Effects of timber harvest intensity on macronutrient cycling on oak-dominated sandy soils of northwest Wisconsin

Background/Question/Methods

In Wisconsin there is concern that timber harvesting on nutrient-poor, sandy soils could lead to depletion of nutrients. This concern could become exacerbated by the increasing demand for biofuels, which could result in intensified utilization practices. This project measured dry matter and macronutrient (N, P, K, Ca, Mg) pools and major inputs (bulk precipitation, weathering + mineralization) and outputs (leaching losses) on four treatment plots at each of five sites on sandy soils at the Governor Knowles State Forest in northwest Wisconsin: a control, conventional <10 cm diameter (live branch and top remaining) biomass removal, a <5 cm diameter (live branch and top remaining) treatment, and a removal of all aboveground biomass.  All treatments were applied during the early winter, leaf-off period. Data were collected over two years (2010 and 2011) were applied to a nutrient-balance equation to determine if the treatments would lead to an accumulation or net depletion of nutrients.

Results/Conclusions

The soil was the leading pre- and post-harvest pool for P, K, Ca and Mg in the ecosystem.  Biomass removed during whole-tree harvests constituted ~50% of the ecosystem nitrogen and contributed significantly to the loss of the other macronutrients.  N (NH₄ and NO₃), K, Ca, and Mg were calculated to be accumulating on all harvest treatments. During the first year after harvest, N mineralization was significantly higher and leaching losses of N were significantly lower on all harvest treatments than on controls. During the first year, there were no significant differences in mineralization rates and leaching losses among treatments for the other macronutrients; however, during the second year, differences in N mineralization and leaching had largely disappeared. Phosphorus was the only nutrient that showed a net depletion, and this depletion was detected for all harvested treatments. However, the depletion rates were so low that it would take several rotations for any P deficiencies to become apparent. The results of this study may not apply to other sandy soils in Wisconsin.