Straw returning and potassium fertilization affect clay mineralogy and available potassium

Clay-size particles are rich in phyllosilicate minerals and iron oxides, which strongly influence the availability of soil potassium. This study investigated the clay minerals in three particle sizes (450-2000, 100-450, and 25-100 nm) and their relationships with soil K under nitrogen and phosphorus fertilizers (NP); NP plus straw returning (NP + RS); N, P, K fertilizers (NPK); and NPK + RS in paddy soils. With decreasing particle size, the vermiculite and illite contents decreased, kaolinite was dominant in the 25-100 nm fraction, and the molar ratios of SiO2/Al2O3 decreased. Exchangeable K (EK) in the 25-100 nm fraction was significantly higher than that in the 450-2000 and 100-450 nm fractions, while non-exchangeable K (NEK) showed the opposite trend. The most pronounced changes in clay minerals were in the 25-100 nm fraction, where the K addition (Straw returning and/or K fertilization) showed a significant increase in illite content of 51.6-129.0% compared to the NP treatment. K addition maintained a positive K balance and increased the soils' EK, NEK, and amorphous Fe oxide contents. Structural equation modeling showed a strong direct effect of straw returning and K fertilization on both soil EK and NEK (p < 0.05). In the 25-100 nm fraction, EK was associated with changes in Fe oxides, and NEK was associated with the conversion of vermiculite to illite due to K addition. In all, phyllosilicate minerals and iron oxides in soil clay fractions changed under straw returning and K fertilization from micron to nanoscale, which were the driving factors affecting soil EK and NEK.