The effect of organic matter manipulations on site productivity, soil nutrients, and soil carbon on a southern loblolly pine plantation

Forest harvesting intrinsically removes organic matter and associated nutrients; these exports may impact soil productivity and soil carbon stores of managed forests. This study examined the effect of manipulating forest floor and harvest residue inputs on nutrient availability and carbon content in the context of intensive forest management. Treatments were applied 15 years prior to this study and included removal and addition of forest floor and harvest residues, and a reference. We examined stand volume, litterfall, root biomass and foliar N and P at year 14 or 15. Soil moisture and temperature (0-10 cm) and available N and P in the O and 0-20 cm depths were measured once per month during year 15. Soil carbon and nitrogen were measured on whole soils as well as two density fractions in the O-horizon, 0-20, 20-40, and 40-60 cm soil depths at year 15. In general, many of the initial responses found by an earlier study (age 10) have dissipated. Standing volume in the added treatment was 31% higher than the removed, but no significant difference was found between the removed and reference treatments. The added treatment resulted in higher concentrations of N in the light and heavy density fractions of the 0-20 cm depth, which led to higher mass of N in both of these fractions. The added treatment had the greatest whole soil heavy fraction N mass. There were no differences in available N in the O-horizon or 0-20 cm depth as tested using ion exchange membranes; however available P was significantly lower in the O-horizon of the removed treatment (37% lower than the reference). Bole volume was correlated with some measures of total and available N and P in the O and 0-20 cm soil horizons, suggesting that increases in growth found in the added treatment were a result of additional nutrients. There were no significant differences between C concentration or mass of the 0-20 cm or 20-40 cm soil depths between the treatments; however the added treatment had significantly more (51% more than the reference) carbon at the 40-60 cm soil depth. The added treatment had a significantly higher C:N relative to the reference in the 20-40 cm (21.0 and 14.5, respectively) and 40-60 cm (18.0 and 11.4, respectively) depths, suggesting that relatively fresh, undegraded organic matter had enriched this depth. This additional carbon sequestered at depth could contribute to a long-term soil carbon pool. The results of this study suggest that higher intensity use, such as forest floor removal and whole tree harvest, of these forests may not impact long term productivity at this site with typical soil nutrient status; however, more research is necessary to determine the mechanism(s) of this resilience.