The impacts of drainage, nutrient status and management practice on the full carbon balance of grasslands on organic soils in a maritime temperate zone

Abstract

Abstract. Temperate grasslands on organic soils are diverse due to edaphic properties but also to regional management practices and this heterogeneity is reflected in the wide range of greenhouse gas (GHG) flux values reported in the literature. In Ireland, most grasslands on organic soils were drained several decades ago and are managed as extensive pastures with little or no fertilisation. This study describes a 2-year study of the net ecosystem carbon balance (NECB) of two such sites. We determined GHG fluxes and waterborne carbon (C) emissions in a nutrient-rich grassland and compared it with values measured from two nutrient-poor organic soils: a deep-drained and a shallow-drained site. Carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) fluxes were determined using the chamber technique, and fluvial C fluxes were estimated by combining drainage water concentrations and flows.The nutrient-rich site was an annual source of CO 2 (233 g C m −2 yr −1 ), CH 4 neutral, and a small source of N 2 O (0.16 g N 2 O-N m −2 yr −1 ). Net ecosystem exchange (NEE) at the shallow-drained nutrient-poor site was −89 and −99 g C m −2 yr −1 in Years 1 and 2 respectively, and NEE at the deep-drained nutrient-poor site was 85 and −26 g C m −2 yr −1 respectively. Low CH 4 emissions (1.3 g C m −2 yr −1 ) were recorded at the shallow-drained nutrient-poor site. Fluvial exports from the nutrient-rich site totalled 69.8 g C m −2 yr −1 with 54 % as dissolved organic C. Waterborne C losses from the nutrient-poor site reflected differences in annual runoff totalling 44 g C m −2 yr −1 in Year 1 and 30.8 g C m −2 yr −1 in Year 2.The NECB of the nutrient-rich grassland was 663 g C m −2 yr −1 with biomass exports being the major component accounting for 53 %. The NECB of the nutrient-poor deep-drained site was less than half of the nutrient-rich site (2-year mean 267 g C m −2 yr −1 ). Although NEE at the nutrient-poor shallow-drained site was negative in both years, high biomass export meant it was a net C source (2-year mean NECB 103 g C m −2 yr −1 ). While the impacts of the nutrient and drainage status on NEE, biomass exports and fluvial C losses were confirmed, inter-regional differences in management practice and climate were also significant factors which impacted on the overall NECB of these ecosystems. Contrary to expectation, the NECB of nutrient-poor drained organic soils under grasslands is not necessarily a large C source and this has implications for Ireland's choice of national GHG inventory reporting methodologies. This study can also aid the development of strategies to deliver reduced emissions tailored to local grassland types.

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

“…Indeed respiration appreciably exceeded Gross Primary Production (GPP) at Site B, but was similar to GPP at Site A ( Fig. 5; Renou-Wilson et al 2014).…”

“…Thus compared to DOC, fluvial CO 2 fluxes and evasion have generally accounted for smaller fractions of the net terrestrial C balances of undisturbed temperate peatland systems, however this was not the case at our sites. The component fluxes of the terrestrial C balance of the study sites given by Renou-Wilson et al (2014) are shown in Fig. 5 for comparison with the fluvial C fluxes.…”

“…The combination of grasslands over peat in concert with artificial drainage may therefore play a disproportionate role in regional C dynamics but has yet to receive study. To address this gap and provide fluvial components for an allied terrestrial C balance study (Renou-Wilson et al 2014) we determined organic and inorganic fluvial C fluxes at small spatial scales for two such sites. In addition we examine the potential for remineralisation of dissolved organic matter (DOM) exports and the associated timescales using high resolution biological oxygen demand measurements over 90 days.…”

Divergent NEE balances from manual‐chamber CO2 fluxes linked to different measurement and gap‐filling strategies: A source for uncertainty of estimated terrestrial C sources and sinks?

“…This problem can be overcome by adequately pooling flux data to the respective plant phenological stages (cluster‐wise approaches). Although the improvement is small compared with the overall variability and therefore may not be significant, this result fits well with the recommendations of other studies that have suggested the use of vegetation proxies to account for plant phenological stages for improved R ECO and NEE modeling ( Burrows et al, ; Kandel et al, ; Renou‐Wilson et al, ).…”

The role of waterborne carbon in the greenhouse gas balance of drained and re-wetted peatlands

“…Wallin et al, 2010;Billett and Harvey, 2013), which could explain lower direct measurements of CO2 emissions from drainage ditches in the studies described above. Comparing two drained peatlands under grassland, Renou-Wilson et al (2014) recorded mean excess CO2 in drainage water of 4.3 g C m -2 yr -1 in a nutrient rich site, and 16 g C m -2 yr -1 in a nutrient-poor site, the latter representing around 60% of the total fluvial C flux during a dry year.…”