Fall Conservation Deep Tillage Stabilizes Maize Residues into Soil Organic Matter

Efforts for increasing soil organic matter (SOM) content under agricultural systems have primarily focused on management practices that reduce exposure of SOM to decomposition via minimum tillage. We assess an alternative approach, termed "fall conservation deep tillage" (FCDT), to SOM stabilization through fall incorporation of crop residues into the soil profile together with N fertilizer. In an eastern Nebraska field under irrigated maize (Zea mays L.), we measured total soil C and N stocks on an equal soil mass basis (0- to 30-cm depth) and the composition of four SOM fractions after 14 yr of previous no-till management and 1, 2, and 3 yr after conversion to FCDT. After 3 yr of FCDT, redistribution of soil C, N, and soil organic matter fractions occurred within the soil profile; however, total soil C and N stocks remained unchanged. An increase in the soil-crop residue contact surface through FCDT increased free light fraction by 170% in deeper strata in the soil profile. Change from no-till to FCDT led to initial reduction of C and N stocks as mobile and calcium bound humic acid fractions after 1 yr of tillage, suggesting enhanced decomposition and/or condensation. By the second and third years of FCDT, stabilization of crop residues into these humified fractions was significant. The trends observed after 3 yr of FCDT at field-scale match prior plot-scale experiments from the same region and suggest that a positive balance of soil C and N accrual and loss may be achieved by enhancing the soil-residue contact surface in these soils.