Earthworms regulate ability of biochar to mitigate CO2 and N2O emissions from a tropical soil

Abstract, Applied Soil Ecology, 2019

Soils account for >80% and 20% of the total agricultural N₂O and CO₂ emissions respectively. Soil management activities that target improved soil health, such as enhancing earthworm activity, may also stimulate further emissions of CO₂ and N₂O. One recommended strategy for mitigating these soil emissions is biochar amendment. However greater clarity on the interaction between earthworm activity and biochar, and subsequent impact on CO₂ and N₂O are needed to evaluate the environmental impacts of management practice. We measured N₂O and CO₂ emissions from a kaolinitic Acrisol in the presence or absence of earthworms, with and without application of two different biochars in a microcosm study. The two biochars were derived from indigenous trees; Zanthoxylum gilletii and Croton megalocarpus, and were tested at three application rates of 5 Mg ha⁻¹, 10 Mg ha⁻¹ and 25 Mg ha⁻¹. Emissions of CO₂ and N₂O increased by 26% and 72% respectively in the presence of earthworms. In microcosms with biochar and earthworms however, emissions depended on type of biochar and rate of application. With C. megalocarpus, CO₂ emission increased with increasing rates of biochar application with 25 Mg ha⁻¹ resulting in higher CO₂ fluxes compared to no-biochar control (p = 0.002), while no change was observed with Z. gilletii at the same rate. Nitrous oxide emissions were suppressed at 25 Mg ha⁻¹ for both C. megalocarpus (p = 0.009) and Z. gilletii (p = 0.011). Reduction in N₂O flux was however not consistent across biochar types. No change in N₂O was observed with 5 Mg ha⁻¹ and 10 Mg ha⁻¹of C. megalocarpus. Biochar from Z. gilletii at 5 Mg ha⁻¹ however led to increase in N₂O emissions (p < 0.001). Our findings suggest that earthworms may moderate the effect of biochar, with suppression of N₂O emissions occurring at only high biochar application rates, which may occur at the cost of increasing CO₂ emissions. These findings contrast with biochar suppressing effect on N₂O emissions even at moderate biochar rates of (10 Mg ha⁻¹) when in absence of earthworms, an outcome typical of many laboratory experiments. These findings highlight new interactions among application rate, source of biochar (and hence properties) and earthworms.

Keywords: Biochar, Earthworm, Soil N₂O emission, Soil CO₂ emission