Long-term Effects Of Biochar Application On Fluvo-aquic Soil Carbon Pool And Crop Yields

It is necessary to understand the effect of biochar on different fractions of soil carbon pool before evaluating its role in terrestrial carbon sink.Most of the prviouse studies about the effect of biochar on soil carbon pool were conducted by incubation experiments,which could not reflect the real process of field soil.This study was conducted at the Shang-zhuang Experimental Station of the China Agricultural University in Haidian District,Beijing,China.We applied a range of biochar(0,30,60 or 90 t ha-1,equals to 0,1.11,2.22 and 3.33%(w/w),respectively)to the field soil in June 2009.The biochar was produced using waste mushroom growth substrate at 400? for 4 h in a sealed oven.Combining wheat and maize straws incorporation,the soil organic and inorganic carbon content,soil organic carbon ?13C,soil water content and soil dissolved organic carbon content were measured after biochar application for 5 years,and the quantity and quality changes of biochar were analyzed,then soil chemical and physical properties and crop yields were determined after biochar application for 7 years.The results showed that:(1)Long-term biochar application increased soil total carbon content for 31.4%,65.7%and 101.8%at the biochar application rate of 30,60 and 90 t ha-1,respectively.B30,B60,and B90 enhanced soil total organic carbon content(including biochar organic carbon)for 2.53,5.29 and 8.21 g kg-1,respectively,and they increased soil total inorganic carbon content(including biochar inorganic carbon)for 0.12,0.26 and 0.38 g kg-1,respectively.For the soil carbon pool(excluding biochar carbon),biochar reduced soil organic carbon content for 11.1%,21.4%and 31.3%,while they increased soil inorganic carbon content for 7.8%,20.2%and 28.2%,respectively.(2)Biochar application significantly decreased soil labile organic carbon content,while it did not affect soil recalcitrant organic carbon content.Soil organic carbon was derived from wheat and maize,and biochar decreased wheat-derived organic carbon content,while it did not influence maize-derived organic carbon content.(3)During the 6th and 7th year of biochar application,biochar did not affect soil water content,dissolved organic carbon content and fraction contents.Soil dissolved organic carbon content was positively related to soil water content,dissolved organic carbon fractions(reducing sugar,soluble phenol and aromatics).(4)Biochar mass loss was nearly 40%after it was applied in the 0-20 cm soil layer for 5 years,and combined with the biochar C content variation,the biochar C loss was nearly 1/3,regardless the biochar application rates.There were new formed small pores existed in the biochar particles after aging,and the biochar surface became smoother.The specific surface area of aged biochar increased more than 100%relative to the fresh biochar,and the aged biochar total pore volume also increased relative to the fresh biochar.However,the chemical properties of biochar had very limited changes.(5)Long-term biochar application had no effect on soil physical properties.Biochar increased soil total organic carbon and available potassium contents but had no effect on other soil chemical properties.Application biochar did not increase crop yields in a single season or in short term,however,from the average yields,it was found that biochar increased yields for 6.5%.In summary,the long-term filed study results showed biochar application decreased wheat-derived organic carbon accumulation,however,it promoted soil inorganic carbon formation.As the transportation of biochar vertically or horizontally and the aging process of biochar,the effect of biochar on soil water content,soil dissolved organic carbon content and compositions,soil nutrients and crop yields were not significant.This study proved a powerful evidence for the evaluation of long-term benefits of biochar on a Fluvo-aquic soil.