Agricultural land use affects nitrate production and conservation in humid subtropical soils in China

To date, very few studies have been conducted to investigate the characteristics of gross nitrogen (N) transformations in subtropical agricultural soils. In this study, 12 natural woodland and 10 agricultural soils were collected to investigate the effects of land use on soil gross N transformations in the humid subtropical zones in China. The results showed that gross autotrophic nitrification rates (average 0.19 mg N kg−1 d−1) in the woodland soils were significantly lower than those determined in the agricultural soils (average 1.81 mg N kg−1 d−1) (p < 0.01). However, the NO3− immobilization rates (average 0.10 mg N kg−1 d−1) in the agricultural soils were significantly lower than in the woodland soils (average 0.47 mg N kg−1 d−1) (p < 0.01). On average, 98% of the total NO3− produced could be immobilized into organic-N in the woodland soils, while, it accounted for only 10% in the agricultural soils. These differences in gross N transformations resulted in the inorganic N being dominated by NH4+ in the woodland soils; however, NO3− dominated the inorganic N in the agricultural soils. The risk of N leaching and runoff from soil sharply increases after woodland soils are converted to agricultural soils. Application of organic fertilizers with high C/N ratios to agricultural soils in subtropical regions to increase soil organic C content and the C/N ratio is expected to improve NO3− immobilization capacity and reduce the risk of N leaching and runoff from soil.