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Module 7 Suggestions for Guidance on the Likely Effects of Changing Land Use from Grazing or Semi-Natural Vegetation to Forestry on Soil C and N in Organic Soils

7.1 An assessment of the likely effects of changing land use from grazed or semi-natural vegetation to forestry on carbon stores and fluxes in upland organo-mineral soils in the UK

The following report is a summary of an extensive review (Reynolds, 2007) exploring the likely effects on organic carbon stores and fluxes of converting semi-natural and extensively grazed natural vegetation to forestry on upland organo-mineral soils, and does not include deep peats.

As a party to the United Nations Framework Convention on climate change, the UK is required to protect and enhance the sinks and reservoirs for greenhouse gases. Within the UK, approximately 30% (1357 Tg to a depth of 1 m) of the soil organic carbon ( SOC) stock is held in peat soils, with a further 22% in organo-mineral soils (Bradley, et al., 2005). In relation to land use, only 9% of the UKSOC stock resides in forest and woodland soils, although the carbon density of these soils is relatively high (25 kg m ^-2 or 250 tC ha ^-1) compared to pasture and arable soils (16 and 12 kg m ^-2 or 160 and 120 tC ha ^-1 respectively; Bradley et al., 2005). Organo-mineral soils may well become increasingly important for the future expansion of forestry in the uplands if the extensive grazing agriculture they currently support becomes economically marginal because of CAP reform. An assessment of the likely effects on SOC stocks of this potentially major change in land use is therefore required.

Assessments of the effects of afforestation on soil organic carbon stocks in the UK have relied heavily on modelling approaches (Dewar & Cannell, 1992; Cannell et al., 1993; Cannell and Dewar 1995) which have subsequently contributed to the national carbon inventories for the UK (Cannell et al., 1999; Milne et al., 2000). As far as SOC is concerned, Cannell et al., (1999) consider three types of soil; peats, upland organo-mineral soils and mineral soils. For the purposes of the carbon inventory, the latter are assumed to be planted with broadleaves and are predicted to gain SOC as the trees add subtantial quantities of litter to soils with relatively low organic matter content, including former agricultural land (Cannell and Dewar 1995). For peats, information from field measurements of net ecosystem carbon exchange suggest that these will be a small net source of carbon resulting from the effects of drainage and soil drying by the trees (Hargreaves et al., 2003). Upland organo-mineral soils under plantation conifer forest are considered to be 'carbon neutral' having gained as much carbon from forest litter as has been lost through accelerated decomposition during site preparation and drainage. This assumption has been evaluated in this module.

Planting trees can affect the ecosystem carbon balance in two opposing ways. Drainage and disturbance during site preparation and planting may lead to higher SOC losses as increased microbial respiration rates, aeration and disruption of soil aggregates lead to higher rates of organic matter decomposition. Conversely, carbon will be sequestered by tree growth while leaf and needle fall will contribute to the accumulating litter layer at a faster rate than for the pre-existing ground vegetation. Tree roots and root litter will also contribute to the below ground carbon stock.

Two main approaches were taken to assess the likely effects of changing land-use from grazed acid grassland or semi-natural vegetation to forestry on SOC in organo-mineral soils of the UK. These were:

1) Direct measurement of change in soil organic carbon
2) Measurement of biogeochemical fluxes to produce a carbon budget

As there are relatively few directly relevant studies, the review has drawn on studies of related systems to draw inferences and conclusions about the likely effects of afforestation on the SOC balance of organo-mineral soils

7.1.1 Evidence from direct measurement of SOC stocks

In the context of organo-mineral soils in the UK uplands, the evidence from direct measurements of SOC was inconclusive about a major effect on SOC stocks following a change in land use from semi-natural grassland / moorland to forestry. International studies suggest that planting broadleaf trees will have little effect on SOC stocks, whereas conifer planting, especially in high rainfall areas may lead to depletion of SOC stocks (Guo and Gifford 2002) However, it is important to note that the meta-analysis conducted by Guo and Gifford (2002) was dominated by studies from New Zealand and the conifer.species considered were mainly pines. Few relevant UK data sets exist which address the issue and the experimental data presented in Module 2 of this report appear to contradict the findings of the international meta-analysis. The inherent problems of soil heterogeneity, vertical gradients in organic matter content and bulk density, contribute uncertainty to making these types of comparison. Furthermore, the extent to which results from other systems, for example peatlands and lowland agricultural soils can be extended to upland organo-mineral soils is also very uncertain. Rooting patterns and tree growth rates will differ considerably, and the effects of tree species must also be taken into account.

