Summary
Carbon sequestration in soils that have previously been depleted of organic matter due to agriculture is an important component of global strategies to mitigate rising atmospheric CO2 concentrations. Extensive areas of low productivity farmland have been abandoned from agriculture in eastern North America and elsewhere over the past century, and are naturally regenerating to temperate forests. We investigated the soil carbon sequestration potential of such lands by sampling adjacent mature forest and agricultural field sites, and replicated chronosequences of forest succession on Podzol, Brunisol, and Luvisol soil types that are considered ‘marginal’ for agriculture and have been abandoned extensively across southeastern Ontario, Canada. Total soil organic carbon and nitrogen stocks to 10 cm depth were approximately 32% and 18% lower, respectively, in agricultural fields compared to mature forests. Furthermore, carbon stocks across our 100-year chronosequences increased most within the 0–5 cm soil depth interval, tended to increase within the 5–10 cm interval, and were unaltered within the 10–20 cm interval. Soil type had little effect on the potential magnitude or rates of soil carbon sequestration (~10 g C m−2 y−1 in the top 10 cm), perhaps because all sites shared a common vegetation successional pattern. Finally, our investigations of the ‘labile’ free-light carbon and nitrogen fractions in the Brunisol soil type indicated no increases across the chronosequence, implying that soil carbon accumulation was primarily in more recalcitrant pools. Our results indicate that each of these low productivity soil types can be moderate carbon sinks for a century following agricultural abandonment, and strongly suggest that time since abandonment is more important than soil type in determining the potential magnitude of carbon sequestration within this climatic region.
Methodology
This study was conducted near Kingston in southeastern Ontario, Canada (44°14′N 76°30′W) in the summers of 2004–2007. Warm summers and cold winters (mean temperatures of 16 and −4°C, respectively) are common within this region, and on average there are 222 days with a mean temperature above 0°C. Mean annual precipitation ranges from 700 to 900 mm and is fairly evenly distributed throughout the year with at least half occurring as snow between December and March (Environment Canada 2009). The study area is in the Mixedwood Plains ecozone where agriculture is a major land use, but there is also a considerable amount of mixed coniferous–deciduous forest (Wiken 1996). Mature forests are dominated by sugar maple (Acer saccharum Marsh.) and red oak (Quercus rubra L.), with some white oak (Quercus alba L.), red maple (Acer rubrum L.), basswood (Tilia americana L.), eastern hemlock (Tsuga canadensis L.), white pine (Pinus strobus L.), ironwood (Ostrya virginiana (P. Mill) K. Koch), and blue beech (Carpinus caroliniana Walt.) (Ecological Stratification Working Group 1995; Wiken 1996).
Our study focused on three common soil series that are characteristic of the northern portion of the Mixedwood Plains ecozone, and that represent three different soil orders (Podzolic, Brunisolic [equivalent to W.R.B. ‘Cambisol’], and Luvisolic) within the Canadian Classification System. These soil types correspond to Spodosol, Inceptisol, and Alfisol orders, respectively, in the United States soil classification system. The Podzol (Monteagle soil series) (Gillespie and others 1966) is a well-drained sandy loam that is common across the siliceous Canadian Shield (Figure 1). This shallow and stony soil (typically 20–30 cm) is derived from glacial till that occurs amongst the numerous bedrock outcrops on the landscape, and is of very limited agricultural use (Gillespie and others 1966). The Brunisol (Farmington soil series) is a well-drained loam or sandy loam (Gillespie and others 1966) of about 20 cm depth that occurs extensively on limestone in Eastern Ontario (Figure 1). The shallowness of this soil type is the primary restriction on its agricultural productivity, although it is farmed more intensively where the soil depth permits (Gillespie and others 1966). The Luvisol (Lansdowne soil series) is an imperfectly drained clay soil formed on late glacial lacustrine sediments close to Lake Ontario (Figure 1). The Luvisol is generally deeper than the other two soil types (typically ~30–50 cm of gleyed non-calcareous material above 20–30 cm of highly calcareous sub-soil (Gillespie and others 1966)), and is considered the most productive of the region’s soil types, mostly likely due to its superior water holding capacity during the summer months (Gillespie and others 1966).