Carryover Effects On Above and below Ground Carbon and Nitrogen Pools in Douglas-Fir Forests in the Pacific Northwest.

Recent studies have shown past N fertilization of forestland soils, formed on glacial deposits, in the Pacific Northwest region has long-term effects on site productivity lasting up to 15-25 years after application. The phenomenon is called the “Carryover” effect. This study reexamines the Carryover effect on the same five study sites used in the previous study by re-measuring productivity of second rotation Douglas-fir trees (10-12 years old) and understory vegetation. In addition, above and belowground C and N pools were measured. The results show strong evidence that Carryover effects significantly impact above ground carbon (C) and nitrogen (N) pools of second rotation crop trees, understory vegetation and forest floor material, as well as below ground soil N and C pools. Total above ground biomass (tree, understory and forest floor) was significantly greater (p = .068) on the Carryover sites than on the paired controls. Nitrogen content in the above ground component (understory and forest floor only) was significantly greater (352 kg N ha-1) (p = .027) on the Carryover sites. Soil C and N pools was examined by depth. Total N content of the soil, was 5% greater (290 kg N ha-1) (p = 0.392) on the Carryover sites. Concluding that 642 kg N ha-1 of the original 884-1120 kg N -1 that was applied to these five Carryover sites has remained in the ecosystem 15-25 years after application. Total soil C was 11% less (210 Mg ha-1) on the Carryover sites. The results of the this study show that in general the long-term effects of N fertilization decrease below ground C pools while increasing above ground C and N. Because this study has a limited number of sites and studies like this are so few more investigation may be necessary.