Molecular evidence for pervasive riverine export of soil organic matter from the Central Himalaya

Abstract

Soil erosion in high mountain ranges plays an important role in redistributing soil organic carbon across landscapes and may influence the global climate on different timescales [1, 2]. Here, we investigate the dynamics of soil organic matter export in the steep mountain belt of the Himalaya by tracing the provenance of soil-derived lipids in riverine sediments from nested catchments with areas ranging from 370 to 57700 km2. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a suite of lipids that occur ubiquitously in soils [3, 4]. Their isomer distribution depends on environmental parameters such as the mean annual temperature of the local environment [3]. In this study, we explore the use of brGDGT distributions as a proxy for the altitudinal provenance of soil organic matter in riverine sediments of the Central Himalaya of Nepal. BrGDGT distributions in soils collected along an altitudinal profile, spanning elevations from 200 to 4450 m asl, yield a robust calibration of soil signatures as a function of elevation. This calibration is then used to trace the provenance of soil organic matter exported from their catchments and entrained in suspended sediments of rivers draining the Central Himalaya. We show that brGDGT compositions of fluvial sediments accurately reflect the mean elevation of the soil-cover in their respective watersheds. The type of land-cover does not seem to have a significant influence on the export of organic matter at a catchment scale. We, therefore, conclude that soil organic matter mobilization in the Himalaya occurs pervasively, and is currently insensitive to anthropogenic perturbations.