EARTHWORMS AND NITROGEN MINERALIZATION IN CORN AGROECOSYSTEMS WITH DIFFERENT NUTRIENT AMENDMENTS

To investigate the influence of earthworms on net rates of N mineraliz ation in agricultural soils, we manipulated earthworm populations for three years in three corn agroecosystems that differed in nutrient inp uts (inorganic N, legume, or manure). Each spring and fall, earthworms were added to, removed from, or left unmodified in field enclosures w ithin each agroecosystem In the second and third year, we measured soi l concentrations of mineral N (0-15 cm depth) in these enclosures, as well as net rates of N mineralization using an in situ soil core incub ation method. In the inorganic N-fertilized agroecosystem, mineral N c oncentrations were often greatest in the enclosures with earthworm add itions or least in enclosures with earthworm reductions. The largest d ifferences were seen following inorganic fertilizer application, sugge sting either greater net N immobilization or increased N loss from the surface soil in the enclosures with earthworm reductions. Subsequent net rates of N mineralization were,greater in the enclosures with redu ced earthworm populations, possibly due to the turnover of N in microb ial biomass. In the legume-fertilized agroecosystem, overall rates of net N mineralization were greater in enclosures with earthworm additio ns than in enclosures with earthworm reductions. In this agroecosystem , cumulative net N mineralization during the two-year study was signif icantly higher in enclosures with earthworm additions than in enclosur es with unmodified or reduced populations. Earthworms can significantl y influence the net rates of N mineralization in agricultural soils, b ut the timing, direction, and the magnitude of this influence appears to be dependent on the nature of the nutrient inputs. (C) 1998 Elsevie r Science B.V.