article · Applied Soil Ecology
Agricultural soils are major sources of nitrous oxide and carbon dioxide emissions. Although promoting earthworm activity improves soil health, it can also stimulate greenhouse gas releases. Applying biochar is often proposed to mitigate these emissions, but the combined effects of earthworms and biochar remain complex. A laboratory microcosm study evaluated a kaolinitic Acrisol amended with biochars from two indigenous tree species, Zanthoxylum gilletii and Croton megalocarpus, applied at three different rates with and without earthworms. Earthworms alone increased carbon dioxide and nitrous oxide emissions by 26 percent and 72 percent respectively. When earthworms were present, the impact of biochar depended heavily on tree feedstock and dosage. Nitrous oxide emissions decreased only at the highest biochar application rate of 25 megagrams per hectare for both species. However, that same rate of Croton megalocarpus biochar increased carbon dioxide emissions, while a lower rate of Zanthoxylum gilletii biochar actually increased nitrous oxide emissions.
Managing agricultural soils to capture carbon and improve fertility can produce unintended environmental trade-offs. Soil organisms like earthworms influence greenhouse gas emissions, meaning that biochar practices shown to work in simple laboratory tests might perform differently in living soils. Understanding these biological interactions helps ensure that soil management interventions genuinely lower greenhouse gas emissions rather than shifting or accelerating them.
This research provides baseline data for agricultural land managers, carbon offset developers, and biochar producers seeking to optimise soil amendment formulations for emission reductions. Because the findings derive from a small-scale laboratory microcosm experiment, the work represents early-stage research that requires further field testing before guiding commercial biochar application rates or formal carbon credit protocols.
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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.
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DOI: 10.1016/j.apsoil.2019.04.001
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