Effects of long-term nitrogen addition on phosphorus cycling in organic soil horizons of temperate forests

High atmospheric nitrogen (N) deposition is expected to impair phosphorus (P) nutrition of temperate forest ecosystems. We examined N and P cycling in organic soil horizons of temperate forests exposed to long-term N addition in the northeastern USA and Scandinavia. We determined N and P concentrations, enzyme activities and net N and P mineralization rates in organic soil horizons of two deciduous (Harvard Forest, Bear Brook) and two coniferous (Klosterhede, Gardsjon) forests which had received experimental inorganic N addition between 25 and 150kgNha(-1)year(-1) for more than 25years. Long-term N addition increased the activity of phosphatase (+180%) and the activity of carbon (C)- and N-acquiring enzymes (cellobiohydrolase: +70%, chitinase: +25%). Soil N enrichment increased the N:P ratio of organic soil horizons by up to 150%. In coniferous organic soil horizons, net N and P mineralization were small and unaffected by N addition. In deciduous organic soil horizons, net N and P mineralization rates were significantly higher than at the coniferous sites, and N addition increased net N mineralization by up to 290%. High phosphatase activities concomitant with a 40% decline in P stocks of deciduous organic soil horizons indicate increased plant P demand. In summary, projected future global increases in atmospheric N deposition may induce P limitation in deciduous forests, impairing temperate forest growth.