Response of soil organic matter to cover cropping in water-limited environments

Abstract

Cover crops are promoted to improve soil health and soil carbon (C) sequestration in agroecosystems, yet responses of various soil organic carbon (SOC) and nitrogen (N) components to cover cropping have not been quantified for water-limited environments. This study evaluated the response of SOC and N components to different cover crops and mixtures in limited irrigation winter wheat (Triticum aestivum L.)-sorghum (Sorghum bicolor L. Moench)-fallow rotation. Cover cropping treatments included pea (Pisum sativum L.), oats (Avena sativa L.), and canola (Brassica napus L.); mixtures of pea + oats (POmix), pea + canola (PCmix), pea + oats + canola (POCmix), and a six-species mixture (SSmix) of pea + oats + canola + hairy vetch (Vicia villosa Roth) + forage radish (Raphanus sativus L.) + barley (Hordeum vulgare L.); and a fallow. Cover crop treatments were arranged in a randomized complete block design within each phase of the crop rotation. Soil samples were collected in the summer of 2019 and 2020 from the 0–15 cm depth of study plots established in fall 2015 and analyzed for various soil organic matter (SOM) components. Soil inorganic N was 7–36% lower with cover crops than fallow. The PCmix had 48–73% greater 24-h-carbon dioxide-carbon (CO2-C) than fallow, canola, and SSmix at termination time. Thirty-six days after termination, particulate organic carbon (OC) content was 61–69% higher with pea, SSmix, and POCmix than fallow. The SOC content was 9.3–22% greater with oats than pea, canola, POmix, and SSmix. Similarly, total N content with oats was 10% and 22% higher than with SSmix and canola, respectively. The increase in SOC and total N were primarily observed in intermediate-size aggregates (250 μm–2 mm and 53–250 μm). However, the minimum data set of soil health included SOC, soil pH, labile organic nitrogen (LON), mineral-associated organic nitrogen (MAON), and microbial biomass carbon (MBC). While this study showed a diverse response of SOC and N components to various cover crop treatments, oats and their mixture as cover crops had greater SOC and total N than other cover crops. Cover cropping could improve soil health in crop-fallow rotations in water-limited environments.