Earthworm Contribution to Nitrogen Cycle in No-till Corn and Soybean Agroecosystems Determined with the Secondary Production Method.

Earthworms are abundant in temperate agroecosystems, where they contribute to nutrient cycling due to the turnover of earthworm biomass and stimulation of microbially-mediated processes of decomposition and mineralization. The specific contribution of earthworms to nitrogen cycling can be determined with the secondary production method, which considers nitrogen released from the earthworm during its lifespan and due to mortality. The objective of this study was to determine the secondary production of earthworms in no-till corn and soybean agroecosystems and to estimate the nitrogen contribution from earthworm populations in both phases of the corn-soybean rotation. This study was conducted for two consecutive years (2014 and 2015) in two adjacent agricultural fields at the Macdonald Research Farm of McGill University in Ste-Anne-de-Bellevue, Quebec, Canada (45°25’ N, 73°56’ W). Fields were under a no-till corn-soybean rotation with both phases present each year. The secondary production, defined as the accumulation of biomass through growth and reproduction, was estimated by sampling the earthworm population at regular intervals from April to November and inferring the change in earthworm biomass between sampling dates using the size frequency method.  Earthworms were collected by handsorting and formalin extraction at approximately biweekly intervals from five replicate points in the sampling grids established in each field. Earthworms were preserved in 5% formalin until they could be identified, counted and weighed in the laboratory. About 70% were juvenile earthworms, likely of A. turgida, A. tuberculata, L. terrestris and A. chlorotica, the dominant sexually-mature species in these fields. There were significant (P<0.05, HSD) seasonal fluctuations in earthworm numbers and biomass, probably related to soil moisture and temperature conditions, as well as food availability. Results of the secondary production and estimated nitrogen contribution from earthworm populations will be presented and discussed in relation to the biophysical conditions in corn-soybean rotations.