Effects Of Biochar Combined With Phosphate Fertilizer On Soil Nutrient Availability And Cotton Growth

The lack of phosphorus（P）mineral resources in China and low efficiency of soil P utilization in farmland have restricted the increase in crop yield.The deficient in P nutrition of cotton will also lead to serious yield reduction.As a new type of soil amendment,biochar has attracted much attention due to its great potential in improving soil fertility,increasing crop yield and soil organic carbon sequestration.The effects of biochar on soil nutrient availability and crop yield are closely related to the application of chemical fertilizer.However,the combined effects of biochar produced from crop straw with P fertilizer on soil P supply and cotton growth is still unclear.At the same time,the difference between straw biochar and straw in impacting soil nutrient availability is not fully understood.To answer this question,a pot experiment was carried out at the Pailou research station of Nanjing Agricultural University in Nanjing,Jiangsu Province from 2019 to 2020.In this experiment,CCRI 425 was used as the cotton variety,and two P fertilizer levels of 0kg P₂O₅hm^-2and 200 kg P₂O₅hm^-2were set（0 g/pot、18.52 g/pot）.Two wheat straw biochar rates（5 t/hm² and 30 t/hm²,0.056 kg/pot、0.333 kg/pot）,two wheat straw rates（5 t/hm² and 15 t/hm²,0.056 kg/pot、0.167 kg/pot）and one control without straw or biochar（CK）were compared at each P application level to forming a total of 10 treatments.The goals of this study were to determine the responses of cotton root morphology,growth and yield,as well as soil nutrient content and enzyme activities related to soil carbon and P conversion to the combined application of straw or straw biochar and P fertilizer;and to clarify the effects of straw biochar on soil nutrient availability and cotton growth by comparing the response differences between straw biochar and straw treatments.The study should provide a theoretical basis for the rational application of biochar in agricultural production and for increasing cotton yield and reducing chemical fertilizer input.The main results are as follows:1.Effects of biochar combined with phosphate fertilizer on cotton growth and yieldRegardless of P application,the addition of straw biocharat the low rate（5 t/hm²）significantly increased total root length,total root surface area,biomass,P uptake of cotton and seedcotton yield from 2019 to 2020.Compared with the control,the seedcotton yield was increased by 11.1%averagely over the two years when straw biochar was added at the low rate.Adding straw biochar at a high rate（30 t/hm²）also promoted the cotton growth,and increased seedcotton yield averagely by 7.1%,which was significantly lower than that of straw biochar added at a low rate.The impacts of straw addition at a low rate（5 t/hm²）on cotton growth and yield were similar to those of the low rate of straw biochar treatment,but high rate of straw treatment（15 t/hm²）negatively affected cotton growth and significantly reduced cotton yield.Straw biochar and straw combined with P fertilizer had certain interaction effects on cotton yield,root morphology and biomass.The combined application of biochar or straw with P fertilizer decreased the beneficial effects on cotton growth and yield over single application of biochar or straw.Correlation analysis showed that seedcotton yield significantly positively correlated with total root length,total root volume,root P accumulation and root biomass,among which seedcotton yield had the closest relationship with total root length and root P uptake.This indicates that the increase in cotton yield induced by straw biochar or straw could be greatly attributed to the improvement of root morphology and then the promotion of phosphorus absorption.2.Effects of biochar combined with phosphate fertilizer on soil fertilityThe responses of soil pH,available P,available K and organic matter contents to straw biochar and straw treatment were basically the same.No matter whether the application of P fertilizer or not,they were significantly increased under straw biochar and straw treatment,and the increments of straw biochar treatment were higher than those of straw treatment.The increments of high rate of biochar or straw were greater than those of low rate of biochar or straw.Soil dissolved organic carbon content also increased with biochar and straw treatment,but the increase magnitud of straw biochar treatment was significantly lower than that of straw treatment.Soil available nitrogen content changed greatly in response to treatments at different growth stages of cotton,and low rate of straw biochar inhibited soil available nitrogen content in some stages.Straw biochar and straw combined with P fertilizer had certain interaction effects on soil p H,available nitrogen,available P,available potassium,organic matter and dissolved organic carbon contents at different growth stages of cotton.Among them,biochar combined with P fertilizer had significant interactions on soil available P content in two years and three periods.Straw biochar or straw combined with P fertilizer had a great effect on improving soil available nitrogen and organic matter contents,but the effect of single application was decreased.Correlation analysis showed that soil available N,available P,available K,organic matter and soluble organic carbon were significantly positively correlated with P accumulation and total root length of cotton.Soil available N,available P,available K and organic matter were significantly positively correlated with cotton seedcotton yield.Among them the soil available P content had the most close relationship with seedcotton yield.These results indicated that straw biochar or straw improved soil nutrient supply,which had a great relationship with the improvement of root morphology,the promotion of nutrient absorption and the increase of cotton yield.3.Effects of biochar combined with phosphate fertilizer on soil carbon and phosphorus conversionRegardless of P application or not,straw biochar significantly increased soil microbial biomass P content,alkaline phosphatase activity,cellulase activity and sucrase activity,and the increase high rate of biochar was higher than low rate of biochar.Low rate of biochar had little effect on soil microbial biomass carbon.Straw biochar treatment at two dosage levels significantly reduced soil acid phosphatase activity,while high rate of biochar reduced soilβ-glucosidase activity.Straw treatment significantly improved the soil carbon and P conversion indexes.In general,compared with low rate of straw treatment,the soil treated with low rate of biochar had lower the contents of soil microbial biomass carbon and dissolved organic carbon,and the activities of soil sucrase,cellulase andβ-glucosidase.This indicated that the effect of straw biochar on soil carbon conversion was less than that of straw.Compared with the high rate of biochar,the soil treated with high rate of straw had higher alkaline phosphatase activity.Correlation analysis showed that,except for soil pH,soil nutrient indexes were significantly or extremely significantly positively correlated with soil C and P conversion related enzymes,indicating that soil microecological environment was sensitive to the changes of soil nutrient supply induced by biochar and straw,and biochar and straw played an important role in soil C and P conversion.In conclusion,a moderate amount of straw biochar can significantly promote the growth of cotton,and the effect is equal to that of straw under short-term addition.The single application of biochar promoted the growth and yield of cotton greatly,while the combined application of biochar reduced the effect.The beneficial effect of biochar on yield was closely related to the improvement of soil P and K nutrient availability,and the improvement of nutrient availability was related to the microbial biomass and enzyme activities of soil carbon and phosphorus conversion.