Biochar Increases Soil C Sequestration but Warming Temperatures May Increase Soil Temperature Sensitivity and N2O Flux

Abstract

Global atmospheric carbon dioxide (CO2) and nitrous oxide (N2O) concentrations have increased rapidly in recent decades and in the United States agriculture accounts for 10% of greenhouse gas emissions (GHG). One potential method to combat GHG emissions is the application of biochar to agricultural soils. Biochar is organic matter that has undergone pyrolysis (i.e. combustion under low to no oxygen conditions), which results in a recalcitrant, carbonaceous material. In addition to direct CO2 mitigation through pyrolysis, biochar can further reduce GHG emission and increase soil carbon (C) sequestration, soil quality and crop yields. To examine the impact of biochar in Hawaii soils, two important agricultural soils (Oxisol and Mollisol) with contrasting fertility under two different cropping (zero-tillage napiergrass and conventional sweet corn) with and without biochar were analyzed both post biochar amendment and after 1 year (two crop harvests for both napiergrass and sweet corn) for a suite of soil properties and microbial community composition. Additionally, individual bags of biochar were buried within the field and removed at year one to assess how the physical and chemical properties of biochar changed over time. Overall, biochar increased soil C by 47% compared to the control and influenced soil microbial community abundance but had little impact on other soil properties and crop yields. The biochar itself began to breakdown within the soil and become coated in clay particles. While this represents a static view of how biochar influences soils in agronomic systems, given how difficult it is to remove biochar once amended, it is also important to establish the impacts of increasing global temperatures within these systems. To assess the response to temperature, the soils collected after one year were incubated across an eight-point gradient to determine temperature sensitivity for both soil respiration and N2O flux. At 26°C there were no treatment effects in soil respiration, but for N2O the Mollisol had increased flux (p<0.01). Using the full gradient, the temperature sensitivity of the soils was assessed; almost all treatments had an increase in soil respiration with temperature. However, biochar nearly doubled the temperature sensitivity of soils (p=0.017). Most soils were temperature insensitive for N2O flux, with the exception of the Mollisol napiergrass biochar. Given the importance of N2O in the context of climate change, the gene encoding nitrous oxide reductase (nosZ) was enumerated in concert with the total microbial community (16S rRNA gene) using quantitative PCR. While total microbial abundance and the abundance of genes involved in denitrification did not change with biochar and actually decreased with temperature (p=0.0088), the ratio of denitrifying bacteria to total bacteria nearly doubled in the 31°C compared to the 23°C (p=0.0144). The soils were then provided with an addition of a labile C source, similar to the addition of organic inputs or root exudates to see how the temperature-adapted communities responded. The biochar amended soils at 31°C had respiration 47% greater compared to soils with biochar at 23°C while the microbial abundance increased by 65% in the 31°C, although it was not significant. Conversely, no differences were found in N2O flux or in nosZ genes for the glucose amended soils. However, the denitrifying bacteria had greater abundance in the day 60 soils than compared to the glucose amended soils. Conversely, the overall microbial abundance was increased in the glucose amended soils compared to the day 60 soils. These results suggest that in some cases, while biochar increases soil C sequestration, it may exacerbate effects of climate change by increasing the temperature sensitivity of soil respiration in both more stable and more labile C pools as well as increase the temperature sensitivity of soil N2O flux in a bioenergy crop with a ratoon harvest in a Mollisol soil. This indicates a need for a better understanding of how biochar alters the soil environment and a risk assessment for the use of biochar as a climate change mitigation strategy.