Insight Into The Mechanism Of Low Molecular Weight Organic Acids-mediated Release Of Phosphorus And Potassium From Biochars

The application of biochar in soil improvement and plant growth promotion partially depends on the direct release of nutrients from biochar.Low molecular weight organic acids(LMWOAs)are ubiquitous in the environment.When biochar is amended into soil,the release of nutrients from biochar will be inevitably affected by LMWOAs.The mechanism of LMWOAs–mediated release of nutrients from biochars remains little known.In this study,a series of biochars with different properties derived from bamboo,corn straw,and wheat straw were produced in air-limitation and nitrogen-flow atmospheres at various pyrolysis temperatures(300-750°C).Firstly,the effects of feedstock types and pyrolysis conditions on the retention and release of nutrients in biochars were investigated;secondary,the release of phosphorus and potassium from different types of biochars was performed in four different LMWOAs(acetic,glycolic,tartaric and citric acid)solution,according to the p H changes of the solution,we systematically enunciated the influence mechanism of LMWOAs and biochars properties on the release of phosphorus and potassium from biochar.The main results are listed as follows:(1)Firstly,the types of biomass posed a significant effect on the content and retention and release of nutrients including K,Ca,Mg and P in biochars,and the retention and release of nutrients in biochar increased with the content of nutrients in their corresponding biomass,the content and retention of nutrients in biochar followed the order:wheat straw derived biochar>corn straw derived biochar>bamboo derived biochar;secondary,biochars pyrolyzed in different pyrolysis temperature and pyrolysis atmospheres(nitrogen-flow and air limitation)had an insignificant effect on the retention of nutrients.But low pyrolysis temperature supported the release of nutrients from biochar,and the release of P from biochar produced in air-limitation environemnt was higher than that from nitrogen-flow environment;finally,the low p H promoted the release of P and K.(2)Acetic acid suppressed the release of P from biochars,and glycolic acid showed a negligible effect on the release of P from biochars,while tartaric acid and citric acid facilitated the release of P from biochars produced in nitrogen-flow atmosphere at high pyrolysis temperatures(biochars with high atomic ratios of polyvalent metals to P).Firstly,the deprotonated LMWOAs combined with the dissolved HPO₄^2-or H₂PO₄^-to form a complex through the hydrogen bond and cation bridging.The hydrophobic methyl group of acetic acid was exposed outside of the complex,which decreased the water-solubility of phosphate.Glycolic acid,tartaric acid,and citric acid contain hydrophilic hydroxyl groups,which can stabilize the complexes in the aqueous solution.Secondary,biochars had a high ratio of polyvalent metal to P,which indicates insoluble P-containning minerals;tartaric acid and citric acid contain more than two carboxyl groups,which formed multivalent anions after dissociation,the multivalent anions of LMWOAs can exchange with P bound with polyvalent metals in biochar,thereby promoted the release of P from biochar.(3)Different types of LMWOAs posed a similar effect on the release of K from biochars,and LMWOAs promoted the release of K from biochars produced at low pyrolysis temperatures.Biochars produced at high pyrolysis temperature had a high K/(P+S)ratio,most of K⁺were not wrapped or covered by P or S containing minerals and could be directly released,thus,the release of K⁺from these biochars was less affected by LMWOAs.However,biochars produced at low pyrolysis temperature had a low K/(P+S)ratio,most of K⁺ions were wrapped or covered by P or S containing minerals and and was not able to release directly.The undeprotonated carboxyl group in LMWOAs served as a hydrogen bond donor to displace K⁺wrapped or covered by the P or S containing anions and promoted the release of K.This study systematically elucidated the influence of preparation conditions on the retention and release of nutrients in biochar and the mechanism of LMWOAs-mediated release of P and K from biochars.The results provide theoretical support for optimizing the application of biochar in improving soil fertility,having an important practical significance in the agricultural application of biochar.