Effect of passive warming (soil translocation) on 18O CUE and soil enzyme activity along a tropical forest elevational gradient

The impact of global warming on soil processes is a critical area of concern. Limited studies have investigated soil organic carbon (SOC) dynamics' adaptation to warming. This poses a great challenge in assessing and understanding terrestrial C cycle response to climate change. Carbon Use Efficiency (CUE), indicating the proportion of metabolized organic C allocated to microbial biomass growth, is a pivotal regulator governing the fate of soil C. Moreover, our understanding of fundamental drivers of microbial CUE is largely elusive and inconclusive, especially in tropical ecosystems.

To address these knowledge gaps, we translocated top soil samples (10 cm deep soil cores) from two higher elevation sites (Vuria, 2000 m a.s.l, and Ngangao, 1800 m a.s.l) to a lower site (Macha, 1600 m a.s.l) along a moist montane rain forest gradient in Taita Hills, Kenya. Utilizing an ¹⁸O-water tracing approach, we examined the changes in microbial CUE in response to approximately three years of experimental warming. We also measured enzyme activities and conducted a 6-month laboratory incubation (15°C and 25°C) to study temperature sensitivity in native and translocated samples.

Our hypotheses were: (i) Both microbial CUE and C related enzyme activities would decrease, however, N- and P- cycle enzyme would increase along an altitudinal gradient toward the top of the Taita Hills, primarily governed by soil C and N availability; (ii) passive warming by soil translocation would result in higher CUE in translocated soils compared to native soils; (iii) At lower temperatures, soil microbial CUE is expected to decrease due to microbes allocating increased energy towards synthesis of enzymes involved in nutrient acquisition, while reducing C investment towards their growth.

Initial findings have revealed significant distinctions in enzyme activity profile due to elevation and temperature effects. Specifically, β-glucosidase and acid phosphatase activities increased and decreased along the elevation, respectively. Consistent with our hypothesis, enzyme activities and microbial CUE were higher in translocated soil than native soil. The six-month incubation had a similar effect on translocated soils and lower temperature increased the microbial CUE. In summary, our study indicates that passive warming alters microbial temperature adaption and underscores the influential role of soil enzyme activities in regulating microbial CUE. We suggest that soil microbiome at lower temperature indicates greater need for nutrients and energy. Our results highlight the need to investigate a wide variety of temperature influence on tropical soils in order to better understand and predict how the changing climate will affect C and nutrient cycling.