The activity of microorganisms and their turnover strongly influence soil organic carbon (SOC) accumulation. However, microbial life-death cycles mediate the influence of fertilization on SOC storage across soil depths escapes mechanistic clarity. We analyzed 0–15 cm (surface) and 30–50 cm (deep) soils from a 45-year long-term fertilization field experiment, which included no fertilization (Control), mineral fertilization (NPK), pig manure (M), and a combination of mineral and pig manure (NPKM). Microbial carbon use efficiency (CUE) was measured using a H 2 18 O labelling approach and microbial necromass was indicated by amino sugars. Over 45 years, both M and NPKM increased microbial growth by 1.8- to 5.9-fold, CUE by 65-100% compare with NPK in surface soils by alleviating microbial nutrient stoichiometric imbalances (reduced C:N imbalance) and improving soil physical structure (lower bulk density). This consequently caused greater microbial necromass yield by 38-61% as compared to NPK. While microbial biomass, necromass, and CUE declined with soil depths, manure enriched Fe oxides and Ca 2 that stabilized microbial necromass, and subsequently induced a higher contribution of total microbial necromass to SOC in deep (74-76%) compared to surface soils (34-43%). Consequently, subsoils under manure shifted from C-limited zones to dominant SOC sinks. Thus, improving microbial CUE, especially via organic manure, promotes SOC sequestration by efficiently converting fresh inputs into persistent, microbially-derived organic matter across the soil profile. • Fertilization increased microbial growth rates and CUE with higher values under manure than mineral fertilization. • CUE decreased with soil depth due to nutrient limitation with longer turnover rates and lower carbon uptake in deeper soils. • Higher soil carbon availability in the surface soil led to greater microbial growth and faster turnover. • Manure fertilization caused 38-61% higher microbial necromass than mineral fertilization. • Manure fertilization increased microbial necromass contribution to soil organic C in deep (74-76%) than surface (34-43%).
Chen et al. (Thu,) studied this question.