Long-term peanut cultivation alters soil organic C stocks and aggregate stability: insights from coupled iron oxide, microbial, and lignin phenol analyses.

INTRODUCTION: The microbial and mineral C pumps play crucial roles in soil organic C (SOC) stabilization via complicated organo-mineral interactions. Yet, how both of them regulate SOC dynamics during peanut cultivation remains elusive.

METHODS: The surface soils from five peanut fields of varying ages (0-, 2-, 5-, 10-, and 20-yr.) were selected to elucidate the temporal variations in SOC dynamics and its potential mineral-microbial synergistic regulation patterns, based on PLFAs, eco-enzymatic activity, amino sugars, lignin phenols and Fe oxides analysis, etc.

RESULTS: With increasing peanut cultivation ages, SOC, labile C pools, microbial biomass, hydrolase activities, plant‑ and microbial‑derived C (PC and MRC), all exhibited unimodal patterns, peaking at 2-yr. peanut field before subsequently declining. Furthermore, the decreased Cy/Pre (i.e., the ratio of cyclopropyl PLFAs to their monoenoic precursor) and elevated fungi/bacteria values in older peanut fields indicated intensified bacterial physiological stress and a community shift favoring fungal dominance in these fields. Long-term peanut cultivation (i.e., 10 and 20-yr.) decreased SOC lability, enhanced aggregate stability, and altered SOC origins (i.e., enlarged PC but reduced MRC contribution to SOC). As peanut cultivation ages increased, the aggravated soil acidification (evidenced by a decline in pH from 6.10 to 4.57) concurrently enhanced the activation and complexation of Fe oxides, which was attributed to elevated levels of reactive Fe oxides (i.e., Feo and Fep). This transition altered the Fe-C coupling modes from 'weak' adsorption-driven (C:Fe molar ratios < 1) in 0- and 2-yr. peanut fields, to 'strong' complexation (1 to 6) in the, 5-, 10-, and 20-yr. peanut fields. The PLS-PM and Mantel test further suggested that intensified soil acidification promote reactive Fe oxides accumulation, while labile C pools and microbial biomass positively affect the accumulation of PC and MRC, respectively. Notably, reactive Fe oxides play a significant role only in MRC, rather than in PC accumulation.

DISCUSSION: Overall, these results elucidate the coupled mineral‑microbial influences on SOC dynamics over the peanut cultivation chronosequence, thereby enhancing our understanding of SOC sequestration processes in China's peanut-producing regions.