Articles | Volume 14, issue 8
Biogeosciences, 14, 2069–2088, 2017
Biogeosciences, 14, 2069–2088, 2017

Research article 24 Apr 2017

Research article | 24 Apr 2017

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

Stephanie K. Jones1,2, Carole Helfter1, Margaret Anderson1, Mhairi Coyle1, Claire Campbell1, Daniela Famulari1, Chiara Di Marco1, Netty van Dijk1, Y. Sim Tang1, Cairistiona F. E. Topp2, Ralf Kiese3, Reimo Kindler4, Jan Siemens5, Marion Schrumpf6, Klaus Kaiser7, Eiko Nemitz1, Peter E. Levy1, Robert M. Rees2, Mark A. Sutton1, and Ute M. Skiba1 Stephanie K. Jones et al.
  • 1Centre for Ecology and Hydrology, Edinburgh, Bush Estate, Penicuik, Midlothian, EH26 QB, UK
  • 2Scotland's Rural College, King's Buildings, West Mains Road, Edinburgh, EH9 3JG, UK
  • 3Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU), Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany
  • 4Department of Waste Management and Environmental Research, Technische Universität Berlin, Franklinstr. 29, 10587 Berlin, Germany
  • 5Chair of Soil Resources, Institute of Soil Science and Soil Conservation, iFZ Research Centre for BioSystems, Land Use and Nutrition, Justus Liebig University Giessen, Heinrich-Buff-Ring 26–32, 35392 Giessen, Germany
  • 6Department for Biogeochemical Processes, Max Plank Institute for Biogeochemistry, Hans-Knöll-Str. 10, 07745 Jena, Germany
  • 7Soil Science and Soil Protection, Martin Luther University Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle (Saale), Germany

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 NOx 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.

Short summary
We assessed the nitrogen (N), carbon (C) and greenhouse gas (GHG) budget from an intensively managed grassland in southern Scotland using flux budget calculations as well as changes in soil N and C pools over time. Estimates from flux budget calculations indicated that N and C were sequestered, whereas soil stock measurements indicated a smaller N storage and a loss of C from the ecosystem. The GHG sink strength of the net CO2 ecosystem exchange was strongly affected by CH4 and N2O emissions.
Final-revised paper