The impact of land use on soil carbon in Miombo Woodlands of Malawi
Introduction
Recently there has been a growing interest in understanding the carbon stocks of each ecosystem worldwide. As negotiations of the Kyoto Protocol progress, knowing the size of these carbon stocks and the factors that impact them is becoming economically important to countries. Estimates of aboveground biomass stocks exist for most ecosystems, however carbon belowground in roots and soil is less well characterized. The ancient soils of the Basement Complex in south-central Africa covered by the Miombo Woodlands present local farmers with nutrient poor soils and crop yields in this part of Africa are some of the lowest worldwide. Currently this region is enduring extensive deforestation and land degradation owing to population increases, escalating agricultural production, and woodfuel demands. Much of the newly cropped land is unsuitable for agriculture and degrades quickly, thereby forcing the farmer to convert even more land to agriculture. The impacts of agricultural conversion and shifting-cultivation on nutrient cycling and ecological health, however, have not been studied extensively in the Miombo Woodlands Region.
Owing to slow soil organic matter turnover rates, as compared to aboveground vegetation, soil carbon levels do not react as quickly to changes in land use. Due to this property, soil carbon levels measured through time can establish the long-term productivity and possible sustainability of that land use system. In a nutrient poor system, soil organic matter (SOM) can play an important role in the stability, quality, and fertility of the soil. Farmers and land use planners are therefore interested in land use management that will enhance soil carbon levels.
In most ecosystems worldwide the conversion of land to agriculture will drastically change natural internal nutrient cycling and nutrient loss will exceed nutrient gain. Under agriculture, biomass litter inputs become minimal and tillage will split up soil aggregates, increasing decomposition (Allen, 1985, Tate, 1987). Soil carbon is reduced most drastically in the plow layers of the soil. Generally, as the labile carbon from the previous land use is decomposed, agricultural conversion results in soil carbon loss that tends to be rapid in the years immediately after conversion. The rate of loss then diminishes over time and Soil Organic Carbon (SOC) may reach a new equilibrium (Houghton et al., 1983, Schlesinger, 1986, Davidson and Ackerman, 1993).
As is common in drier systems (Woomer et al., 1997), in the Miombo Woodlands ecosystem of south-central Africa roughly 60% the total carbon stock is found belowground (Campbell et al., 1998a, Campbell et al., 1998b). The range of soil carbon levels across the Miombo region and the main environmental regulators are somewhat known, however there has been little research on how land use conversion will alter these carbon stocks. This study builds on our understanding of how the dominant land use pattern impacts soil carbon levels and the soil carbon vertical structure by comparing soil carbon stocks within the dominant land cover types: Miombo Woodland, agricultural fields, and fallow fields. Differences in soil carbon stocks as agricultural fields increased in age are explored by sampling fields of differing ages. Likewise, the possible recovery of soil carbon stocks within abandoned fields reverting to woodlands is estimated in fallow fields of increasing age.
Section snippets
Study area
The Miombo region spans 2.8 million km2 of south-central Africa (Scheme 1). Precipitation ranges from 650 to 1500 mm and 95% of annual precipitation occurs during the hot wet season (Campbell, 1996, Desanker et al., 1997). The natural ecosystem is an open woodland (20–60% canopy cover) with a grass understory (Trapnell, 1959, Rodgers, 1996). The aboveground biomass of the Miombo Woodlands ranges from 37,000 to 95,000 kg/ha (Malaisse et al., 1975, Chidumayo, 1990; Tietema, 1993; Chidumayo, 1995;
Explanation of sites
The five Miombo woodland sites measured contained mixed aged trees that did not show signs of coppicing or firewood collection. Grass grows to about 1 m in height and is often collected for thatch at the end of the dry season. Maize is the dominant crop sown at the agricultural sites but other crops included tobacco, millet and groundnuts. Fertilizers were only used when tobacco was planted and would be applied at very small levels due to the substantial cost for the farmer. Age of agricultural
Carbon within the profile
The regression type found to most appropriately describe carbon changes with depth in this study also significantly characterized 76% of the 2700 worldwide soil profiles compiled in a study by Jobbagy and Jackson (2000). All sites could accurately be portrayed by the log carbon, log depth equation type except one site that had unusually high carbon levels at depth. The Miombo surface SOC values in this study are slightly higher than most previous studies in the Miombo Region (Fig. 6a). Below
Conclusions
The rapid changes taking place in the populations of south-central Africa are resulting in a greater area of land under human control and more intense use of the land than ever before. Processes or management practices that alter the inputs of organic matter into the soil carbon pool or the decomposition rate of SOM will affect soil carbon levels.
This study examined a number of both natural and human driven factors influencing soil carbon levels in the Miombo Woodland Region. By removing the
Acknowledgements
This work has taken place as part of the ‘Coupling Land Use and Land Cover Changes, and Ecosystem Processes in Miombo Woodlands’ project funded by NASA's LCLUC (Land-Cover and Land-Use Change) program (NAG5-6384). This work was conducted in association with Steve Makungwa and Alex Mangulana at the Forestry Research Institute of Malawi and Richard Chatchuka of the Kasungu District Forestry Office. We owe much gratitude to the farmers surrounding the Chimaliro forest reserve for their warm
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