Testing potassium limitation on soil microbial activity in a sub-tropical forest

Because potassium (K) is a rock-derived essential element that can be depleted in highly-weathered tropical soils, K availability may limit some portion of soil microbial activity in tropical forest ecosystems. In this paper we tested if K limits microbial activity in the condition of sufficient labile C supply. An incubation experiment was performed using surface soil samples (0–10 cm depth) obtained from four permanent ecological research plots in a natural sub-tropical forest in southern China. Soil samples were taken in September 2016. Heterotrophic soil respiration rates and microbial biomass were measured after the addition of glucose (both D and L) with and without K (potassium chloride). We did not observe any effects of K addition on soil microbial respiration, suggesting that K does not limit the microbial activity in the condition of sufficient labile C supply. The lack of microbial response to added K can be attributed to the high mobility of K in forest ecosystems, which may have provided sufficient K to microbes in our soil samples (already provided at the beginning of the incubation). However, at the present stage, we cannot conclude that K is not a limiting factor of soil microbial activity in other tropical forest ecosystems because of the heterogeneity of tropical forest ecosystems and few observations. The hypothesis needs to be tested in larger numbers of tropical forests.