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.

Summary1. 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. 2. 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. 3. 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. 4. 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.

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.

Abstract. 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 used 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 C 4 grasses and legumes. These results suggest that the productivity of diverse plant communities was partly dependent on belowground plant interactions that caused roots to be distributed more deeply in soil.

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