Interaction between water, crop residue and fertilization management on the source-differentiated nitrogen uptake by rice

Alternate wetting and drying (AWD) is considered as an effective water-saving practice for rice cultivation widely applied across the world. Although AWD can reduce global warming potential compared to continuous flooding (CF), it may have negative effects on N availability for crop by promoting N losses (nitrification-denitrification, leaching) and immobilization, due to the frequent soil redox cycling. By means of a growth chamber pot experiment and a ¹⁵N stable isotope approach we investigated the interactions between water, crop residue and fertilizer N management on the contribution of different N sources (i.e. fertilizer, rice straw, soil) to rice plant N uptake. We hypothesized that with respect to CF, AWD will decrease plant uptake of fertilizer (FDN), straw (StDN), and soil (SDN) derived N due to greater losses, greater microbial N immobilization during residue turnover under oxic conditions, and less N supply from soil organic matter (OM) desorbed under reducing conditions. Moreover, we hypothesized that the underlying processes will be influence by the timing of straw addition with respect to flooding and the temporal distribution of mineral N application.

Rice was grown for 60 d in a factorial setup including: (i) two water regimes: CF for 60 d vs. AWD (30 d of flooding followed by 30 d of alternating conditions involving 3 drain-flood cycles), and (ii) three straw and fertilizer managements that involved a combination of straw addition (10 Mg ha^-1) 30 or 60 d before seeding (S30 and S60, respectively), and N fertilization (ammonium sulfate) split between pre-seeding and tillering in 60+60 or 80+40 kg N ha^-1, such that treatments compared were S30-N60-N60, S30-N80-N40 and S60-N60‑N60. ¹⁵N-enriched fertilizer and straw were used in separate replicated setups to quantify the relative contribution of FDN, StDN and SDN to plant N uptake, as well as fertilizer use efficiency (FUE).

Plant N was mainly soil and fertilizer-derived (≈ 58 and 40%, respectively), while straw only contribute a minor amount (< 3%). Although AWD reduced total N uptake by about 10-13% with respect to CF, FDN and FUE were only slightly affected by water management, suggesting that differences in N nutrition did not depend exclusively on fertilizer N losses. SDN contributed more to plant nutrition in CF than in AWD, particularly when straw was incorporated in proximity to flooding. The combination of a fresh OM supply and reducing conditions under CF favoured the reductive dissolution of Fe oxides and desorption of soil OM that increase soil N supply via mineralization. StDN contributed less to plant nutrition in AWD than in CF albeit the higher mineralization rates we expected with more frequent oxic conditions. We attributed this to a higher microbial N demand under aerobic conditions that leads to a greater immobilization SDN during decomposition. The higher SDN and StDn for N60-N60 treatment with respect to N80-N40 suggested that an equilibrated splitting of N fertilizer between pre‑seeding and tillering stages could favor microbial activity under AWD improving straw degradation and soil N release.

This research was funded by the Lombardy Region through the project RISWAGEST