Fire exclusion and nitrogen mineralization in low elevation forests of western Montana

Little is known regarding how fire exclusion influences nitrogen (N) cycling in low elevation forests of western Montana. Nor is it clear how the change in fire frequency that has resulted from forest management has influenced ecosystem function in terms of plant-soil-microbe interactions. A fire chronosequence approach was used to examine the influence of forest succession on soil biochemical properties and microbimal activity at 10 sites with varying time since fire (2-130 years). The rate of decomposition of buried tongue depressors and cotton strips, was found to decrease significantly (R²=0.410, P=0.087 and R²=0.761, P=0.003, respectively) with time since fire (TSF). Net N mineralization and nitrification, as estimated by resin sorbed NH₄⁺ and NO₃^- concentrations, both exhibited significant non-linear decreases (R²=0.870, P=0.000 and R²=0.620, P=0.007, respectively) with TSF. Nitrification potential measured using an aerated soil slurry method, also decreased significantly (R²=0.595, P=0.009) with TSF. These decreases in N availability along with an increase in the metabolic quotient and a decrease in labile C pools with TSF indicated a decline in substrate quality and microbial activity with secondary forest succession. The concentration of total phenols in mineral soil showed no significant trend with TSF, but was negatively correlated (R²=0.486, P=0.025) with resin sorbed NO₃^- concentration indicating either enhanced immobilization or perhaps chemical inhibition. These results imply that biochemical processes (decomposition and N transformations) may be limited by the lack of available substrate and potentially as a result of rapid immobilization, chemical inhibition or a combination of both at least partially induced by changes in vegetation with TSF. Our results suggest that N availability in ponderosa pine ecosystems of the inland Northwest are directly dependent upon fire history and secondary successional stage.