Annual emissions of CH<sub>4</sub> and N<sub>2</sub>O, and ecosystem respiration, from eight organic soils in Western Denmark managed by agriculture

Abstract

The use of organic soils by agriculture involves drainage and tillage, and the resulting increase in C and N turnover can significantly affect their greenhouse gas balance. This study estimated annual fluxes of CH₄ and N₂O, and ecosystem respiration (<i>R</i>_eco), from eight organic soils managed by agriculture. The sites were located in three regions representing different landscape types and climatic conditions, and three land use categories (arable crops, AR, grass in rotation, RG, and permanent grass, PG) were covered. The normal management at each site was followed, except that no N inputs occurred during the monitoring period from August 2008 to October 2009. The stratified sampling strategy further included six sampling points in three blocks at each site. Environmental variables (precipitation, PAR, air and soil temperature, soil moisture, groundwater level) were monitored continuously and during sampling campaigns, where also groundwater samples were taken for analysis. Gaseous fluxes were monitored on a three-weekly basis, giving 51, 49 and 38 field campaigns for land use categories AR, PG and RG, respectively. Climatic conditions in each region during monitoring were representative based on 20-yr averages. Peat layers were shallow, typically 0.5 to 1 m, and with a pH of 4–5. At six sites annual emissions of N₂O were in the range 3 to 24 kg N₂O-N ha⁻¹, but at two arable sites (spring barley, potato) net emissions of 38 and 61 kg N₂O-N ha⁻¹ were recorded. Both were characterized by fluctuating groundwater with elevated SO₄^2&minus; concentrations. Annual fluxes of CH₄ were generally small, as expected, ranging from –2 to 4 kg CH₄ ha⁻¹. However, two permanent grasslands had tussocks of <i>Juncus effusus</i> (soft rush) in sampling points that were consistent sources of CH₄ throughout the year. Emission factors for organic soils in rotation and permanent grass, respectively, were estimated to be 0.011 and 0.47 g m⁻² for CH₄, and 2.5 and 0.5 g m⁻² for N₂O. This first documentation of CH₄ and N₂O emissions from managed organic soils in Denmark confirms the levels and wide ranges of emissions previously reported for this region. However, the factorial approach also identified links between gaseous emissions and site-specific conditions with respect to soil, groundwater and vegetation which point to areas of future research that may account for part of the variability and hence lead to improved emission factors or models

Multiyear greenhouse gas flux measurements on a temperate fen soil used for cropland or grassland

“…, a summer crop with bare soil before and after cropping. On the contrary, a fen in Denmark, cultivated with barley undersown by grass, revealed a higher cumulated annual R eco than our cropland site ( Elsgaard et al, 2012; Petersen et al, 2012). The difference between the Danish studies was possibly also due to a higher autotrophic respiration of the under sown grass compared to the system without under seed.…”

“…Nevertheless, there are few direct measurements of greenhouse gas emissions from arable fens ( Maljanen et al, 2007), especially on deeply drained temperate fens, using the chamber technique ( Petersen et al, 2012; Elsgaard et al, 2012; Kandel et al, 2013). Most studies were either conducted for a time period of only one or two years or not all three GHGs were considered.…”

Regional greenhouse gas emissions from cultivation of winter wheat and winter rapeseed for biofuels in Denmark

“…However, it was considered doubtful to treat the present organic soils according to the fixed IPCC EF, because the definitions of organic soils in the Danish JB (jordbonitet) soil classification system and the IPCC guidelines are very different: in the Danish system, soils are classified as organic if they have a soil organic carbon (SOC) content higher than 5.9% (Greve & Breuning-Madsen, 2005), whereas the IPCC definition of organic soils (IPCC, 2006) implies a SOC content higher than 12% (this latter definition applies to soils that are never saturated with water for more than a few days). Thus, whereas high N 2 O emissions have indeed been found for Danish agricultural peat soils (with 16Á47% SOC in the top soil; Petersen et al, 2012), the area of such soils is much lower than suggested from the occurrence of soils classified as organic in Denmark …”

Evaluating effects of land management on greenhouse gas fluxes and carbon balances in boreo-temperate lowland peatland systems

“…Whilst no heterogeneity was found across studies, the studies by Petersen et al [40] showed much higher variability than the other two studies, possibly because they used nationwide survey data rather than site-based experimental comparisons. The Petersen et al [40] studies were classed as 'unclear' in their susceptibility to bias, and more information on the methodology Figure 14 Forest plot of meta-analysis of effect sizes (raw mean difference) comparing N 2 O flux from wet versus dry peatlands. The dashed line represents no effect.…”