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. 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 (CO₂), methane (CH₄) and nitrous oxide (N₂O) 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 CO₂ (233 g C m⁻² yr⁻¹), CH₄ neutral, and a small source of N₂O (0.16 g N₂O-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 CH₄ 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.