Abstract
Tea-planted soils generally receiving high nitrogen (N) fertilizer doses are more vulnerable to acidification, and turn into significant sources of the potent greenhouse gas nitrous oxide (N2O). However, little is known about the magnitude of soil N2O emissions from global tea plantations. Based on a global meta-analysis of field experimental data collected from major tea growing countries, we quantify annual N2O emissions, calculate direct emission factors (EFd) and identify key environmental controls of emissions from tea plantations. However, most data are from China and Japan, which is to be expected given that tea plantations in these countries represent >60% of the global area and the vital environmental research community in both countries. Results suggest that annual N2O emissions from soils of global tea plantations are on average 17.1 kg N ha−1 (or 8008 kg CO2-eq ha−1), being substantially greater than those reported for cereal croplands (662–3757 kg CO2-eq ha−1). The global mean EFd for N applications to tea plantations equals 2.31% (with a 95% confidence interval of 1.91%–2.71%), being two times higher than the Intergovernmental Panel on Climate Change default value of 1%. Across tea plantations worldwide, total N2O emissions are estimated to be 57–84 Gg N yr−1, or 1.5%–12.7% of total direct cropland N2O emissions. Given that tea plantations account for only 0.3% of total cropland area, our finding highlights that tea-planted soils are global hotspots for N2O emissions and that these systems might be prime targets for climate change mitigation in the agricultural sector. Considering that tea is a high price commodity for which consumers may be willing to apply pressure for more climate-smart production, possible mitigation efforts include use of controlled-release fertilizers or nitrification inhibitors, and application of biochar and/or lime for increasing soil pH; i.e. measures that increase N use efficiency while reducing the climate footprint of tea production.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas (GHG) contributing ca. 6% to observed global warming, and is the most important contributor to stratospheric ozone layer destruction in the twenty-first century (IPCC 2013)
Annual N2O emissions and EFds and their controlling factors The GHG balance of tea plantations is almost entirely driven by soil N2O fluxes, while changes in soil C stocks or CH4 fluxes are of minor importance (e.g. Yao et al 2018, Chiti et al 2018)
Our analysis shows that against the assumption of IPCC (2013), soil N2O emissions positively exponentially increase with N application rates, becoming obvious once N fertilization rates are higher than approximately 250 kg N ha−1 yr−1
Summary
Nitrous oxide (N2O) is a potent greenhouse gas (GHG) contributing ca. 6% to observed global warming, and is the most important contributor to stratospheric ozone layer destruction in the twenty-first century (IPCC 2013). Tea (Camellia sinensis L.) is the most widely consumed beverage in the world It is planted over a wide range of climatic (tropical, subtropical and temperate) conditions ranging from 45◦N to 34◦S latitude in as many as 62 countries across the globe. 87% of the world total tea planting area (FAO 2018). Average N application rates used in tea fields have been reported to be >800 kg N ha−1 yr−1, and in some cases even above 2000 kg N ha−1 yr−1 in Japan (Wang et al 2001, Tokuda and Hayatsu 2004). Even for Kenya where fertilizer applications are the second most expensive agronomic inputs in tea plantations, but where international tea companies pay farmers in dependence of tea quality, local farmers do apply up to 800 kg N ha−1 yr−1, with a common rate of 200–350 kg N ha−1 yr−1 (Kamau et al 2000, Owuor et al 2010). N fertilizer inputs to tea plantations are much higher as compared to cereal cropping systems, which are on a global average in a range of 81–117 kg N ha−1 yr−1 (Gerber et al 2016)
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