Fertilization turns a rubber plantation from sink to methane source

Soils, particularly in upland forests, are the largest biological sink for atmospheric methane (CH₄), providing a valuable ecosystem service. Rubber plantations have continually expanded in Southeast Asia, and it is known that converting forests to rubber plantations reduces soil CH₄ uptake. However, the effect of management practices, and in particular fertilization, on the methane balance of a rubber plantation has not yet been studied. Rubber plantations cover almost 10% of the country's surface area and almost all rubber plantations are fertilized, two thirds of them intensively or very intensively.

We measured net soil CH₄ fluxes over more than a year in a 9-ha experimental rubber plantation with four levels of fertilizer application. We observed a strong and significant reduction of net soil CH₄ uptake with increasing fertilisation, which was not explained by differences in CH₄ diffusion related to soil water content. Fertilisation not only decreased the methanotrophic activity but also stimulated methanogenic activities probably related to an increase in the availability of nitrogen and labile carbon substrates.

Our results show that intensive fertilization turned soil from methane sink to source, particularly during the rainy season. Given the areas cultivated with rubber trees in Thailand and more widely in South-East Asia, a transition towards rational fertilization of plantations would have a significant positive effect on national reporting greenhouse gas inventories.