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Plant and Soil 164: 129-145, 1994. (~) 1994 Kluwer Academic Publishers. Printed in the Netherlands.

Nutrient availability in a montane wet tropical forest: Spatial patterns and methodological considerations

W.L. S i lver 1., E N . Sca t ena 1, A .H. Johnson 2, T.G. S i c c a m a 3 and M.J . Sanchez 1 1international Institute of Tropical Forestry, USDA Forest Service, Call Box 25000, Rio Piedras, PR 00928, USA, 2Department of Geology, University of Pennsylvania, Philadelphia, PA 19104, USA and 3 Yale School of Forestry and Environmental Studies, 370 Prospect St., New Haven, CT 06511, USA. * Present address: Yale School of Forestry and Environmental Studies, 360 Prospect Street, New Haven, CT 06511, USA

Received 24 January 1994. Accepted in revised form 5 May 1994

Key words: nutrient cycling, plant-soil interactions, spatial variability, topographic patterns, tropical forest ecology

Abstract

Soils and forest floor were sampled quantitatively from a montane wet tropical forest in Puerto Rico to determine the spatial variability of soil nutrients, the factors controlling nutrient availability to vegetation, and the distribution of nutrients in soil and plants. Exchangeable cation concentrations were measured using different soil extracting procedures (fresh soil with NH4C1, air-dried and ground soil with KC1, and a Modified Olsen solution) to establish a range of nutrient availability in the soil, and to determine the relationship between different, but commonly used laboratory protocols.

The availability of exchangeable Ca, Mg, and K was significantly lower in soils extracted fresh with NHaCI than from soils which were dried and ground prior to extraction with KCI or a modified Olsen solution. Soil nutrient availability generally decreased with depth in the soil. Several soil properties important to plant growth and survival varied predictably across the landscape and could be viewed in the context of a simple catena model. In the surface soils, exchangeable base cation concentrations and pH increased along a gradient from ridge tops to riparian valleys, while soil organic matter, exchangeable Fe and acidity decreased along this gradient. On the ridges, N, P, and K were positively correlated with soil organic matter; on slopes, N and P were positively correlated with organic matter, and Ca, Kg, and pH were negatively correlated with exchangeable Fe. Nutrient availability in the upper catena appears to be primarily controlled by biotic processes, particularly the accumulation of organic matter. The Ca, K, and P content of the vegetation was higher on ridges and slopes than in the valley positions. Periodic flooding and impeded drainage in the lower catena resulted in a more heterogeneous environment.

A comparison of the Bisley, Puerto Rico soils with other tropical montane forests (TMF) revealed that the internal heterogeneity of soils in the Bisley Watersheds is similar to the range of average soil nutrient concentrations among TMF's for Ca, Mg, and K (dry/ground soils). Phosphorus tended to be slightly higher in Bisley and N was lower than in other TMFs.

Introduction

Humid tropical forests are diverse and complex environments, where plant species composition and distribution overlay a mosaic of spatially heterogeneous resource availability and ecosystem dynamics (Richards, 1952). In many ways, the complexity of humid tropical vegetation is a reflection of the spa-

tial and temporal variability in environmental conditions. For example, the composition and distribution of species in tropical forests have been related to the distribution of light (Chazdon and Fetcher, 1984; Chaz- don and Pearcy, 1991; Denslow etal., 1990; Richards, 1983), and disturbances (Connell, 1978; Denslow, 1987; Hartshorn, 1978). Only a few studies, however, have examined the scale of spatial variability in

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soil nutrient availability and other soil characteristics (Proctor et al., 1983, 1988).

Soil chemical and physical properties are controlled by a combination of biotic and abiotic phenomena that vary across the landscape (Jenny, 1980). Individual plants alter soil properties in their immediate vicin- ity creating a positive feedback between vegetation and soils (reviewed by Hobbie, 1992). Soil nutrient pools are also influenced by the variability imposed by topography, drainage characteristics, disturbance history, and parent material (Jenny, 1980). The occur- rence of well developed soil catenas in the humid tropics (Milne, 1935; Sanchez, 1976; Young, 1976), and toposequences in vegetation have been document- ed, particularly for montane forests (Basnet, 1992; Edwards and Grubb, 1982; Shreve, 1914; Weaver, 1991). However, the relationship between spatial patterns in soils and plants is less well understood (Proctor et al., 1983, 1988).

