Patterns of nitrogen cycling were studied in N—fertilized (N: 95 kg/ha, P: 45 kg/ha, K: 135 kg/ha) and “unfertilized” (no N, but the same levels of P and K) agroecosystems with heavy weed infestations. To calculate monthly standing crop nitrogen levels, nutrient concentrations were multiplied by biomass values for each month. Soils were also sampled monthly at three depths; precipitation was sampled weekly and analyzed for N inputs. Nitrification and denitrification enzyme activities were measured, as well as rates of denitrification from unaltered soil cores. Nitrate leaching was measured with porous cup lysimeters. Weeds completed more vigorously with the crop in fertilized treatments than in unfertilized treatments, resulting in greater crop growth in unfertilized treatments. Nitrification, denitrification, and leaching losses of N were greater in fertilized treatments than in unfertilized treatments, especially after residue input following harvest of the summer crop. Soil organic nitrogen tended to be lower in unfertilized treatments than in fertilized treatments after summer cropping, whereas soil organic carbon showed an opposite trend. As a result the carbon: nitrogen ratio of soil organic matter was significantly higher in unfertilized treatments than in fertilized treatments. Fertilized treatments showed a positive N balance (47 kg/ha) while unfertilized treatments showed a net loss of N (57 kg/ha). Nitrogen use efficiency by plants was higher in unfertilized treatments than in fertilized treatments, but was low relative to many unmanaged ecosystems.