Spatial and temporal trends of soil total nitrogen and C/N ratio for croplands of East China

Adequate soil total nitrogen (TN) is critical to crop productivity, and soil carbon-to-nitrogen ratio (C/N) is an important indicator of soil fertility. Accurate knowledge of soil TN content and C/N ratio is essential for precision agriculture and soil biogeochemical modeling. This study aims to characterize the spatial and temporal trends of soil TN level and C/N ratio for cropland soil across Zhejiang province of East China. A total of 29,927 topsoil (0–20 cm) samples were collected during 2007–2008 for analyzing soil properties, on the basis of which we mapped the soil TN content at a 250 m resolution using a random forest model (RF). With 22 predictors covering soil properties, fertilization, meteorology, locations, and population density, the RF model showed good predictive performance with cross-validation R2 = 0.65 and RMSE = 0.43 g kg−1. The soil pH, cation exchange capacity (CEC), and fertilization played important roles in determining the spatial variations of soil TN and C/N ratio. The overall level of cropland soil TN within the study area was estimated to be 1.75 ± 0.43 g kg−1 (μ ± σ), varying from 0.25 to 3.64 g kg−1. By comparing our estimates with the previous extensive survey for 1979–1985, we found that the soil TN has slightly increase trend and the soil C/N ratio slightly decreased, which was attributed to intensified N fertilization in the last decade. This work provides up-to-date knowledge of soil TN content and soil C/N ratio in Zhejiang province of East China, indicating the importance of updating soil property databases for better regional cropland management.