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

Recent studies of SOC storage and turnover have employed 13C natural abundance ( 13C) as an in situ Soil organic carbon (SOC) is sensitive to management of tillage, marker of relic and recent SOC pools. Mass concentraresidue (stover) harvest, and N fertilization in corn (Zea mays L.), tions of SOC and the 13C signature are sufficient to but little is known about associated root biomass including rhizodeposition. Natural C isotope abundance ( 13C) and total C content, meacalculate the amount of SOC originating from a C4 crop sured in paired plots of stover harvest and return were used to estimate (e.g., corn) or from a C3 crop [e.g., soybean, Glycine corn-derived SOC (cdSOC) and the contribution of nonharvestable max (L.) Merr.] when the initial soil organic carbon biomass (crown, roots, and rhizodeposits) to the SOC pool. Rhizo(SOCi) has a different 13C signature than the current deposition was estimated for each treatment in a factorial of three crop (Balesdent et al., 1987). The 13C technique has tillage treatments (moldboard, MB; chisel, CH; and no-till, NT), two shown that tillage influences the depth distribution of N fertilizer rates (200 and 0 kg N ha 1), and two corn residue manageSOC (Angers et al., 1995; Layese et al., 2002), storage ments. Treatments influenced cdSOC across a wide range (6.8–17.8 of SOC stock (Balesdent et al., 1988, 1990), and CO2 Mg C ha 1). Nitrogen fertilization increased stover C by 20%, cdSOC efflux from decomposing crop residue (Rochette et al., by only 1.9 Mg C ha 1, and increased rhizodeposition by at least 110% 1999a). Gregorich et al. (1996) reported significant SOC compared with that with no N fertilizer. Stover harvest vs. stover return reduced total source carbon (SC) by 20%, cdSOC by 35%, turnover as influenced by long-term N fertilization of and total SOC. The amount of stover source carbon (SSC) responded continuous corn. Soil organic C and 13C measurements to tillage (MB CH NT), but tillage affected the amount of cdSOC indicated that N fertilized soils contained more SOC differently (NT CH MB). Total SOC was maintained only by from recent crops than unfertilized soils; the difference both N fertilization and stover return during the 13-yr period. The was accounted for by more C4–derived C in fertilized ratio of SC in the nonharvestable biomass to SSC ranged from 1.01 soils (Gregorich et al., 1996). From 22 to 30% of the to 3.49; a ratio of 0.6 conforms to a root-to-shoot ratio of 0.4 when remaining total SOC in the Ap horizon was replaced the root biomass includes 50% rhizodeposits. Tillage controlled the by cdSOC in fertilized soils, whereas in unfertilized soil, fraction of SC retained as cdSOC (i.e., humified; 0.26 for NT and 0.11 only 15 to 20% was replaced during a 30-yr period. The for MB and CH), even though N fertilization, stover harvest, and 13C technique has allowed recent-crop inputs to the tillage all significantly influenced SC. Decomposition of labile rhizodeposits was a major component of the nonhumified fraction. Rhizototal SOC pool to be quantified as caused by differences deposition was as much as three times greater than suggested by in soil management, even though the total SOC pool laboratory and other controlled studies. To understand and manage itself may decrease, increase, or not change during the the entire C cycle, roots and rhizodeposition must be included in the recent-crop period. analysis at the field level. Many factors are known to influence the quantity of C retained by the soil, including mass of C inputs (Buyanovsky and Wagner, 1997; Huggins et al., 1998), T illage greatly influences SOC storage (Angers initial amount of SOC (Campbell et al., 1991), soil texet al., 1995; Reeves et al., 1997; Dao, 1998; Needelture (Needelman et al., 1999), soil temperature and waman et al., 1999; Clapp et al., 2000). Storage of SOC in ter regimes (Rendig and Taylor, 1989; Kaspar and soil depths 7.5 cm is usually greater with no-tillage Bland, 1992; Goss and Watson, 2003), soil N content than in annually tilled systems when sweep, CH, disc, (Gregorich et al., 1996), fertilizer applications (Balaor MB are used for the primary tillage operation (Allbane and Balesdent, 1992), crop residue contact with maras et al., 2000). However, SOC storage below 7.5 cm soil (Clapp et al., 2000), composition of the residue C can be greater in annually tilled systems (Jastrow, 1996; source (Martens, 2000), and the presence of living roots Clapp et al., 2000). Depth distribution of SOC has been (Cheng and Coleman, 1990). Measurement problems linked to tillage tool control over the burial depth of may also influence the quantity of C retained, such as crop residues (Allmaras et al., 1988). Numerous factors uncertainty in the conversion from specific mass conceninteract with tillage to influence changes in SOC storage, trations to a volumetric or field area basis because of such as soil texture and sampling depth (Ellert and Betincomplete sampling depth and bulk density determinatany, 1995), time since treatments were initiated (Liang tion (Ellert and Bettany, 1995) and unspecified spatial et al., 1998), and N fertilizer rate and placement (Gregorich et al., 1995, 1996; Wanniarachchi et al., 1999). 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 USDA-ARS, Dep. of Soil, Water, and Climate, Univ. of Minnesota, harvested; HI, harvest index; MB, moldboard plow; NT, no-tillage; St. Paul, MN 55108. Received 9 June 2003. *Corresponding author R, ratio of USC to SSC; r, residues returned; S, stover as a portion of ([email protected]). SOC or cdSOC; SC, source carbon for input to soil; SOC, soil organic carbon; SOCi, initial soil organic carbon; SOCR, relic soil organic Published in Soil Sci. Soc. Am. J. 68:1366–1375 (2004).  Soil Science Society of America carbon; SY, stover yield; U, unharvestable material as a portion of SC or cdSOC. 677 S. Segoe Rd., Madison, WI 53711 USA