Effects Of Biochar Addition On Soil Phosphorus Availability And Cotton Phosphorus Uptake

Phosphorus(P)is one of the nutrients necessary for the growth of plants,but most P in the soil is difficult to be absorbed and utilized by plants,and plants often grow slowly due to P deficiency.Although phosphate fertilizer can alleviate the lack of P in the soil,it is easily fixed by soil minerals to form insoluble P.The characteristic of soil P Results in the lower availability of P,and excessive application of phosphate fertilizer can easily cause water eutrophication.Biochar as an amendment from organic waste,due to its diversity of nutrient elements and special porous structure,can be added to the soil to change the supply of soil P,affecting plant growth and P uptake.Up to now,multiple studies have shown that biochar can increase the content of soil available P.However,due to differences in soil properties,biochar characteristics and management conditions,the influence degree of biochar on soil available P and plant P content varies greatly among studies.The combination of nitrogen fertilizer and biochar,as the main management measure,is how to affect the availability of P,which is not clear.A meta-analysis was conducted with 507 data from 95 eligible literatures in this study to quantitatively assess the response of plant P absorption,soil available P and soil phosphatase to biochar addition,and then by grouping and regression analysis,the relationship between the response differences and factors from different biochar characters(raw material,C:N ratio,pyrolysis temperature,application rate),soil traits(texture,pH,organic carbon content),and fertilizer application was reveal.Meanwhile,the pot experiment of wheat straw biochar addition was conducted at the Pailou Experimental Station of Nanjing Agricultural University,Nanjing,Jiangsu Province from 2016 to 2018.The cotton cultivar is CCRI50.Two biochar addition levels(0t ha-1,15 t ha-1)and 4 nitrogen application levels(0,75,150,300 kg N ha-1)was set.The aims of studying the impact of biochar on cotton P uptake,soil available P content,phosphatase activity,microbial biomass P and the transformation of P fractions were to provide a theoretical basis for the improvement of soil fertility and the rational utilization of straw biochar.The main findings are as follows:1.Biochar addition improves soil P availability:A meta-analysisThe results of Meta-analysis revealed that irrespective of biochar characters(raw material,C:N ratio,pyrolysis temperature,application rate),soil traits(texture,pH,organic carbon content),and fertilizer application,biochar addition significantly improved the average soil available P content by 57.6%over control.Meanwhile,biochar addition promoted the P utilization of crops,but the response ratios of plant P concentration of biochar under different adding conditions were almost lower than those of soil available P and even were not significant under some conditions.This average response ratio of plant P concentration was 30.6%.The biochars,derived from livestock manure,low-temperature pyrolysis,with lower C:N ratio,alkaline,or higher application rate,were relatively more effective to upsurge soil available P content and plants P concentration in sandy and loamy soils.For main enzymes impacting P cycle,the average activity of alkaline phosphatase improved(2.8%)whilst the acid phosphatase activity decreased(17.8%).Overall,biochar addition had a significant positive effect on soil available and plant P concentration,but had a minute effect on soil phosphatase.The improvement in soil P availability might mainly be ascribed to a great amount of active P fractions carried by biochar itself.2.Effects of biochar and nitrogen application rate on soil P availability and cotton P absorptionUnder the four nitrogen application levels,the addition of biochar significantly increased the P uptake of cotton by 3.8%-11.6%,and the response of cotton plant P uptake to biochar weakened with the increase of the application period.There was no significant influence in the third year.The interaction effect between biochar and nitrogen application rate on the P absorption by cotton was significant only in the reproductive organs,not in other parts.The addition of biochar increased soil pH,organic carbon content,C:N ratio,phosphatase activity,and microbial biomass P,but decreased soil available nitrogen content.These changes in soil properties caused a significant increase in the content of soil active P and intermediate active P,and a significant decrease in the content of stable P.The effects of biochar and nitrogen application rate on soil active P were significant,but the medium active P and stable P were significantly affected,and the trend with the year forward of biochar was consistent with that of cotton P uptake content.To sum up,the Meta-analysis and pot experiments have shown that biochar can significantly increased the soil available P content and the plant P content,which contributed to the improvement of plant biomass.However,it should be noted that the effect of biochar on increasing the soil available P content and the plant P content was very different under different types of biochar,soil conditions and management measures.When biochar is utilized to improve soil fertility,the above factors should be considered comprehensively,so as to add more efficiently biochar to soil in a targeted manner,in order to achieve the purposes of efficient and sustainable utilization of soil P and improvement of crop yields.