Infrared-based analysis of organic matter composition in liquid and solid runoff fractions collected during a single erosion event

Abstract

Heavy rain events can cause soil material to be widely transported by surface runoff in arable soil landscapes. While soils of the eroded hilltop and backslope areas are affected by a loss of material, soil properties in deposition areas are affected by sedimentation of particulate and dissolved mineral and organic substances. During the event, the composition of mineral and organic matter (OM) fractions changes dynamically with runoff velocity. However, changes in OM composition during runoff have mostly been neglected. The objective was to identify changes of OM composition of runoff fractions with time and flow velocity during a single rainfall-runoff event. By the comparison of runoff from a conventionally-tilled (CT) plot and a no-till (NT) plot, we aimed at discussing process-based explanations for dynamic OM composition in runoff. Surface runoff was collected from CT and NT plots at the toe-slope of an experimental hillslope. Time-dependent runoff samples were compared with cumulative bulk samples obtained by Coshocton wheels and separated in a coarser sediment (sed>63 µm) and a finer fraction of < 63 µm-sized particles plus dissolved OM (<63 µm&diss). The OM composition was determined with Fourier transform infrared (FTIR) spectroscopy. It was found to change with runoff volume due to contact with litter (NT) or soil minerals (CT) mainly for finer fractions from both CT and NT plots. Even during a single runoff event, the OM mass and composition in runoff changes characteristically. The OM composition dynamics was dominated by OMcat/C-O-C ratios in runoff from CT and by C-H/CO ratios in runoff from NT due to contact with soil minerals (CT) and litter (NT). The time-dependent collection schemes suggested that OM composition in runoff reflected the surface properties and could help to further improve explanations for spatial distribution of OM-related soil properties such as the potential wettability in arable soil landscapes.