Optimizing combination of chemical nitrogen fertilizer and manure can increase yield and economic benefits of dryland wheat while reduce environmental risks

Partial substitution of chemical fertilizer with organic fertilizer is emerging as a promising measure to achieve sustainable agriculture with high crop yields and low environmental risks. How much synthetic nitrogen fertilizer can be replaced by each unit of organic fertilizer? How to balance the economic benefits against the environmental risks? There is still a lack of long-term field observations to address these challenges. An 8-year field fertilization experiment (initiated in 2014) was conducted using a split-plot design with five nitrogen rates (N: N0, N75, N150, N225, N300) as main plots in combination with two manure rates (M: M0 and M1) as subplots. The results indicated that the grain yield and economic benefits slowly increased or even decreased after the N rate exceeded 150 kg ha⁻¹. The N rate required for M1 to reach the highest yield of M0 was 105 kg ha⁻¹, which was 123 kg ha⁻¹ less than the 228 kg ha⁻¹ required for that of the M0. At this point, each ton of manure can replace 4.1 kg of synthetic N. Manure application considerably increased the net economic benefit by 10.2 %. The nitrate residue in the 0−200-cm soil layer sharply increased with the N rate, particularly when the N rate exceeded 150 kg ha⁻¹. An N rate exceeding 150 kg ha⁻¹ was more likely to cause nitrate leaching to the deeper soil layer (below 200 cm) during the summer fallow season. N₂O emissions and NH₃ volatilization gradually increased with the nitrogen and manure rates, and both exhibited nonlinear distribution curves with the N rate. Pursuing higher grain yields reduced economic benefits and caused more N pollution. The Structural equation modeling (SEM) results showed that manure application significantly increased the water storage rate during the summer fallow period by 4.1 %, which would promote wheat N uptake and ultimately reduce reactive nitrogen losses and improve economic benefits. Taken together, M1N150 was the optimal fertilization scheme to synergistically achieve high yield, high economic benefits and low environmental risks in Guanzhong Plain of China.