Earthworm additions increased short-term nitrogen availability and leaching in two grain-crop agroecosystems

Abstract

Earthworms were added to enclosures in two agroecosystems to determine their influence on soil nitrogen availability and microbial activity, and to quantify their effect on the leaching of water and nitrogen through the surface soil. The two agroecosystems were a corn-soybean rotation with chisel-plow-disk tillage following corn (CS), and a corn-soybean-wheat-vetch rotation with ridge-tillage (CSW). In both agroecosystems, earthworm additions in the fall (100 m −2) led to more abundant deepdwelling earthworms and less abundant surface-dwelling earthworms than in enclosures with no additions, but had little effect on total earthworm abundance after 5 months. In the CS system, earthworm additions led to greater concentrations of potentially mineralizable nitrogen (PMN) and microbial biomass nitrogen (MBN) in the surface soil than in control enclosures in the following spring. In the CSW system, earthworm additions led to greater overall concentrations of dissolved organic nitrogen (DON), but only to localized changes in PMN and MBN at different positions within crop rows and at different soil depths. Soil mineral nitrogen concentrations were not influenced by earthworm additions. Earthworms significantly influenced soil microbial activity in both agroecosystems; earthworm additions generally increased soil dehydrogenase activity (DHA) in the CS system, and reduced it in the CSW system, compared to controls. Earthworm additions led to 4- to 12-fold increases in the volume of soil leachate collected during 1 week in zero-tension pan lysimeters buried 45 cm deep. For both agroecosystems, nitrogen flux in leachate was increased by nearly 10-fold in response to earthworm additions. Most of this response was due to increased flux of DON. We conclude that the composition of earthworm communities can strongly influence the availability and leaching of nitrogen in the surface soil of some grain-crop agroecosystems, at least in the short-term. More work is needed to predict the longer-term consequences of earthworms on crop productivity, nitrogen use efficiency and groundwater quality.