Evaluation of biological and chemical nitrogen indices for predicting nitrogen-supplying capacity of paddy soils in the Taihu Lake region, China

Abstract

Simple and rapid chemical indices of soil nitrogen (N)-supplying capacity are necessary for fertilizer recommendations. In this study, pot experiment involving rice, anaerobic incubation, and chemical analysis were conducted for paddy soils collected from nine locations in the Taihu Lake region of China. The paddy soils showed large variability in N-supplying capacity as indicated by the total N uptake (TNU) by rice plants in a pot experiment, which ranged from 639.7 to 1,046.2 mg N pot−1 at maturity stage, representing 5.8% of the total soil N on average. Anaerobic incubation for 3, 14, 28, and 112 days all resulted in a significant (P < 0.01) correlation between cumulative mineral NH 4 + -N and TNU, but generally better correlations were obtained with increasing incubation time. Soil organic C, total soil N, microbial C, and ultraviolet absorbance of NaHCO3 extract at 205 and 260 nm revealed no clear relationship with TNU or cumulative mineral NH 4 + -N. Soil C/N ratio, acid KMnO4-NH 4 + -N, alkaline KMnO4-NH 4 + -N, phosphate–borate buffer extractable NH 4 + -N (PB-NH 4 + -N), phosphate–borate buffer hydrolyzable NH 4 + -N (PBHYDR-NH 4 + -N) and hot KCl extractable NH 4 + -N (HKCl−NH 4 + -N) were all significantly (P < 0.05) related to TNU and cumulative mineral NH 4 + -N of long-term incubation (>28 days). However, the best chemical index of soil N-supplying capacity was the soil C/N ratio, which showed the highest correlation with TNU at maturity stage (R = −0.929, P < 0.001) and cumulative mineral NH 4 + -N (R = −0.971, P < 0.001). Acid KMnO4-NH 4 + -N plus native soil NH 4 + -N produced similar, but slightly worse predictions of soil N-supplying capacity than the soil C/N ratio.