Understanding the spatiotemporal dynamics of soil organic C (SOC) quantity and origins, along with the potential microbial-mineral mechanisms influencing these changes, is crucial for long-term SOC preservation in agroecosystems; however, this knowledge is lacking during apple cultivation. By analyzing the temporal variations in SOC, microbial-derive C (MNC), plant-derived C (PC), iron oxides, exo-enzymes, and microbial communities across soil profiles over apple plantation establishment time-series (6, 14, 28, 35, and 42 years), this study investigated the synergistic roles of iron oxides and microbial attributes in regulating SOC cycling. Results showed that the SOC contents ranged from 5.5 to 12.3 g kg −1 , increased with increasing plantation ages; meanwhile, the pH values showed opposite trends with SOC during orchard development. Averaged across all soil profiles (0−60 cm), the elder orchard (i.e., 35 and 42 years) soils exhibited greater C resource levels (i.e., 45.0%−83.1% higher MNC and 91.4%−202.4% higher PC) than the younger orchard (i.e., 6 and 14 years) soils; meanwhile, long-term apple orchard cultivation (i.e., 35 and 42 years) enlarged PC but reduced MNC contribution to SOC. Notably, the above-mentioned C resources exhibited pronounced ‘surface aggregation’ patterns across soil profiles, with PC and MNC contents in the 0–20 cm layer being 42.0%–78.9% and 70.8%–302.0% higher than those in deeper layers. Besides, the contents of amorphous (Feo) and complex iron-oxides (Fep) increased with increasing plantation ages, accompanied by elevated iron-bound SOC contents and C:Fe molar ratios, but microbial biomass (evidenced by total PLFAs) and hydrolase activities firstly increased, peaking at 28 years orchards with mean values of 11.6 nmol g –1 and 36.7, and subsequently decreased across all soil profiles. Partial least squares path model (PLS-PM) verified that increased soil acidification impedes microbial growth while promoting iron oxides accumulation. Random forest model revealed that the contribution of PC (or MNC) to SOC were mainly regulated by microbial biomass, nitrate-N, and Feo (or Fep, pH, and microbial biomass). The PLS-PM and Mantel test jointly confirmed that MNC was mainly regulated by mineral indices (i.e., iron oxides), rather than microbial attributes; meanwhile, the PC was primarily governed by mineral indices (i.e., Feo and activation index) and biotic variables (e.g., hydrolase activities). Collectively, these findings underscore the crucial role of pH in modulating mineral-microbial interactions for SOC preservation during orchard cultivation, thereby deepening our understanding of SOC dynamics within orchard ecosystems in China.
Zhang et al. (Fri,) studied this question.