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

Abstract

AbstractThe aim of this study was to identify the parameters that affect the nitrogen (N) isotope ratio (δ15N) in soils of temperate and boreal forests. We measured the δ15N in 30 soil profiles of temperate and boreal forests in Sweden and analyzed the relationships between δ15N in the soils and soil chemical properties as well as site characteristics. In addition, we conducted a meta-analyses of δ15N in the organic layer of European forests. We identified two types of δ15N patterns; in type D soils, the δ15N in the mineral soil decreases with depth, whereas in type C soil, the δ15N in the soil profile is almost constant. Type D soils had a significantly higher δ15N 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 δ15N was negatively correlated with the N:P ratio of the organic layer (R2 = 0.21, p < 0.001). Across Europe, the δ15N of the organic layer was positively correlated with latitude (R2 = 0.58, p < 0.001), and negatively with mean annual temperature (R2 = 0.52, p < 0.001) and atmospheric N deposition (R2 = 0.42, p < 0.001). In conclusion, our results show that the δ15N 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.