A guide to conservation land management and greenhouse gas emissionsLeft Undamaged bogs sequester carbon very slowly, but hold large quantities of carbon in their peat.The draining of bogs therefore leads to the release of large quantities of CO2 through oxidation of thepeat. Right During the first hundred years following establishment, 1ha of native broadleaved woodlandremoves an average of around seven tonnes of CO2 per year. Iain Masterton/Alamy Stock Photo (left); JohnMorrison/Alamy Stock Photo (right)Semi-natural ‘dry’ habitats and farmlandSemi-natural ‘dry’ habitats on soils that arenot derived from drained peat usually exert acooling effect on the climate. These habitatsinclude lowland heathland and sand dunes, andalso dry grassland, upland heath, dry scrub anddry woodland where they occur on mineral ororgano-mineral soils. The cooling produced bythese habitats is due to their net removal of CO2from the atmosphere through photosynthesis byplants, and the subsequent accumulation of aproportion of the carbon from this in vegetationand soil. Their net fluxes of methane and nitrousoxide are generally negligible, unless subject toartificial fertilisation and/or grazing by ruminants(i.e. cattle and sheep).A number of factors can affect the rate thatthe vegetation and soil accumulate carbon,and thereby the effect of a given habitat on theclimate. For instance, the GHG flux of woodlandvaries in relation to its age, the species, densityand growth rates of trees and any habitatmanagement that takes place (above a certainlevel, planting higher densities of trees willhave little or no effect on the rate of carbonsequestration, because it will not increase theoverall rate of tree growth per unit area).In the case of unmanaged woodland, the netrate of uptake of CO2 is low when the treesare small, while soil disturbance caused whenplanting trees can even initially result in a netrelease of CO2 to the atmosphere. The rate ofuptake of CO2 then increases during the maingrowth phase of the trees, only to slow as thewoodland matures – vegetation biomass per unitarea does not continue to increase indefinitely.Carbon will, though, still tend to continueaccumulating in the soil. Because of this largevariation in GHG flux over time, the GHG fluxof woodland is often estimated over a long periodand then expressed as a mean annual rate. Forexample, in Figure 1 we show the mean annualGHG flux of dry broadleaved woodland over 30years and 100 years from establishment.Arable and drained grassland on organic soils(i.e. those derived from peat) both produce a bigwarming effect per unit area (see Figure 1). Thisis due mainly to the release of large quantitiesof CO2 through oxidation of the dried-out peat,with this being especially high on arable dueto cultivation repeatedly exposing the peat tomicrobial action. In addition, intensive arablefarming on both organic and mineral soils leadsto significant emissions of GHGs from themanufacture and use of nitrate fertilisers appliedto them, and also from the use of machinery (e.g.Williams et al. 2010). Intensive grassland onboth organic and mineral soils also results in the20 Conservation Land Management Summer 2022 | Vol. 20 No. 2
It seems that your browser's pop-up blocker has prevented us from opening a new window/tab. Please click the button below to open the link manually.
