Soil organic matter stability in intensively managed ponderosa pine stands in California

Forest soils comprise a large portion of the global terrestrial C pool, and soil organic matter (SOM) is essential to soil function and forest productivity; however, responses of SOM quality to changes in fertility, moisture availability, or management are not well understood. We tested the effects of two common forest management practices, fertilization and competing vegetation control using herbicides, on surface SOM distribution and stability characteristics in three ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson) plantations of differing productivity, soil type, and parent material in northern California by using density fractionation and aerobic laboratory incubation of the surface soils. The treatment effects on pine productivity revealed responses dominated by moisture limitation at the less productive sites and by nutrient limitation at the most productive site. The stability of surface S0M increased with increasing site productivity. Fertilization increased N concentrations and decreased C/N ratios in whole soils and light fractions at the less productive sites, and the effects persisted for more than a decade. Furthermore, fertilization increased soil C mineralization from the intermediate-productivity site during incubation. In contrast, fertilization decreased C mineralization from the most productive site, suggesting that fertilization increased SOM stabilization at this site. Controlling understory vegetation with herbicides reduced N availability, as evidenced by reduced light-fraction N at the poorest site and decreased N mineralization during incubation. Our study demonstrates the importance of site characteristics and the use of a combination of indices in determining the effects of forest management practices on SOM characteristics and dynamics. Abbreviations: DF, dense fraction; DIN, dissolved inorganic nitrogen; DOC, dissolved organic carbon; DOM, dissolved organic matter; DON, dissolved organic nitrogen; LF, light fraction; SOM, soil organic matter; SPT, sodium polytungstate; TDN, total dissolved nitrogen. doi: 10.2136/sssaj2009.0062