Charosphere Effects Of Biochar And Its Associated Key Processes Of Soil Nitrogen Transformation

Biochar,as a multifunctional soil amendment,is widely used in improving soil structure,ameliorating soil acidification,reducing nutrient losses,and mitigating greenhouse gas emissions due to its unique physical and chemical properties.The effects of biochar in soil radiate outwards from biochar particles,forming a special "charosphere" micro-zone.The structure and properties of charosphere soil are different from the bulk soil and biochar itself,leading to the spatial heterogeneity of the biochar effects.However,it is still unclear which factors have predominant effects on soil properties in the charosphere,or how far these effects radiate from biochar,which need in-depth discussion.In this study,field trial and incubation experiment were conducted to investigate the effects of biochar and its components on soil physicochemical properties in charosphere for the first time.Meanwhile,this study clarified the changes of microbial abundance,diversity and community structure along the charosphere gradient,and further assessed the effects of biochar on soil nitrogen(N)transformation and its related functional microorganisms.Finally,a pot experiment was conducted to explore the N distribution and transformation in soil-rice-biochar system,providing a theoretical basis for application of biochar in agricultural production and greenhouse gas emission reduction.The main findings are as follows:(1)The charosphere effects of biochar.The benzene polycarboxylic acids were analyzed in soils sampled from different distances to concentrated-applied biochar in a field trial.The results showed that biochar only slightly increased the content of benzene polycarboxylic acids in biochar-adjacent soil,because the migration of biochar was very slow in the soil.The concentrations of benzene polycarboxylic acids were not related to soil propertis,further indicating that the spatial heterogeneity of biochar effects did not resulted from the biochar migration but the dissolved substances released from biochar.The changes of soil properties along the millimeter-scale charosphere gradient were investigated using a multi-sectional box in a short-term incubation experiment.It showed that biochar pyrolyzed at 300? mainly increased dissolved organic carbon in the charosphere,while the biochar pyrolyzed at 700? mainly increased soil p H in the charosphere.Both types of biochars significantly increase the content of available phosphorus and various cations in the charosphere,improving the nutrient availability.The differences between soils in various distance were significant,which was caused by the rapid release of dissolved substances from biochar at the initial stage.(2)The changes of microbial diversity and community structure in the charosphere.The high-throughput sequencing technology was used to explore the effects of intact biochar,HCl-modified biochar and washed biochar on the microbial abundance,diversity,and community structure in the millimeter-scale charosphere.In the charosphere,the abundance and composition of soil microorganisms were significantly affected by the charosphere distances.The number of responsive microorganisms in the near-charosphere zone was the highest,which decreased with the increasing distance from biochar.However,the intact biochar did not alter the soil?-diversity(Chao1 and Shannon-Wiener index),but it significantly affected the microbial community structure.The abundance difference analysis of OTUs indicated that this was mainly due to the special physicochemical properties of biochar.Biochar promoted the growth of specific microorganisms in the charosphere,thereby limiting the growth of other microorganisms.The number of OTUs sensitive to intact biochar in the charosphere was higher than that of HCl-modified and washed biochar,indicating that acidification and washing treatments reduced the specificity of biochar.The three types of biochars all changed the microbial abundance and composition along the charosphere gradient.Biochar mainly promoted the relative abundance of copiotrophs(Proteobacteria),which was higher in near charosphere;The abundance of oligotrophic microorganisms(Acidobacteria)decreased with biochar.(3)The N-related functional microoganisms in the charosphere.The effects of biochar pyrolyzed at different temperatures or intact,HCl-modified and washed biochar on the soil N transformation and related functional microorganisms over a multisectional gradient were investigated by q PCR and high-throughput sequencing technologies.The changes in the ratio of archaeal and bacterial amoA genes in the charosphere were negatively correlated with the soil p H,dissolved organic carbon and nutrient contents,while genes for denitrification,which were not closely correlated with environmental factores,were also not affected by biochar as significantly as the amoA genes.In addition,biochar promoted the microbial assimilation of ammonium into glutamate,so it significantly reduced the concentration of nitrate and the emission of N2 O,preventing soil N leaching and gaseous N losses.Meanwhile,it showed that the functional microorganisms responsible for N-fixation,ammonia oxidation and denitrification were more abundant in the near-charosphere,indicating that the Nrelated functional microorganisms were affected along the charosphere gradient.The different kinds of response of the functional microorganisms in different processes altered the environmental function,thereby affecting the spatial distribution of N in the charosphere.(4)The charosphere effects on the rice growth and N distribution.The effects of intact biochar,washed biochar and washed biochar + biochar extract on rice growth and N transformation and distribution were investigated by double-layer rice pot experiment.Biochar and its extract promoted rice growth in the near-char zone and the total N in the rice roots and grains,but reduced the total N of the rice in the far-char zone.The washed biochar without dissolved substances had almost no effect on rice growth and total N in rice and grains,soil dissolved organic carbon,organic N,ammonium and available phosphorus,indicating that the residual effect of biochar after aging was different from that of fresh biochar.However,the three types of biochar have significantly reduced N2O emissions.The types of biochar,near-/far-char zones,and tillering/maturity stages all had significant effects on the functional genes related to N transformation.Soil with intact biochar had a higher abundance of 16S rRNA,bacterial amoA and nar G genes than the washed biochar.The washed biochar + extract increased the abundance of nir S and nir K genes,but washed biochar alone had no significant effect compared with control.The near-/far-char zones only affected the abundance of amoA and nif H genes,and had little effect on denitrification genes,which indicated that the spatial heterogeneity of biochar effects played different roles in N transformation processes,highlighting the need to consider the biochar aging and spatial heterogeneity of the charosphere in future studies.To sum up,the spatial heterogeneity of biochar effects was mainly because of charosphere effects rather than migration of biochar.Differential distribution of soil microoganisms was driven by the changes of soil properties along the charosphere gradients,thus affecting the nitrogen transformation processes and crop growth.