Decomposition Dynamics Of The Mixture Of Chinese Milk Vetch And Rice Straw And Their Influences On Soil Nutrients And Greenhouse Gas Emission

The release of nutrients from plant residues is critical to the sustainability of agricultural systems and that can meet the requirements of microorganisms and plants for carbon and nutrients.The continuous mineralization from plant residues is a major source of available nutrient for plants in the soil.The addition of plant residues to the soil also affects the carbon cycle and its stock,thereby increasing the emission of greenhouse gases（CO₂ and CH₄）from soil to the atmosphere,which contributes to the increase of global warming.The type of plant residues,their initial content of nutrients,mixing ratios,incorporation time,availability of soil water,and the temperature take control of the release of nutrients from plant residues.The chemical and physical properties of soil may also affect the pattern of nutrient release and mineralization from plant residues and the emission of greenhouse gases from soil.To understand how nutrient release from plant residues,dynamics of mineralization in the soil,effect on the carbon cycle,and the emission of greenhouse gases from soil,three incubation experiments were carried out in two different soils,Red paddy soil and Fluvo-aquic paddy soil,under laboratory conditions for 180 days.Two types of plant residues were used as singular or mixed species:1)Chinese milk vetch（Astragalus sinicus L.）,2)Rice（Oryza sativa L.）straw.The mixing ratios of plant residues were 100%:0,75%:25%,50%:50%,25%:75%,and0:100%of Chinese milk vetch and rice straw,respectively(as M₁₀₀,M₇₅S₂₅,M₅₀S₅₀,M₂₅S₇₅,and S₁₀₀,respectively).The first experiment was done in the glasshouse using litterbag method to determine the rates of decomposition and release of nutrients from plant residue.There were seven sampling times（1,5,15,30,0,120,and 180 days）from flooding soil（Red paddy soil）.Results showed that the decomposition of plant residues mostly occurred in the early stages of incubation and the average cumulative decomposition of residues during the first month of incubation were 8.7,4.1,58.,50.3,and 42.4%for M₁₀₀,M₇₅S₂₅,M₅₀S₅₀,M₂₅S₇₅,and S₁₀₀00 treatments,respectively.The decomposition rate and nutrient release rate were directly based on the initial content of nutrients in plant residues.Also,the high rate of release of nutrients occurred in the early stages of incubation.The highest rates of cumulative release of carbon,nitrogen,and phosphorus were from milk vetch M₁₀₀00 treatment followed by mixed treatments M₇₅S₂₅,M₅₀S₅₀,M₂₅S₇₅5 and then pure rice straw S₁₀₀.The second experiment was conducted in order to determine the effects of the type of residue,the mixing ratios,and the soil properties on mineralization rates and soil fertility.Plant residue was added to flooding soil at 2%and there were five times for sampling（1,15,30,0,and 180 days）,then the concentrations of nutrients were measured.Results showed a decrease in the total carbon and nitrogen contents over time in all treatments.Mineralization of N was based on the initial N content in residues,where mineralization increased by increasing nitrogen in residues.Phosphorus mineralization was affected by soil properties and there was an increased concentration in the red paddy soil over time and decreased or steady in fluvo-aquic paddy soil.For potassium,the highest release was from rice straw and the lowest was from milk vetch treatment in the two types of soil.The concentration of nutrients（TC,NH₄,NO₃,P,K）in the flooded water was measured and the results showed all concentrations were low except potassium.The objective of the third experiment was to determine the effect of plant residue decomposition on the emission of methane and carbon dioxide from the soil.2 g plant residue was added to 100 g flooding soil and there are nine sampling times（1,3,5,10,15,30,0,120,and 180 days）.The results showed a significant increase in CH₄ and CO₂ emissions due to the addition of plant residues compared to the untreated soils.The type of residues and the initial characteristics of the soil also affected the pattern and the cumulative emission rates.In red paddy soil,the highest cumulative CO₂ emission rate at the end of the incubation period was from M₁₀₀（5781.7 mg CO₂–C kg^-11 soil）,while in fluvo-aquic paddy soil the highest rate was from S₁₀₀00 treatments（551.7 mg CO₂–C kg^-11 soil）.The highest cumulative emission rate for CH₄was from rice straw treatments S_100,while the lowest cumulative emission was in treatment M₅₀S₅₀0 in both types of soil.Overall,results showed mixing of residues reinforce the decomposition of rice straw and the release of nutrients;Mineralization pattern of nutrients was affected by initial properties of residues and soil;The mixing ratio of residues（50:50）reduced methane emission from soil.