Alkali-Enhanced Biochar Production And Its Potential As An Alternative Silicon Source

Silicon?Si?is increasingly recognized as a beneficial nutrient for crop growth and plays a pivotal role in prevention of plant disease.At present,conventional Si fertilizer includes wollastonite with limited reserves and high cost,and industrial by-product slag containing excessive heavy metals.The earth surface contains various types of Si-rich plants,however,whether these plants could be recycled as Si sources,and how to maximize their bioavailability are what draw our attention.The process of preparing biochar by plant feedstocks could transform and change the morphology and availability of numerous mineral elements.However,research of biochar Si availability is still rare and not deep enough.Therefore,bio-based silicon fertilizer was prepared,based on biomass pyrolysis and alkali pretreatment,to alleviate the shortage of silicon fertilizer and the defects of products performance,and conserve resources and reuse agricultural waste.Four Si-rich agricultural wastes such as rice straw,sugarcane harvest residue,miscanthus and switchgrass were selected to blend with alkali reagents?KOH,K₂CO₃,CaO?in different ratios and prepare alkali-enhanced bicohars at 350,450 and 550?,respectively.Based on physical-chemical properties and structural characteristics of alkali-enhanced biochar,the official method published by AAPFCO in 2013 was chosen to evaluate plant available Si content of alkai-enhanced bicohar prepared at different condition and treatments.Simultaneously,wollastonite,the conventional Si fertilizer,was chosen as a Si standard.An indoor incubation experiment was conducted to investigate the effect of alkali-enhanced biochars amendent on soil pH improvement and plant available Si content.At the same time,two pot studies were conducted to verify the effect of Si uptake by Si-accumulator plants,perennial ryegrass and rice,after the amendment of alkai-enhanced biochar,moreover,the effect on suppression of gray leaf spot development in perennial ryegrass was studied.The main results showed as follows:?1?Four Si-accumulator plants?rice straw,sugarcane harvest residue,miscanthus and switchgrass?were selected to prepare biochar at 550?,the research evaluated the effect of KOH modification on total Si content,plant available Si content and phytolith structure of biochars.The results showed that KOH pretreatment generally decreased the percentage of total Si in biochar.Among each feedstock-derived biochar,plant available Si content of KOH-enhanced rice straw biochar?RS-10KB treatment?,miscanthus biochar?M-10KB treatment?,sugarcane harvest residue biochar?SHR-5KB treatment?and switchgrass biochar?S-1KB treatment?reached the highest and were 4.67,2.13,2.20 and 0.85%respectively.The plant available Si content of KOH-enhanced rice straw biochar RS-10KB was significantly greater than wollastonite?WO?.Pyrolysis process and KOH treatment vigorously destroyed phytolith structure of biomass,therefore released more phytolith-Si.Rice straw,sugarcane harvest residue and miscanthus were screening as the best feedstocks for preparation of alternative Si source.?2?The research studied the effect of different alkali treatments?KOH,K₂CO₃,CaO?on physical-chemical properties,total Si content,plant available Si content.The results showed that with increasing ratios of alkali:biomass,total Si content of alkali-enhanced biochar decreased,and plant available Si content increased and then decreased.All alkali treatments could increase plant available Si content of feedstocks,and KOH showed the best efficiency followed with K₂CO₃ and CaO.Thus,for three feedstocks of rice straw,sugarcane harvest residue and miscanthus,KOH and K₂CO₃ were chosen as alkali treatments for preparing high-efficiency Si source.?3?The research evaluated the effect of different pyrolysis temperature?350,450,550??on total Si content,plant available Si content of rice straw and husk derived biochars.The results showed that plant available Si content of KOH and K₂CO₃-enhanced rice straw biochar and rice husk biochar?except 5K2B?increased with increasing pyrolysis temperature.During the 30 days incubation of alkali-enhanced biochar in weak acid solution and neutral saline solution,addition of K₂CO₃ promoted the release of Si in both solutions,and the increasing pyrolysis temperature accelerated the dissolution of Si in biochar.Generally,Si dissolution of rice straw biochar was greater than rice husk biochar.Therefore,550?was selected to be the best pyrolysis temperature for the preparation of alternative Si sources.?4?Based on the incubation experiment of two acidic soils?Commerce,Briley?and application rate of alkali-enhanced biochar?0,1,3%?,the research evaluated the effect of alkali-enhanced biochar?550??amendment on soil pH improvement,plant available Si content.The results showed that in acidic soils of Commerce and Briley,improvement effect of KOH-enhanced rice straw biochar?RS-10KB and 25KB?,miscanthus biochar?M-10KB and 25KB?,sugarcane harvest residue biochar?SHR-10KB and 25KB?on soil acidity were stronger than that of other biochar treatments,and