DYNAMICS OF SOIL ORGANIC-MATTER AND CORN RESIDUES AFFECTED BY TILLAGEPRACTICES

This study was conducted to determine the influence of tillage practic es on the decomposition of corn (Zea mays L.) residues and turnover of soil organic matter (SOM). Measurements of delta(13)C were made of th e whole soil, the microbial biomass, and two particle-size fractions ( <50 and >50 mu m) in soils that had been under corn production for 11 yr and from an adjacent meadow. Meadow-derived C in total SOM (0-24 cm ) decreased under corn cropping at the same rate under all tillage tre atments. Corn-derived C was evenly distributed with depth in the moldb oard plow treatment and accumulated at the surface in the shallow, red uced-tillage treatments. The incorporation of corn residue C into SOM in the 0- to 24-cm layer was not significantly affected by tillage and was estimated to be approximate to 41 g C m(-2) yr(-1), which represe nts approximate to 30% of the corn residue inputs. Both the macroorgan ic matter (>50 mu n) and microbial biomass had a greater enrichment (u p to 35%) in corn-derived C than either the whole soil or the microorg anic matter (<50 mu m), indicating that these pools are important reci pients of plant residue inputs. Nevertheless, the microorganic matter was also a significant sink for C input, acounting for approximate to 50% of the total corn-derived C remaining In the surface (0-8 cm) soil . Under the conditions of this study, tillage practices influenced the vertical distribution of SOM and corn residues but had no detectable effect on SOM turnover and on the fate of corn residues when the whole Ap horizon was considered.