Hydrologic control of dissolved organic matter concentration and quality in a semiarid artificially drained agricultural catchment

Abstract

Agricultural practices have altered watershed-scale dissolved organic matter (DOM) dynamics, including in-stream concentration, biodegradability, and total catchment export. However, mechanisms responsible for these changes are not clear, and field-scale processes are rarely directly linked to the magnitude and quality of DOM that is transported to surface water. In a small (12 ha) agricultural catchment in eastern Washington State, we tested the hypothesis that hydrologic connectivity in a catchment is the dominant control over the concentration and quality of DOM exported to surface water via artificial subsurface drainage. Concentrations of dissolved organic carbon (DOC) and humic-like components of DOM decreased while the Fluorescence Index and Freshness Index increased with depth through the soil profile. In drain discharge, these characteristics were significantly correlated with drain flow across seasons and years, with drain DOM resembling deep sources during low-flow and shallow sources during high flow, suggesting that DOM from shallow sources bypasses removal processes when hydrologic connectivity in the catchment is greatest. Assuming changes in streamflow projected for the Palouse River (which contains the study catchment) under the A1B climate scenario (rapid growth, dependence on fossil fuel, and renewable energy sources) apply to the study catchment, we project greater interannual variability in annual DOC export in the future, with significant increases in the driest years. This study highlights the variability in DOM inputs from agricultural soil to surface water on daily to interannual time scales, pointing to the need for a more nuanced understanding of agricultural impacts on DOM dynamics in surface water.

Water and nitrogen movement through a semiarid dryland agricultural catchment: Seasonal and decadal trends

“…The mean of the yearly artificial drain NO 3 − fluxes (Figure ) was 15 kg N ha −1 year −1 (Table ). Bellmore, Harrison, Needoba, Brooks, and Keller () estimated dissolved organic N (DON) fluxes from the AD over the same interval as this research and reported a mean yearly value of 1.3 kg N ha −1 year −1 .…”

“…Regarding chemostatic behavior of N reservoirs in agroecosystem catchments, our temporally detailed data show that within each WY, N concentrations are much larger during wintertime peak flows than earlier and later (Figure ). Bellmore et al () observed similar concentration–efflux behavior for AD DON. Thus, at the CAF, both NO 3 − and DON behave chemostatically interannually but not seasonally, implying limited reservoir size and/or availability for export on the seasonal timescale.…”

“…Preferential flow, reported to occur in the AD catchment in recent work (Bellmore et al, ), could also in principle produce a skewed distribution of water and solute residence times. The mean AD‐catchment water residence time of months to a year could be attained by the combination of very rapidly moving parcels of soil water, conducted by preferential pathways, with water parcels moving much more slowly through the soil matrix.…”

Variation of organic matter quantity and quality in streams at Critical Zone Observatory watersheds

“…Processes impacting stream water DOM characteristics are varied and complex, and despite the fact that our observed relationships between atmospheric S and N deposition, Ca and Mg concentrations, and DOM quantity and quality are consistent with previous studies linking DOM response to atmospheric deposition and divalent cation concentrations (Figure ), we acknowledge that other processes play a role. For example, time‐variable streamflow has been shown to influence DOM quantity and quality [ Hood et al ., ; Inamdar and Mitchell , ; Sebestyen et al ., ; Perdrial et al ., ; Bellmore et al ., ]. While the effects of hydrology on stream water quality cannot be ruled out, many of the effects of time‐variable discharge on stream water quality were accounted for by collecting samples during nonstormflow conditions.…”

Sources and subsurface transport of dissolved reactive phosphorus in a semiarid, no‐till catchment with complex topography

“…The decline in DRP concentration over the water year could either be the result of the depletion of P pools, and/or the increasing contribution of P‐depleted areas to the drain line discharge (Reid et al., 2012). The DRP response during individual storm events exhibited a similar flushing behavior to nitrate and dissolved organic carbon (Bellmore et al., 2015; Kelley et al., 2017), presumably because enriched near‐surface soils contributed preferentially to DRP leaching during those periods (Christianson et al., 2016; King et al., 2015).…”