Effects of Biochar- and Hydrochar-Amended Organic Fertilizer on Crop Production, NH3 Loss, and Fertility of Coastal Saline–Alkali Soil

Biochar- and hydrochar-amended organic fertilizers are widely used to enhance saline-alkali soil fertility and crop production. However, their effects on ammonia (NH₃) volatilization from saline-alkali soils remain unclear. Here, we conducted a pot experiment to investigate the impacts of organic fertilizer (OF), and of that with biochar (BC-OF) and hydrochar (HC-OF) amendments at a rate of 15% on crop production, on soil fertility and NH₃ loss in saline-alkali soil with rice-wheat rotation, using chemical fertilizer alone as the control (CK). Compared with CK, OF, BC-OF, and HC-OF increased the rice and wheat yields. In particular, HC-OF harvested a significant 16.6% more grain yield than OF (<i>p</i> < 0.05). Organic fertilizer amendments exerted a general mitigating effect on the NH₃ volatilization during different observations after nitrogen (N) fertilization. Correspondingly, they reduced total NH₃ volatilization during the rice season compared to CK. Compared with CK, HC-OF significantly reduced NH₃ emissions by 61.6% during the wheat season and 28.3% annually (<i>p</i> < 0.05). Moreover, HC-OF treatment reduced total NH₃ volatilization in the wheat season by 55.8% and 64.7% compared to OF and BC-OF, respectively. Compared with CK, HC-OF treatment significantly reduced soil pH, while both the BC-OF and HC-OF treatments increased soil total N and ammonium N contents, even surpassing those in the OF treatment. However, no significant differences were observed among treatments in soil electrical conductivity, nitrate, available phosphorus and potassium, as well as organic matter content. In conclusion, HC-OF is more suitable for enhancing crop yield and reducing soil N loss in saline-alkali soils.