| Biochar is a solid product obtained via pyrolysis of agricultural wastes under high temperature and oxygen-limited conditions.Biochar soil amendment has been shown not only to enhance the stable stock of organic carbon,contributing to carbon capture and sequestration,but also to improve soil fertility and crop growth and health.Questions have been raised so far including the relations of organic composition in biochar to their biological effects,whether and how soil organic matter could be modified by the biochar incorporation to soil?These become the critical issues in understanding biochar's role in sustainable soil and environment management.In this paper,we studied the agriculture derived biochar,analyzed the molecular composition of biochar soluble fraction and its biological effect of seed germination.Meanwhile,we also analyzed the structural characteristics and molecular composition of soil organic matter based on the long-term field trial of biochar amendment in rice paddy soil,in order to clarify the possible mechanism of organic matter affecting the soil carbon stability and soil fertility.The main findings are as follows:1.Molecular identification of biochar soluble components and its seed germination bio-effects test.Wheat straw biochar(abbreviated as wheat biochar)and maize straw biochar(abbreviated as maize biochar)were chosen as the experimental materials which have been studied for a long time in our lab.The labile organic soluble components(AQU1)was obtained by dilute acid solution,and the seed germination effect was conducted by applying different soluble C concentration gradient of AQU1.In order to explore the link between the molecular organic composition and the seed germination bio-effects,the characteristic organic molecules of AQU1 were analyzed and interpreted by infrared spectroscopy(FTIR),nuclear magnetic resonance(13C-NMR and 1H-NMR),and high resolution liquid chromatography-mass spectrometry(LC/MS).The two biochar AQU1 components contained a number of beneficial elements for plant growth,including zinc,manganese,calcium,potassium and iron,but none of the toxic heavy metal elements were detected.Carbon and nitrogen contents of maize biochar AQU1 were 7.7%and 0.5%,respectively,while those of wheat biochar AQU1 were 0.7%and 0.3%,respectively.The C/N ratio of the former was significantly higher than the latter.Potentiometric titration of maize biochar AQU1 revealed a humic-like character,while this was not obvious for wheat biochar AQU1 in which the inorganic contents were dominant.LC/MS shown that both AQU1 fractions,contained a certain amount of heterocyclic nitrogen compounds with small molecular sizes which are more homogenous in maize biochar AQU1 than in wheat biochar AQU1,as revealed by their Van Krevelen plots.The results of maize seed germination test with different carbon concentration showed that maize biochar AQU 1 diluted with different carbon concentrations significantly promoted the elongation of coleoptiles,and the effect of the stimulated salt at the corresponding carbon concentration on the seed germination was not significant.However,wheat biochar AQU1 showed inhibition on the elongation of coleoptiles.The homogeneity of the humic molecules contained in the maize biochar AQU1 may stimulate the growth of coleoptiles,which is not obvious compared to the bio-stimulation of wheat biochar AQU1.We have proven for the first time that the positive or negative bio-stimulation effect of biochar may be not only related to its organic matter concentration,but also with the type of the humic suprastructures.2.A long-term field trial of biochar amendment was conducted in a rice paddy soil in Guanghan,Sichuan China;wheat straw biochar was applied once after wheat harvest in 2010.The topsoil in 0-15 cm depth were collected twice in 2014 and 2015 respectively,from the treatment with different ratio of biochar amendment(0,20 and 40t·ha-1,abbreviated as C0,C20 and C40).Elemental analysis and off-line Pyr-TMAH-GC/MS analysis were chosen to analyze the distribution of soil organic matter and the molecular composition of organic matter in paddy soil after biochar application.To understand the stability of biochar in paddy fields,and the mechanism of biochar soil organic carbon sequestration.Bulk wheat biochar's extractable material,by off-line Pyr-TMAH-GC/MS,found to be consisted mainly by amines,aromatic compounds,alkanes and aliphatic esters.Compared wheat biochar amended soil with bulk wheat biochar and control soil,as well as the low and high application ratio of biochar amendment soil in both year,we found that the molecular composition of organic matter components in biochar amended soil were consisted by labile molecules held by wheat biochar,stable molecules which were not influenced by biochar amendment,molecules from wheat biochar organic components,and molecules produced by the transformation of soil organic matter due to the biochar application.The organic carbon content of C20 and C40 from the soil collected in 2015,were increased by 27.4%and 40.6%,respectively;soil organic matter components showed increase of phenolic characteristics and steroid products.Applying a higher rate of biochar,SOM was found to be enriched with alkanes and amines deriving from the biochar.C20 had a better effect than C40 since the stability of labile SOM was higher.However,C40 enriched soil with more biochar derived molecules and this possibly influenced the microbial community and thus its activity of SOM transformation.3.Humeomics technique was applied to analyze organic components in bulk wheat biochar and its amended soil from long-term field trial in rice paddy.Humeomics techniques could obtain unbound free components(AQU1,ORG1),weak ester bound organic components(AQU2,ORG2),strong ester bound organic components(AQU3,ORG3),ether bound organic components(AQU4),and a certain amount of residual organic matter(RESOM)in organomineral residue,through organic and inorganic solutions with the stepwise extracting ability varying from mild to strong.AQU fractions and RESOM were the hydrosoluble components and ORG fractions were the organo-soluble components.RESOM component can be used to further understand the role of biochar in the stabilization and maintenance of paddy soil organic matter.Mass yields and Elemental Analysis revealed the carbon and nitrogen contents of Humeomic fractions in soils.The mass and nitrogen content of Humeomic residual fraction(RESOM)in the soils with different biochar amended ratio(20 t·ha-1 and 40 t·ha-1)were lower than that of control soil.However,compared with the control,in C20 treatment,organic components of the strong ester bound and the Humeomic residual organic fractions showed an increase.In C40 treatment,it showed a decrease of the weak ester bound hydrosoluble fractions.The mass and carbon content of soil organic matter obtained from Humeomics and alkali extraction in 2014 sample was higher than that in 2015 sample,suggesting that biochar may promote organic matter sequestration in soil.This is in agreement with the Humeomic findings of decreasing the unbound hydrosoluble labile component yields and increasing the strong ester-bound organic and the residual organic fractions yields.In summary,this study examined the molecular characteristics of biochar extracts and the potential link of their aqueous labile fraction to their bio-promotion effect,revealing molecular changes in a rice paddy soil receiving a single biochar amendment 4 years ago.The biological effects of biochar extracts vary with the type of the biochar feedstock.High contents of soluble organic carbon fractions in maize straw biochar with homogenous heterocyclic N compounds could be beneficial to plant growth.Following the amendment,biochar could increase the SOM content but also modify the OC composition of the SOM fractions in soil.The employment of the novel cutting-edge molecular characterization technique of Humeomics methodology,allowed the understanding of a reduced extracting ability through the enhancement of and residual humin OC bound fraction under the biochar amendment.This could explain,in part at least,why soil OC persistence and carbon sequestration is promoted with biochar in rice paddies.Nevertheless,the long term trends of these molecular changes and the potential impacts on C cycling and soil fertility and plant growth requires additional field research and examination. |
PDF Full Text Request