Effect Of Milk Vetch Planting And Its Straw Application On Potassium Availability Of Paddy Soil

More attention should be paid to potassium（K）availability of paddy soil in south China under the condition of K deficiency and leaching.As the most common green manure in paddy soil of southern China,the advantages of milk vetch on soil carbon,nitrogen and phosphorus fertility have been widely recognized.However,studies on the effect of milk vetch as green manure on soil K fertility are controversial.In fact,there are few in-depth studies on the effects of milk vetch as green manure on K fertility in paddy fields,which just make a simple discussion on the changes of soil available K content due to the straw application of milk vetch.In addition,the effect of milk vetch planting on K supply capacity of paddy soil is rarely mentioned.In this paper,by studying the different K forms,the ability of soil K release and K fixation,the potassium Q/I curve and K-bearing minerals of field experiments or indoor simulation experiment,we discussed the ability of K absorbing of milk vetch,how milk vetch planting influence the K availability,the impact of long-term straw application of milk vetch and the NH₄⁺that significantly increased in this process on the K availability of paddy soil,and make clear the effect and mechanism of milk vetch as green manure on the K supply capacity of paddy soil,to provide theoretical basis for improving K availability of paddy soil in southern China.The main results are as follows:（1）In the pot experiment,illite was used as the K nutrition source of growth,then the milk vetch grew well and had developed roots,and the K content of illite decreased by14.0–17.8 g/kg,and the layer structure of illite was destoried,indicating that milk vetch is able to absorb the K of illite when planting.In the field experiments,compared with the soil before planting,the reduction of soil non-exchangeable K（NEK）and lattice K（LK）of low K fertility treatment was 85 mg/kg and 50 mg/kg,respectively after one growing season of milk vetch planting,which was more than the high K fertility treatment and close to high K,better integrate soil fertility treatment.The above results indicated that the milk vetch root system has a strong ability of soil K uptake.（2）After short-term（5 years）milk vetch as green manure treatment,the contents of NEK and LK in the subsurface paddy soil（10–25 cm）were 488.1mg/kg and 8.77g/kg,respectively,which were significantly lower than the the no-milk vetch treatment（CK）.Also,the maximum release of available potassium in the subsurface of paddy soil was 1989 mg/kg,which was lower than the CK.The content of NEK and the maximum release of available potassium in topsoil（0–10 cm）of paddy soil were 761.2 mg/kg and 2504 mg/kg,respectively,which were significantly higher than the CK.The above results indicated that in the short-term milk vetch as green manure system,the milk vetch planting mainly absorbs the K in the subsurface of paddy soil.（3）The LK and total K content of the milk vetch treatment（MV）in topsoil（0–10 cm）of paddy soil was 8.65 g/kg,which was significantly lower than that of the winter follow treatment（WF）.The NEK in topsoil was significantly higher than that of WF,which was104.8mg/kg.Compared with the WF,the diffraction peaks of feldspar and mica of MV weakened significantly.However,the content of illite was significantly higher than that of WF,increased from 40%to 47%.The potassium Q/I curve showed that,compared with WF,the equilibrium K concentration ratio（CR_K⁰）and critical exchangeable K（EK^r）were lower,which were 0.0035（mol/L）^1/2 and 0.0938 cmol/kg,respectively.The potential buffering capacity of NEK（PBKⁿ）was 6.67 cmol/kg/（mol/L）^1/2,with an obvious increasing trend compared with WF.The magnitude of the conversion of added K to nonexchangeable pool（β）was increased from 9.98%to 14.35%.The above results indicated that after long-term straw application of milk vetch,more added K⁺would be stored in the NEK sites of illite（4）Indoor simulation data showed that,NH₄⁺can promote the release of NEK of illite.Further studies showed that with the temperature increasing from 20℃to 40℃,the average rate of illite potassium releasing under the conditions of concentration of 0.01,0.1 and 1mol/L NH₄⁺was increased from 79.1,82.3 and 87.3 to 113.1,114.4 and 123.0 mg/kg/h,respectively.The promotion effect of NH₄⁺on NEK release between illite layers will improve the bioavailability of NEK of paddy soil.In conclusion,the milk vetch has a strong ability of soil K absorbing.In a short-term,the milk vetch could absorb the NEK and LK in the subsurface of paddy soil and store in the straw during planting period,then returning them back as dissolved K⁺during the decomposition of straw,thus improving the K availability in topsoil of paddy soil.In a long-term,the LK and NEK in topsoil of paddy soil could be activated by the cumulative effect of straw decomposition process,so as to improve the K availability in topsoil of paddy soil.In addition,as a nitrogen fixation crop,the long-term milk vetch as green manure treatment can also promote the release efficiency of NEK by significantly increasing NH₄⁺.It is clear that both the milk vetch planting and its straw application have a positive effect on the K supply capacity of paddy soil.The results provided a theoretical basis for improving K availability of paddy soil in southern China.