Nitrous oxide emissions from white clover based grassland used for dairy production under moist maritime climatic conditions
W. Burchill1, 2, D. Li3, G. Lanigan4, M. Williams2 and J. Humphreys1
1Animal & Grassland Research and Innovation Centre, Teagasc, Moorepark,, Ireland;
2Department of Botany, TrinityCollegeDublin, College Green, Dublin 2, Ireland;
3Department of Botany and Microbiology, the University of Oklahoma, USA;
4JohnstownCastle Environment Research Centre, Teagasc, Johnstown Castle, Ireland.
There is uncertainty about the extent of annual nitrous oxide (N2O) emissions from grazed white clover-based grassland used for dairy production. This is due to the large temporal variation attributable to varying climatic conditions and heterogeneity of N concentrations in the soil due to random distribution of excreta by grazing livestock. The objective of this study was to measure the annual N2O emissions from grazed white clover-based grassland used for dairy production.
The study was conducted at Solohead Research Farm (52˚51’N, 08˚21’W). Poorly drained gley (90%) and grey-brown podzolic (10%) soils with a clay loam texture (28.3% clay, 35 % silt) and low permeability predominate. The soils are seasonally wet, waterlogged or flooded due to impeded drainage.
N2O emissions from (i) white-clover based dairy production system and (ii) white clover-ryegrass plots receiving no input of N and not grazed (background emissions) were measured over three years between October 2009 and November 2011. The dairy system was rationally grazed by dairy cows and received annual fertilizer N inputs of 100 kg ha-1. Herbage mass was measured before each grazing event and the N content of herbage was subsequently determined and used to calculate annual N uptake in herbage dry matter (DM). Annual surplus rainfall (mm) was 701 mm in 2008/09, 687 in 2009/10 and 394 in 2010/11.
Annual N uptake (kg ha-1) in herbage from the dairy system was 249, 267 and 408 for the three years respectively; there was substantially higher mineralization of N in the soil in the year with low surplus rainfall (2010/11) compared with the two wetter years. This has a direct impact on annual N2O emissions (kg ha-1 of N), which were (mean ± SE): 2.7 ± 0.16, 6.4 ± 1.14 and 28.0 ± 5.01 for the three years respectively.N2O emissions in year three was significantly higher (P<0.001) than in year one and two, which were not significantly different from each other.
Annual background N2O emissions (kg ha-1 of N) followed a similar trend with substantially higher emissions in 2010/11 (mean ± SE): 1.0 ± 0.50, 2.5 ± 0.12 and 6.3 ± 1.28 for the three years respectively. Differences between years in annual rainfall impacted directly on the mineralization of N on this heavy soil with impeded drainage. The extent of mineralization of N in the soil was reflected in uptake of N in herbage DM and in N2O emissions from the soil. The sharp contrast in annual emissions found in this study also suggests the need for long term studies when quantifying annual N2O emissions from pasture-based livestock production systems.