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Increasing the quantity and quality of plant biomass production in space and time can improve the capacity of agroecosystems to capture and store atmospheric carbon (C) in the soil. Cover cropping is a key practice to increase system net primary productivity (NPP) and increase the quantity of high-quality plant residues available for integration into soil organic matter (SOM). Cover crop management and local environmental conditions, however, influence the magnitude of soil C stock change. Here, we used a comprehensive meta-analysis approach to quantify the effect of cover crops on soil C stocks from the 0–30 cm soil depth in temperate climates and to identify key management and ecological factors that impact variation in this response. A total of 40 publications with 181 observations were included in the meta-analysis representing six countries across three different continents. Overall, cover crops had a strong positive effect on soil C stocks (P < 0.0001) leading to a 12% increase, averaging 1.11 Mg C/ha more soil C relative to a no cover crop control. The strongest predictors of SOC response to cover cropping were planting and termination date (i.e., growing window), annual cover crop biomass production, and soil clay content. Cover crops planted as continuous cover or autumn planted and terminated led to 20–30% greater total soil C stocks relative to other cover crop growing windows. Likewise, high annual cover crop biomass production (>7 Mg·ha−1·yr−1) resulted in 30% higher total soil C stocks than lower levels of biomass production. Managing for greater NPP by improving synchronization in cover crop growing windows and climate will enhance the capacity of this practice to drawdown carbon dioxide (CO₂) from the atmosphere across agroecosystems. The integration of growing window (potentially as a proxy for biomass growth), climate, and soil factors in decision-support tools are relevant for improving the quantification of soil C stock change under cover crops, particularly with the expansion of terrestrial soil C markets.

AimsHalf of field crop nitrogen (N) is often derived soil organic matter (SOM) mineralization, yet we do not fully understand the extent to which plant genotypic differences influence SOM mineralization dynamics across different soil N contexts. We explored the effects of rapeseed (Brassica napus) genotypic diversity on N uptake from organic and inorganic N sources.MethodsIn a greenhouse study, we applied dual 15N labeled ammonium-nitrate fertilizer to examine N uptake patterns of rapeseed in different N environments. Ten varieties were grown in a full factorial experiment with four treatments, including combinations of high and low N fertilizer and SOM.ResultsWe found limited varietal differences in total biomass or N uptake across soil environments. Across all varieties, SOM was an important, additive N source even as N fertilizer availability increased. High SOM/High Fertilizer treatment plants obtained 64% of N from SOM, while plants grown with High SOM/Low Fertilizer obtained 89% of total N from SOM. Under low SOM availability, the high fertilizer addition increased overall N uptake from SOM by 42% relative to the low N fertilizer treatment.ConclusionsIntegrating plant reliance on SOM-N sources into crop breeding and NUE estimates has potential to improve crop productivity and improve overall system N use efficiency.

Interest in planting mixtures of cover crop species has grown in recent years as farmers seek to increase the breadth of ecosystem services cover crops provide. As part of a multidisciplinary project, we quantified the degree to which monocultures and mixtures of cover crops suppress weeds during the fall-to-spring cover crop growing period. Weed-suppressive cover crop stands can limit weed seed rain from summer- and winter-annual species, reducing weed population growth and ultimately weed pressure in future cash crop stands. We established monocultures and mixtures of two legumes (medium red clover and Austrian winter pea), two grasses (cereal rye and oats), and two brassicas (forage radish and canola) in a long fall growing window following winter wheat harvest and in a shorter window following silage corn harvest. In fall of the long window, grass cover crops and mixtures were the most weed suppressive, whereas legume cover crops were the least weed suppressive. All mixtures also effectively suppressed weeds. This was likely primarily due to the presence of fast-growing grass species, which were effective even when they were seeded at only 20% of their monoculture rate. In spring, weed biomass was low in all treatments due to winter kill of summer-annual weeds and low germination of winter annuals. In the short window following silage corn, biomass accumulation by cover crops and weeds in the fall was more than an order of magnitude lower than in the longer window. However, there was substantial weed seed production in the spring in all treatments not containing cereal rye (monoculture or mixture). Our results suggest that cover crop mixtures require only low seeding rates of aggressive grass species to provide weed suppression. This creates an opportunity for other species to deliver additional ecosystem services, though careful species selection may be required to maintain mixture diversity and avoid dominance of winter-hardy cover crop grasses in the spring.

