Soil C and Nutrient Pools Five Years After Manipulating Biomass and Competing Vegetation At Two Contrasting Sites In the Pacific Northwest.

Utilization of forest biomass in intensively managed forests has potential to alter soil productivity with the likelihood dependent on site-specific characteristics and the suite of management practices employed.  We assessed changes in mineral soil total C and nutrient (total N and exchangeable Ca, K, Mg) pools with replicated treatments of biomass removal (whole-tree (WT) and bole-only (BO) harvests) and competing vegetation control (VC, initial or annual application) at two Douglas-fir plantations.  The sites represented contrasts in potential productivity due to large differences in soil total nutrient pools and available water holding capacity.  There were main effects of biomass removal and VC on the absolute change in soil pools of some elements at both sites, with effects most prevalent in the top 15 cm of soil.  At the less-productive site, total soil pools (0-60 cm) of C, N, and exchangeable Ca significantly increased in the BO and initial VC treatments when compared with WT and annual VC treatments. Exchangeable magnesium increased and K decreased regardless of treatment.  At the higher-productivity site, soil C and nutrient pools did not change in response to treatments, but total soil exchangeable Mg increased during the study period.  Correlation analyses indicated little influence of soil nutrient pools on early growth at the low-productivity site likely because soil water is more limiting than nutrient availability at this site.  At the higher-productivity site, vegetation growth was positively correlated to nutrient pools, indicating changes in pools associated with harvesting and treatment could influence crop development in the future.  The influence of changes in soil nutrient pools on future tree growth will likely be dependent on the relative degree to which water and nutrient availabilities limit growth, highlighting the need to assess site-specific limitations to growth when considering use of intensive forest management practices.