Effects of earthworms on soil aggregate stability and carbon and nitrogen storage in a legume cover crop agroecosystem

We investigated the effects of earthworms on soil aggregate size-distribution, water-stability, and the distribution of total C and N among aggregates of different sizes. Earthworm populations were experimentally manipulated (reduced, unaltered or increased) in field enclosures cropped to soybean (Glycine max). Soil samples were collected and sieved (10 mm) field-moist, then air-dried and re-sieved to obtain dry-sieved aggregates (DSA) of six size classes. The percentage water-stable aggregates (WSA) in each size class was determined by wet-sieving DSA. There was no effect of earthworm manipulations on the depth-distribution of total soil C and N or the distribution of DSA of different size classes. In the earthworm addition treatment, DSA in 1–2, 2–4, and 4–10 mm size classes contained more WSA than DSA of the same size class from the unaltered earthworm treatments. WSA had higher C and N contents than DSA of the same size class. Our results suggest that earthworm activity can increase soil structure stability and the storage of soil C and N in large WSA.