, stemflow, and throughfall fractions. However, variation in patterns and drivers of rainfall partitioning across global biomes remains poorly understood, which limited the ability of climate models to improve the predictions of biome hydrological cycle under global climate change scenario. Here, we synthesized and analyzed the partitioning of incident rainfall into interception, stemflow, and throughfall by trees and shrubs at the global scale using 2430 observations from 236 independent publications. We found that (1) globally, median levels of relative interception, stemflow, and throughfall accounted for 21.8%, 3.2%, and 73.0% of total incident rainfall, respectively; (2) rainfall partitioning varied among different biomes, due to variation in plant composition, canopy structure, and macroclimate; (3) relative stemflow tended to be driven by plant traits, such as crown height:width ratio, basal area, and height, while relative interception and throughfall tended to be driven by plant traits as well as meteorological variables. Our global assessment of patterns and drivers of rainfall partitioning underpins the role of meteorological factors and plant traits in biome-specific ecohydrological cycles. We suggest to include these factors in climate models to improve the predictions of local hydrological cycles and associated biodiversity and function responses to changing climate conditions. ispartof: GLOBAL CHANGE BIOLOGY vol:27 issue:14 pages:3350-3357 ispartof: location:England status: published