Soil organic carbon and C-13 abundance as related to tillage, crop residue, and nitrogen fertilization under continuous corn management in Minnesota

Long-term field experiments are among the best means to predict soil manage ment impacts on soil carbon storage. Soil organic carbon (SOC) and natural abundance C-13 (delta(13)C) were sensitive to tillage, stover harvest, and nitrogen (N) management during 13 years of continuous corn (Zea mays L,), g rown on a Haplic Chernozem soil in Minnesota. Contents of SOC in the 0-15 c m layer in the annually-tilled [moldboard (MB) and chisel (CH)] plots decre ased slightly with years of corn after a low input mixture of alfalfa (Medi cago sativum L.) and oat (Avena sativa L.) for pasture; stover harvest had no effect. Storage of SOC in no-till (NT) plots with stover hal vested rema ined nearly unchanged at 55 Mg ha(-1) with time, while that with stover ret urned increased about 14%. The measured delta(13)C increased steadily with years of corn cropping in all treatments; the NT with stover return had the highest increase. The N fertilization effects on SOC and delta(13)C were m ost evident when stover was returned to NT plots. In the 15-30 cm depth, SO C storage decreased and delta(13)C values increased with years of corn crop ping under NT, especially when stover was harvested. There was no consisten t temporal trend in SOC storage and delta(13)C values in the 15-30 cm depth when plots received annual MB or CH tillage. The amount of available corn residue that was retained in SOC storage was influenced by all three manage ment factors. Corn-derived SOC in the 0-15 cm and the 15-30 cm layers of th e NT system combined was largest with 700 kg N ha(-1) and no stover harvest . The MB and CH tillage systems did not influence soil storage of corn-deri ved SOC in either the 0-15 or 15-30 cm layers. The corn-derived SOC as a fr action of SOC after 13 years fell into three ranges: 0.05 for the NT with s tover harvested, 0.15 for the NT with no stover harvest, and 0.09-0.10 for treatments with annual tillage, N rate had no effect on this fraction. Corn -derived SOC expressed as a fraction of C returned was positively biased wh en C returned in the roots was estimated from recovery of root biomass. The half-life for decomposition of the original or relic SOC was longer when s tover was returned, shortened when stover was harvested and N applied, and sharply lengthened when stover was not harvested and N was partially mixed with the stover. Separating SOC storage into relic and current crop sources has significantly improved our understanding of the main and interacting e ffects of tillage. crop residue, and N fertilization for managing SOC accum ulation in soil. Published by Elsevier Science B.V.