Tillage and Cover Cropping Effects on Aggregate‐Protected Carbon in Cotton and Tomato

Abstract

Conservation tillage (CT) and cover cropping (CC) are agricultural practices that may provide solutions to address water and air quality issues arising from intensive agricultural practices. This study investigated how CT and CC affect soil organic matter dynamics in a cotton(Gossypium hirsutum L.)–tomato (Lycopersicon esculentum Mill.) rotation in California's San Joaquin Valley. There were four treatments: conservation tillage, no cover crop (CTNO); conservation tillage with cover crop (CTCC); standard tillage, no cover crop (STNO); and standard tillage with cover crop (STCC). After 5 yr, the top 30 cm of soil in CTCC had an increase of 4500 kg C ha−1, compared with an increase of 3800 kg C ha−1 in STCC from initial soil C content in 1999. To enhance our understanding of C dynamics in CT systems, we pulse‐labeled cotton with 13CO2 in the field and followed the decomposition of both the roots and the shoots through three physical fractions: light fraction (LF), which tends to turnover quickly, and two relatively stable C pools—intraaggregate LF (iLF) and mineral‐associated carbon (mC). Soil under CT treatments retained more of the cotton‐residue‐derived C in LF and iLF than ST 3 mo after placement in the field. These differences disappeared after 1 yr, however, with no discernable differences between CT and ST regardless of CC. In California's Mediterranean climate, CT alone does not accumulate or stabilize more C than ST in tomato–cotton rotations, and the addition of cover crop biomass is more important than tillage reduction for total soil C accumulation.