This is due, in part, to our limited understanding of the ecological relevance of currently used soil extraction techniques to mature forest vegetation. In most cases, soil extraction methodology was developed for agricultural systems assuming relatively homogeneous material, and was tested on potted greenhouse plants which may have little or no relationship to mature forest trees. Soils are often air-dried and ground prior to extraction significantly altering both chemical properties and physical structure (Bartlett and James, 1980; Haynes and Swift, 1989), particularly for humid tropical soils which do not reach an air-dried state under natural conditions. Drying soils can both increase and decrease soil nutrient pools through the breakdown of organic matter (Bartlett and James, 1980), and the sorption of certain nutrients, such as P, to clay particles and organic matter (Haynes and Swift, 1989). Grinding releases nutrients stored in fine roots, organic matter, and soil organisms. Extraction of fresh soils also has limitations. Fresh soils are difficult to sieve and homogenize, and must be stored in refrigeration and processed in a timely manner to reduce the effects of fungal and microbial growth.

In this study, we address these problems by quantitatively sampling soil on two adjacent, steepland montane watersheds in a humid tropical forest in Puerto Rico. Our specific goals were to: 1) determine a range of exchangeable soil nutrient pools on a watershed-scale using multiple extraction techniques, 2) determine the distribution of selected soil chemical and physical properties horizontally across the landscape, and vertically within the rooting zone, and 3)

explore patterns in the allocation of nutrients among soil exchange sites, forest floor, and the vegetation within distinct topographic zones and on the watersheds as a whole.

Materials and methods

Study site

The study site is in the tabonuco (Dacryodes excelsa Vahl) forest zone of the Bisley Research Area in the Luquillo Experimental Forest (LEF), Puerto Rico (18 ° 18' N, 65050 ' W). The study was conducted on two adjacent watersheds (approximately 6 ha each) that occur between 250--450 m above sea level (Scatena, 1989), and are classified as subtropical wet forest (sen- su Holdridge, 1967) receiving 3000--4000 mm of rain per year with most months receiving a minimum of 200 mm (Scatena, 1989).

The parent material underlying the watersheds is volcanoclastic sandstone rich in ferromagnesium min- erals, and which weathers to a soil high in clay, free Fe and A1, but low in silica and free bases (Bonnet, 1939; Scatena, 1989). Soils are mapped as part of the Humatus-Zarzal-Cristal complex (Johnston, 1992), and are classified as clayey, mixed isothermic, Epi- aquic Tropohumults or Palehumults (Ultisols) (Bein- roth, 1982). The dominant silicate clays are degraded illites which have lost a large portion of K (Jones et al., 1982). Within this complex, four distinct soil types can be identified which correspond to the major topographic features of the watersheds (Table 1).

Soil sampling and analyses

In 1988, prior to soil sampling, the watersheds were surveyed and permanently marked with stakes on a 40 m grid. A description was made of each grid point including information about tree species composition, basal area, and classification of the topography as either ridge, side slope, valley, or riparian valley (Scatena, 1989; Scatena et al., 1993).

In June 1988, forest floor and surface soils were sampled on Watersheds 1 and 2. Two 15 x 15 × 10 cm pits, one for chemistry and one for root and rock volume, were dug at each grid point, 1 m east and 1 m west of the stake. Pits were excavated by first securing a 15 × 15 cm template (inside area) with long nails and removing all live vegetation. Forest floor material (composed of all recognizable dead plant material)

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Table 1. Soil and landscape features in Bisley Experimental Watersheds based on topographic units defined by Scatena 1989, and soil units defined by Johnston 1992

