Fertilization turns a rubber plantation from sink to methane source

Abstract. The rapid expansion of rubber cultivation, driven by the demand for natural rubber in the tire industry constitutes a significant land-use change in Southeast Asia. This significant land-use change has reduced soil methane (CH₄) uptake, thereby weakening atmospheric CH₄ removal over extensive areas. While fertilization is a widespread practice in rubber plantations, its role in further weakening the soil CH₄ sink remained poorly understood. Over 1.5 years, we measured soil CH₄ fluxes biweekly in an experimental rubber plantation with four distinct fertilization treatments to evaluate their impact on the soil CH₄ budget. Our findings revealed that fertilization not only reduced soil CH₄ consumption, but also increased soil CH₄ production. The difference in soil CH₄ uptake between unfertilized plots (-2.9 kg CH₄ ha^-1 yr^-1) and those with rational fertilization (-2.1 kg CH₄ ha^-1 yr^-1) was moderate. Recommended fertilization rates reduced soil CH₄ uptake by 60 % (-1.1 kg CH₄ ha^-1 yr^-1), and heavy fertilization transformed the soil into a net source of CH₄ (+0.3 kg CH₄ ha^-1 yr^-1). The suppression of soil CH₄ oxidation was likely driven by increased mineral nitrogen in the soil solution and soil acidification, while elevated dissolved organic carbon likely stimulated CH₄ production in the topsoil. Most rubber tree trunks emitted CH₄, likely of internal origin. Trunk CH₄ fluxes ranged from -0.10 to 0.51 nmol s^-1 per tree, with no significant fertilization effect. At the national level, adopting rational fertilization practices in Thailand could enhance the net soil CH₄ sink by 5.9 Gg CH₄ yr^-1. However, this mitigation strategy would have a limited impact on the overall greenhouse gas budget of the agricultural sector in Southeast Asia, unless it is extended to other tree plantations and cropping systems.