Long-Term Corn Residue Effects: Harvest Alternatives, Soil Carbon Turnover, and Root-Derived Carbon

studies have shown surprisingly little response of SOC to differences in C inputs. These results suggest an upper A better understanding of C turnover, with estimates of rootlimit on C sequestration in mineral soils independent derived C, is needed to manage soil C sequestration. The objective was to evaluate the long-term treatment and environmental effects of the input rate, as demonstrated by Campbell et al. on unharvestable soil C components. Two N fertilizer treatments and (1991a, 1991b) and Soon (1998), who showed no effect a control were imposed during 29 yr of continuous corn (Zea mays L.) of varying C inputs on soil organic matter (SOM). with stover removal as silage vs. stover return during grain harvest with Huggins and Fuchs (1997) suggested that SOM changes moldboard plow (MB) tillage. Soil organic carbon (SOC) declined and were dependent on the unharvested corn biomass (abovenatural 13C abundance ( 13C) increased during the 29-yr period. Field ground and root) response to N. Balesdent and Balaaverages of SOC and 13C (0–30 cm) were 96.4 Mg ha 1 and 17.3‰ bane (1996) found that corn root-derived C contributed in 1965; respective values in 1995 were 78.9 Mg ha 1 and 16.6‰. about 1.6 times more C to SOC than stover-derived C. Loss of SOC was greater with stover removed or no fertilization, but Natural 13C abundance techniques have significantly 13C increased for all treatments. Stover yield (SY), SOC, and 13C improved knowledge about C sequestration. The 13C 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 values of SOC represent the relative contribution of C3 189 Mg ha 1 when stover was harvested and 268 to 284 Mg ha 1 when and C4 plant species to NPP (Collins et al., 1997; Boutton stover was returned. The SC input from unharvestable sources was et al., 1998). The 13C values from plants with C3 photo1.8 times more than SC from aboveground stover when N was added synthesis typically range from 40 to 23‰, while those and 1.7 when N was not added. The root-to-shoot ratio was 1.1 when with C4 photosynthesis range from 19 to 9‰ (Collins N was added and 1.2 with no N. Only 5.3% of the SC was retained et al., 1997; Boutton et al., 1998). For corn and soybean as SOC. Unharvestable C contributions to rhizodeposition are much [Glycine max. (L.) Merr. ] in southern Minnesota, these larger than suggested from controlled studies including C-enriched values are 12 and 26‰, respectively (Huggins et CO2 followed by soil respiration or CO2 efflux measurements. al., 1998b). When vegetation becomes compositionally stable for a long period of time, the 13C values of SOC in the upper soil profile (0–20 cm) approach that of the T management of crop residues and SOC is of plant community (Nadelhoffer and Fry, 1988) because primary importance for maintaining soil fertility and only slight isotope fractionation may occur during early productivity as well as environmental quality. Managestages of SOM decomposition in well-drained, mineral ment practices are needed to optimize net primary prosoils (Boutton, 1996). duction (NPP) for improved soil C sequestration (RobinSince these unique 13C values persist during decomson et al., 1996; Paustian et al., 1997). position and SOM formation, the SOM turnover rate Reicosky et al. (1995) reviewed tillage and biomass can be determined by the rate at which the 13C value production impacts on SOC in agricultural systems. of SOC changes to approach that of the new plant comThey concluded that SOC was controlled by crop resimunity (Balesdent et al., 1987; Boutton et al., 1998). due input that was largely determined by crop choice, Collins et al. (1999) found that 13C values in MB-tilled fertilization, and climate. Crop residue return or resoil were higher than those from noncultivated soil unmoval, biological oxidation rates, and soil erosion conder 8 to 35 yr of continuous corn. The proportion of trolled the SOC losses from agricultural systems. C4–derived corn C ranged from 22 to 40% of the total Under continuous corn with MB tillage, the removal C and decreased with soil depth. Gregorich et al. (1996) of stover plus grain decreased SOC of Mollisols in Iowa reported that N fertilized soils accumulated more corn(Larson et al., 1972; Robinson et al., 1996), Indiana derived soil organic carbon (cdSOC) than unfertilized (Barber, 1979), Michigan (Vitosh et al., 1997), Wisconsoils. After MB tillage, Angers et al. (1995) found that sin (Vanotti et al., 1997), and Minnesota (Bloom et al., C4–derived SOC was distributed evenly in the upper 3