TILLAGE-INDUCED DIFFERENCES IN ORGANIC-MATTER OF PARTICLE-SIZE FRACTIONS AND MICROBIAL BIOMASS

Tillage can modify the quantity and vertical distribution of organic matter in soils. Its effects on the composition of organic matter is less well documented, however. The objective of this study was to determine the effects of tillage practices in silage corn (Zea mays L.) production on some characteristics of organic matter of a Neubois silt loam (fine-loamy, mixed, frigid Aeric Haplaquept). Carbon and N contents of particle-size separates and microbial biomass C were determined on soil samples collected in three soil layers (0-8, 8-16, and 16-24 cm) from three tillage systems (moldboard plow, minimum tillage, and ridge tillage) that had been established for 11 yr. There were no detectable differences in total organic C among tillage treatments at any depth. The organic C of the sand-size separates, however, accounted for 7% of the total C in the moldboard plow system and up to 19% in the reduced tillage treatments. There was little difference in the organic C content of the silt- and clay-size fractions among the tillage treatments. Nitrogen contents of the size fractions generally mirrored those observed for C. Microbial biomass C accounted for 1.2 to 1.4% of the organic C in the moldboard plow treatment, 2.4 to 3.9% in the ridge tillage treatment, and 3.5 to 5.1% in the minimum tillage treatment. This study demonstrated that, even in a cropping system that involves almost no return of aboveground residue to the soil, such as silage corn production, reducing tillage intensity can result in the maintenance or increase of the more labile fractions of soil organic matter.