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DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

Corn-Residue Transformations into Root and Soil Carbon as Related to Nitrogen, Tillage, and Stover Management

USDA-ARS, Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108

^* Corresponding author (eclapp{at}umn.edu).

Soil organic carbon (SOC) is sensitive to management of tillage, residue (stover) harvest, and N fertilization in corn (Zea mays L.), but little is known about associated root biomass including rhizodeposition. Natural C isotope abundance (¹³C) and total C content, measured in paired plots of stover harvest and return were used to estimate corn-derived SOC (cdSOC) and the contribution of nonharvestable biomass (crown, roots, and rhizodeposits) to the SOC pool. Rhizodeposition was estimated for each treatment in a factorial of three tillage treatments (moldboard, MB; chisel, CH; and no-till, NT), two N fertilizer rates (200 and 0 kg N ha^–1), and two corn residue managements. Treatments influenced cdSOC across a wide range (6.8–17.8 Mg C ha^–1). Nitrogen fertilization increased stover C by 20%, cdSOC by only 1.9 Mg C ha^–1, and increased rhizodeposition by at least 110% compared with that with no N fertilizer. Stover harvest vs. stover return reduced total source carbon (SC) by 20%, cdSOC by 35%, and total SOC. The amount of stover source carbon (^SSC) responded to tillage (MB > CH > NT), but tillage affected the amount of cdSOC differently (NT > CH > MB). Total SOC was maintained only by both N fertilization and stover return during the 13-yr period. The ratio of SC in the nonharvestable biomass to ^SSC ranged from 1.01 to 3.49; a ratio of 0.6 conforms to a root-to-shoot ratio of 0.4 when the root biomass includes 50% rhizodeposits. Tillage controlled the fraction of SC retained as cdSOC (i.e., humified; 0.26 for NT and 0.11 for MB and CH), even though N fertilization, stover harvest, and tillage all significantly influenced SC. Decomposition of labile rhizodeposits was a major component of the nonhumified fraction. Rhizodeposition was as much as three times greater than suggested by laboratory and other controlled studies. To understand and manage the entire C cycle, roots and rhizodeposition must be included in the analysis at the field level.

Abbreviations: ¹³C, ¹³C natural abundance • cdSOC, corn-derived soil organic carbon • CH, chisel plow • f, fraction of soil organic carbon derived from corn • F, ratio of ^ScdSOC to total C in stover • h, residues harvested • HI, harvest index • MB, moldboard plow • NT, no-tillage • R, ratio of ^USC to ^SSC • r, residues returned • ^S, stover as a portion of SOC or cdSOC • SC, source carbon for input to soil • SOC, soil organic carbon • SOC_i, initial soil organic carbon • SOC_R, relic soil organic carbon • SY, stover yield • ^U, unharvestable material as a portion of SC or cdSOC

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