Influences of nitrogen on carbon dynamics in forest soil and density fractions

Application of N fertilizer is a common forest management practice in the Pacific Northwest, yet the long-term influence of fertilization on forest soil properties is not well known. Although elevated N often increases mineralization of C and N from labile organic matter, negative effects have been documented in recalcitrant organic matter and whole soil. Using a series of paired plots in which one of each pair had undergone longterm N fertilization, I investigated the effects of elevated N on C and N mineralization in forest soils and organic fractions. The 02 horizons (02), whole soils (WS), light fractions (LF), heavy fractions (HF), and hysically recombined fractions (RF), from the paired plots were incubated in the laboratory for 300 d. For control soils, an additional "summed" fraction (SF), was computed from LF and HF results. Prior to analysis of the effects of elevated N, a general test of the density fractionation technique was conducted in the control soils. The LF and HF were hypothesized to represent labile and recalcitrant fractions in soil, but C and N were not substantially more stable in the HF during the incubation. Total cumulative respiration and N mineralization were similar for both the SF and the WS, but C and N mineralization in both fractions were higher than in the RF. The depressed respiration in the RF might be explained by an antagonistic interaction between the varied microbial communities that degrade LF and HF; in the heterogeneous WS, these communities may be spatially separated. The density separation technique appears to be a viable method for isolating and studying different soil fractions, but these fractions should be considered more carefully in the context of microbial interaction and soil spatial heterogeneity. Elevated N depressed cumulative respiration to a similar extent in all substrates. The mechanisms most involved in degrading these substrates are negatively affected by elevated N, but may not be the same in each substrate. While laboratory results may not withstand the variability of the natural environment, the potential for elevated N to stabilize C in soil suggests the need for more detailed field measurements.