Effects of Straw Returning and Potassium Fertilizer on Soil Aggregate and Potassium Distribution Under Rapeseed-Rice Rotation

The study aimed to explore effects of different fertilization on paddy soil aggregate characteristics and potassium distribution under rapeseed-rice rotation and winter fallow by the located experiment test. It was hoped that the study would provide a basis for the sustainable utilization of potassium resources in paddy-upland rotation areas of Southern China.

Based on the location experiment of potassium fertilizer in different rotation patterns started in 2016, five treatments were selected, including CK(F) (no fertilization and winter fallow), NPK(F) (NPK fertilizer and winter fallow), NPK(R) (NPK fertilizer and winter rapeseed), NPK (SR+R) (NPK fertilizer with rice straw returning and winter rapeseed), and NP50%K (SR+R) (50% reduction of K fertilizer with rice straw returning and winter rapeseed). The soil samples were collected from the 0-20 cm soil layer of each treatment after rape harvest of the third year for analysis of physical and chemical properties, such as soil aggregates distribution, and exchangeable and non-exchangeable potassium content variation in soil aggregates. The stability of soil aggregates and the distribution of potassium in aggregates were further investigated.

The soil aggregate fractions of all treatments was the highest in <0.053 mm. Compared with NPK(F), NPK(R) increased the proportion of aggregates of 1-2 mm, 0.5-1 mm and 0.25-0.5 mm by 26.2%-82.6% under the same fertilization treatment. Also the stability of soil aggregates was increased, the proportion of aggregates of >0.25 mm, mean weight diameter (MWD), and geometric mean diameter (GMD) were significantly increased by 30.6%, 31.2% and 82.0%, respectively. Under rapeseed-rice rotation, the proportion of aggregates of >2 mm was increased under NPK(SR+R), which was 69.7% higher than that under NPK(R). The exchangeable potassium content in soil aggregates decreased with decreasing particle size under all treatments. NPK(SR+R) significantly increased exchangeable potassium content in all aggregate fractions by 22.2%-46.0% compared with NPK(R) under rapeseed-rice rotation. NP50%K(SR+R) significantly reduced the exchangeable potassium content in aggregates of >0.5 mm by 19.4%–20.6% than NPK(SR+R). Compared with NPK(F), three fertilization treatments under rapeseed-rice rotation all reduced the non-exchangeable potassium content in aggregate fractions. Under all treatments, the contribution rate of different fractions of soil aggregates to the bulk soil in potassium was the highest in <0.053 mm. Compared with NPK(F), NPK(R) significantly increased the contribution rates of 1–2 mm and 0.5–1 mm aggregates to bulk soil in potassium by 82.6%, 52.1% (exchangeable potassium) and 105.5%, 36.9% (non-exchangeable potassium), respectively.

The rapeseed-rice rotation could increase MWD, GMD, macroaggregate proportion and the contribution rate of macroaggregate to the bulk soil in potassium and improve soil structure. Under this rotation pattern, the chemical fertilizer combined with rice straw returning could increase the exchangeable potassium content in all aggregate fractions, which improved the potassium supply in paddy soil. However, the rapeseed-rice rotation reduced the content of non-exchangeable potassium in soil aggregates due to high potassium demand, and the input of potassium fertilizer should be appropriately increased.