COMPARISON OF BURIED BAG AND PVC CORE METHODS FOR IN-SITU MEASUREMENTOF NITROGEN MINERALIZATION RATES IN AN AGRICULTURAL SOIL

We compared estimates of soil nitrogen (N) mineralization rates using the buried bag and PVC core methods in an ongoing investigation of the effects of earthworms and N fertilizer sources on agroecosystem N dyn amics. Over a Seven-month period, we paired monthly buried bag and PVC core soil incubations within research plots receiving one of three N treatments (inorganic, legume, or manure fertilizers) and with manipul ated earthworm populations (reduced, ambient, or increased numbers). S oil moisture within both the buried bags and the PVC cores fluctuated in response to changes in the surrounding soil, violating assumptions of the buried bag method that soil moisture remains constant during in cubation. For both methods, overall CV's for net ammonification, nitri fication, and N mineralization rates were very high (104 - 628%). Over all, results for the two methods were significantly correlated for net ammonification (r = 0.89), net nitrification (r = 0.58), and net N mi neralization (r = 0.24). In general, the two methods yielded similar s easonal estimates of net N mineralization and nitrification. However, on one occasion in the plots with the inorganic N treatment, buried ba g estimates of net N mineralization were significantly higher than the PVC core estimates (1.5 versus -0.4 mg N . kg(-1) soil . d(1), respec tively). Under some conditions, the two methods may lead to quite diff erent interpretations of soil N mineralization processes.