Assessment of depth‐dependent microbial carbon‐use efficiency in long‐term fertilized paddy soil using an 18O–H2O approach

Abstract

AbstractActivity of microorganisms and their turnover strongly affect soil organic carbon (SOC) accumulation. However, what remains unclear is how long‐term fertilization affects the microorganisms growth strategies, and how they change with the soil depth. Thus, microbial carbon‐use efficiency (CUE) and microbial biomass (MB) turnover in paddy soil were determined using a novel substrate‐independent H218O labelling approach, and the effects of long‐term fertilization (>30 years) with mineral (NPK) or combined (NPK + OM [manure]) amendments in 0–10, 10–20, and 20–30 cm depths were investigated. Long‐term fertilization increased microbial C uptake, CUE, and growth rates, and all indexes were the highest in the NPK + OM treatment. The CUE ranged between 0.07 and 0.23 and showed variable behaviour with depth: it reduced in the control treatment, indicating that more C was allocated to energy production than biomass growth, and increased in fertilized soils, showing the shift of C usage for biomass growth. The highest CUE was observed at 20–30 cm in NPK and NPK + OM, and indicated that microorganisms overcome the nutrient deficiency in deep soil layers by keeping high C uptake rates at a constant CUE. The MB turnover was more rapid in NPK (70–10 d) and NPK + OM (65–40 d) compared to the control (80–90 d) and intensified with the depth. These findings highlight that the decrease in C uptake under long‐term fertilization with soil depth compensated by an increase in CUE. These shifts in the strategies of microorganisms functioning can explain the accumulation of SOC in heavily fertilized paddy soils.