Effects Of Different Biomass And Pyrolysis Technology On Biochar Characterization And Methane Emissions In Paddy Soil

In recent years, the conversion of biomass to biochar through pyrolysis under limited O2 condition provides a new way for resource utilization of agricultural residues and it has been widely noticed and researched. However, because of the diversity of raw material and differences in pyrolysis conditions, it is still very controversial about the application effects.In this research, biochars were fabricated from biomass bamboo, pecan shell, rice and rape straw through pyrolysis in a vacuum environment at 300～700℃ for 30 min, 90 min,3 h and 6 h, respectively. The relationship of material properties, pyrolysis characteristics and biochar yield was investigated by thermal gravimetric analysis (TGA). The obtained biochars were characterized in terms of its pH, EC, elemental composition, oxygen-containing functional group composition and macroporous structure. Moreover, the influence of raw material, carbonization temperature, carbonization time on basic properties of charcoal were estimated, and the variation trend of biochar properties under different conditions preliminarily were also evaluated. Some material were chosen as matrix to be added into paddy soil for indoor incubation experiments, to study the influence of biochar types on soil methane emission. The main conclusions of this paper are as follows:(1) Different biochars showed different acid-base properties, pH ranged from 4.83 to 9.62, and most were alkaline. Thereinto, the pH of rice straw biochar was relatively high, while that of pecan shell biochar showed lower pH. The EC of both rice and rape straw biochar were higher than others, respectively reaching 597 us-cm’1 and 923.5 μs·cm-1. However the maximum value of bamboo biochar and pecan shell biochar were only 274 μs·cm-1 and 34.46 us·cm-1, significantly lower than those of straw biochar.(2) The yields of biochar fabricated by different raw material were quite different. The rice and rape straw which had higher ash content showed higher yield than those of bamboo and pecan shell biochar. Carbon, oxygen, nitrogen and hydrogen contents of biochar were positively associated with those of raw material. And as the carbonization temperature increased, carbon content increased, oxygen content decreased, aromaticity enhanced and polarity weakened. As biochar gradually tended to be the graphitization, the structure tended to be more stable. Bamboo and pecan shell biochar had relatively large amount of C/N, showing that they were more difficult for decomposition and mineralization than rice and rape straw. As the infrared spectrogram of biochar showed, with the carbonization temperature increased, absorption peaks of most functional groups started to weaken, similar to graphite structure. Therein, because of the relatively large amount of Si content, the rice straw biochar showed lower stability, distinctly different from other three kinds of biochar.(3)The carbonization time had similar influence on biochars’properties. Firstly, in terms of yield and element composition, in the range of low temperature (300～400℃), the increasing of carbonization time brought the decreasing of yield. However in high temperature range, the influence of time extension was not significant. The increasing residence time cannot enhance the stability of biochars all the time, while the carbonization exceeded a certain limit time, the element contents decreased. In this research, biochar carbonized for 3 h and 6 h most showed these trends. In addition, as carbonization time extended, biochar’pH and EC increased, especially for bamboo chip and pecan shell biochar.(4) The paddy soil incubation with washed biochar was conducted for assessing the soil CH4 emission. Results indicated that carbonization at 300℃,500℃,700℃ kept 3 hours under rice straw and bamboo charcoal could inhibit soil methane emissions. The bamboo biochar pyrolysised at 500℃ was more effective than others. Carbonized rice straw at lower temperature could reduce the emission of methane in paddy soil.(5) Adding washed biochar to the rice soil could change the soil microbial community structure, and impact the microbial abundance of methanogens and methane-oxidizing bacteria.