Long-term straw returning improve soil K balance and potassium supplying ability under rice and wheat cultivation

The aims of the present study were to provide scientific bases for rational use of crop straw to substitute chemical potassium (K) input. The effects of potassium fertilization and straw incorporation on soil K balance and K supplying in a long-term (14 years) field experiment. Five treatments were examined: (1) no fertilization (CK); (2) mineral fertilizing (NPK); (3) straw 6000 kg h m^-2 (S); (4) NPK with straw 3000 kg h m^-2 (NPK_1/2S); and (5) NPK with straw 6000 kg h m^-2 (NPKS). K composition, K balance and quantity-intensity (Q/I) relationship were studied. Under no fertilization or low straw returned conditions, soil K was unbalanced and deficienct seriously. Straw return at 6000 kg h m^-2 per season with fertilization improved the soil potassium supply and K balance. Long-term K surplus (4 or 5 years), compared with NPK, the NPKS significantly increased non-special K adsorption (K_nsa) and non-exchangeable K (K_ne) by 5.7-11.2 mg kg^-1 and 65.7-128.1 mg kg^-1, respectively. Q/I relationship showed cropping without straw K or without fertilizer K resulted in lower quantity (nonspecifically and specifically held K i.e. - ∆K₀ and K_x) and intensity (equilibrium activity ratio i.e. CR₀^K) of K in tested soils. K-fertilization with straw maintain higher exchangeable K (EK₀) and a higher difference between EK₀ and minimum exchangeable K(EK_min), and would help to prevent depletion in non-exchangeable pool of soil K under intensive cropping. Additionally, The straw return mainly decreased potential buffering capacity for exchangeable pool (PBC^K_n), 43.92-48.22% of added K in soil might be converted to exchangeable pool while it was 25.67-29.19% be converted to non-exchangeable pool. The contribution of exchangeable K towards plant K uptake would be higher in the soil with straw than the soil without straw and the non-exchangeable K would be the long-term fixed K as a supplement to the potassium pool. K fertilizer with 6000 kg h m^-2 straw return in each crop season increased soil available K and slowly available K. The findings underlined importance of the straw return and contribution for sustain K supplying ability of soils.