Soil Organic Matter in Secondary Forests of Puerto Rico

Abstract

Soil organic matter content and loose litter were studied in secondary forests of the subtropical moistand wet-forest life zones of Puerto Rico to observe patterns due to life zone (climate), type of forest cover, soil group, and topography. Soil organic matter content in the top 23 cm ranged from 8.99 kg/m) in the moist-forest life zone to 7.30 kg/m' in the wet-forest life zone, a highly significant difference. Greater amounts of soil organic matter were found under higher timber volumes, but the trend was less pronounced in the wet forest. Significant interactions with life zone were found for organic matter among soil groups (deep volcanic, shallow volcanic, granitic, and limestone) and among forest classes (young secondary, late secondary, abandoned coffee, and active coffee shade). In secondary forests on granitic soils, highly significant differences of 12.98 and 7.32 kg/m2 of organic matter were found in wet and moist forests, respectively. Moreover, highly significant differences were observed by landform and slope in active coffee shade in wet forests, with the lowest values on convex land forms and slopes >45 percent. Mean loose litter storages in moist and wet forests were not significantly different, nor was there a significant interaction with life zone. Significant differences, however, were detected by timber volume class, soil group, and forest class. Within these highly disturbed wet and moist forests, greater timber volumes tended to be associated with greater loose litter accumulation. AFTER DISCOVERY OF THE ISLAND, the forests of Puerto Rico were cut for timber, cropland, and fuelwood, and by the mid1940s, undisturbed forests accounted for < 1 percent of the land area. Loss of soil organic matter and exposure of subsoils occurred under the widespread practice of slash and burn agriculture (Smith et al. 195 1). In the late 1940s, industrialization and the accompanying rural-tourban migration left much abandoned cropland and pasture in the mountainous interior. Today, a little over 30 percent of the island is covered by forest, much of it on these abandoned sites. Organic matter influences the physical and chemical properties of soil, commonly accounting for much of the cation-exchange capacity. It also influences the stability of soil aggregates, provides sustenance for soil microorganisms, and is a critical factor in soil fertility (Buckman & Brady 1960). In undisturbed conditions, soil organic matter in moistand wet-forest life zones (that is, with the temperature-to-precipitation ratio approaching unity) is roughly 40 kg/m2 (Brown & Lugo 1982). Clearing for agriculture leads to a decline in soil organic carbon, particularly in soils developed on highly weathered parent I Received 22 October 1984, revision accepted 15 October 1985. ma erials (Allen 1985). Although forest clearing continues at a rapid pace (Sommer 1976; Lanley & Clement 1979) and nearly 1.1 million km2 in the Caribbean, Central, and South America have been classified as forest fallow (Lanley & Gillis 1980), the dynamics of organic matter recovery in tropical soils have not been well documented on a regional basis. The initiation of a secondary forest inventory provided a unique opportunity to sample soils throughout the central part of Puerto Rico to relate loose litter and soil organic matter content to climate, vegetation cover, soil type, and topography. This information is needed for comparison with undisturbed soils under natural cover and to provide insights on soil organic matter dynamics after abandonment of farming in tropical regions.