Topographic Geomorphic Soil Soil unit features associations features

Ridges Convex flow Humatus

lines, drainage (Aquic Hapludox) away from site

Yellowish-brown at surface

yellowish-red, friable clay that lacks well developed mottles

Slopes Parallel flow Zarzal

lines, drainage (Typic Tropohumult)

through site

Well developed red mottles and

Fe-Mn concretions at depth

Upland valleys Convergent Zarzal and Cristal

flow lines, includes (Aquic and Typic

small valleys that Tropohumults) have intermittent

surface runoff

Moderately to poorly drained,

low chroma to red mottles,

rocky, variable substrate

Riparian valleys Convergent flow Cristal and Coloso

includes valleys (Typic Tropohumults

with perennial and Tropic Fluaquents) streams

Poorly drained, low chroma mottles to gley, structureless

was collected from within the square and placed in a separate bag for processing. A 10 cm deep block of mineral soil was then carefully excavated and placed in a bag. Multiple soil samples from the 10-35 cm and 35-60 cm depths were collected from inside the excavated pit using a 2.5 cm diameter soil corer. At a few sites, it was not possible to collect samples from one or both of these lower horizons due to large rocks or roots.

To determine bulk density of the 10-35 cm and 35-60 cm depths, eight large quantitative pits (50 x 50 cm) were located in four elevation bands along the ridge dividing the two watersheds. Forest floor was removed and soils were excavated by depth (0-10 cm, 10-35 cm, 35-60 cm) using a technique outlined in Hamburg (1984). Soil from each depth of the big pits was weighed in the field using a spring balance and subsampled for dry weight conversions (105 °C).

Fresh samples were stored in air-tight bags at approximately 4 °C for less than one week prior to extractions and pH determinations. A subsample from all chemistry pits and the two lower depths from the 'roots and rocks pit' was given to the International Institute of Tropical Forest (IITF) laboratory in Puerto Rico.

Laboratory analyses

To determine the influence of soil processing and extracting solutions on exchangeable cations, and to develop data sets that were comparable to others pro- duced by our respective labs, soils were extracted both at field moisture conditions (fresh), and after being air-dried, ground, and passed through a 2-mm sieve (&y/ground).

Approximately 4 g of fresh soil was extracted with approximately 55 mL of 1 M NHaC1 (exact volume determined gravimetrically) for 12 hr using a vertical vacuum extractor (Johnson et al., 1991). Large roots and rocks were manually removed prior to extraction and the soil was stirred in solution with a clean glass rod to break up large aggregates. Samples were analyzed on an ICP for Ca, Mg, K, and A1 at Yale University. No data for fresh soil P extractions are reported here. Pre- liminary tests show that concentrations of extractable P are very low in fresh soils from this area and in most cases are below the detection limit of the analytical instrumentation (<0.09 mg L - l ) (Siegal, 1990; M J Sanchez, personal communication.).

Five g of air-dried and ground soil was extracted with separately 50 mL 1 M KCI and 50 mL of modi-

132

fled Olsen solution (0.01 M NH4-0.01 M EDTA-0.25 M NaHCO3) for 12 hr each using a vertical vacuum extractor (Hunter, 1982; Wilde et al., 1979). Concen- trations of exchangeable Ca and Mg were determined on a DCP-Spectraspan V spectrophotometer from the KCI solutions. Modified Olsen solutions were analyzed for exchangeable K, Fe, Mn, and extractable P also on a DCP at the IITF. All extraction techniques included blanks and replicate samples for quality control. For determination of moisture content, an additional subsample of soil from both fresh and air-dried sets was dried at 105 °C. All data reported here are on an oven dry weight basis.

To determine the relative effects of chemical extractants and drying and grinding on exchangeable Ca, Mg, and K, 15 test samples were collected from the 0-10 cm soil depth in the Bisley Research Watersheds using a 2.5 cm soil corer. One quarter of each sample was extracted fresh with KCI, fresh with NHaC1, air-dried and ground prior to extraction with KCI, and air dried and ground prior to extraction with NH4CI. All test samples were analyzed at IITF for exchangeable Ca and Mg, and soils extracted with NH4CI were also analyzed for exchangeable K. No testing was done using a modified Olsen extraction. Several soil samples and bottles were exchanged between Yale University and IITF for a cross-lab check, and yielded excellent agreement.

