A new net mineralizable nitrogen assay improves predictions of floristic composition

Question: Can a simple measurement of nitrogen (N) availability be related to an ecologically relevant response, i.e. mean Ellenberg N indicator value (EN)? Location: UK (England,Wales and Scotland). Methods: Soil cores from a stratified sample of UK habitats were analysed for mineralizable N with a conventional incubation and a new flushing method, which uses a single mineral N extraction. Predictions of mean EN using mineralizable N and other soil measurements were assessed by fitting linear mixed-effect models, using the Akaike information criterion (AIC) as a measure of model parsimony. Results: Mineralizable N measurements using the flushing method described a component of the variation in mean EN that was more orthogonal to bulk soil properties such as moisture content, total N/C ratio and pH than that described by conventionally measured mineralizable N. Mineralizable N as measured using the flushing method improved the accuracy of predictions obtained using only bulk soil measurements, and appeared in the best two-term and threeterm models. Conclusions: Much of the variation in mean EN can be related to soil N/C ratio, pH or moisture content, but mineralizable N distinguishes variation in mean EN that is independent of these bulk soil properties. The new measure will be useful for studies of the exposure of plants to N, in particular when assessing N pollution effects on plant species composition.