Nitrogen loss due to nitrate reduction in the soil profile depends on the type of cropland

Abstract

The N cycle in shallow soil is affected by the cropland type; however, little is known about nitrate loss in deeper soil layers in upland and paddy fields. We employed a soil slurry-based ¹⁵N paired tracer technique to investigate denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the 0–200 cm soil profile of paddy and uplands (40 cm per layer) in the Ningxia Yellow River irrigation district. The rates of N loss of denitrification and anammox ranged from 0.29 to 9.06 nmol N g⁻¹·h⁻¹. In the paddy soil, the rates of N loss decreased from the 0–40 cm layer to the 160–200 cm layer, and the rates ranged from 0.74 to 9.06 nmol N g⁻¹·h⁻¹, in the upland soil, the rate of N loss exhibited a decreasing trend from the 0–40 cm layer to deeper layers, followed by an increase from the 80–120 cm layer to the 160–200 cm layer, with values ranging from 0.29 to 1.99 nmol N g⁻¹·h⁻¹. Denitrification was the major contributor to N loss in paddy soil, accounting for 66.59–85.47 % of nitrate loss. In addition, paddy field soils presented a more diverse N-reducing bacterial community than uplands soil did, with Chao1, ACE and Shannon index in the bacterial diversity analysis being greater in paddy fields than upland soils. Desulfobacca, Fimbriimonas, Thiobacillus, Anaerolinea, Anaeromyxobacter and Syntrophobacter presented relatively high abundances in the 0–40 cm soil profile in the paddy fields. The nitrate reduction rate in the soil profile was determined by crop type. The nitrogen reduction loss in deep soil layers cannot be ignored, and long-term crop types can impact soil texture and microbial community structure, ultimately leading to differences in the diversity of nitrogen-transforming microorganisms.