Optimization of fertilization scheme based on sustainable wheat productivity and minor nitrate residue in organic dry farming: An empirical study

The substitution of chemical nitrogen fertilizer with manure holds the potential for a synergistic rise in wheat grain yield and protein concentration, while minimizing residual nitrate in soil. We conducted a 6-year field fertilization experiment including two manure treatments (with or without) and five nitrogen applications rates (0, 60, 120, 180 and 240 kg ha-1). The study investigated the impact of single chemical nitrogen (CN) and manure substitution for nitrogen fertilizer (MN) on the grain yield (GY), grain protein concentration (GPC), plant nitrogen uptake (PNupt) and plant nitrogen requirement (PNR) of wheat, and the dynamic change of soil nitrate-N. The findings revealed that: (1) the MN demonstrated a greater advantage over CN, as evidenced by a 13.4-16.0 % increase in GY, a 2.6-3.8 % increase in GPC, a 7.2-15.7 % increase in PNupt and a 1.5-4.7 % reduction in PNR. (2) Soil nitrate accumulation (SNA) significantly increased when fertilizer rates ≥180 kg ha-1 and the peak annually shifted to deeper layer. The MN increased the SNA0-100 by 20.9-21.8 %, but significantly reduced SNA0-200 by 11.8-13.5 % compared with the CN. Topsoil nitrate content (SNC0-20) can be adopted as a substitute for SNA0-100 to make the fertilization schedule convenient. (3) Regression analysis revealed (taking the MN for example) that the optimum N rates for the maximum GY (5417 kg ha-1) and GPC (15.3 %) were 164 and 211 kg N ha-1, respectively. The nitrate-N safety threshold was 62 kg ha-1 at the fertilizer rate of 89 kg N ha-1. Based on this, nitrogen fertilizer input reduced by 44.8-57.2 % and SNA0-200 by 17.9-33.6 %, with achieving 91.8-95.0 % of maximum GY and 89.7-92.9 % of maximum GPC. Substituting manure for nitrogen fertilizer achieved the potential of maintaining the grain yield and protein concentration while the minimization in soil nitrate residue. This study offers a feasible way for fertilization recommendation and nitrate residue controlling in dry farming.