Effects And Mechanisms Of Straw Control On Soil Potassium Supply

Potassium(K) is one of the essential nutrition elements for crop, and the absorption of potassium mainly comes from the soil. The application of K fertilizer can be an effective supplement for soil K status, but, the relative shortage of water-soluble K resources in China has hindered the potash consumption considering the domestic production and international price. Therefore, on the one hand, it is necessary to optimize the application of K-fertilizer and improve the K-fertilizer efficiencies; on the other hand, to broaden the approach of K resources has important practical significance, such as crop straw and other K-bearing rock materials. In this paper, we carried out data analysis, field experiments and laboratory simulation system to study the types and distribution of straw potassium resources in Hubei province, to assess the effects of straw potassium substitution for chemical K-fertilizer and also to discuss the interaction between organic acid and K-bearing mineral(e.g., biotite). All of them would provide the basic knowledge for sustainable use of straw potassium resources, and the main conclusions are as follows:(1) The results showed that the average amount of crop straw and straw K resources were 28.24 Mt/yr and 483 Kt/yr, respectively in Hubei province. The rice straw, oilseed rape straw, wheat straw, corn straw and cotton straw, which were the main crop types, accounted for 92.40 % of the total crop straw resources. According to the statistical yearbook in 2013, the crop straw was mainly distributed in Jingzhou, Yichang, Xiangyang, Xiao Gan, Jingmen and Huanggang, where the straw and straw K resources accounted for 74.6% and 74.0% of the total resources in Hubei province. Therefore, Jianghan plain and Eastern region play an important role in the production of crop straw in Hubei province. Based on economic and social development of Hubei province present situation, straw incorporation is the most fundamental way to the comprehensive utilization of straw, and an important measure to increase grain yield. Moreover, a thorough technical system should be established in accordance with local condition.(2) Results of laboratory incubated experiments showed that in the water-soil system, straw K and chemical K mainly existed as exchangeable K, followed by soluble K. With the increment of extra potassium, soil fixation-K began to increase, and under the condition of equal K rate, the proportion of exchangeable K was higher in the treatment with rice straw K than the treatment with chemical K whereas the proportion of non-exchangeable K of them was on the contrary. There were no significant difference on the fixation K of soil between straw K and chemical K when the amount of added potassium was within the threshold. However, when the amount of extra potassium was beyond the threshold, adding straw to the soil could reduce K fixation. Therefore, straw K and chemical K have similar chemical property and can be the alternative use. During the processes of straw decomposition and K release, crop residues could adsorb K+ from the ambient environment, which was subject to decomposition periods and extra K+ concentration. Crop residues could absorb a large amount of aqueous solution to preserve K+ indirectly during the initial decomposition period. These crop residues could also directly adsorb K+ via physical and chemical adsorption in the later period, allowing part of this K+ to be absorbed by plants for the next growing season.(3) In the early rice-late rice-winter oilseed rape cropping system, straw returned(incorporation or mulch) could significantly increase the crop yield and nutrient uptake, especially in the winter oilseed rape season for 7 years. Whether conventional tillage or no-till with straw could improve the sustainable productivity in double rice-rapeseed rotation system, and play positive effects on the soil fertility. Straw returned could significantly increase the surplus amount of nitrogen and potassium in the agricultural systems. Therefore, the rates of nitrogen and potassium fertilizer may be reduced with an appropriate proportion, in order to make full use of the straw nitrogen and potassium. Field trials for one year showed that the K fertilizer and straw returned could increase yield of rice and K accumulation amount of shoots at varying degrees. The straw incorporation with K-fertilizer treatment got the best effect compared with CK treatment. Using the fertilizer efficiency model and considering straw returning to field, the optimal amount of K fertilizer was 20.0% ~ 49.1% less than the recommended amount in Hubei province of High- and Middle-K soil(Avail. K>100 mg/kg). Whereas for Low-K soil(Avail. K<100 mg/kg), the current recommended amount of K-fertilizer was deficient. In High-K soil level, the application of K fertilizer and straw returned could increase crop yield and K uptake, especially for rapeseed after one crop rotation. The straw incorporation with K-fertilizer treatment also got the best effect compared with CK treatment. Straw incorporation had no significant impact on the KfREs of rice and winter oilseed rape, but increased the KfAE of rapeseed. Compared with the KfE without straw incorporation, both the KRE and KAE of rice and rapeseed decreased. Under the condition of continuous straw returned, the optimal amounts of K fertilizer for rice and rapeseed could reduce 42.2% and 31.2% compared to the recommended amount. Herein, under the high soil K level condition, straw potassium can not only replace partial chemical potassium and get a higher yield, but also improve soil available K, maintain the soil system in nutrient balance, and assure the virtuous circle of straw potassium resources.(4) In situ AFM observations indicate that the dissolution of biotite from the biotite(001) surface occurred on the terrace, segment, and fringe of pits. The dissolution rates of terraces and the release rates of K, Si and Al could be greatly accelerated with the help of citric acid, but both of them would decrease as the p H increasing. In p H 4.0 citric acid solution, 70 min dissolution reactions of biotite(001) surfaces result in more etch pits than in p H 6.0 and 8.0 solutions. For a long reaction time, in acidic aqueous solution, 4.0 in p H, 24 hours of incubation left some tiny etch pits, about 0.1~0.9 nm in depth, on the surface of biotite(001), making the surface rough. After 96 h of incubation, pits became more apparent, averaging 0.957 nm in depth, and more in number, but only covering 4.8% of the surface. After 140 h, an unstable coating precipitated on the surface, hindering the process of dissolution. However, in citric acid solution(CA) 4.0 in p H, after 24 h of incubation, large numbers of etch pits appeared on the surface of biotite(001) and dissolution of the surface layer became apparent. After 48 hrs, the surface terrace dissolution rate of the biotite increased significantly, with dissolved area reaching 48.7% of the surface. And 140 hrs later, circular spallings were observed on the(001) surface, leading to breakage of the layer(1~2 nm in thickness) into fragments and promoting further rise in dissolution rate. In CA solution containing Na+, rising Na+ concentration increased the dissolution rate of the surface and was a secondary coating was formed, too. Meanwhile, K+-Na+ ion exchange on the solid-fluid interface was enhanced and spalling in the(001) surface structure, about 2~10 nm in depth became more and more apparent. With the incubation going on(140 hrs), biotite expanded in depth(~20 nm), causing cracks in the perimeters(0.1~1.9 nm in depth) and eventually zonal hydration of the surface microstructure, and formation of hydrous mica(illite) on the surface of the biotite. Therefore, in the complex rhizospheric environment, on the earlier stage, organic acids could promote the mineral dissolution and element release rates, however, on the later period the secondary coating would hinder the dissolution process. In the consideration of long-term soil management, this process has a positive impact on maintaining the stability of soil structure.