N Fertilization Impacts on C in Shallow Vs. Deep Soil Horizons: Observations and Mechanisms.

Fertilization is an important management tool to increase the productivity of many forests. Fertilization can be done for many reasons but is typically done to increase the volume growth of the merchantable portion of a forest. The impacts of fertilization on C sequestration is variable, with observations of negative, positive and no impacts on forest C. The soil profile below about 20 cm is typically ignored. We studied seven forests that had received (up to 20 years previously) a range of N additions from 0-1120 kg N/ha previously. Typically, about half of the soil C was located below 20 cm, though it decreased greatly with depth. The soil of unfertilized and fertilized stands were compared for differences in C by horizon. For the seven PNW Douglas-fir forests studied, we would have predicted an average decrease in soil C due to the fertilization treatment in 0-20 cm soil of 7 Mg C/ha. However, the 20-100 cm depth increment showed an average increase of 10 Mg C/ha from the addition of fertilizer for an overall increase 0-100 cm of 3 Mg C/ha. Though the increase in these seven studies are modest, these results show the importance of including sampling of deeper soil horizons in these studies. Current studies in PNW forests show that N fertilization increases the movement of DOC into the soil profile. This may be one mechanism for the observations of differential response of soil C to N by depth.