Biochar decreases methanogenic archaea abundance and methane emissions in a flooded paddy soil
Graphical abstract
Introduction
Globally, agricultural management practices are responsible for substantial emissions of greenhouse gasses (GHGs), especially methane (CH4) emissions from rice production (van Loon et al., 2019). Rice is one of the world's most important staple food crops, and the global demand for rice will increase by 24% over the next 20 years (Haque et al., 2015). China is a major rice-cultivation country, producing 28.1% of the world's rice grain (Ding et al., 2018). As a result, Chinese rice paddies are a significant source of CH4 emissions, and techniques need to be developed to reduce CH4 emissions from rice paddies. A promising technique to reduce CH4 emissions from rice paddies is applying biochar to paddy fields (Feng et al., 2012; Wang et al., 2018). Biochar is produced from the pyrolysis of biomass, which can be a waste material or a byproduct from agriculture and forestry (Hagemann et al., 2017), and its application can increase soil porosity and the soil's sorption capacity for low molecular weight substances (Gul et al., 2015), and change soil redox properties (Klüpfel et al., 2014) and nutrient transformations (Hagemann et al., 2016). The application of biochar in rice cultivation has been widely used due to its potential for increasing carbon (C) sequestration in and reducing GHG emissions from agricultural soils (Lehmann, 2007; Wu et al., 2019b).
Effects of biochar on soil CH4 emissions were dependent on the application rate (Wang et al., 2018; Zhang et al., 2010); for example, CH4 emissions were not affected when biochar was applied at 10 Mg ha−1, but were increased at 40 Mg ha−1 in a paddy soil (Zhang et al., 2010). Our previous studies found that CH4 emissions were decreased when biochar was applied at 40 Mg ha−1 but were not affected when applied at or less than 20 Mg ha−1 in a paddy soil in a pot experiment (Qi et al., 2020b). In addition, we found that CH4 emissions were higher in biochar application than in chemical fertilizer application alone in the rice season following vegetable cultivation in a 2-year field study when biochar was applied at 10 Mg ha−1 every year (Qi et al., 2020a). The effect of biochar application rate on CH4 emissions may be attributed to changes in soil bulk density (Laird et al., 2010), surface area and presence of micropores in biochar-amended soils (Mukherjee and Lal, 2013), and microbial properties (Wang et al., 2019). In our previous studies, we also found that the CH4 emissions were affected by water irrigation management practices (Qi et al., 2018), soil temperature and labile organic C (Qi et al., 2020b) and land-use change (Qi et al., 2020a) in biochar-amended soils. However, in those earlier studies, we did not investigate the effect of interannual variation in CH4 emissions and the microbial control of CH4 emissions in biochar-amended rice paddy soils.
Biochar can reduce CH4 emissions by changing the microbial community (Feng et al., 2012; Liu et al., 2011b; Qin et al., 2016). Methanogens and methanotrophs regulate CH4 emissions in paddy soils (Conrad, 2007). Feng et al. (2012) and Qin et al. (2016) found significant decreases in CH4 emissions by biochar addition and explained the result by increases in methanotrophic bacteria biodiversity and abundance. Nevertheless, biochar application effects on methanotrophic abundance change over time following a single biochar application in a paddy field, and the reduction in CH4 emissions by biochar addition can be caused by the decrease in methanogenic relative to methanotrophic gene abundances (Wang et al., 2019). In addition, Cai et al. (2018) found that CH4 emissions were closely related to changes in methanogenic archaea genes rather than methanotrophic bacteria genes in a 45-day incubation experiment. Microbial properties should be studied for more than one rice season due to interannual variability resulting from changes in soil properties, including soil temperature (Brockett et al., 2012; Liu et al., 2010), and other factors that have large temporal variations (Sato and Sugimoto, 2013; Wang et al., 2018; Wang et al., 2019). However, few people have studied the effect of interannual variation on methanogenic and methanotrophic community structures in biochar-amended rice soils (Table S1).
The objectives of this study were to (1) assess the effect of biochar application rates on CH4 emissions, and the abundance and community structure of methanogenic archaea (mcrA) and methanotrophic bacteria (pmoA) genes that are related to CH4 emissions in a pot experiment of a paddy soil in two rice seasons; (2) investigate the effect of interannual variation in CH4 emissions, and the gene abundance and community structure of methanogens and methanotrophs.
Section snippets
Experimental design
The pot experiment was conducted over two rice seasons from March 2017 to September 2018 in a greenhouse at Southwest University. Each pot had a diameter 24.4 cm on the top end, and 21.0 cm on the bottom, and a height 23.0 cm. The soil used in this pot experiment was collected from an arable soil, which had a WHC of 30–50%, from an agricultural farm of Southwest University. Vegetables (beans and chili) were planted in the farm before the soil was collected. Each pot was filled with 6 kg
Effects of biochar application rates on soil properties and CH4 emissions
Soil pH was higher in BC3 than in BC0 and BC1 in both 2017 and 2018 (Table 2). The mean redox potential across each growing season was not different among treatments in 2017 and 2018. The SOC was higher in BC2 and BC3 than in BC0 in both 2017 and 2018. The DOC was not different among BC0, BC1, BC2, and BC3 in 2017 but was lower in BC3 than in BC0, BC1, and BC2 in 2018. The MBC was lower in BC0 and BC3 than in BC1 and BC2 in 2017 but was not different among BC0, BC1, BC2, and BC3 in 2018. The NH4
Discussion
Methane emissions decreased in all treatments with biochar addition as compared with the BC0 treatment in two rice seasons; such effects were linked with decreased methanogenic mcrA archaea abundance (Fig. 3a) rather than changes in the methanotrophic pmoA bacteria abundance (Fig. 3b); this is consistent with biochar having the potential to mitigate CH4 emissions in flooded soils reported in a meta-analysis (Jeffery et al., 2016). The continuous flooding water regime for rice cultivation in the
Conclusions
Methane emissions were decreased by 22.2–95.7% in biochar addition treatments compared with BC0 in the two rice seasons, which were linked with reductions in mcrA archaea abundance, and changes in rice aboveground biomass production, MBC, NH4+-N, NO3−-N, AP, and soil temperature. The abundances of methanogens were lower in higher biochar addition (9 and 18 g kg−1) treatments than in BC0 in both rice seasons. The abundance of methanotrophs was increased in the first but decreased in the second
CRediT authorship contribution statement
Conceptualization: Le Qi and Ming Gao; Formal analysis and Methodology: Le Qi and Zilong Ma; Investigation and Data curation: Le Qi, Peng Zhou, Rong Huang and Yingyan Wang; Resources and Supervision: Ming Gao, Scott X. Chang and Zifang Wang; Writing: Le Qi, Zilong Ma and Scott X. Chang. The manuscript was prepared by Le Qi with the assistance of Zilong Ma and was revised and reviewed by Scott X. Chang and Ming Gao.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge funding from the National “Five-Year” Key Research and Development Program (No. 2017YFD0800101), National Key Water Conservancy Construction Fund of the Three Gorges Follow-up Research Program (No. 5001022019CF50001), the State Cultivation Base of Eco-agriculture for Southwest Mountainous Land, the China Scholarship Council, Chongqing Graduate Student Research Innovation Program (CYB18091), and the National College Students Innovation and Entrepreneurship Training
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