Influence of tillage on the dynamics of loose- and occluded-particulate and humified organic matter fractions

This study was carried out at a site (fine silty mixed mesic Argiaquic Argialboll) where use of no-tillage (NT) practices, for over a decade, had not increased soil organic carbon (SOC) sequestration relative to plots that had been moldboard plowed (MP). Even though the total SOC contents of these soils were known to be similar, we expected input and decay rates of residue-derived C, conservation of root-derived C, and the importance of aggregate protection of particulate organic matter (POM) to differ among these tillage treatments. Corn (Zea maize L.) was pulse-labeled with 13CO2 repeatedly during the 1995 growing season to allow the fate of residue-derived C retained in loose-POM (LPOM), aggregate-occluded POM (OPOM) and in mineral associated humified (HF) fractions, to be tracked through April 1997. Tillage practices were related to fundamental differences in the depth and rate at which residues decayed and the distribution of those residues among SOC fractions. In December 1995, approximately 50% of the C derived from labeled residues was recovered in the LPOM, OPOM, and HF fractions of the NT plots, while only 22% was recovered in those fractions from MP plots. After initial rapid losses of label-derived C, C turnover rates were relatively slow in the MP plots compared to C turnover rates observed at the surface of the NT plots. As a result, after 1.5 years the MP and the NT plots retained similar amounts (approximately equal to 10%) of label-derived C in the 0-20 cm depth. Shifts in the percent label recovery suggest that newly assimilated C was rapidly lost from the LPOM fraction as it accumulated in the OPOM and HF fractions. Increases in the fractional abundance of label-derived C in the OPOM and HF fractions accounted for approximately half of the label lost from LPOM. Trends in both the fractional abundance and percent label recovery in the OPOM and HF fractions indicated that C derived from 1995-residues was concentrated at 0-5 cm depth in NT plots and was more evenly distributed in the MP plots. In December 1995, the fractional abundance of OPOM and HF was greater in the root than shoot labeled plots, indicating that root-derived C was incorporated into SOC more rapidly than shoot-derived materials. By spring, the fractional abundance of OPOM and HF had increased in tilled plots amended with labeled shoots. Our fractionation scheme revealed the influence of aggregation on the decay dynamics of C introduced by newly incorporated residues and identified fundamental differences in the depth, decay dynamics and distribution of C, newly assimilated into the SOC fractions of NT and MP soils.