EARTHWORM EFFECTS ON CARBON AND NITROGEN DYNAMICS OF SURFACE LITTER IN CORN AGROECOSYSTEMS

We examined the influence of earthworms on surface litter decompositio n in corn (Zea mays) agroecosystems in Wooster, Ohio. We employed a sp lit-plot experimental design with 12 main plots, each 20 x 30 m and co ntaining three 4.5 x 4.5 m held enclosures in which earthworm populati ons were (1) increased, (2) decreased, or (3) unmodified. The main plo ts 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 held enclos ures within each main nutrient-treatment plot. We added corn litter to the soil surface in each of the treatment combinations in the field e nclosures 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 Lumbric us terrestris from the rest of the litter to determine if they differe d from each other in C and N content and microbial activity. We initia ted a second study in the following year (November 1993), adding mecha nically shredded corn litter to the field enclosures and harvesting af ter 184 d, but without analyzing earthworm middens separately. Earthwo rms increased the rate of decomposition of surface litter in both year s, and we estimated that they could have consumed 840 kg.ha(-1).yr(-1) of surface litter. The C/N ratio of litter remaining on the soil surf ace (midden plus nonmidden) was significantly greater in plots with un modified or increased populations than in enclosures with decreased po pulations. However, litter in earthworm middens, which comprised simil ar to 10% of the total surface litter, had a lower C/N ratio and great er microbial activity than litter in the surrounding area, suggesting that earthworms incorporated litter with the low C/N into their midden s. Because the earthworms feed mainly on litter in their middens, we p ropose that they selectively removed litter fractions with a low C/N r atio, thereby increasing the overall C/N ratio of remaining surface li tter. 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 p rocess in systems with large populations of this earthworm species. Th ese earthworms may exert major control over litter decomposition in mi nimally tilled or no-till agroecosystems where they may influence atte mpts to manage surface litter.