Tropical lowland rainforests :rapid recyclers or efficient storers of carbon?

Abstract

Amazonian rainforests are important systems that vigorously cycle carbon and water at the global and regional scale. However, despite their importance, dynamics of organic matter cycling in tropical rainforests within the global carbon cycle is poorly understood. New field-based hydrology and geochemistry data is presented from a three-year study (2009-2011) in the pristine rainforest of central Guyana at the northern rim of Amazonia. The study shows that two commonly used satellite- and interpolated-based models to estimate rainfall greatly overestimate and underestimate in the wet and dry season, respectively. This misrepresentation of hydrology at local and regional scales greatly affects our ability to understand and predict the connections and feedbacks between the hydrological and geochemical cycles. River water δ2H and δ18O isotopes from peak wet and dry seasons suggest that the majority of rainfall has a residence time of at least one month, which provides regions of water saturated zones for organic matter (OM) to be preserved in deeper rainforest soils (Δ14C ages of 360-1200 years). In contrast, Δ14C values of surface soils and dissolved organic matter (DOM) in river water recycle within ~60 years or less. Carbon normalised yields of lignin phenols, used as tracers of vascular plant material, are abundant across the terrestrial-aquatic interface. However, lignin appears to accumulate in river bed sediments. Soil leachate experiments confirm that during mobilisation from soil particulates into the dissolved phase, desorption processes change the composition of lignin biomarker ratios. Surface soil δ13C signatures show that the majority of carbon is fixed through the C3 pathway (-26.4 to -32.0‰). However, a strong variability of up to 10‰ in riverine δ13C of dissolved organic carbon suggests that changing δ13C of DOC reflects changing contributions of degraded and fresh organic compounds within the total OM pool. Superimposed on seasonal cycling, short intense rain events cause rapid mobilisation of large amounts of DOC (up to 114 mg/L) that is divided into two main fractions, humic substances and ‘invisible’ DOM, or ‘iDOM’. The latter group is characterised by non UV-absorbing organic compounds of mono- and oligosaccharides, alcohols, aldehydes, ketones and amino sugars. Importantly, iDOM contributes up to 89% to the total river OM pool during peak DOC supply