Soil pH was determined with a Beckmann pH meter and electrode in a slurry solution of 10 mL deionized H20 with 8 g of fresh soil (adapted from McLean, 1982). For determination of exchangeable acidity, soils were extracted for 12 hr on a vertical vacuum extractor both fresh and dry/ground using 1 M KC1. Total exchangeable acidity was determined by titration with 0.1 M NaOH to a phenolphthalein endpoint (Thomas, 1982).

Total N was determined using a modified Kjeldahl method to include nitrate and ammonium (Bremner and Mulvaney, 1982). Approximately I g of air-dried and ground soil was digested in a microwave in 10 mL H2SO4 and 15 mL H202. Soil organic matter content was determined in 0.25 g of soil using the Walkley- Black procedure (Nelson and Sommers, 1982).

Forest floor samples were dried to a constant weight at 70 ° C, weighed to determine mass, and ground using a Wiley mill. Approximately 0.3 g of ground forest floor was predigested in 8 mL H2SO4 and LiSO4 and then digested with a LiSO4 solution using a block digestor (Parkinson and Allen, 1975). Tbe solutions were analyzed on an Thermal Jarrel Ash ICP for total

AI, Ca, Mg, K, and P and on a Technicon auto analyzer for NH 4.

Statistical analyses

Originally, data were analyzed using a geostatistical technique that examines spatial auto-correlation using semi-variagrams (Robertson, 1987). However, due to the highly dissected nature of the topography, it quick- ly became evident that for most soil properties the strongest relationships existed at the scale of the dominant topographic feature. Hence, we compared soil characteristics across the topographic categories that had been based on geomorphologic factors and were already shown to be well correlated with plant species distribution (Scatena et al., 1993).

Statistical analyses were performed using Systat 5.0 (Wilkinson, 1990). Data were log transformed when appropriate to meet the assumptions for analysis of variance (ANOVA). Factorial ANOVAs were used to determine: 1) if soil chemical and physical properties changed significantly by depth (depth); 2) if soil chemical and physical properties differed between topographic zones (topography); 3) if there were significant interactions between depth and topography in soil chemical and physical properties (depth • topography). The Tukey-Kramer hsd multiple comparison test was used to determine where differences occurred (Steel and Torrie, 1980). Similarity between wet and dry processing and extraction techniques was examined using regression analyses. Pearson correlation coefficients were calculated to determine if there were relationships between soil properties. Significance was set at p <0.05 unless otherwise noted.

Results

The extractions using fresh soils from the Bisley Water- sheds yielded significantly lower exchangeable Ca, Mg, and K concentrations than the dry/ground soil extractions (Table 2). Test samples revealed that drying and grinding significantly increased exchangeable Ca, Mg, and K (NH4CI only) concentrations for both chemical extractants, and that NH4CI extracted significantly more Ca and Mg than KC1 for both fresh and dry/ground samples (Table 3). On the watersheds, the KC1 and modified Olsen extractions of dry/ground soils increased the average concentrations of Ca, Mg, and K by a factor of 3, 2, and 7 respectively in the 0-10 cm soil depth when compared to fresh soils (Table 2).

133

Table 2. (a) Exchangeable cations and exchangeable acidity in fresh soil, pH and bulk density and (b) exchangeable cations, exchangeable acidity (0-10 cm depth only), extractable P in dry and ground soil, total N and organic matter by depth and topographic position in the Bisley Research Watersheds, LEF, Puerto Rico. Standard errors in parentheses. Dashed lines represent data below the detection limit of the analytical instrumentation. Different lower ease letters within a depth indicate significant differences (p < 0.05) between topographic zones. Different upper case letters indicate significant differences (p < 0.05) between depths (whole watershed only), Boldface italic print indicates significant (p < 0.05) differences between extraction techniques

(a)

Exchangeable cations and exchangeable acidity (cmol + kg - t)

Topographic Exchange- Bulk density

position n Ca Mg K AI able acidity pH (gcm -3)

