Influence of cover crops at the four spheres: A review of ecosystem services, potential barriers, and future directions for North America

Cover crops have been studied for over a century, but the recognition of a complex interaction of cover crop on the Earth's biosphere, lithosphere, hydrosphere, and atmosphere is relatively recent. Furthermore, previously published cover crop research has largely focused on evaluating cover crop impacts on subsequent crop yield. Understanding the cover crop-induced benefits on soil organic carbon (SOC) sequestration, nitrous oxide (N₂O) emissions, wind and water erosion, weed control, and soil microbial communities has gained considerable attention in the last few decades, which is crucial to make progress towards developing sustainable agricultural production systems. New research is continuously published to gain a comprehensive understanding of the multiple ecosystem services provided by cover crops. Here, in this review, we aimed to (a) summarize current knowledge related to cover crop impacts on agroecosystem functioning and explore the potential mechanisms responsible for those effects, and (b) identify the key factors limiting the adoption of cover crops into agroecosystems and the conspicuous knowledge gaps in cover crop research. Overall, the review results suggest that cover crops increased subsequent crop yield, increased SOC storage, increased weed suppression, mitigated N₂O emissions, reduced wind and water erosion, suppressed plant pathogens, and increased soil microbial activity and wildlife biodiversity. However, the magnitude of benefits observed with cover crops varied with cover crop type, location, and the duration of cover cropping. Notably, cover crop termination methods, designing crop rotations to fit cover crops, additional costs associated with cover crop integration, and uncertainty related to economic returns with cover crops are some of the major barriers limiting the adoption of cover crops into production systems, particularly in North America. In addition to long-term effects, future research on cover crop agronomy, breeding cover crop cultivars, and interactive effects of cover crops with other sustainable land management practices is needed.

Cumulative impact of cover crops on soil carbon sequestration and profitability in a temperate humid climate

Although soil C sequestration with cover crops (CCs) has been linked with the potential of CCs in climate change mitigation, the long-term usage of CCs on soil C storage and farm-based economics have been widely overlooked. Therefore, in a CC experiment established in 2007 in a temperate humid climate, four CCs and a no-CC control were compared to evaluate their potential to sequester C and provide economic returns. Total amount of plant C added to soil with CCs translated into greater soil organic carbon (SOC) content by 10-20 Mg C ha-1 than the no-CC control across both sites. Greater crop yield and reduced yield variability with CCs suggest the long-term potential of CCs in increasing agroecosystem resiliency. Moreover, greater profit margins with CCs in processing vegetable crops but not grain and oilseed crops indicate CC effects on crop profitability are dependent on the production system. Our study results indicated that the loss in profit margins with CC usage in grain and oilseed crops might be overcome with C pricing (at $50 Mg-1) on quantity of C sequestered after 9 years of CCing; thus, providing financial compensation to growers may be a mechanism to encourage CC adoption.

Cover crop and crop residue removal effects on temporal dynamics of soil carbon and nitrogen in a temperate, humid climate

Quantification of seasonal dynamics of soil C and N pools is crucial to understand the land management practices for enhancing agricultural sustainability. In a cover crop (CC) experiment established in 2007 and repeated at an adjacent site in 2008, we evaluated the medium-term impact of CC (no cover crop control (no-CC), oat (Avena sativa L.), oilseed radish (OSR, Raphanus sativus L. var. oleoferus Metzg. Stokes), winter cereal rye (rye, Secale cereale L.), and a mixture of OSR+Rye) and crop residue management (residue removed (-R) and residue retained (+R)) on soil C and N dynamics and sequestration. Labile and stable fractions of C and N were determined at seven different time points from 0-15 cm depth during tomato (Solanum lycopersicum L.) growing season in 2015 and 2016 (referred to as site-years). As expected, over the tomato growing season in both site-years, organic C (OC) and total N did not change while the labile C and N fractions changed with greater concentrations observed at 2 weeks after tillage (WAT) and greater treatment differences observed for seven out of eleven soil attributes at tomato harvest. Therefore, 2WAT (early June) and tomato harvest (early September) are reasonably optimum sampling times for soil C and N attributes. Seasonal variation of labile fractions suggested the potential impact of substrate availability from crop residues on soil C and N cycling. Medium-term CC usage enhanced the surface soil C and N storage. Overall, this study highlights the positive and synergistic influences of CCs and maintaining crop residues in increasing both labile and stable fractions of C and N and enhancing soil quality in a temperate humid climate.