Cover crops have the potential to be agricultural nitrogen (N) regulators that reduce leaching through soils and then deliver N to subsequent cash crops. Yet, regulating N in this way has proven difficult because the few cover crop species that are well-studied excel at either reducing N leaching or increasing N supply to cash crops, but they fail to excel at both simultaneously. We hypothesized that mixed species cover crop stands might balance the N fixing and N scavenging capabilities of individual species. We tested six cover crop monocultures and four mixtures for their effects on N cycling in an organically managed maize-soybean-wheat feed grain rotation in Pennsylvania, USA. For three years, we used a suite of integrated approaches to quantify N dynamics, including extractable soil inorganic N, buried anion exchange resins, bucket lysimeters, and plant N uptake. All cover crop species, including legume monocultures, reduced N leaching compared to fallow plots. Cereal rye monocultures reduced N leaching to buried resins by 90% relative to fallow; notably, mixtures with just a low seeding rate of rye did almost as well. Austrian winter pea monocultures increased N uptake in maize silage by 40 kg N ha-1 relative to fallow, and conversely rye monocultures decreased N uptake into maize silage by 40 kg N ha-1 relative to fallow. Importantly, cover crop mixtures had larger impacts on leaching reduction than on maize N uptake, when compared to fallow plots. For example, a three-species mixture of pea, red clover, and rye had similar maize N uptake to fallow plots, but leaching rates were 80% lower in this mixture than fallow plots. Our results show clearly that cover crop species selection and mixture design can substantially mitigate tradeoffs between N retention and N supply to cash crops, providing a powerful tool for managing N in temperate cropping systems.

Background Cover cropping is an agricultural practice that uses secondary crops to support the growth of primary crops through various mechanisms including erosion control, weed suppression, nutrient management, and enhanced biodiversity. Cover crops may elicit some of these ecosystem services through chemical interactions with the soil microbiome via root exudation, or the release of plant metabolites from roots. Phytohormones are one metabolite type exuded by plants that activate the rhizosphere microbiome, yet managing this chemical interaction remains an untapped mechanism for optimizing plant-soil-microbiome interactions. Currently, there is limited understanding on the diversity of cover crop phytohormone root exudation patterns and our aim was to understand how phytochemical signals selectively enrich specific microbial taxa and functionalities in agricultural soils. Results Here, we link variability in cover crop root exudate composition to changes in soil microbiome functionality. Exudate chemical profiles from 4 cover crop species ( Sorghum bicolor , Vicia villosa , Brassica napus , and Secale cereal ) were used as the chemical inputs to decipher microbial responses. These distinct exudate profiles, along with a no exudate control, were amended to agricultural soil microcosms with microbial responses tracked over time using metabolomes and genome-resolved metatranscriptomes. Our findings illustrated microbial metabolic patterns were unique in response to cover crop exudate inputs over time, particularly by sorghum and cereal rye amended microcosms. In these microcosms, we identify novel microbial members (at the genera and family level) who produced IAA and GA 4 over time. Additionally, we identified cover crop exudates exclusively enriched for bacterial nitrite oxidizers, while control microcosms were discriminated for nitrogen transport, mineralization, and assimilation, highlighting distinct changes in microbial nitrogen cycling in response to chemical inputs. Conclusions We highlight that root exudate amendments alter microbial community function (i.e., N cycling) and microbial phytohormone metabolisms, particularly in response to root exudates isolated from cereal rye and sorghum plants. Additionally, we constructed a soil microbial genomic catalog of microorganisms responding to commonly used cover crops, a public resource for agriculturally relevant microbes. Many of our exudate-stimulated microorganisms are representatives from poorly characterized or novel taxa, revealing the yet to be discovered metabolic reservoir harbored in agricultural soils. Our findings emphasize the tractability of high-resolution multi-omics approaches to investigate processes relevant for agricultural soils, opening the possibility of targeting specific soil biogeochemical outcomes through biological precision agricultural practices that use cover crops and the microbiome as levers for enhanced crop production. BPG5Z_iDvqR4KrAs2TcSTi Video Abstract

A global transformation in semi-arid cropping systems is occurring as dryland (non-irrigated) farmers in semi-arid regions shift from crop rotations reliant on year-long bare fallows, called summer fallow, to more intensively cropped systems. Understanding the rate of cropping system intensification at the landscape scale is critical to estimating the economic and environmental implications of this movement. Here, we use high-resolution satellite data to quantify dryland cropping patterns from 2008 to 2016 in the US High Plains. We use these estimates to scale up our previous field-level research in this region on soil carbon, herbicide use, yields, and profitability. Over the nine year study period, the High Plains witnessed a profound shift in cropping systems, as the historically dominant wheat-fallow system was replaced by more intensified rotations as the dominant systems by land area. Out of the 4 million hectares of non-irrigated cropland in the study area, this shift coincided with a 0.5 million-hectare decline in summer fallow and a concurrent increase in alternative (non-wheat) crops. We estimate that, from 2008 to 2016, these patterns resulted in a 0.53 Tg (9%) increase in annual grain production, 80 million USD (10%) increase in annual net farm operating income, substantial reductions in herbicide use, and an increase in C sequestration that corresponds to greenhouse gas reductions of 0.32 million metric tons of CO2 equivalents per year (MMTCO2e yr−1). We project each of these implications to a scenario of potential maximum 100% intensification and estimate that, relative to 2016 levels, herbicide use would be reduced by more than half, grain production would increase by 25%, net operating income would increase by 223 million USD (26%), and greenhouse gases would be reduced by an additional 0.8 MMTCO2e yr−1. The scale of cropping intensification in the High Plains and its environmental and economic impacts has important implications for other regions undergoing similar transformations, and for policy that can either support or hinder these shifts toward more sustainable cropping systems.

