Effects of Forest Conversion on the Carbon Budget of a Tropical Soil

Abstract

To determine how the conversion of mature tropical forests to secondary forests affects the soil carbon budget, major soil carbon storages, inputs, and CO2 evolution from a tropical inceptisol were measured over a six-month period in both a mature lowland Tropical Premontane Wet Forest and a nearby secondary site located on the same soil type. Total carbon storage in and on the mature forest soil was composed of 9330 gC/m2 in soil organic matter, 1850 gC/m2 in litter and 340 gC/m2 in small roots (diameters <5 mm); larger roots were not measured. Average daily inputs to the mature forest soil included 1.3 gC/m2 in litterfall and 0.10 gDOC/m2 in precipitation (throughfall + stem flow). The evolution of CO2 from the mature forest soil averaged 3.4 gC/m2 d, or 2.6 times the average rate of litterfall. Total carbon storage in and on the soil of the second-growth was composed of 8600 gC/m2 in soil organic matter, 700 gC/ m2 in litter and 157 gC/m2 in small roots, or 2060 gC/m2 less than in the mature forest. Litterfall in the young second-growth averaged 0.7 gC/m2 d, and precipitation averaged 0.12 gDOC/m2 d. Soil-CO2 evolution averaged 4.6 gC/m2d, or 1.4 times the rate in the mature forest. Measured inputs of carbon to the soil were considerably less than soil-CO2 evolution rates in both sites; part or all of these differences can be attributed to root production, which was not measured. It was found that forest conversion to young secondary vegetation resulted in losses of soil organic matter, fewer small roots, less non-woody and large wood litter, lower rates of litterfall, and increased rates of soil-CO2 evolution. TROPICAL FORESTS ARE BEING RAPIDLY CONVERTED to a variety of secondary plant communities. Potential consequences of tropical deforestation range from local soil degradation to changes in the world's climate, yet the specific changes that accompany forest conversion are not well documented. The soil is of particular importance after forest removal because it supplies many of the innocula, nutrients, and materials used to rebuild the aboveground community. Traditional cropping systems often lead to soil nutrient and organic matter losses (Nye and Greenland 1960, 1964; Brams 1971; Krebs 1975). Natural fallows, in contrast, tend to increase the fertility of depleted soils (Nye and Greenland 1960). Understanding soil processes under natural secondary vegetation may therefore aid in the development of soil-conserving production systems. I have focused upon soil carbon because carbon is initimately involved in virtually all biological processes, and organic matter, even when present in small amounts, is an extremely important soil constituent. In highly weathered tropical soils organic matter provides most of the cation exchange sites (Nye and Greenland 1960, Sanchez 1976), improves soil aggregation and stability (Allison 1973, Sanchez 1976, Cheng 1977), and is a major source of plant nutrients (Bornemisza 1966, Allison 1973, Sanchez 1976). In addition, soil carbon is dynamic, and reflects changes in the aboveground community. The purpose of this study was to determine how conversion of a mature forest to young secondary vegetation affected the carbon balance of a tropical inceptisol. The major components of a soil carbon budget are shown in Figure 1. I quantified the carbon storages in soil organic matter, litter, and small roots (diameters < 5 mm); carbon inputs in litterfall, throughfall, and stem flow; and carbon release, in the form of CO2, from the soil surface in both a mature forest and a young secondary site in lowland, humid Costa Rica. I then compared the carbon budgets of the two sites to determine the effects of forest conversion on soil carbon.