Effects of process water obtained from hydrothermal carbonization of poultry litter on soil microbial community, nitrogen transformation, and plant nitrogen uptake

Process water (PW) obtained from hydrothermal carbonization of nitrogen-rich (N-rich) biowaste is proposed to be a renewable resource utilized as a liquid N fertilizer. However, its effects on soil microbial community, N transformation, and plant N uptake are unclear or controversial. In this study, fertilizers were prepared with different percentages of PW (poultry litter, 220 °C 1 or 8 h, PW-S or -L) and urea to supply 160 mg kg^-1 total N in a barren alkali soil. Results showed that the addition of PW relative to pure urea decreased organic N mineralization by low bio-accessibility, increased N loss by high soil pH, and decreased NO₃^--N by low nitrification substrate. It supported the lettuce in health but decreased plant N uptake by low NO₃^--N. It significantly increased the gram-positive bacteria that responded to resistant organic matter, changed the bacterial community to enhance decomposition, detoxification, ureolysis, and denitrification, and to decrease nitrification. Its inhibition effect on nitrification activity was stronger than that on nitrifiers growth. Different from PW-S, the addition of PW-L seriously and significantly decreased seed germination index and fungal biomass that responded to N retaining capacity, respectively. The best fertilizer was 50% urea +50% PW-S that supported the seed germination and seedling growth, and mildly affected microbial community.