Graduation date: 1997

Nitrogen availability in agricultural soils from fertilizer, plant residue inputs, and soil organic matter has important implications beyond crop yield. Legume winter cover crops and one fourth the recommended N rate on sweet corn resulted in yields equivalent to those at the recommended rate in the Willamette Valley of western Oregon. Cereal rye winter crops absorbed an average of 40 kg N/ha that otherwise would have been leached, but did not effectively replace fertilizer N. Cereal rye as a cover crop therefore shows an ability to immobilize N from fertilizer. This was further confirmed in an experiment with "N labeled urea where results showed that N derived from fertilizer in sweet corn or cereal rye plant residue was less available for crop uptake and loss from the system than inorganic N or N directly immobilized from fertilizer. Losses of N from fertilizer ranged from 40 to 73% of that which was in the soil over winter. Mineralization of organic matter N is an important process in N availability, especially when cover crops are used to replace fertilizer. Finding a general indicator or predictor of N mineralization in soils would help in reducing fertilizer N costs and leaching of inorganic N that is applied in excess of crop needs. In a screening of 17 biological and chemical properties of 19 differently managed soils from around the state of Oregon, a model using total soil N and ��-glucosidase activity provided the best model of mineralized N uptake by ryegrass. Biological activity is primarily responsible for the transformations that result in N availability in soils. Management of soils directly impacts soil biology, and results from multivariate analyses of biological and chemical parameters in differently managed soils showed that disturbance creates an overriding common biochemical state in soils. Beyond disturbance, vegetation and the nature of organic inputs also impart recognizable multivariate patterns in soils managed differently. These results suggest that indicators independent of soil type may be used to discern effects of management on agricultural soils. / Graduation date: 1999