Manure-nitrogen substitution for urea leads to higher yield but increases N2O emission in vegetable production on nitrate-rich soils

Large amounts of organic manure are applied globally to maintain vegetable production, but this practice also leads to a high accumulation of nitrate in soils used in vegetable cultivation. It is unclear whether the increased carbon input from manure, applied with the goal of improving nitrogen (N) supply, might stimulate nitrous oxide (N₂O) emissions from such high-nitrate soils. We here conducted a two-year field experiment in a typical soil (classified as an Anthrosol) used for vegetable cultivation, with high nitrate residue, in China, to investigate the comprehensive effects of various proportions of manure N and urea N (0–100 % manure N) on yield, N₂O emission, and global warming potential (GWP). 25 % and 50 % manure-N substitutions increased the yield, over five vegetable-growing seasons, by 5–31 %, compared with 100 % urea-N. Cumulative N₂O emissions and the sum of GWP (mGWP) also increased with the ratio of manure N to urea N. Compared to 0 %M, cumulative N₂O emissions increased by −20–41 %, 20–98 %, 10–151 %, and 20–235 % in 25 %M, 50 %M, 75 %M, and 100 %M, respectively, and mGWP increased by 8 %, 19 %, 28 %, and 40 %, respectively. Moreover, the average yield-scaled GWP across five treatments ranged from 78 to 191 kg CO₂-eq Mg⁻¹ year⁻¹. The manure-N substitution affected the abundance of nirK, nirS, and nosZ genes, significantly increasing the value of (nirK+nirS)/nosZ, by 2.9, 10.3, 24.7, and 21.0 times, in 25 %M, 50 %M, 75 %M, and 100 %M, respectively, compared to 0 %M, which may explain the increase in cumulative N₂O emissions and the sum of GWP at higher manure percentages. Our study shows that higher relative manure-N quantities can be beneficial to yield but increase N₂O emission, and highlights the importance of carefully balancing manure substitution ratios in high-nitrate vegetable soils.