Soil aggregate stability improves greatly in response to soil water dynamics under natural rains in long-term organic fertilization

Soil structure in highly-weathered soils is adjusted by soil water content (SWC) under natural precipitation, and is also driven by the soil chemical properties. Such a relationship may be further dependent on types of fertilization. The objectives of this study were to investigate the effects of long-term fertilization (NPK, NPK + straw, manure, and control) after 17 continuous years on the SWC, temperature, aggregate stability, and the associated chemical properties (oxides, organic matter, and zeta potential), and to elucidate the relationship between SWC and aggregate. The aggregate stability (MWD), consecutive SWC, temperature, and chemical properties from 0 to 10 and 10 to 25 cm were measured during one year. Our results showed that the manure and NPK + straw treatments significantly improved the soil aggregate stability (P < 0.05) (e.g., average MWD 1.13 vs. 0.74 mm for manure vs. control at 0-10 cm) due to increase in the fraction of macroaggregate (>2 mm). The SWC values and degree of variation at the same time were also improved more in manure and NPK + straw treatments than others. Soil aggregate MWD was significantly negatively correlated the SWC at the moment of soil sampling (WC0) for all treatments. But the aggregates under two organic treatments was not significantly correlated with the antecedent SWC before half day and 4 days (WC0.5 and WC4), which indicated that the aggregates under manure and NPK + straw treatments were relatively stable when SWC changed before 0.5 and 4 days. The high stability of aggregates in the two organic-treated soils was also confirmed by very low relative slaking index values. The fertilization positive effect on soil aggregation to resist slaking was also attributed to the decline in soil clay zeta potential and improvement in soil OM and amorphous Fe oxides compared to control. The results suggest that long-term manure and NPK + straw application can improve aggregate stability and resistance to slaking under SWC variation whereas application of NPK only did not show such an effect.