Long-Term Cultivation Reduces Influence Of Flowpaths On Dissolved Organic Matter and Nitrate Dynamics.

Dissolved organic matter (DOM) is increasingly recognized due to its role in C and N cycling within and between ecosystems and delivery of available substrate to decomposing microbial community. Shift in land-use and management has been shown to change the size and properties of this highly dynamics organic matter pool. We compared dissolved organic C and N (DOC and DON) and dissolved inorganic N (DIN) concentrations as well as DOM properties, including hydrophilic-OM (%), UV absorption (SUVA₂₅₄), humification index and C/N ratio, along fast and slow flowpaths at three soil depths (20, 40, 60 cm) for a paired land-use (forest vs. cultivated cropland). The effect of land-use on DOC and DON concentrations was largely limited to 20 cm depth where there were greater DOC concentrations in the forest (only fast flowpath) and DON concentrations in the cropland (both flowpaths). However, land-use effect on DOM properties persisted to soil depth and suggested that DOM in the cropland was more decomposed regardless of flowpath. Flowpath affected DOM only in the forest topsoil, where fast flowpaths contained greater DOC concentrations and represented properties of less decomposed OM. Despite further decomposed status of DOM in the cropland, there were strong negative exponential relationships between the C/N ratio of DOM and nitrate concentration for fast (r²=0.59, P<0.01) and slow flowpaths (r²=0.69, P<0.01), suggesting the maintenance of stoichiometric sinks for nitrate. In contrast, in the forest we were unable to detect any links between nitrate and DOM dynamics. Our data suggest long-term cultivation reduced the delivery of relatively undecomposed DOM from topsoil to subsoil through the reduction of fast flowpaths. In the long-term, these processes may affect the accumulation and stability of subsoil OM which is recognized as large but potentially sensitive reservoir of organic carbon and nitrogen.