Permanent grassland on organic soils

Organic soils under permanent grassland play a critical role for carbon and nutrient cycling, locally and globally. However, their environmental impact – whether positive or negative – is to the largest extent de-pending on soil water conditions.

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3.2.1 Drained organic soils

Drainage of organic soils accelerates decomposition of organic matter, further enhancing mineralization of organic nitrogen (N) and phosphorus (P), hence leading to leaching of e.g. carbon (C), N and P into bodies of water. However, the actual effect and magnitude of leaching is strongly depending on catchment hy-drology and site-specific biogeochemical conditions (Tuukanen et al., 2017).

With most organic soils being environmental sensitive areas, harvesting and removal of biomass results in an export of excess N and P from soil, mitigating nutrient leaching with efficiencies of up to 92% (Jabłońska et al., 2020). However, organic soils are very inhomogeneous, with some areas delivering a high amount of nutrients from peat mineralization and some showing a lack of potassium (K). Fertilization should hence be restricted in areas characterized by nutrient losses, while in some areas site-specific fertilization with deficit-nutrients has shown to significantly increase biomass yields as well as removal of excess N and P (Nielsen et al., 2013). In conclusion, general estimates of nutrient losses from drained organic soils cannot be made due to site-specific hydrological and biogeochemical interactions. However, biomass harvest and removal under appropriate fertilization clearly reduces the risk of nutrient leaching (Jørgensen & Schelde, 2011).

In addition, drained organic soils are hotspots of greenhouse gas emissions (GHG) due to stimulation of oxidative processes, enhancing emissions of carbon dioxide (CO2) and nitrous oxide (N2O).

The estimated global warming potential by GHG emissions of permanent grassland in temperate climate is reported within the range of 17 – 30 t CO2eq/ha/year for drained fens (lowland areas), depending on drainage depth, and 25 t CO2eq/ha/year for bog peatlands (Wilson et al, 2016). Specifically for Denmark, the range of reported GHG from agriculturally used drained organic soils is between 3.5 to 13.6 t C/ha/year and up to 61 kg N2O-N/ha/year, respectively (Elsgaard et al., 2012, Petersen et al., 2012). Biomass produc-tion on drained organic soils does not mitigate these GHG emissions.

3.2.2 Rewetting of organic soils

Paludiculture is the term for a production system that combines rewetting and biomass production with flood-tolerant crops (Tanneberger & Wichtmann, 2011). Rewetting of formerly drained peatlands is a sug-gested mitigation option in terms of reducing CO2 emissions and restoring the ecosystem carbon sink func-tion (Joosten et al., 2012). In this context, rewetting of drained peatlands has been included as a potential target for climate change mitigation in the Kyoto protocol (IPCC, 2014). Paludiculture has further been sug-gested as a promising option to reduce anthropogenic CO2 emissions from peatlands, while at the same time facilitating continued agricultural biomass production (Tanneberger & Wichtmann, 2011). In Germany, emission reductions following rewetting were reported with up to 89% for fen (lowland) organic soils, and 70% for bogs (Drösler et al., 2013). In a Danish context, it is estimated that rewetting of drained peatlands will reduce GHG emissions by approximately 3-15 t CO2eq/ha/year (Wilson et al., 2016, Nielsen et al., 2021b). Even though rewetting of organic soils will in most circumstances lead to increases in methane

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(CH4) emissions, these will be offset by significant reductions of CO2 and N2O emissions (Günther et al., 2020). It is not expected that biomass harvest will have a significant negative effect on the net GHG miti-gation potential of rewetting (Günther et al., 2015).

In addition to effects of rewetting and paludiculture on GHG emissions, associated effects on potential nu-trient discharges to water bodies are likely. The environmental effects of a raised water table will lead to changes in leaching of nutrients as soil redox conditions are decreased due to restricted oxygen (O2) diffu-sion. In this context N and P biogeochemical processes are of special interest. Anaerobic conditions favour denitrification, i.e., microbial removal of nitrate (NO3-), possibly in competition with plant NO3- uptake (e.g., Kaye and Hart, 1997). On the other hand, anaerobic conditions decrease the adsorption of P to iron (Fe) and manganese (Mn) oxides due to microbial reduction of these minerals (Hoffmann et al., 2009). Conse-quently, P may be released to the soil solution and discharged to downstream vulnerable recipients. Yet, the P uptake by harvested and exported crops in paludiculture may mitigate the high P mobilisation at least during the growing season (Zak et al., 2014). However, the majority of results on the prevention of nutrient leaching by paludiculture currently is known from macrophyte species such as cattail (Typha spp.) and common reed (Phalaris spp.), being not suitable as biomass feedstock for biorefineries if the focus is on protein. Hence, the effect of grass species in paludiculture on the mitigation potential of nutrient leaching remains to be more evaluated in the future.

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One possibility to mitigate high nutrient discharges on organic soils is topsoil removal (e.g. Zak et al., 2017;

Huth et al., 2020) prior to the cultivation of suitable perennial grasses. Topsoil removal has shown the po-tential to avoid elevated amounts of nutrients in topsoil, otherwise prone to leaching, while simultaneously mitigating methane emissions by a factor of up to 400 (Huth et al, 2020). However, the feasibility of such measures needs to be documented.

In conclusion, rewetting of organic soils and the subsequent conversion to permanent grassland for biomass production is a promising option to mitigate adverse environmental impacts such as GHG emissions and nutrient leaching into bodies of water, while keeping up biomass production. However, detailed site-spe-cific magnitudes of reductions in a Danish context remain to be documented.