Importance of the vegetation-groundwater-stream continuum to understand transformation of biogenic carbon in aquatic systems – A case study based on a pine-maize comparison in a lowland sandy watershed (Landes de Gascogne, SW France)

Abstract

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O 2). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO 3 − ; on average

Carbon dioxide dynamics in an agricultural headwater stream driven by hydrology and primary production

“…Based on high-frequency data, CO 2 concentrations in streams draining nutrient-poor forest and peatlands, as well as tropical forests, are often found related to variations in stream discharge but with site-specific response patterns, with CO 2 found to be either positively or negatively related to stream discharge (Crawford et al, 2017;Dinsmore et al, 2013;Johnson et al, 2007). These response patterns have often been connected to the catchment characteristics and changes in hydrological pathways, which in turn control the dominant source areas (both from a vertical and lateral point of view) of CO 2 in the catchment soils (Campeau et al, 2018;Leith et al, 2015;Dinsmore and Billett, 2008). In contrast, other catchments lack a strong hydrological control and instead display clear diel cycles in stream CO 2 concentration, indicating a metabolic control (Crawford et al, 2017).…”

Partitioning carbon sources in a tropical watershed (Nyong River, Cameroon) between wetlands and terrestrial ecosystems – Do CO<sub>2</sub> emissions from tropical rivers offset the terrestrial carbon sink?

“…The Awout River was dry during the previous base flow period: its riverbed was completely invested by large macrophytes (up to 2 m tall) and many small pockets of stagnating water remained (visual observations). During base flow period, DOC could accumulate in these stagnating waters and then it is remobilized when the water flows again, as observed in temperate rivers (Deirmendjian et al, 2019;Sanders et al, 2007). As we do not see a general increase of DOC in base flow period in the whole watershed we believe that these local vegetation regrowth in river beds and riverbanks have a minor impact of the whole carbon balance in the Nyong watershed.…”

Floods increase carbon dioxide and methane fluxes in agricultural streams

“…For streams draining low‐relief agricultural areas, fine sediment deposits and nutrient enrichment typical of these systems can favour high rates of production and concentrations of both gases during baseflow periods (e.g. Crawford & Stanley, 2016; Deirmendjian et al., 2019; Romeijn et al., 2019), but evasion may be constrained by relatively low gas exchange velocities associated with modest channel slopes (Bodmer, Heinz, Pusch, Singer, & Premke, 2016; Borges et al., 2018). A key finding of this study is that while gas sources may have shifted to include inputs from soil flushing, evasion during flood periods was not supply limited, nor was it strongly transfer limited because of higher stream flow velocities.…”