Controls on Surface and Subsurface Soil Organic Matter Composition and Carbon Storage Along a Mean Annual Temperature Gradient in Hawaiian Tropical Montane Wet Forest

The formation and stabilization of soil carbon (C) in tropical forests are key processes influencing the global C cycle. Both climate and mineralogy are known to exert important control over these processes, especially at depth for tropical soils. However, recent work suggests plant litter quality-controlled microbial assimilation also plays an important role in the stabilization of soil C. Cotrufo et al. (2013) proposed the Microbial Efficiency Matrix Stabilization (MEMS) framework that describes the formation and stabilization of soil organic matter (SOM) by identifying significant C pools within soil and linking those pools to plant inputs. The MEMS framework, however, fails to incorporate important concepts such as mineralogical content. In this study, we utilize a highly constrained (constant vegetation, geology/substrate/soils, soil moisture, and disturbance history), long-term, whole-ecosystem mean annual temperature (MAT) gradient spanning 4.3°C located in Hawaiian tropical montane wet forests to evaluate the controls on soil C storage and formation. Here, we measure above- and belowground plant litter quantity and quality, and SOM pool sizes, mineralogy, and age to 1 m depth across this MAT gradient.

Initial results show that elevation/changes in MAT and depth has a strong impact on the mass balance of the SOM pools. In surface soils (0-15 cm), there is a strong inverse correlation between the mass balance for mineral-associated organic matter (MAOM) and light-particulate organic matter (L-POM) (P<0.001, r2=0.94). Specifically, with increasing elevation there is less MAOM and more L-POM. Therefore, we hypothesize that C from L-POM transfers to MAOM with increasing MAT in surface soils. Furthermore, in the deepest soils (60-100 cm) there is a strong inverse correlation between heavy-POM (H-POM) and MAOM (P<0.001, r2= 0.98), where MAOM increases, and H-POM decreases with increasing elevation. We further hypothesize that more C from H-POM transfers to MAOM with decreasing MAT in deeper soils. Mineral and 14C analyses will reveal if changes in elevation/MAT result in changes to the capacity of these soils to store C. Overall, results from this research will contribute to better understanding of how soil C is cycled and stored at depth in tropical forest ecosystems.