Digital mapping of the global soil δ15N at 0.1° x 0.1° resolution using random forest regression with climate classification

Spatial information of the natural abundance of soil nitrogen stable isotope (delta N-15) is beneficial for deeply understanding the terrestrial nitrogen (N) cycle. However, to date, the precise global map of soil delta N-15 still lacks. In this study, based on the measured soil delta N-15 data by Craine et al. (2015) (https://doi.org/10.1038/srep08280) and related environmental variables including soil, topography, vegetation, and climate, we constructed the optimal relationship model between soil delta N-15 and environmental variables, and mapped the global pattern of soil delta N-15 at 0.1 degrees x 0.1 degrees resolution (in natural terrestrial ecosystem). Results indicated that satisfied performance was achieved (R-2 = 0.68 and RMSE = 1.26 parts per thousand) by separately building the optimal relationship models for soil delta N-15 in each of five climate zones (Tropical, Arid, Temperate, Cold and Polar) using the random forest regression algorithm. In addition, critical controls of the soil delta N-15 in different climate zones were thus identified based on the variable importance calculated by each random forest regression model. In the Tropical zone, soil delta N-15 might be primarily regulated by microbial N loss, and soil pH and organic matter were identified as two most important factors. In the Arid zone, abiotically gaseous N loss regulated by solar radiation would be the critical controls of soil delta N-15. In the Temperate zone, temperature-related variables were identified as the critical controlling factors, and in the Cold zone, soil water and heat conditions had the equally greater importance, and bulk density was the dominated factor in the Polar zone. Furthermore, the predicted global soil delta N-15 ranged from -0.44 parts per thousand to 12.59 parts per thousand, with the mean value of 5.06 parts per thousand, and the standard deviation of 1.74 parts per thousand. Significantly higher soil delta N-15 (P < 0.05) were observed in the Tropical and Arid zones with mean values of 6.52 parts per thousand and 6.11 parts per thousand, respectively. This indicated that the soil N cycles were more open than those in the Temperate, Cold and Polar zones (mean soil delta N-15 of 4.37 parts per thousand, 3.67 parts per thousand and 2.76 parts per thousand, respectively). This study provides clues for potential environmental regulations on terrestrial N cycle in different climates, and the global soil delta N-15 map can be a reliable data support for future research.