Effects of Ammonium Addition on Methane Oxidation in a Paddy Soil: Insights Into Microbial Interactions

Rice paddies are a major anthropogenic source of methane and a key target for reducing emissions of the greenhouse gas to the atmosphere. The delicate equilibrium between the production and oxidation of methane in paddy soils is shaped largely by the abundances and compositions of different microbial communities within the soil ecosystem and the interactions between them. Ammonium addition can alleviate nitrogen deficiency for methanotrophs, but ammonium can also inhibit their growth when present in excess. However, the threshold concentration for this switch is not currently known. Here we report the results of a nine-day laboratory incubation experiment that sought to examine the effects of increasing ammonium concentrations on methane oxidation in rice paddy soil at refined concentration intervals. We measured methane oxidation rates and analysed the gene abundances and community compositions of methanotrophs and ammonia oxidizers in the incubated soils to decode interactions between these communities. Our results showed that an ammonium concentration of 10 mg ${\mathrm{NH}}_4^{+}$-N d.w.s stimulated methane oxidation, but concentrations above 30 mg ${\mathrm{NH}}_4^{+}$-N kg⁻¹ d.w.s inhibited the oxidation rate. At the lower ammonium concentration, type Ia methanotrophs appeared to outcompete ammonia oxidizers for nitrogen; however, this was reversed at higher concentration where the proliferation of ammonia oxidizers was stimulated. We show how ammonium stimulated the ammonia-oxidizing bacteria (AOB) to a greater extent than ammonia-oxidizing archaea (AOA), but with much smaller changes in the specific AOB community composition, when compared to the AOA communities. Our findings highlight ammonium concentration as a key factor regulating the interaction between methanotrophs and ammonia oxidizers in paddy soils and identify the threshold where ammonium shifts from promoting to inhibiting methane oxidation.