Effects of applying organic manure and biochar on microbial necromass accumulation and the underlying microbial processes in tea plantations

Fertilization management approaches influenced soil organic carbon (SOC) content and microbial community composition in tea plantations. However, the impacts of organic manure mixed with biochar application on microbial necromass buildup and the underlying microbial mechanisms are still unclear. This study used amino sugars as biomarkers to examine the variations in the accumulation of microbial necromass carbon (C) after 4 years of application of different fertilizers, including chemical fertilizer (NPK), organic manure substituted for 50% of NPK (OM50), and OM50 plus 20 t hm -2 biochar (OM50+BC). Simultaneously, the stable isotope probing technique targeting phospholipid fatty acids ( 13 C-PLFA) and amino sugars ( 13 C-AS) was utilized to characterize microbial processes, such as microbial anabolism and necromass production, in soils treated with various fertilizers, including 13 C-labeled glucose. Compared with NPK, OM50 significantly increased bacterial necromass C content and its contribution to SOC by 32.38% and 20.49%, respectively; OM50+BC significantly increased bacterial, fungal, and total necromass C by 36.43%, 21.72%, and 25.40%, but reduced the contributions of fungal and total necromass C to SOC by 13.32% and 10.67%, respectively. OM50+BC had stronger 13 C assimilation into living microbial biomass, which was 1.14–1.47 times more than that in the NPK on average. Similarly, the newly generated microbial necromass in OM50+BC was 1.03–1.40 times more abundant than that in NPK. However, the retention efficiency of 13 C-glucose from the living microbes to microbial necromass in OM50+BC was lower than that in NPK and OM50, leading to a reduced proportional contribution of 13 C-AS to 13 C-SOC in OM50+BC. Our findings showed that OM50+BC increased microbial anabolism, necromass production, and accumulation, but was less efficient than the NPK and OM50. These findings give a mechanistic basis for improving carbon sequestration and sustainable fertilization in tea plantation soils.