Trends of increasing agricultural trade, increased concentration of livestock production systems, and increased human consumption of livestock products influence the distribution of nutrients across the global landscape. Phosphorus (P) represents a unique management challenge as we are rapidly depleting mineable reserves of this essential and non-renewable resource. At the same time, its overuse can lead to pollution of aquatic ecosystems. We analyzed the relative contributions of food crop, feed crop, and livestock product trade to P flows through agricultural soils for 12 countries from 1961 to 2007. Due to the intensification of agricultural production, average soil surface Ñ balances more than tripled from 6 to 21 kg P ha⁻¹ between 1961 and2007 for the 12study countries. Consequently, countries that are primarily agricultural exporters carried increased risks for water pollution or, for Argentina, reduced soil fertility due to soil P mining to support exports. In 2007, nations imported food and feed from regions with higher apparent P fertilizer use efficiencies than if those crops were produced domestically. However, this was largely because imports were sourced from regions depleting soil P resources to support export crop production. In addition, the pattern of regional specialization and intensification of production systems also reduced the potential to recycle P resources, with greater implications for livestock production than crop production. In a globalizing world, it will be increasingly important to integrate biophysical constraints of our natural resources and environmental impacts of agricultural systems into trade policy and agreements and to develop mechanisms that move us closer to more equitable management of nonrenewable resources such as phosphorus.

Organic amendments like compost can enhance soil health and climate change mitigation in managed grassland ecosystems. We previously demonstrated in a northern Colorado cool‐season pasture that infrequent compost applications support net soil organic carbon sequestration. Here, we examined plant and soil biota responses over three growing seasons to better understand how plant–soil feedbacks support net sequestration under compost. Compost doubled soil P and increased soil K by one‐third, but slightly decreased soil pH in the top 10 cm of soil. Differences in plant production and plant community composition were immediate after application and sustained over the experimental period while soil biota were slower to respond. A path analysis (χ2 = 14.0, p = 0.23) suggests that the plant effect on soil organic carbon sequestration (R2 = 0.67) was fully mediated by the soil microbial community, especially bacteria. Our work supports the importance of microbially derived inputs for building soil organic matter in grasslands receiving organic amendments.

Background and aims The selection of legume species and species mixtures influences agroecosystem nitrogen (N) and carbon cycling. We utilized a fertility gradient to investigate the effects of plant species interactions on biological N fixation of an annual and perennial legume in response to shifting soil resource availability. Methods Legume N fixation of annual field pea (Pisum sativum) and perennial red clover (Trifolium pratense) grown in monoculture and mixtures with oats (Avena sativa) or orchardgrass (Dactylis glomerata) was estimated using the 15N natural abundance method across 15 farm fields and we measured six soil N pools ranging from labile to more recalcitrant. Results Evidence of complementary and facilitative species interactions was stronger for the perennial red clover-orchardgrass mixture than for the annual field pea-oat mixture (N Land Equivalency Ratios were 1.6 and 1.2, respectively). We estimated that the transfer of fixed N from red clover to orchardgrass increased aboveground N fixation estimates by 15% from 33 to 38 kg N ha−1. Despite a more than 2-fold range in soil organic matter levels and more than 3-fold range in labile soil N pools across field sites, the N fertility gradient was not a strong predictor of N fixation. While grass N assimilation was positively correlated with soil N pools, we found only weak, inverse correlations between legume N fixation and soil N availability. In grass-legume mixtures, soil N availability indirectly influenced N fixation through plant competition. Conclusions These results suggest that increasing diversity of cropping systems, particularly through the incorporation of perennial mixtures into rotations, could improve overall agroecosystem N cycling efficiency.

Food systems are under increasing pressure to produce sufficient food for the global population, decrease the environmental impacts of production, and buffer against complex global change. Food security also remains elusive for many populations worldwide. Greater emphasis on food system resilience could reduce these vulnerabilities. We outline integrated strategies that together could foster food system resilience across scales, including (a) integrating gender equity and social justice into food security research and initiatives, (b) increasing the use of ecological processes rather than external inputs for crop production, (c) fostering regionalized food distribution networks and waste reduction, and (d) linking human nutrition and agricultural production policies. Enhancing social–ecological links and fostering adaptive capacity are essential to cope with short-term volatility and longer-term global change pressures. Finally, we highlight regional case studies that have enhanced food system resilience for vulnerable populations. Efforts in these areas could have dramatic impacts on global food system resilience.