Evaluation of the soil aggregate stability under long term manure and chemical fertilizer applications: Insights from organic carbon and humic acid structure in aggregates

Abstract

The stability of soil aggregates is crucial for sustainable agricultural development, but the contribution of organic carbon (OC) and humic acid (HA) to aggregates remains elusive. A 22–year field experiment was conducted to examine the impact of variations in HA properties and structure within aggregates under various fertilization management schemes on OC levels and aggregate stability at depths of 0–20 cm and 20–40 cm. Five treatments included non–fertilizers (CK), chemical fertilizers (CF), and chemical fertilizers with three manure application rates (CM, 7.5, 15.0, and 22.0 t ha^–1). The results showed that the mean weight diameter (MWD) increased with increasing application rate of manure, which was 3.40 %–14.20 % higher than CK. Small macro–aggregates (2–0.25 mm) exhibited notable carbon sequestration with increasing manure application rate, showing changes 1.25–2.64 times faster than the other aggregates. Moreover, different fertilization managements facilitated the conversion of protein–like substances into humic–like substances, enhancing the aromaticity, humification, and molecular weight of HA in macro–aggregates (>0.25 mm). Two-dimensional correlation spectroscopy confirmed that the carboxylic acid COO^— of HA in aggregates (>0.053 mm) showed a preferential affinity for manure at depths of 20–40 cm, whereas the polysaccharides C–OH were more associated with other aggregates. The partial least squares path model (PLS–PM) revealed that the fluorescence composition of HA primarily indirectly influenced aggregate stability by affecting the OC content of aggregates at depths of 0–20 cm. Conversely, the change in functional groups directly influenced stability at depths of 20–40 cm. Overall, our findings suggested that the structural properties of HA could serve as sensitive indicators of fertilization management in black soil, thereby influencing OC sequestration and aggregate stability.