0-10 m

Ridge 24 0.6 (0.2) a 1.2 (0.2) a a 0.07 (0.02) a 3.5 (0.3) a 3.8 (0.4) 4.7 (0.1) a 0.60 (0.05)

Slope 41 0.9 (0.2) a 1.4 (0.2) a a 0.15 (0.02) a 3.2 (0.3) ab 3.5 (0.4) 4.8 (0.1) ab 0.68 (0.06)

Upland Valley 13 0.9 (0.5) a 1,7 (0.3) ab a 0.02 (0.02) a 2.8 (0.4) ab 3.5 (0.7) 5.1 (0.1) bc 0.56 (0.07)

RiparianValley 9 2.0 (0.8) a 2,6 (0.2) b a 0.12 (0.04) a 1.8 (0.5) b 2.7 (1.0) 5.3 (0.1) c 0.61 (0.11)

Watershed mean 87 1.0 (0.2) A a 1,5 (0.1) A a 0.07 (0.01) A a 3.5 (0.3) 3.1 (0.2) 4.8 (0.1) 0.63 (0.04) A

10-35 em Ridge 22 0.4 (0.3) a 2.1 (1.5) a 0.02 (0.01) a 3.2 (0.3) 1.5 (0.3) 4.9 (0.1) a

Slope 36 0.6 (0.3) a 1.8 (0.8) a 0.06 (0.04) a 2.9 (0.2) 3.0 (0.2) 5.0 (0.1) a

Upland Valley 13 0.5 (0.3) a 1.6 (0.4) ab - 3.0 (0.5) 3.5 (0.5) 5.1 (0.1) ab

Riparian Valley 9 0.9 (0.5) a 2.0 (0.2) b 0.03 (0.02) a 2.9 (0.7) 4.7 (1.5) 5.4 (0.1) b

Watershed mean 80 0.6 (0.2) B a 1.9 (0.5) B 0.04 (0.02) AB a 2.4 (0.2) 3.0 (0.2) 5.1 (0.0)

35-60 cm

Ridge 20 0.3 (0.3) 2.3 (1.9) - 3.5 (0.3) 3.9 (0.3) 4.2 (0.1)

Slope 23 0.4 (0.4) a 1.6 (0.7) - 3.7 (0.4) 3.6 (0.4) 5.0 (0.1)

Upland Valley 9 0.5 (0.4) 1.3 (0.5) 0.01 (0.01) 3.5 (0.7) 3.9 (0.7) 5.1 (0.1)

Riparian Valley 2 0.1 (0.1) a 1.7 (0.7) - 8.0 (0.5) 8.2 (0.5) 5.3 (0.2)

Watershed mean 54 0.4 (0.2) B a 1.6 (0.7) C 0.01 (0.01) B 3.9 (0.3) 3.8 (0.3) 5.0 (0.1)

0.98 (0.03) B

1.13 (0.06) C

There were no significant effects of drying and grinding on exchangeable acidity, and wet and dry extractions were highly correlated for the watersheds as a whole (Table 2, 4). The two extraction techniques to determine Ca and Mg were also significantly correlated in the 0-10 cm depth (Table 4). Within distinct topographic zones, significant correlations between extraction techniques occurred on ridges (n = 23; Ca: R 2 = 0.96; p <0.01; Mg: R 2 = 0.74; p <0.01, and Exchange- able Acidity: R 2 = 0.88; p <0.01) and on slopes (n = 40; Ca: R 2 = 0.32p <0.01; Mg: R 2 = 0.87; p <0.01; Exchangeable Acidity: R 2 = 0.90; p <0.01), but not in the upland or riparian valleys. The two extracting procedures used to estimate K were not significantly correlated within the watersheds, due at least in part to the abundance of fresh samples with very low concentrations (Table 2).

In general, the concentrations of nutrient cations, extractable P, total N, and soil organic matter decreased significantly with depth (Table 2). Concentrations of exchangeable Ca (dried/ground samples), Mg (both techniques), Mn, and pH increased significantly from ridge tops to riparian valleys in the surface soils, while exchangeable Fe, acidity, total N, and soil organic matter decreased (Table 2). The same trend was observed for Ca, Mg, and Mn in the 10-35 cm soil depth (dried and ground soils). No statistically significant topographic pattern was observed for bulk density. Bulk density was very low (0.63 g cm -3) in the surface soils and increased significantly with depth (Table 2).

