Impacts of felling, mulching, and burning on budgets of C, N, S, P, K, Ca, and Mg; rates of C0 2 evolution from the soil; soil seed storage; and plant growth were evaluated. The felled tropical evergreen forest was 8-9 yr old, interspersed with patches of 70-yr-old forest and had a leaf area index of 6 and aboveground biomass of 5.2 kg/rn 2 • Harvest of the largest trees removed 18% of the S, and > I 0% of all other elements except N. During the 11-wk mulching and drying period, 33% of the K and 13% of the P disappeared, but losses of other elements were <6%. Burn temperatures were >620°C in surface fuels, but soil temperatures were seldom > l00°C at I ern or >50°C at 2 ern. The burn volatilized 1600 g/rn 2 C, 49 g/rn 2 N, and 13 g/rn 2 S. Postburn wind and water erosion of ash, plus leaching, removed 34 g/rn 2 N, 20 g/rn 2 K, I g/ rn 2 P, 39 g/rn 2 Ca, and 7 g/rn 2 Mg, but insignificant amounts of C and S. After the burn and onset of the rains, 57% of the initial amount of N and 39% of the initial C still remained because of conservation of the organic-rich upper 3 ern of soil.Soil C0 2 evolution was greater from beneath the 11-wk-old slash (3.6 gC· rn-2 • d-1 ) than from beneath the forest (2.5 gC · rn-2 • d-1 ), probably because the slash conserved soil moisture better than the actively transpiring forest. After the burn both the burned field and forest soil evolved C0 2 at =4.5 gC· rn-2 ·d-•. At this rate, 154 d of decomposition and respiration would release as much C into the atmosphere as did the burn.Soil seed storage dropped from =8000 seeds/rn 2 (67 species) in the forest, to 6000 seeds/rn 2 (51 species) after II wk of mulching, to 3000 seeds/rn 2 (37 species) after the burn. The seeds not killed by the burn, the survival of mycorrhizal fungi, and the release of nutrients resulted in vigorous and diverse postburn regrowth.

For 5 yr we monitored the fertility of a volcanic-ash derived Inceptisol at a site in the humid tropics of Costa Rica. After forest felling and burning, we established four treatments in a randomized block design with six blocks: a sequence of monocultures (two crops of maize [Zea mays] followed by cassava [Manihot esculenta], then the tree species Cordia alliodora), successional vegetation, a mimic of successional vegetation that was physiognomically similar to the model but shared no species with it, and a species-enriched version of successional vegetation. In addition, one plot was maintained free of vegetation. Species-rich successional vegetation was effective at maintaining soil fertility, although we observed general trends of soil-nutrient decline beneath all treatments, presumably because of plant uptake. It proved possible to imitate the fertility-maintaining characteristics of successional vegetation by creating an equally species-rich community of different floristic composition, but the maintenance of fertility was not enhanced by further species enrichment. Successive peaks of nitrate-nitrogen in soil solution, extractable phosphorus, and extractable potassium occurred during the 1st yr, perhaps driven by an early increment of organic matter from postburn debris and roots. Organic matter, total nitrogen, and extractable sulfur were remarkably stable during the 5-yr period. Depletions of cations, decreases in effective cation exchange capacity (CEC ), and increases in acid saturation were related to treatment in the following order: bare soil > monocultures > the three diverse, successional communities. In the bare-soil plot, fertility decreased dramatically: there was a net loss of exchangeable cations and inorganic nitrogen, the phosphorus-fixation capacity increased, and acid saturation reached a potentially toxic 86%. At the start of the study, three of the blocks had soil with lower pH, lower CEC , and higher acid saturation. During the study this less fertile soil lost proportionally more cations and increased more in acid saturation and phosphorus-fixation capacity. The less fertile soil under monocultures proved exceptionally vulnerable to loss of fertility; after 5 yr under monocultures, for example, acid saturation reached 38% in the more fertile soil and 75% in the less fertile soil. In the species-rich communities, however, changes in soil fertility were far less marked.

No abstract

Total root biomass and surface area of fine roots (<5 mm diameter) were determined to a depth of 85 cm in 1-, 8-, and 70-year-old tropical forests on the same soil type (Typic Dystrandept) at Turrialba, Costa Rica. Root biomass and root area index (RAI) increased with age: biomass = 219, 1291, and 1555 g/m2, and RAI = 1.34, 4.66, and 5.28 in the 1-, 8-, and 70-year-old stands, respectively. Fine roots were most concentrated in the surface 5 cm of soil at all three sites, but especially so in the 70-year-old stand. The 1- and 8-year-old vegetations had proportionally more fine roots below 25 cm than the 70-year-old vegetation, but lacked the large-diameter roots characteristic of the 70-year-old stand.

Twenty‐seven soil profiles distributed within a 2.5‐ha site were described as Typic Dystrandepts of the Colorado soil series. Leaf‐cutter ant influence was noted in 85% of the pedons. The influence on each profile was estimated, and out of all profiles, 37% had low, 26% medium, and 22% high disturbance. The surface area covered by leaf‐cutter mounds was 38.9% of the study area; only 1% of the aboveground disturbed area was active, however. Leaf‐cutter ants transport material from the AB and B horizons to the soil surface, producing a new A1 horizon and, in addition, some subsoil chambers are filled with plant material. It is proposed to use a subindex (i) to denote insect influence as an agent on the formation of a subsoil horizon (ABi, B2i) and also, for the overburden (Ai).