Long-term corn residue effects: Harvest alternatives, soil carbon turnover, and root-derived carbon

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 C-13 abundance (delta(13)C) increased during the 29-yr period. Field averages of SOC and delta(13)C (0-30 cm) were 96.4 Mg ha(-1) and -17.3parts per thousand in 1965; respective values in 1995 were 78.9 Mg ha(-1) and -16.6parts per thousand. Loss of SOC was greater with stover removed or no fertilization, but VC increased for all treatments. Stover yield (SY), SOC, and VC 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 CO2 followed by soil respiration or CO2 efflux measurements.