When considering the watersheds as a unit (i.e. not separating by topographic zone), exchangeable K, extractable P, and total N concentrations were significantly and positively correlated with soil organic matter (Table 4). Exchangeable acidity was significantly

134

Table 2. Continued

(b)

Exchangeable cations and exchangeable acidity (cmol + kg - l) Topographic position n Ca Mg K Fe Mn

Exchange- Total able acidity p ( # g g - t ) N(%)

Organic matter (%)

0-10 em Ridge 24 2.3 (0.5) a a 2.2 (0.4) a b 0.5 (0.04) b 7.2 (0.8) a 0.3

Slope 41 3.0 (0.5) a b 3.2 (0.4) ab b 0.6 (0.05) b 5.2 (0.5) ab 0.6

Upland Valley 13 3.7 (1.3) ab b 4.5 (1.5) ab b 0.4 (0.03) b 3.5 (0.6) b 0.8

Riparian Valley 9 6.2 (1.0) b b 4.3 (0.6) b b 0.6 (0.07) b 3.1 (0.5) b 0.6

Watershed mean 87 3.3 (0.4) A b 3.2 (0.3) A b 0.5 (0.03) A b 5.7 (0.4) A 0.5

(0.1) a 5.5 (0.5) a 34 (7) 0.34 (0.07) a 9.6 (2.2) a

(0.1) b 4.2 (0.5) a 23 (2) 0.31 (0.02) ab 7.1 (0.6) ab

(0.1) b 3.5 (0.5) ab 21 (4) 0.25 (0.02) ab 4.4 (0.5) b**

(0.1) b 2.0 (0.6) b 26 (2) 0.23 (0.02) b* 5.2 (0.6) b**

(0.1) A 4.2 (0.3) 26 (2) A 0.31 (0.02) A 7.3 (0.7) A

10-35 em Ridge 22 0.7 (0.2) a b 1.1 (0.4) a 0.2 (0.12) b 3.8 (0.6) a 0.3 (0.1) a

Slope 36 1.5 (0.5) a b 2.4 (0.7) ab 0.2 (0.04) b 2.0 (0.3) b 0.3 (0.1) a

Upland Valley 13 3.0 (1.5) ab b 2.8 (0.8) ab 0.2 (0.02) 2.1 (0.7) b 0.9 (0.2) b

Riparian Valley 9 4.2 (1.1) b b 3.1 (0.6) b 0.3 (0.04) b 2.8 (0.6) ab 0.9 (0.2) b

Watershed mean 80 3.3 (0.4) B b 2.2 (0.4) B 0.2 (0.02) B b 2.6 (0.3) B 0.5 (0.1) A

35-.-60 cm Ridge 20 0.6 (0.3) 1.1 (0.5) 0.1 (0.03) 0,4 (0.5) 0.1 (0.1) a

Slope 23 1.1 (0.4) b 2.1 (1.0) 0.1 (0.01) 0.9 (0.1) 0.3 (0.1) ab Upland Valley 9 3.0 (2.6) 3.0 (1.6) 0.1 (0.03) 1.1 (0.5) 0.3 (0.1) b

Riparian Valley 2 2.4 (0.0) b 3.9 (1.4) 0.2 (0.03) 1.I (0.3) 0.8 (0.1) ab

Watershed mean 54 1.3 (0,5) C b 1.9 (0.5) B 0.1 (0.01) C 1.1 (0.2) C 0.3 (0.1) B

7 (1) a 0.14 (0.01) 3.1 (0.3)

6 (1) a 0.13 (0.01) 2.6 (0.2) 9 (1) a 0.13 (0.02) 2.4 (0.4)

20 (4) b 0.19 (0.04) 3.7 (1,0)

