The impact of land use on soil carbon in Miombo Woodlands of Malawi

Abstract

In the Miombo Woodlands Region of south-central Africa, it is estimated 50–80% of the total system's carbon stock is found in the top 1.5 m belowground. Deforestation and rapid population growth rates have led to reduced fallow periods and widespread land degradation in the south-central Africa area of the Miombo Woodlands. The impact of this land use conversion on belowground carbon and nitrogen stocks within the Miombo Woodlands has not been examined extensively in the past. We addressed how the soil carbon profile reacts to conversion to agriculture, the continuation of agriculture, and the ability of the soil carbon budget to recover following abandonment within the Chimaliro Forest Reserve and surrounding villages in Kasungu, Malawi. Protected natural Miombo Woodlands sites, agricultural fields of increasing ages, and fallow fields of increasing ages were sampled. Surface carbon levels in Miombo soils varied from 1.2 to 3.7%. Agricultural soil carbon was significantly depressed with surface layers ranging from 0.35 to 1.2% carbon. Unexpectedly, fallow carbon and nitrogen levels continued to be significantly repressed (surface soils 0.65–2.3% C), pointing out the possible unsustainability of the current agricultural management cycle dominant in the area. On average, agricultural soils contain 40% less soil carbon than the natural Miombo Woodlands. Soil carbon declined logarithmically with depth within all land use types. Clay content was significantly positively correlated with soil carbon in the top 40 cm and therefore areas of higher clay content contained elevated carbon levels. Although a common attribute to many agricultural systems, bulk densities were not significantly altered by land use changes.

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.