Soil organic matter and nitrogen cycling in response to harvesting, mechanical site preparation, and fertilization in a wetland with a mineral substrate
Forested wetlands are becoming an important timber resource in the Upper Great Lakes Region of the US. However, there is limited information on soil nutrient cycling responses to harvesting and post-harvest manipulations (site preparation and fertilization). The objective of this study was to examine cellulose decomposition, nitrogen mineralization, and soil solution chemistry four years after a forested, mineral soil wetland in Northern Michigan was whole-tree harvested, site prepared, and fertilized: (N, P, N + P). Organic matter decomposition was greatest in the site preparation bedding treatment and lowest in whole-tree harvested with no mechanical site preparation treatment. Both N and P additions, alone and in combination resulted in increased cellulose decomposition regardless of site preparation treatment (15-38% for the harvest-only treatment, 20-40% for the bedded treatment, and 15-44% for the trenched treatment). However, based on dissolved organic carbon concentrations in the soil solution, organic matter decomposition was inhibited on an overall plot basis; that is, outside the area of cellulose strip placement. The site preparation bedding treatment resulted in a net mineralization of N (9.2 g-N m-2) over a 10 week incubation period. The disc trench and harvest-only treatments resulted in a net immobilization of N (3.1 g-N m-2 and 1.5 g-N m-2, respectively). Nitrogen, P, and N + P inhibited N mineralization in the bedded treatment by 10-25% over the control. There was a fertilizer-induced increase in N immobilization of 50-40% and 25-50% in the harvest-only and trenched treatments, respectively. It appears that soil microorganisms at this site are limited by soluble C more than N or P. By adding cellulose strips to the soil, the soluble C limitation was, in part, overcome. Once the soluble C limitation was alleviated, then the soil microorganisms responded positively to N and P additions.