[AHW32-12] コーヒーかすバイオ炭およびナノバイオ炭施用による農地土壌からの一酸化二窒素および二酸化炭素量放出量の低減
キーワード：バイオ炭、一酸化二窒素、二酸化炭素
Denitrification is one of important pathways responsible for nitrous oxide (N2O) emissions from soil. Biochar can decrease N2O emissions because it acts as an electron shuttle that would enhance the denitrification processes. It also provides favorable conditions for denitrifers by increasing soil pH and water retention capacity. However, research on effects of biochar with different particle sizes on N2O emissions is currently limited. The objective of the study was to determine effects of biochar and nano-biochar applications on N2O and CO2 emissions in the denitrification processes with reference to surface structures of the biochar materials, soil pH, electrical conductivity (EC), mineral nitrogen, and soil type.
Topsoil soil samples, collected from a paddy field in Ushimado, Okayama (TC 14.7 g kg-1, CN ratio 10.2) and a greenhouse at Kochi University (TC 62.6 g kg-1, CN ratio 16.0), Japan, were air-dried and 2-mm sieved for the following experiment. Soil NO3--N contents were 3.5 and 373 mg N kg-1 for Ushimado (U) and Kochi (K) soils, respectively and it increased in subsamples of U soil to 400 mg N kg-1 (U400) to eliminate the effect of initial NO3--N content. Coffee grounds waste was pyrolyzed for biochar at 600oC for 1 hour and that mixed with Fe(NO3)3•9H2O for nano-biochar at 900oC for 1 hour. Values of pH and EC (1:10) were 10.0 and 0.4 dS m-1 for biochar and 10.1 and 2.8 dS m-1 for nano-biochar. Five grams of air-dried soil amended with or without 5% biochar materials were adjusted to 100% water holding capacity and anaerobically incubated at 25oC for 4 days in 125-mL glass bottles in triplicate. Concentrations of N2O and CO2 in the headspace of bottles were analyzed with gas chromatograph (GC-8A, Shimazu, Japan) equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD), respectively.
Fluxes of N2O were generally the largest from K soil, followed by U400 and U soils. The lowest N2O emissions in U soil occurred due to much lower initial NO3--N content. Cumulative N2O emissions decreased in K and U400 soils amended with biochar and nano-biochar. These are probably due to the electron shuttle effect and pH increase by application of these biochar materials. Moreover, nano-biochar reduced more cumulative N2O emissions than biochar, which can be explained by Fe added in nano-biochar production and a larger specific surface area of nano-biochar (325 m2 g-1) than that of biochar (1.7 m2 g-1). Trends of NO3-N content decreases in each soil were similar among treatments with and without biochar materials, indicating the last reduction process of N2O to N2 in denitrification was more enhanced by application of nano-biochar. Emissions of CO2 decreased in K and U400 soils amended with biochar and nano-biochar, which is generally more distinct in the soils amended with nano-biochar than with biochar. Nano-biochar with a larger specific surface area would have absorbed more CO2 than biochar. Nitrogen mineralization in soils amended with nano-biochar was lower than with biochar, which is indicated by more increases in NH4+-N content in nano-biochar amended soils.
Applications of biochar and nano-biochar reduced N2O and CO2 emissions from different two agricultural soils when NO3-N contents were as high as 400 mg kg-1. Nano-biochar reduced N2O emissions more than biochar did. This is because nano-biochar had a larger specific surface area and increased more soil pH than biochar. Further research is needed to investigate effects of biochar and nano-biochar on soils with different microbial and chemical properties before applying them to a real field condition.
Topsoil soil samples, collected from a paddy field in Ushimado, Okayama (TC 14.7 g kg-1, CN ratio 10.2) and a greenhouse at Kochi University (TC 62.6 g kg-1, CN ratio 16.0), Japan, were air-dried and 2-mm sieved for the following experiment. Soil NO3--N contents were 3.5 and 373 mg N kg-1 for Ushimado (U) and Kochi (K) soils, respectively and it increased in subsamples of U soil to 400 mg N kg-1 (U400) to eliminate the effect of initial NO3--N content. Coffee grounds waste was pyrolyzed for biochar at 600oC for 1 hour and that mixed with Fe(NO3)3•9H2O for nano-biochar at 900oC for 1 hour. Values of pH and EC (1:10) were 10.0 and 0.4 dS m-1 for biochar and 10.1 and 2.8 dS m-1 for nano-biochar. Five grams of air-dried soil amended with or without 5% biochar materials were adjusted to 100% water holding capacity and anaerobically incubated at 25oC for 4 days in 125-mL glass bottles in triplicate. Concentrations of N2O and CO2 in the headspace of bottles were analyzed with gas chromatograph (GC-8A, Shimazu, Japan) equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD), respectively.
Fluxes of N2O were generally the largest from K soil, followed by U400 and U soils. The lowest N2O emissions in U soil occurred due to much lower initial NO3--N content. Cumulative N2O emissions decreased in K and U400 soils amended with biochar and nano-biochar. These are probably due to the electron shuttle effect and pH increase by application of these biochar materials. Moreover, nano-biochar reduced more cumulative N2O emissions than biochar, which can be explained by Fe added in nano-biochar production and a larger specific surface area of nano-biochar (325 m2 g-1) than that of biochar (1.7 m2 g-1). Trends of NO3-N content decreases in each soil were similar among treatments with and without biochar materials, indicating the last reduction process of N2O to N2 in denitrification was more enhanced by application of nano-biochar. Emissions of CO2 decreased in K and U400 soils amended with biochar and nano-biochar, which is generally more distinct in the soils amended with nano-biochar than with biochar. Nano-biochar with a larger specific surface area would have absorbed more CO2 than biochar. Nitrogen mineralization in soils amended with nano-biochar was lower than with biochar, which is indicated by more increases in NH4+-N content in nano-biochar amended soils.
Applications of biochar and nano-biochar reduced N2O and CO2 emissions from different two agricultural soils when NO3-N contents were as high as 400 mg kg-1. Nano-biochar reduced N2O emissions more than biochar did. This is because nano-biochar had a larger specific surface area and increased more soil pH than biochar. Further research is needed to investigate effects of biochar and nano-biochar on soils with different microbial and chemical properties before applying them to a real field condition.