9 (1) B 0.14 (0.01) B 3.0 (0,2) B

3 (1) 0.08 (0.01) 1.4 (0.2)

3 (0) 0.07 (0.01) 1.7 (0.2) 5 (1) 0.05 (0.01) 1.2 (0.3)

9 (1) 0.06 (0.01) 1.3 (0.0)

3 (0) C 0.07 (0.01) C 1.5 (0.1) C

* p = 0.09

** p = 0.07

and positively correlated with exchangeable Fe, while both were significantly and inversely correlated with pH, exchangeable Ca, Mg, and Mn. Total N was also negatively correlated with soil pH (Table 4). When separated by topographic zones, similar relationships existed on ridges and slopes (Fig. 1), but not in the valleys.

Forest floor mass, nutrient concentrations (%), and nutrient content (kg ha -1) did not vary significantly between different topographic zones. The mean forest floor mass on the watersheds was 7 Mg ha -1. There were 154 Mg ha -1 of soil organic matter down to a depth of 60 cm on the watersheds. The riparian valleys had significantly less soil organic matter than ridge tops. Soil nutrient pools (kg ha -1) generally exhibited similar patterns with depth as nutrient concentrations (Fig. 2). Both exchangeable Ca and Mg pools were significantly larger in the riparian valleys than on ridges for all soil depths (Fig. 2).

Discussion

Spatial variability in soil properties: The catena model

Several soil properties in the Bisley Research Water- sheds varied across the landscape, in more or less predictable patterns from ridge tops to riparian valleys. Exchangeable bases increased along this gradient, while soil organic matter, exchangeable acidity, and Fe decreased. Similar catenas in soil chemical and physical properties occur throughout the humid tropics (Edwards and Grubb, 1982; Moniz and Buol, 1982; Sanchez, 1976; Young, 1976), and result primarily from soil mineralogical properties, the distribution of soil organic matter, and water movement (Moniz and Buol, 1982; Sanchez, 1976). In this section, we discuss the spatial patterns in soil nutrient availability observed in the Bisley Watersheds in the context of a simple catena model.

135

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136

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K kg ha-1

UDDD I i I I

Fig. 2(,4). The distribution of nutrient pools (kg ha -1) in the vegetation and forest floor compared with exchangeable nutrients in soils exl~acted fresh with NH4CI (a) and, dried and ground with KCI and a modified Olsen solution (b), and total nitrogen in the Bisley Research Watersheds, Luquillo Experimental Forest, ER. Nutrients in the vegetation are taken from Scatena et al. (1993).

137

3000

2000 0

1000 <

1000 r~

(D

2000

3000

1000 K kg ha-1

800

o 6 0 0

~ 4 0 0

< 2 0 0

200

4 0 0 o

6 0 0

800

Ca kg ha-I b S000

4O00

o 3000

2OOO 0

< lO00

0

~ 0 0 0

_ 2000

3000

4OO0

5000 Ridge Slope

Mg kg ha-1

l i I 1

Valley Rip.Valley

'Jij 1 I l I

Ridge Slope Val ley Rip.Valley

1000 ~ ' ' ' Ridge Slope Valley Rip.Valley

Fig. 2(B).

100

8O

60

40

20

0

20

4O

60

80

100

8000

6000

4000

20O0

0

2OOO

4OO0

6000

8000

(Continued)

P kg ha-1

I I I

Ridge Slope Valley Total N kg ha-1

I

Rip.Valley

O Live Vegetation • Forest Floor [] 0 -10 cm [ ] 10-35 cm

35-60 crn

!ji! I I

Ridge Slope Valley Rip.Valley

The soil catena

In the upper section of the Bisley catena, soils are well drained, flooding is rare, and the lateral flow of nutrient rich waters acts to remove base cations from the soil exchange complex. Cation loss is facilitated

through the accumulation of Fe oxides which can block a significant proportion of the exchange sites in the well aerated mineral soils of this region (Abruna and Smith, 1953; Fox, 1982). Consequently, soil organic matter becomes responsible for the majority of cation exchange (Abruna and Vicente, 1955), and contributes

