Magnitude, form and bioavailability of fluvial carbon exports from Irish organic soils under pasture

Abstract

Organic soils are widespread in Ireland and vulnerable to degradation via drainage for agriculture. The soil-landuse combination of pasture on organic soils may play a disproportionate role in regional C dynamics but is yet to receive study. Fluvial C fluxes and labile organic fractions were determined for two such sites at nested field (c.4 ha) and subcatchment scales ([40 ha); one relatively dry and nutrient rich, the other wetter and nutrient poor. Field scale flux from the nutrient poor site over 2 years was 38.9 ± 6.6 g C m -2 yr -1 with DIC [ DOC [ POC at 57, 32 and 11 % respectively, and 72 % DIC was comprised of above equilibrium CO 2 . At the nutrient rich site, which overlies limestone geology, field scale export over an individual year was 90.4 g C m -2 with DIC [ DOC [ POC at 49, 42 and 9 %, but with 90 % DIC as bicarbonate. By comparison with the nutrient poor site, the magnitude and composition of inorganic C exports from the nutrient rich site implied considerable export of soil-respiratory C as bicarbonate, and lower evasion losses due to carbonate system buffering. Labile DOC determined using dark incubations indicated small fractions (5-10 %) available for remineralisation over typical downstream transit times of days to weeks. These fractions are probably conservative as photolysis in the environment can increase the proportion of labile compounds via photocleavage and directly remineralise organic matter. This study demonstrates that monitoring at soil-water interfaces can aid capture of total landscape fluvial fluxes by precluding the need to incorporate prior C evasion, although rapid runoff responses at field scales can necessitate high resolution flow proportional, and hydrograph sampling to constrain uncertainty of flux estimates.

The full carbon balance of a rewetted cropland fen and a conservation-managed fen

“…Dissolved CO 2 was ~18% of total fluvial C export, larger than the POC flux, whilst CH 4 (which has a low solubility in water) made a negligible cont %). Our dissolved CO 2 fluxes of 1.2 g C m -2 yr -1 and 1.8 g C m -2 yr -1 are similar to that reported from UK a raised bog (1.3 g C m -2 yr -1 , Dinsmore et al, 2010) but smaller than those from blanket bogs (3.8 g C m -2 yr -1 , Worrall et al, 2003) and drained Irish grasslands (2.4-4.4 g C m -2 yr -1 , Barry et al, 2014). It is likely that this is because slow water movement in ditches results in the majority of aquatic CO 2 being lost on-site as gaseous fluxes, rather than exported off-site fluvially.…”

Minimal climate change impacts on natural organic matter forecasted for a potable water supply in Ireland

“…Present day simulated average yearly DOC export from the Boyne catchment (5.8 ± 1.3 g C m −2 year −1 ) falls in the lower spectrum of values reported for peat catchments (Barry et al, 2016;de Wit et al, 2016b). However, the Boyne catchment has approximately 4% peatland and therefore with respect to agricultural catchments falls in the higher spectrum of values reported (Kronholm and Capel, 2012).…”

The historical dependency of organic carbon burial efficiency

“…data) and up‐scaled to account for missing sources (Carvalho et al ), plus a small amount released from the lake sediment (Scott ), giving a total DOC loading of 3.67 g C m −2 if all DOC was sedimented to the lake floor. Terrestrial POC (TPOC) input is more difficult to estimate but there is consensus that loadings are less than for DOC (Worrall and Moody ; Barry et al ). Even if catchment losses are set as high as 50% of values for DOC, given significant mineralization in transit to and while in the lake (Worrall and Moody ), we conservatively estimate a TPOC loading of 6.88 g C m −2 to the lake floor.…”