Effects Of Summer Green Manure Plant-returning On Soil Phosphorus Of Fallow Farmlands

Phosphorus?P?is one of the essential nutrients for crop growth,but because it's easily immobilized in soil by sorption and precipitation with calcium,iron,aluminum and other cations,the lack of soil available P is an important factor to limit plant growth.Phosphate fertilizer is generally used to supplement soil P in farmland,however,the low efficiency of phosphate fertilizer and risks of the resource depletion,also with eutrophication caused by its excessive application,making the P management difficulty.Improving the availability of P in soil may be key to balance the reduction of phosphate fertilizer application and crop yield.Green manure is generally planted for land conservation as a fallow solution.Returning of green manure plants can improve the soil P availability to a certain extent,but need further study,especially the effect of field management after plant-returning.At present,farmlands in Shanghai plant rice in summer,then followed by winter green manure.For the farmlands need long-term fallow,the effect of summer green manure on soil nutrients also needs to be further studied.In this study,both field experiments and pot studies were conducted with common paddy soil from Fengxian,Shanghai.The decomposition of soybean?Glycine Max?Linn.?Merr.?,sweet potato?Ipomoea batatas?L.?Lam.?,mung bean?Vigna radiata L.?and weeds?mixture of Leptochloa chinensis?L.?Nees and Ammannia baccifera L.?as summer green manure and their effects on soil P availability were compared.The underlying mechanisms of soil P availability affected by the four species with different water conditions?60%field water-holding capacity,100%field water-holding capacity,flooding?were explored,also with their influences on growth of succeeding crops.The results are expected to provide some theoretical basis for the P management of summer fallow farmland in Shanghai.The main findings are as follows:?1?The results of field experiments showed that the decomposition and nutrient release of different plants were different,and thus affected the availability of soil P.Soybean,sweet potato and mung bean were all decomposed rapidly in the early stage and then slowed down in late time after returning,while weeds decomposed slowly but evenly all the way.After 182 days of decomposition,sweet potato was decomposed the most thoroughly with the greatest P release,respectively reached 73.44%and 86.66%.Plant-returning also affected the soil nutrient.Soybean increased soil available P most effectively,increased about 4.42 mg/kg,while mung bean increased soil alkaline phosphatase activity most significantly.Mung bean and weeds significantly reduced soil Ca₂-P and Fe-P,but increased the accumulation of Ca₈-P,showing a great influence on soil P composition.Soybean,sweet potato and weeds maintained high availability of soil P,while mung bean decreased the availability of soil P.?2?Pot experiments showed that the effect of plant-returning on soil P availability and its mechanism were varied by water conditions.At 60%and 100%field water-holding capacity?relatively aerobic conditions?,available P of plant-returning soil gradually increased with time,and soil phosphorus with less availability(i.e.O-P,Ca₁₀-P,Fe-P)was gradually transformed to forms more available?i.e.Ca₂-P,Ca₈-P?.After plants returning for 182 days,the Ca₂-P and Ca₈-P ratios at 60%field water-holding capacity were higher,directly increasing the availability of soil inorganic P.The soil TP,TOC and Or-P ratios at 100%field water-holding capacity were higher,showing an increased potential pool for P availablility.Under these conditions,soybean-returning mainly increased the soil Ca₂-P ratio,while sweet potato decomposed quickly and released P rapidly in early stage.Mung bean-returning mainly increased soil Or-P ratio and alkaline phosphatase activity,while weeds continuously increased soil Ca₈-P ratio and alkaline phosphatase activity by maintaining slow decomposition.Plant-returning continuously affected DIP in soil solution and soil available P.The DIP in soil solution of sweet potato,mung bean and weeds reached the highest level after returning for about 1 month,while soybean did for about 45 days.The concentration of soil available P of mung bean and sweet potato reached the peak value after about 4 months returning at the relatively aerobic condition.The soil available P of soybean and weeds reached the highest after about 4 months returning at 100%field water-holding capacity,but continued to improve the soil available P after 4 months returning at 60%field water-holding capacity.The flooding environment resulted in relatively high DIP in soil solution.The fraction of Ca₂-P,Ca₈-P and Or-P were reduced while the ratios of O-P and Ca₁₀-P were improved.Soybean and mung bean-returning for more than 5 months while flooding may increase DIP in soil solution.Weeds-returning increased ratios of soil O-P and Ca₁₀-P,which are difficult to be used by plants,thus effectively immobilized soil P.?3?The growth of the succeeding soybean indicated that 100%field water-holding capacity was more beneficial to its growth and P uptake,effectively promoting pod development.60%field water-holding capacity was more helpful to the development of stalk and leaf,while flooding was better to root development.Under all field water-holding capacity conditions,soybean-returning was shown to benefit the growth of succeeding soybean most.Sweet potato-returning was beneficial to the development of pod,while weeds-returning to that of the root.?4?The growth of succeeding soybean represented the improvement of soil P,and the results of principal component analysis?PCA?showed that soybean and mung bean-returning at 100%field water-holding capacity during fallow was most beneficial to the improvement of soil P in all treatments,while weeds-returning under all the water conditions was poor.However,since weeds-returning could effectively immobilize P in flooding soil,it may be helpful in keeping P in paddy fields.It is also necessary to pay more attention to the weed control during the growth of succeeding crops.