There is evidence that organic matter accumulates at the surface of organo-mineral forest soils, but the long-term fate of this material is uncertain and depends on the extent to which it is incorporated into the long-term, stable organic carbon pool within the soil profile (Pataki et al., 2003). This in turn is likely to depend on the tree species, soil type, site nutrient status, site hydrology and climate. Some studies reported in a review by Post and Kwon, (2000) recorded a loss of organic carbon at depth in the profile, whilst observing accumulation at the surface. In these cases, organic carbon inputs during early growth of the forest were insufficient to replenish decomposition losses lower in the profile. A decrease in the recalcitrant soil organic carbon pool was predicted by the MERLIN model in a simulation of planting and growth of a 30 year old Sitka spruce plantation which replaced moorland vegetation growing on acid peaty podzols in north Wales (Emmett et al., 1997). The data used to parameterise the model, which came from a chronosequence of Sitka spruce plantations on similar soils, showed, however, a net accumulation of organic carbon by the ecosystem, both as new wood and in the labile soil organic matter pool mainly in the forest floor. Evidence of a loss of 'old' soil organic carbon during forest development also comes from a study of the ¹⁴C signal in soil solution dissolved organic carbon ( DOC) by Karltun et al. (2005). Samples of DOC were collected from the transition zone between the A and B soil horizons of a forest chronosequence comprising twelve sites in southwest Sweden, ranging from agricultural land to 89 year old, first generation Norway spruce. In order to explain the observed changes in ¹⁴C signal along the chronosequence, the authors proposed that two processes were occurring simultaneously; namely changes in litter input and increased mobilisation of soil organic carbon formed before afforestation. The implication is that any assessment of SOC stock change in response to afforestation must extend down to the base of the rooting zone in order to account for potential losses of SOC from the subsoil as well accumulation at the soil surface.

Leaf and needle litter accumulation is an important process for transferring newly fixed carbon to the the surface of forest soils. Another significant but much less well understood and quantified pathway is the exchange of carbon between plant roots and the soil (Jones et al., 2004). This has the potential to directly transfer newly fixed carbon into the soil at depth where it may be subsequently incorporated into the stable, long-term organic carbon pool. Recent work suggests that carbon sequestration via this pathway may decline as atmospheric CO ₂ concentrations rise, with obvious implications for the terrestrial carbon sink (Heath et al., 2005).

7.1.2 Evidence from flux studies

The questions addressed by flux measurements essentially come down to how much of the carbon exchanged with the atmosphere finds its way into the stable, long-term SOC store, and how much do the other flux pathways offset the carbon sink.

In order to summarise the review findings, a simple matrix was constructed which provides a qualitative indication of the likely effects of forestry development on individual fluxes (Table 1). The summary suggests that overall, there is likely to be little net effect of forest development on many of the carbon fluxes, particularly for DOC and Dissolved Inorganic Carbon ( DIC), although the latter is uncertain. Particulate Organic Carbon ( POC) fluxes are likely to be enhanced at all stages of the forest cycle, but changes in forest management practice in line with current Guidelines should constrain these losses (Nisbet et al., 1997; Forestry Commission, 1998; HMSO, 2003).

The effect of forest harvesting on net CO ₂ exchange may be neutral depending on whether carbon lost to the atmosphere from increased decomposition rates and microbial activity is subsequently balanced by uptake of carbon in re-growing vegetation. This is likely to be very site specific and will be determined to a great extent by harvest residue management and site environmental conditions. However some measurement campaigns similar to those conducted during the afforestation of peatland sites by Hargreaves et al. (2003) would provide useful data.

Table 7.1. Suggested effects of forest development on carbon fluxes in organo-mineral soils .

Symbols:
First symbol represents direction of flux, where negative sign represents depletion of atmospheric carbon.
Second symbol represents the change in the magnitude of the flux with respect to semi-natural or extensively grazed moorland / acid grassland. '0' means that there is no change as a result of changing land use; '+' means an increase in flux with respect to that measured in semi-natural moorland / grassland.

In conclusion, the overall effects on soil organic carbon stocks of land use change from semi-natural, extensively grazed vegetation to forestry on organo-mineral soils can be summarised as follows:

• There have been few directly relevant studies of the effects on soil organic carbon stocks of afforestation of organo-mineral soils, so all conclusions about the likely effects of land use change to forestry are inferential from related studies on UK peatlands and studies from abroad.
• It is not possible yet to identify likely effects on soil organic carbon stocks down to individual soil types or even broad categories of freely and non-freely drained soils.
• The overall conclusion (as assumed by UK carbon balance models) is that afforestation probably has little net effect on soil organic carbon stores in organo-mineral soils, but this is a very uncertain statement.

The main areas of uncertainty for organo-mineral soils are:

• What is the net effect on the soil organic carbon store of planting and disturbance versus subsequent carbon capture by trees and inputs to soil as litter?
• What effects do forest harvesting have on soil organic carbon stores?
• To what extent is the gain in soil organic carbon from litter deposited at the soil surface offset by any loss of pre-existing soil organic carbon from the subsoil?
• How much of the carbon captured by trees and deposited as litter finds its way into the "stable" soil organic carbon store?
• What are the effects of different species and forest management regimes (eg continuous cover forestry vs. conventional patch clearfell) on soil organic carbon storage?

7.2 Provisional management guidance

Some provisional guidance for managing the effects of afforestation on SOC stocks in organo-mineral soils can be suggested from the outcome of the review. These are summarised in Table 7.2. Currently this guidance must be regarded as very preliminary it is based on limited data and has not been tested against modelled scenario outcomes. However the broad principles behind the guidance are to:

i) Minimise disturbance during any site operations so as to limit particulate organic carbon losses and to reduce the opportunities for mineralising the pre-existing soil organic carbon store, for example by limiting soil drying and aeration

ii) Take steps to maintain the net CO ₂ sink strength at all stages of the forestry cycle by maximising vegetative cover

Table 7.2. Guidance for managing the effects of land use change from semi-natural moorland / acid grassland to forestry on upland ogano-mineral soils.

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