Gross and Net Rates of Soil N Transformations in a Southern Pine Forest Managed for Bioenergy Production.

Nitrogen has been identified as one of the most limiting nutrients to terrestrial ecosystem primary production and therefore has received significant attention.  Soil nitrogen (N) cycling and availability is largely driven by the balance between microbially-mediated mineralization-nitrification-immobilization processes. Release of inorganic N via mineralization of soil organic matter (SOM) is the main process by which N is released into the soil in a plant-available form.  Microbial N immobilization in surface soil horizons acts as a large sink for soil N and is a N retention mechanism within ecosystems.  Measuring gross N transformations can provide important information on processes affecting ecosystem N dynamics that net rates may not capture.  Gross N mineralization, gross nitrification, and NH₄⁺ and NO₃^- immobilization were measured during a short-term (2 d) laboratory incubation experiment.  Soils were collected from beds and interbeds of treatment plots by taking multiple cores from the bed and interbed using a 3 cm diameter PVC corer and divided into 0-5 and 5-15 cm depth increments. Treatments included were: 1) pine intercropped with switchgrass with harvest residuals removed; 2) traditional pine establishment with harvest residuals removed; and 3) traditional pine establishment with harvest residuals left in place.  Gross N transformations were measured in April 2012, July 2012, October 2012, and February 2013.  Sampling dates were chosen to reflect phenological stages of switchgrass throughout the growing season as well as seasonal variability.