Fertilization decreases microbial CUE via enhancing soil properties and microbial respiration in coal mine reclamation area

Abstract

Microbial carbon use efficiency (CUE) is a pivotal parameter in regulating soil carbon cycling, however, the effects of different fertilization on microbial CUE in reclaimed soil and its key driving mechanisms remain unclear. To address this knowledge gap, a long-term trial was conducted to explore the differential characteristics of microbial CUE and its microbial mechanism in reclaimed soil under fertilization. Four treatments were included in the trials: 1) normal farmland (NL); 2) no fertilization (NF); 3) balanced mineral NPK fertilization (NPK); 4) NPK plus organic fertilizer (NPKM). Our results showed that both NPK and NPKM significantly decreased the microbial CUE by 29.6 % and 48.1 %, respectively, concurrently, significantly prolonged the microbial biomass turnover time. The microbial growth rates and growth quotient (qGrowth) in all treatments were significantly lower than respiration rates and respiratory quotient (qCO₂) by an order of magnitude. Furthermore, the microbial growth rate in NF and NPKM were significantly lower than NL, and respiration rate in NPK and NPKM were significantly lower than NL. NPK and NPKM significantly enhanced soil particulate and mineral-associated organic carbon (POC and MAOC) contents relative to NF. In addition, NPK and NPKM also significantly increased the phospholipid fatty acid (PLFA) concentrations of gram-positive bacteria, gram-negative bacteria, aerobic bacteria and other bacterial groups, while only the fungal PLFA concentrations in NPKM were comparable to normal field levels. A significant positive correlation was found between microbial CUE and soil pH, as well as the ratio of microbial biomass carbon to nitrogen (MBC/MBN). Conversely, microbial CUE exhibited significant negative correlations with soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), available nitrogen (AN), available phosphorus (AP), available potassium (AK), MBC, and microbial PLFA. Microbial CUE also showed a negative correlation with qCO₂ (R² = 0.48, P < 0.01) and MBC, POC, MAOC (R² = 0.39–0.64, P < 0.05) in reclaimed soil. Random forest model (RFM) further identified MAOC, aerobic bacteria, MBN, TN, MBC/MBN, G⁺ and SOC as the primary factors affecting microbial CUE. The Partial least squares path modeling (PLS-PM) suggested that fertilization enhanced microbial respiration by improving soil properties, ultimately reducing microbial CUE in reclaimed soil. Overall, long-term application of NPK or NPKM reduced the microbial CUE via enhancing soil properties and microbial respiration in coal mine reclamation area, which also provide a theoretical basis for organic carbon sequestration in reclaimed soil.