Soil Nitrogen Transformations and Availability in Upland Pine and Bottomland Alder Forests

Abstract

Soil nitrogen ( N) processes and inorganic N availability are closely coupled with ecosystem productivity and various ecological processes. Spatio- temporal variations and environmental effects on net N transformation rates and inorganic N concentrations in bulk soil and ion exchange resin were examined in an upland pine forest ( UPF) and a bottomland alder forest ( BAF), which were expected to have distinguishing N properties. The annual net N mineralization rate and nitrification rate ( kg N center dot ha- 1 center dot year- 1) were within the ranges of 66.05- 84.01 and 56.26- 77.61 in the UPF and - 17.22- 72.24 and 23.98- 98.74 in the BAF, respectively. In the BAF, which were assumed as N- rich conditions, the net N mineralization rate was suppressed under NH4+ accumulated soils and was independent from soil temperature. On the other hand, in the UPF, which represent moderately fertile N conditions, net N transformation rates and N availability were dependent to the generally known regulation by soil temperature and soil water content. Stand density might indirectly affect the N transformations, N availability, and ecosystem productivity through different soil moisture conditions. The differing patterns of different inorganic N indices provide useful insight into the N availability in each forest and potential applicability of ion exchange resin assay.