Spatial patterns of nitrogen isotope ratios in forest soils are related to latitude and soil phosphorus concentration

The aim of this study was to identify the parameters that affect the nitrogen (N) isotope ratio (δ 15 N) in soils of temperate and boreal forests. We measured the δ 15 N in 30 soil profiles of temperate and boreal forests in Sweden and analyzed the relationships between δ 15 N in the soils and soil chemical properties as well as site characteristics. In addition, we conducted a meta-analyses of δ 15 N in the organic layer of European forests. We identified two types of δ 15 N patterns; in type D soils, the δ 15 N in the mineral soil decreases with depth, whereas in type C soil, the δ 15 N in the soil profile is almost constant. Type D soils had a significantly higher δ 15 N in the organic layer and upper mineral soil than type C soils, which is likely due to N isotope fractionation by ectomycorrhizal fungi in the topsoil. Type D soils were found in boreal forests, but not in temperate forests. They had a significantly lower atmospheric N deposition rate than type C soils, by a factor of 2.3, and a significantly higher phosphorus (P) concentration of the organic layer than type C soils, by a factor of 1.5. We also found that the δ 15 N was negatively correlated with the N:P ratio of the organic layer (R 2 = 0.21, p < 0.001). Across Europe, the δ 15 N of the organic layer was positively correlated with latitude (R 2 = 0.58, p < 0.001), and negatively with mean annual temperature (R 2 = 0.52, p < 0.001) and atmospheric N deposition (R 2 = 0.42, p < 0.001). In conclusion, our results show that the δ 15 N of the organic layer and microbial N (re-)cycling in forest soils is positively related with latitude and the P concentration of the organic layer.