Effects of selection harvest and prescribed fire on the soil nitrogen status of ponderosa pine forests

Abstract

One hundred years of timber harvest and reduced fire frequency have resulted in the conversion of once open stands of ponderosa pine ( Pinus ponderosa) forests to dense forests dominated by Douglas-fir ( Pseudotsuga menziesii). Selection harvest and harvest with prescribed fire have been identified as possible tools to restore ponderosa pine stands to pre-settlement stand structures. Case studies were performed at three separate sites in western Montana to assess the influence of selection harvest and prescribed burning on soil N dynamics. These sites had been exposed to either selection harvest, selection harvest with prescribed burning, or a no-treatment control 0 (Lubrecht Experimental Forest), 2 (E/L Ranch), or 11 (Lick Creek Demonstration Site) years prior to initial soil analyses. Replicate soil samples were collected over at least two growing seasons at each site and analyzed for total C and N, potentially mineralizable N (PMN), short-term soil respiration rates, soil microbial biomass N, extractable NH 4 + and NO 3 −, and soluble sugars (measured as 0.5 M K 2SO 4 extractable anthrone reactive carbon (KARC)). Selection harvest without prescribed burning had little or no influence on levels of available N or microbial activity relative to the control at all three sites. Selection harvest with prescribed fire, however, significantly increased extractable NH 4 +, NO 3 − and KARC immediately following treatment. Such differences were not observed 2 or 11 years following treatment. Potentially mineralizable N was significantly increased immediately following fire, but decreased to levels lower than the control 1 year following treatment. Levels of PMN were also found to be less than the control 2 and 11 years after treatment. Similarly, microbial biomass N was elevated immediately following prescribed burning, but was significantly lower than the control for up to 11 years following prescribed burning. Levels of mineralizable N were lowered within a year of treatment as a result of (1) N loss during soil heating, (2) N loss to plant uptake, and (3) potential leaching losses. The effect of reduced mineralizable N on long-term site productivity is not clear, however, these losses of N from the ecosystem should be considered along with stand mortality and yield when assessing the potential sustainability of forest management strategies.