the effect of low application rate?1%?of KOH-enhanced biochar?10KB and 25KB?was more appropriate than high application rate?3%?.Soil available Si content of KOH and K₂CO₃-enhanced rice straw,miscanthus and sugarcane harvest residue biochar?RS,M,SHR-10KB,25KB and 10K2B,25K2B?were significantly higher than wollastonite?WO?.It was found that 1%could be the suitable application rate of alkali-enhanced biochar in acidic soils,KOH and K₂CO₃-enhanced rice straw biochar?RS-10KB and 10K2B treatments?could be the better Si sources.?5?Based on ryegrass temperature control pot study of 550?prepared alkali-enhanced rice straw biochar?RS-0KB,10KB,10K2B,10CB?,wollastonite?WO?and calcium silicate slag?SL?amendment at different application rate?0,0.22,1%?,the research studied the effect of Si sources amendment on Si uptake and suppression of gray leaf spot development in perennial ryegrass.The results demonstrated that ryegrass Si content of KOH-enhanced rice straw biochar?RS-10KB?was equivalent to wollastonite?WO?and calcium silicate slag?SL?,and was significant higher than that of other biochar treatments,when Si sources application rate was 0.22%(5 t hm^-2).With application rate increasing to 1%,plant Si content of all biochar treatment increased by 8³4%.Alkali-enhanced rice straw biochar efficiently inhibited gray leaf spot development of perennial ryegrass.Compared to control,KOH,K₂CO₃,CaO-enhanced rice straw biochar?RS-10KB,10K2B,10CB?delayed the occurrence of gray leaf spot by 1² days at 0.22%application rate.When application rate increased to 1%,area under disease progress curve?AUDPC?of KOH and CaO-enhanced rice straw biochar?RS-10KB and 10CB?was reduced by 58 and 67%compared with control?CK?,repectively.?6?Based on rice greenhouse pot study of rices straw?RS?,550?prepared alkali-enhanced rice straw biochar?RS-0KB,10KB,10K2B,10CB?,wollastonite?WO?and calcium silicate slag?SL?amendment at different application rate?0,0.22,1%?,the research evaluated the effect of Si sources amendment on Si,P,K,Ca uptake and grain yield of rice aerial part.The results showed that Si amendment significantly increased rice stem,leaf and grain Si content,and were significantly greater than control?CK?.Rice stem and leaf Si content of KOH-enhanced rice straw biocahr RS-10KB increased by 10.54¹8.79%compared to wollastonite?WO?and calcium silicate slag?SL?.Stem and leaf P content of alkali-enhanced rice straw biochar?RS-10KB,10K2B,10CB?were significantly higher than control?CK?,rice straw biochar?RS-0KB?and calcium silicate slag?SL?.Stem and leaf K content of K₂CO₃,KOH-enhanced rice straw biochar?RS-10K2B and 10KB?increased by78.39 and 46.22%as compared to wollastonite?WO?respectively.Grain Ca content of CaO-enhanced rice straw biochar?RS-10CB?was 55.68 and 16.85%higher than control?CK?and wollastonite?WO?,respectively.The grain yield of alkali-enhanced rice straw biochar?RS-10KB,10K2B,10CB?were increased by 17.70²1.88%as compared to calcium silicate slag?SL?.?7?Based on the study of alkali-enhanced biochar as an alternative Si source,the results analyzed the effect of different pyrolysis temperature?350,450,550??and alkali treatments?KOH,K₂CO₃,CaO?on structural characteristics and environmental effect of alkali-enhanced biochar.The research demonstrated that as pyrolysis temperature increased,S_BET,S_mic?<2 nm?,V_T and V _micic of rice straw biochar?0KB?gradually increase;thermal stability increased,absorption peaks of-OH?C-O?C=O in biochar decrease,and the aromaticity of biochar increased,CO₂ release rate of alkali-enhanced biochar decreased.Among rice straw biochars prepared at 550?,the CO₂ cumulative emission of KOH-enhanced rice straw biochar?RS-5KB,10KB?and K₂CO₃-enhanced rice straw biochar?RS-5K2B,10K2B?were reduced by 7.2,17.5 and 71.7,58.7%as compared with control biochar?0KB?repectively,indicating that KOH and K₂CO₃-enhanced biochar prepared at high pyrolysis temperature could inhibit the release of CO₂.Therefore,KOH and K₂CO₃-enhanced rice straw biochar?RS-10KB and 10K2B?prepared at 550?were finally selected as the highly efficient and environment-friendly alternative Si sources.In summary,alkali-enhanced biochars showed different physical-chemical properties and plant available Si content due to the different types of feedstocks,alkali treatments and pyrolysis temperature.In three alkai treatments,KOH and K₂CO₃ treatment could significantly increase plant available Si and water soluble P content in biomass feedstock.From 350 to 550?prepared alkali-enhanced biochars,KOH and K₂CO₃-enhanced rice straw biochar?RS-10KB and 10K2B?not only increased soil plant available Si content and water soluble P content,but also showed lower CO₂ emission and higher thermal stability.Furthermore,KOH and K₂CO₃-enhanced rice straw biochar?RS-10KB and 10K2B?significantly increased rice Si,P and K content;moreover,the increase of Si uptake in ryegrass and suppression of gray leaf spot development were the most remarkable.Therefore,KOH and K₂CO₃-enhanced rice straw biochar?RS-10KB and 10K2B?prepared at 550?could be the best Si sources to replace conventional Si fertilizers?wollastonite and calcium silicate slag?.