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Relations Between Mineral Nutrient Availability and Fine Root Biomass in Two Costa Rican Tropical Wet Forests: A Hypothesis

1987; Wiley; Volume: 19; Issue: 2 Linguagem: Inglês

10.2307/2388741

1744-7429

Stith T. Gower,

Soil Carbon and Nitrogen Dynamics

Fine root biomass was estimated for two Costa Rican broad-leaf evergreen forests with apparent differences in soil nutrient status. Live fine root biomass ( 200 mm of rainfall. Mean annual daily temperature is 20?C and diurnal temperature variation generally exceeds seasonal variation. Site selection was based on soil chemical analyses conducted on the major soil series (River and Arboleda) at La Selva, Costa Rica (Bourgeois et al. 1972). Soils of the selected stands are derived from different parent material. The River soil series is located on a terrace near the confluence of the Rio Puerto Viejo and the Sarapique and is largely alluvium of volcanic origin. The Arboleda soil series is found on an upland position and believed to have derived from basalt (Bourgeois et al. 1972). Classification of these soils according to the U.S. 7th Approximation was hampered by the lack of data on weatherable mineral content. Tentative classification for the River and Arboleda soil series are Fluvaquentic Hapludoll and Oxic Dystrandept, respectively (S. W. Buol, pers. comm.). Dominant canopy vegetation of the River site includes Castilla elastica Cerv., Cedralla mexicana Roem., Cordia allidora (Ruiz & Par.) Chan. ex aken, Cecropia obtusifolia Bert., Pterocarpus officinalis L., Spondias mombin L., and BIOTROPICA 19(2): 171-175 1987 171 This content downloaded from 207.46.13.115 on Sat, 08 Oct 2016 05:34:31 UTC All use subject to http://about.jstor.org/terms TABLE 1. Fine root biomass distribution of two Costa Rican lowland tropicalforest x ? (SE). Root diameter class (mm) Soil depth Live Site (N) (cm) 2-5 Total River 6 0-5 10(5) 11 (6) 23 (17) 127 6 5-10 6 (1) 10 (2) 35 (5) 77 6 10-20 4 (1) 7 (3) 5 (5) 60 6 20-30 1 (0) 6 (3) 1 (1) 81 3 30-40 2 (1) I (1) 0 (0) 14 3 40-50 <1 (0) 3 (2) 0 (0) 10 Total 23 38 64 369 (69) Arboleda 6 0-5 23 (7) 18 (8) 18 (12) 194 6 5-10 12 (1) 20 (8) 15 (6) 156 6 10-20 11 (8) 11 (8) 11 (7) 144 6 20-30 5 (1) 11 (8) 10 (7) 101 3 30-40 3 (2) 0 (0) 19 (19) 49 3 40-50 < 1 (0) 0 (0) 0 (0) 18 Total 54 60 73 662 (105) Terminalia lucida Hoffm. Because this site was a cacao plantation at one time, Theobroma cacao L. is found sporadically in the understory. The Arboleda canopy species include Pentacletbra macroloba Kuntze, Socratea durissma (Derst.) Wendl., Protium copal Engl. and Iriartea gigantea Wendl. (Bourgeois et al. 1972). Roots were sampled during March 3-5, 1983, with a 7-cm-diameter root corer. Sample locations were randomly selected along a transect perpendicular to the slope. Six cores to a 50 cm depth were sampled at each site. Sampling intensity was limited by the amount of freezer space available at the La Selva Research Station. Root cores were separated into 0-5, 5-10, 10-20, 20-30, 30-40, and 40-50 cm depth increments in the field and placed in plastic bags. Samples were frozen upon returning from the field and remained frozen during transportation to the U.S. Root core samples were stored at 10OC to minimize mass loss due to decomposition and respiration. Prior to sorting, root samples were thawed and washed in a nylon mesh sieve (0.5 mm opening) under distilled water. Roots were hand sorted under 10 x magnification from the remaining organic and mineral particles. Roots were separated according to physiological status (live versus dead) and root diameter classes (< 1, 1-2, 2-5 mm) and dried at 70?C to a constant mass. Root tissue was not dry-ashed because further chemical analyses were to be conducted on the remaining tissue. Live roots were distinguished by color and texture criterion. Preliminary work was conducted to substantiate that the root color and texture characteristics chosen to distinguish live and dead roots were correct. Positive identification of the physiological status was determined by 172 Gower preparing cross-sections of the selected roots using a freezing microtome and staining for starch granules with a potassium iodide-iodine solution. Only live roots have starch granules. Six 7-cm-diameter soil cores to a 50 cm depth were collected adjacent to root sampling locations and separated into 10 cm increments. Samples were subsampled. One mineral soil sample was oven-dried to a constant mass at 105?C and weighed to determine moisture content and bulk density. The remaining sample was dried at 70?C to a constant mass and saved for chemical analysis. Soil pH was determined in a 1:1 mixture following Mehlich (1976). Soil nitrogen was determined using a micro Kjeldahl procedure (Nicholson 1984) and analyzed colorimetrically on a Technicon AutoAnalyzer II. Soil to be analyzed for phosphorus and calcium was digested using Mehlich III procedure (Mehlich (1984), phosphorus was determined colorimetrically using a Brinkman PC600 colorimeter, and calcium was determined by atomic absorption using standard procedures. Mineral soil nutrient content was calculated by multiplying mineral nutrient concentration by bulk density and summing by soil depth. Statistical analyses of fine root biomass and soil chemical characteristics were conducted by using the Student's t-test, treating mean values as independent samples with equal variance (Steel & Torrie 1980).