Phosphorus Characteristics Of Biochars Derived From Phosphorus-Enriched Plant And Its Effect On The Replenishment Of Soil Phosphorus

Due to long-term excessive application of phosphate fertilizer,environmental problems such as increasing shortage of phosphate rock resources,surplus of soil phosphorus（P）,and eutrophication of water bodies have been caused.Phytoremediation is an effective way to manage environmental P surpluses.P-enriched plants are a kind of remediation plants that can effectively extract excess P from the environment,but the remediated P-enriched plants have not yet been recycled and utilized in a reasonable and suitable way.Therefore,in this study,Polygonum hydropiper and Polygonum lapathifolium,typical P-enriched plants selected by our group,were used as raw materials for the preparation,and rice straw was used as control material.P-enriched plants were converted into biochars by pyrolysis technology.Based on clarifying the basic material characteristics and P characteristics of P-enriched plant biochars at different pyrolysis temperatures（300℃,400℃,500℃,and600℃）,the effects of P-enriched plant biochars on soil P remediation and related regulatory mechanism were further explored through indoor constant temperature culture experiments and cabbage pot experiments.This study provided a theoretical basis for resource utilization of P-enriched plants and efficient recycling of P resources.The main findings of the study are as follows:（1）The P-enriched plant biochars had typical reticulated pore structures and were alkaline mesoporous materials.As the pyrolysis temperature increased,the yield of biochars derived from P.hydropiper（PHBC）,P.lapathifolium（PLBC）and rice straw（RSBC）decreased significantly,and the p H,ash,conductivity and specific surface area increased gradually.Under high temperature pyrolysis conditions（500℃～600℃）,the carbon morphology of PHBC and PLBC developed towards stable aromatization,with a gradual decrease in polarity and hydrophilicity,and a decrease in the strength of oxygen-containing functional groups.Compared to RSBC,PHBC and RSBC also possessed more abundant functional groups and inorganic minerals,such as P-O,P-O-P,PO₄^3-,and the formation of Ca₃（PO₄）₂ crystals.（2）The P-enriched plant biochars had higher P content,P recovery and P enrichment factor.The P content of PHBC and PLBC was 2.89～3.55 and 2.45～2.68 times higher than that of RSBC,respectively.The highest P content of PHBC was 15.63 mg kg^-1 prepared at600°C.As the pyrolysis temperature increased,the P concentration effect occurred in the P-enriched plant biochars,and almost all the P in the biomass were retained in the biochars,with the organic P chain-breaking and phosphate monoester disappearing,and the inorganic orthophosphate strength significantly increasing.Na HCO₃-P,HCl-P and Res-P were the main P fractions in PHBC and PLBC,indicating that P in P-enriched plant biochar had high effectiveness and stability.In addition,Ca₃（PO₄）₂,P₂O₇^4-and H₃PO₄ analogues were detected in the P-enriched plant biochars,indicating a strong P slow-release capacity.（3）The application of P-enriched plant biochars provided a direct source of P to soil,and the content of total and effective P for the soil increased significantly with increasing application rate（p<0.5）.P-enriched plant biochar prepared at 500℃had the greatest effect on soil available P.The soil available P content reached 137.97 mg kg^-1 after application of4%PH500,which was 39.42 times that of the original soil available P,and the soil P activation capacity was greatly improved.The P-enriched plant biochars drove soil P fractions transformation,significantly changing the distribution characteristics of soil P forms.Under the treatment of PHBC,soil P mainly consisted of Na HCO₃-P,Na OH-P and Res-P,accounting for 84.02%of total P in soil,and under the treatment of PLBC,soil P mainly consisted of HCl-P,Na OH-P and Res-P,accounting for 88.53%of the total P in soil.In addition,the application of P-enriched plant biochar can effectively replenish soil nutrients such as nitrogen,potassium and calcium,effectively alleviate soil acidification and increased soil phosphatase activity,which had a good potential for recharge.（4）Application of P-enriched plant biochars improved the biological effectiveness of soil P by providing available P,enhancing soil phosphatase activity,increasing soil microbial biomass carbon,nitrogen and phosphorus,and alleviating soil acidity.Compared with the treatment without biochar application,the yield of Brassica chinensis significantly increased after applying three types of biochars.The growth of Brassica chinensis was basically the same under the treatment of RSBC and PLBC,and the replenishment effect of PHBC was the best,which was more conducive to Brassica chinensis absorbing P from soil and transporting it to the above-ground parts.The yield of Brassica chinensis reached the maximum of 60.49 g pot^-1 after application of 1%PH500,which was 21.15 times of that without application of biochar.Under this treatment,PHBC can meet the P demand for the growth of Brassica chinensis,and no additional P fertilizer was needed.