The nitrogen, carbon and greenhouse gas budget of a grazed, cut and fertilised temperate grassland

Abstract. Intensively managed grazed grasslands in temperate climates are globally important environments for the exchange of the greenhouse gases (GHGs) carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). We assessed the N and C budget of a mostly grazed and occasionally cut and fertilised grassland in SE Scotland by measuring or modelling all relevant imports and exports to the field as well as changes in soil C and N stocks over time. The N budget was dominated by import from inorganic and organic fertilisers (21.9 g N m⁻² a⁻¹) and losses from leaching (5.3 g N m⁻² a⁻¹), N₂ emissions (2.9 g N m⁻² a⁻¹), and NO_x and NH₃ volatilisation (3.9 g N m⁻² a⁻¹), while N₂O emission was only 0.6 g N m⁻² a⁻¹. The efficiency of N use by animal products (meat and wool) averaged 9.9 % of total N input over only-grazed years (2004–2010). On average over 9 years (2002–2010), the balance of N fluxes suggested that 6.0 ± 5.9 g N m⁻² a⁻¹ (mean ± confidence interval at p > 0.95) were stored in the soil. The largest component of the C budget was the net ecosystem exchange of CO₂ (NEE), at an average uptake rate of 218 ± 155 g C m⁻² a⁻¹ over the 9 years. This sink strength was offset by carbon export from the field mainly as grass offtake for silage (48.9 g C m⁻² a⁻¹) and leaching (16.4 g C m⁻² a⁻¹). The other export terms, CH₄ emissions from the soil, manure applications and enteric fermentation, were negligible and only contributed to 0.02–4.2 % of the total C losses. Only a small fraction of C was incorporated into the body of the grazing animals. Inclusion of these C losses in the budget resulted in a C sink strength of 163 ± 140 g C m⁻² a⁻¹. By contrast, soil stock measurements taken in May 2004 and May 2011 indicated that the grassland sequestered N in the 0–60 cm soil layer at 4.51 ± 2.64 g N m⁻² a⁻¹ and lost C at a rate of 29.08 ± 38.19 g C m⁻² a⁻¹. Potential reasons for the discrepancy between these estimates are probably an underestimation of C losses, especially from leaching fluxes as well as from animal respiration. The average greenhouse gas (GHG) balance of the grassland was −366 ± 601 g CO₂ eq. m⁻² yr⁻¹ and was strongly affected by CH₄ and N₂O emissions. The GHG sink strength of the NEE was reduced by 54 % by CH₄ and N₂O emissions. Estimated enteric fermentation from ruminating sheep proved to be an important CH₄ source, exceeding the contribution of N₂O to the GHG budget in some years.