Nitrogen source and earthworm abundance affected runoff volume and nutrient loss in a tilled-corn agroecosystem

A simulated rainfall event was used to determine the influence of different N sources and earthworm density on surface hydrology and quality of runoff in a tilled-corn cropping system. N was added to spring-disked, continuous-corn plots as chopped legume, manure, or NH4NO3. Earthworm populations were manipulated in fertility treatments by reductions, additions; or left as ambient populations. We found that runoff volume and the NH4+ and NO3- concentration of runoff were greater in inorganic treatments, compared with the organic N sources. Sediment load, and its total C and total N components were lower in manure treatments than chopped legume and inorganic N treatments. Although earthworm manipulations did not affect runoff volume, more sediment and sediment-associated C and N were lost from the reduction earthworm treatment, compared to the ambient treatment. Multiple regression models showed that surface hydrology and sediment flux were associated with incorporated residue levels, earthworm population, macropore and midden density. Our results indicate that water, nutrient and soil conservation could be improved by the use of organic N fertility sources and that earthworm impact varies with earthworm population, species composition, and incorporated residue level.