Water quality and organic matter linkages in waters draining arable fields

Abstract: This study investigated leaching of natural organic matter (NOM) in twelve Swedish arable fields with regards to the quality of dissolved organic matter (DOM) and particulate organic matter (POM), using Absorbance (UV-Vis) and fluorescence spectrometry (EEM). DOM is defined as organic matter (OM) <0.45 μm and POM is defined as OM >0.45 μm. Field sampling was done every two weeks or when flow was available from 2016 until early 2018. Samples were analysed unfiltered and filtered with a 0.45 μm membrane, filtered samples represent DOM properties, whereas by comparing unfiltered and filtered sample results, information about properties of POM was inferred. Nutrients and abiotic parameters were available during the study period, of which nitrogen (N), phosphorus (P), suspended solids (SS), total organic carbon (TOC), dissolved organic carbon (DOC), pH, alkalinity (Alk) and conductivity (Cond) were analysed. Soil texture and climate were found to be the driving factors behind leaching of NOM. To begin with, clay soils had significantly different leaching patterns than loam soils as they had a higher tendency to leach more autochthonous microbial organic matter while loam soils had a higher input of allochthonous humic matter. Some clay soils were determined to have high losses of adsorbed microbial matter to the surfaces of mineral particles as POM. Some loam soils had highly humifide POM in comparison to the DOM, which could be linked to aggregate formation of humic matter with Ca+2 ions. Regarding climate, loam soils in wetter climate had significantly (p<0.05) higher input of humifide terrestrial NOM, while dry climate loam and clay soils had higher input from more microbial sources. Dry periods caused accumulation of labile OM which was rereleased when soils were rewetted. Seasonal changes in POM and DOM indicated by EEM and UV-Vis absorbance was hard to determine due to high variance in the indices, however the overall seasonal pattern indicated a higher input of autochthonous microbial matter during winter and spring that gradually decreased during summer and autumn until November. An increase in leaching of more labile microbial OM when soils were rewetted during November was observed. The water quality of two clay soils (1D and 11M) were determined to have the lowest water quality among the observation fields. The observation fields had high losses of TOC and high input of labile POM, which was caused by the texture and physical slope of the soil, making the soils vulnerable to leaching and high erosion indicated by the high SS.