N and C transformations in long-term N-fertilized forest soils in response to seasonal drought

The aim of this study was to determine the response of N and C transformations in the soil to seasonal drought in repeatedly N-fertilized forest soils. The study sites were long-term N fertilization experiments in two Norway spruce (Picea abies L.) stands growing on rather fertile sites in southern Finland. The treatments were control (C), control exposed to seasonal drought (C + D), N fertilization (N), and N fertilization combined with drought (N + D). To the N-fertilized plots, about 600-1000 kg/ha nitrogen had been applied over a period of 35 years. For the drought treatment, the plots were covered with a plastic roof 1-4 m above ground level for 2-3 months in May-July. Soil was sampled before and after the drought treatments. Net N mineralization, net nitrification, and C mineralization (CO2-evolution) were studied in incubation experiments after soil moisture had been adjusted to constant soil moisture content (60% water-holding capacity (WHC)). Taken as a whole, no major differences were observed in the net formation of mineral N in drought-treated soils compared to untreated soils, but in N-fertilized soil, net formation of mineral N tended to increase due to drought. Previous drought treatment also tended to increase net nitrification, but did not change the average rate of C mineralization, at least not in over the longer term. Soil samples were also incubated at the original soil moisture content. The response of different processes to drought seemed to vary, net nitrification being the most sensitive. Amounts of microbial biomass C and N were lowest in soil samples from the plots that had received N additions, but the drought-treated plots did not differ much from other plots.