Long‐Term Corn Residue Effects

Abstract

A better understanding of C turnover, with estimates of root‐derived C, is needed to manage soil C sequestration. The objective was to evaluate the long‐term treatment and environmental effects on unharvestable soil C components. Two N fertilizer treatments and a control were imposed during 29 yr of continuous corn ( Zea mays L.) with stover removal as silage vs. stover return during grain harvest with moldboard plow (MB) tillage. Soil organic carbon (SOC) declined and natural 13 C abundance (δ 13 C) increased during the 29‐yr period. Field averages of SOC and δ 13 C (0–30 cm) were 96.4 Mg ha −1 and −17.3‰ in 1965; respective values in 1995 were 78.9 Mg ha −1 and −16.6‰. Loss of SOC was greater with stover removed or no fertilization, but δ 13 C increased for all treatments. Stover yield (SY), SOC, and δ 13 C data were applied to a model to estimate unharvestable C and predict total source C (SC) input from corn. The SC for 29 yr totaled 172 to 189 Mg ha −1 when stover was harvested and 268 to 284 Mg ha −1 when stover was returned. The SC input from unharvestable sources was 1.8 times more than SC from aboveground stover when N was added and 1.7 when N was not added. The root‐to‐shoot ratio was 1.1 when N was added and 1.2 with no N. Only 5.3% of the SC was retained as SOC. Unharvestable C contributions to rhizodeposition are much larger than suggested from controlled studies including C‐enriched CO 2 followed by soil respiration or CO 2 efflux measurements.