Replacing nitrogen fertilizer with nitrogen-fixing cyanobacteria reduced nitrogen leaching in red soil paddy fields

Abstract

Excessive chemical nitrogen (N) fertilizer application and flood irrigation in rice production can lead to substantial soil-N losses and low nitrogen-use-efficiency (NUE) in rice due to N leaching from red paddy soils. Although inoculation of irrigated fields with nitrogen-fixing cyanobacteria (NFC) has been shown to increase rice yield, there are few studies on N leaching and NUE in response to NFC when used in place of urea. A two-year field experiment was carried out to examine the concentrations of ammonium-N (NH4+-N), nitrate-N (NO3−-N), total dissolved N (TDN) and dissolved organic N (DON) in percolated irrigation water, as well as cumulative leaching losses after NFC was used to replace different application rates of urea in double cropped rice grown in red paddy soil. Four treatments were established: 1) CK, the control with no application of urea or NFC; 2) N10C0, a conventional full application of urea only; 3) N5C5, addition of a half-application of NFC to replace half of the conventional application rate of urea (N5C5); and 4) N0C10, a full application of NFC only. In 2017 and 2018, although the average rice yield of N0C10 was 25.0 % higher than that of CK, it was 23.8 % lower than that of N10C0. The average NUE of N10C0 was 34.7 %, and the average TDN leaching loss was 13.9 kg ha-1 per crop each year. Correspondingly, NUE of N5C5 was 47.7 %, which was 37.5 % higher than that of N10C0. Replacing 50 % of urea with NFC did not significantly decrease rice yield, but it reduced TDN leaching by 37.2 %. The results showed that the conventional way of fertilization caused the most N loss, while replacing partial urea with NFC significantly reduced the leaching losses of NH4+-N, NO3−-N and DON. Moreover, replacing 50 % of urea with NFC can retain more soil N than conventional fertilization at the end of the rice season because NFC can intercept, fix and delay release of N. Thus, the multiple functions of NFC in N cycling dynamics are more conducive to the retention of N by soil and greatly reduces N leaching.