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 sequestration and stability of organic carbon (OC) in soil aggregates is crucial for sustainable agricultural development. While the contribution of humic acid (HA) to soil 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). Results showed that fertilization management led to a 3.40%–14.20% increase in the mean weight diameter (MWD) of soil aggregates. Small macro–aggregates (2–0.25 mm) exhibited notable carbon sequestration capacity with increasing manure rate, showing changes 1.25–2.64 times faster than other aggregates. Moreover, 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 carboxylic acids COO— of HA in >0.053 mm aggregates had a preferential affinity for manure at depths of 20–40 cm, while the polysaccharides C–OH for other aggregates. The partial least squares path model (PLS–PM) revealed that the fluorescence composition of HA primarily indirectly influenced the aggregate stability by affecting the OC content of aggregates at depths of 0–20 cm. Conversely, the change in functional groups directly influenced the 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.

Keywords: Fertilization management, Soil aggregation, Soil organic carbon, Humic acid