Effects Of Different Biochar On The Distribution Of Reddish Paddy Soil Micro-aggregates And Colloidal Phosphorus Concentration

Aggregates are a crucial soil structural unit that promotes the aggregation of large aggregates and increases soil structural stability,thereby maintaining soil fertility and improving soil productivity.In this 90-day indoor incubation experiment,we investigated the effects of adding two types of biochar made from chicken manure（MBC）and rapeseed straw（SBC）to two types of paddy soil with different phosphorus（P）levels（high P and low P）at mass ratios of 0%,2%,4%,and 8%.We studied the impact of biochar addition on the distribution characteristics and stability of soil aggregates,the P content of aggregates of different sizes,and the release of soil colloidal P.Additionally,we examined the influencing factors of soil colloidal P release by considering aggregate stability and soil P content.The main research results were as follows:（1）The addition of both types of biochar decreased the aggregate with size<53μm,and increased the mean weight diameter and geometric mean diameter,thus had a positive effect on improving the stability of soil aggregates.The addition of biochar had stronger effect on the aggregation of low P soil.The fraction of<2μm particles decreased the most by 15.94%-41.01%,which mainly promoted the aggregates of<2μm size to form larger size aggregates.Both MBC and SBC increased soil calcium,magnesium and dissolved organic carbon（DOC）,and the content of dissolved organic carbon increased as high as338.27%,which promoted the formation of soil large aggregates.（2）The total P content and total P storage of aggregates>250 and 250-53μm increased significantly under the biochar treatment.Adding biochar increased the P contribution rate of larger aggregates（>53μm）,but decreased the P conteibution rate of20-2μm aggregates.Therefore,the addition of biochar promoted P allocating in larger aggregates（>53μm）.The P content of soil increased with the increase of biochar addition.Among the two types of biochar,MBC had a stronger effect on increasing P content in the two kinds of soil than SBC.Compared with the high P soil,the contents of low P soil changed greatly under the treatment of MBC additon,with the contents of H₂O-P and KCl-P increasing by 395.8 times and 153.1 times,respectively.For the soil with low P level,MBC could be used as an adequate external P source.（3）The contents of water-dispersible total P（WTP）and colloidal P（CP）increased under the condition of biochar addition,and the contents increased more significantly under the addition of MBC.Among them,there was a significant difference in WTP among treatments with different levels of MBC,with 8%MBC treatment being 315.8 times（low P soil）and 49.8 times（high P soil）higher than the control treatment,while the WTP of high P soil increased only 1.2 times under 8%SBC treatment.After adding MBC,the content of colloidal P in soil increased,but the CP/WTP ratio decreased,the contribution rate of colloidal P to water-dispersible total P decreased,and the addition of MBC contributed more to the dissolved P in soil.Biochar mainly affects the content of inorganic P in soil.Under the condition of adding MBC,the main form of P in water-dispersed soil solution and dissolved component solutions is molybdic acid reactive P（MRP）.The maximum content of MRP in water-dispersed soil solution of high P soil is 25.79 mg·kg^-1,while the content of molybdic acid unreactive P（MUP）is only 1.89 mg·kg^-1.（4）With the addition of biochar,the stability of soil aggregates was improved,but the content of soil colloidal P did not decrease.The high addition level of high-P biochar（such as MBC）could bring a large amount of P into the soil,resulting in the supersaturation of soil P,promoting the release of colloidal P,and increasing the risk of soil P loss.When applying biochar to soil,it is necessary to comprehensively consider the properties of soil and biochar,to avoid excessive phosphorus brought by biochar and water eutrophication.