Simulating residual effects of animal manures using 15N isotopes

An estimation of the residual nitrogen effects of animal manure is important when optimizing crop rotations and fertiliser applications. The residual effect of animal manure was studied with the dynamic simulation model FASSET. The model was tested on experiments with N-15-labelled animal manures, where the fate of labelled manure N had been studied. Three field experiments differing in animal manure type, the labelled fraction of the manure, soil type and application techniques were analysed. The duration of the experiments varied between one and three years. In general, the model simulations agreed well with the observations. At the end of the experimental periods, the model captured 74% of the observed variation in soil N-15, whereas some systematic difference was found between measured and simulated plant uptake of N-15. The model was used to evaluate the impact of soil types, soil fertility, manure type, climate and crop type on the N-15 residual effect of animal manure. These scenarios showed that wet climates, sandy soils and crops with a short growing season reduce the residual effects of animal manures. The type of the manure also influenced the residual effect of the manure such that the residual effect increased from pig slurry and cattle slurry to deep litter. The soil organic matter level had little influence on the N-15 residual effect of the manure.