CALCULATING GROSS N TRANSFORMATION RATES OF N-15 POOL DILUTION EXPERIMENTS WITH ACID FOREST LITTER - ANALYTICAL AND NUMERICAL APPROACHES

Gross N transformation rates were determined with (NH4+)-N-15 and (NO3-)-N-15 pool dilution experiments in acid litter of a deciduous and a coniferous forest in The Netherlands. Although gross mineralization and NH4+ immobilization rates were comparable in the two litter layers, gross nitrification and NO3- immobilization rates were much lower in the deciduous litter. Gross rates were calculated with different models solved either analytically or numerically. The numerically solved model allowing for recycling of N-15 back to the enriched pool was considered superior. Recycling of N-15 was significant in the (NH4+)-N-15 enrichment experiments. Error analysis with help of the analytical solutions showed that an inherently high error existed in the results due to the sensitivity of the method to errors in the measured variables. The heterogeneity of the samples was the main source of errors in these variables. The design of the method determines the sensitivity for measurement errors. To minimize the sensitivity the duration of the experiment should be maximized and the N-15 abundance of the enriched N pool should be several times the natural N-15 abundance. Application of N-15 above this level does not contribute to the accuracy of the estimations.