LAUSR

We compared the patterns of natural abundance of nitrogen (N) isotope ratio (δ15N) of total soil N among cropland, forest, and grassland soils, with special interests in the effects of farming system on cropland and grassland, and climate zone on forest soils, as well as the general effect of land-use change and site disturbance. We analyzed data on δ15N of terrestrial N sources (n = 532), cropland (n = 168), forest (n = 227 for organic and 428 for mineral soil layers), and grassland soils (n = 624). Forest soils had the lowest δ15N (– 1.0 ± 0.2‰ and + 3.1 ± 0.2‰ for mineral and organic soil layers, respectively), reflecting the influence of the 15N-depleted source N and the more closed nature of the N cycle. Tropical forest soil had higher δ15N than other climate zones, reflecting the influence of the high N availability and loss in tropical forests. The low δ15N in subtropical forest soils likely reflected the influence of the high rate of deposition of 15N-depleted N. The δ15N of cropland (+ 5.0 ± 0.2‰) and grassland (+ 6.2 ± 0.1‰) soils was greater with manure than with synthetic fertilizer applications. Soil δ15N was also affected by land-use change and was often increased (followed by progressive decreases) by site disturbance. Land-use type and land management effects on soil δ15N reflect changes in both the N sources and loss, while land disturbance effects are primarily associated with the degree of N loss. We also conclude that subtropical forest soil δ15N is affected by the high rate of atmospheric N deposition.