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.