Relay-Planted Cover Crops Enhance Soil Carbon Storage and Yield in Continuous Seed Maize

Continuous seed maize (Zea mays L.) can deplete soil organic carbon (SOC) and impair soil function. In a three-year field experiment (2023–2025), hairy vetch (Vicia villosa, Hv) and february orchid (Orychophragmus violaceus, Fo) were relay-planted after male-row removal and incorporated before the next maize season. Soil was sampled once in August 2025, after two completed cover-crop cycles. Compared with monoculture seed maize without cover crops (CK), Hv and Fo had 24.14% and 28.06% higher 0–40 cm SOC stocks. Hv also had a 37.57% and 12.63% higher mineral-associated organic carbon (MAOC) stock and MAOC/particulate organic carbon (POC) ratio. Fo had numerically higher available P and hydrolyzable N and higher activities of several C-, N-, and P-acquiring enzymes. The soil quality index (SQI) was 35.67% and 51.73% higher under Hv and Fo than under CK. The SQI was positively associated with 2025 yield. In 2025, yield was 64.86% and 35.88% greater under Hv and Fo than under CK; the three-year mean yield was 22.86% greater under Hv. Hv was associated with a higher MAOC stock, whereas Fo was associated with greater enzyme activity and numerically greater available N and P. These contrasting responses suggest complementary soil functions, but the mechanisms require testing.