Earthworm effects on carbon and nitrogen dynamics of surface litter in corn agroecosystems

We examined the influence of earthworms on surface litter decomposition in corn (Zea mays) agroecosystems in Wooster, Ohio. We employed a split-plot experimental design with 12 main plots, each 20 x 30 m and containing three 4.5 x 4.5 m field enclosures in which earthworm populations were (1) increased, (2) decreased, or (3) unmodified. The main plots received one of three nutrient treatments (cow manure, legume cover crop, inorganic fertilizer) with four replicates. The three earthworm population treatments were randomly assigned to the three field enclosures within each main nutrient-treatment plot. We added corn litter to the soil surface in each of the treatment combinations in the field enclosures in November 1992 and collected remaining litter after 19, 85, 135, 161, and 191 d. We separated out small piles of surface litter (i.e., "middens") associated with the entrance to burrows of Lumbricus terrestris from the rest of the litter to determine if they differed from each other in C and N content and microbial activity. We initiated a second study in the following year (November 1993), adding mechanically shredded corn litter to the field enclosures and harvesting after 184 d, but without analyzing earthworm middens separately. Earthworms increased the rate of decomposition of surface litter in both years, and we estimated that they could have consumed 840 kg·ha-1·yr-1of surface litter. The C/N ratio of litter remaining on the soil surface (midden plus nonmidden) was significantly greater in plots with unmodified or increased populations than in enclosures with decreased populations. However, litter in earthworm middens, which comprised ∼10% of the total surface litter, had a lower C/N ratio and greater microbial activity than litter in the surrounding area, suggesting that earthworms incorporated litter with the low C/N into their middens. Because the earthworms feed mainly on litter in their middens, we propose that they selectively removed litter fractions with a low C/N ratio, thereby increasing the overall C/N ratio of remaining surface litter. Our results suggest that the redistribution of surface litter by Lumbricus terrestris and its consequences for spatial heterogeneity, microbial activity, and nutrient content of the litter is a critical process in systems with large populations of this earthworm species. These earthworms may exert major control over litter decomposition in minimally tilled or no-till agroecosystems where they may influence