138

Table 3. Comparisons of the effects of soil processing techniques (Fresh Soil and Dry/Ground Soil) and chemical extracting solutions (NH4CI and KC1) on the mean concentrations of exchangeable Ca, Mg, and K (NI-14C1 only) from 15 test samples from the Bisley Research Watersheds, Luquillo Experimental Forest, P.R. Differences between treatments were analyzed using a t-test on log transformed data. Standard errors in parentheses

Exchangeable cations (cmol + k g - 1)

Treatment Ca Mg K a

NH4Cl: Fresh Soil 0.10 (0.05) 0.12 (0.03) 0.10

NH4CI: Dry/Ground Soil 0.15 (0.07) 0.18 (0.04) 0.13

FreshaDry/Ground: t value -8 .9 ** -11 .0 ** -5 .4

(0.04) (0.04)

KCI: Fresh Soil 0.08 (0.04) 0.12 (0.02)

KCI: Dry/Ground Soil 0.10 (0.04) 0.13 (0.03)

Fresh'~Dry/Ground: t value -4 .5 ** -7 .8 **

Fresh NI-LtClaKCl:tvalue -3 .9 ** -4 .2 **

Dry Ground NH4ClaKCl: t value -6 .1 ** -13 .5 **

Fresh NH4C1 a

Dry/Ground KCI:t value -1 .0 ** -1 .1 **

a Test samples were not extracted with modified Olsen

solution so no comparison with NH4C1 is available.

** p < 0.01.

to the availability of extractable P, exchangeable K, and total N concentrations (Table 4). The significant and positive correlation of organic matter with N, P, and K suggests that the availability of these elements to plants is primarily controlled by biotic processes in the upper catena. While this is not surprising for N which is often supplied by biological N2 fixation and microbial N mineralization (Runge, 1983), it is interesting that similar patterns occurred with K and P which are supplied by abiotic sources (precipitation and weathering of parent material). In the mineral soil, K is easily lost from the plant available pool through mobilization and leaching, and P through fixation with Fe and A1 oxides (Sanchez, 1976) resulting in very low exchangeable soil concentrations. In the upper catena, however, the relatively high soil organic matter content helps to conserve these elements and increases their availability to plants.

In contrast, soils in the upland and riparian valleys were relatively high in exchangeable bases, while soil organic matter, exchangeable acidity, and Fe were correspondingly lower (Table 2). In these landscape positions, drainage is impeded, anaerobic conditions occur periodically (Silver, unpublished data), and sat- urated flow moves horizontally through a variably oxi-

dized rooting zone (McDowell et al., 1992). The higher exchangeable cation content of these soils is the result of two different, but related processes. First, cations accumulate in the lower topographic zones from ups- lope input as discussed above. Secondly, the periodic flooding and poor drainage in these zones leads to the reduction of Fe -3 to Fe -2, which is mobilized prefer- entially to other cations resulting in the removal of Fe coatings from soil particles (Jenny, 1980). These soil exchange sites then become available to other cations such as Ca, Mg, and K (Jenny, 1980). The removal of Fe coatings can also have a positive effect on P availability in these soils since P is more strongly sorbed to Fe oxides than to the reduced forms of Fe (Fox, 1988; Jones et al., 1982). The lack of a significant topographic pattern for P may be due to the spatial variability in many of the mechanisms that can affect P availability in these soils, such as low initial concentrations, soil organic matter content, and fixation with Fe and AI oxides (Sanchez, 1976).

Drying and grinding the soil prior to extraction generally increased nutrient concentrations, and the two techniques were significantly correlated for the ridge and slope sites, but not correlated in the valleys (Table 4, Fig. 1). This may reflect the small

139

Table 4. Matrix of Pearson correlation coefficients for comparing soil properties and extraction techniques in the Bisley Research Watersheds, Luquillo Experimental Forest, P.R. Columns marked "Fresh" refer to comparisons made using data from soils extracted fresh with ammonium chloride (Ca, Mg, K, and exchangeable acidity only). Columns and rows marked "Dry" refer to comparisons made using data from dried and ground soils extracted with potassium chloride (Ca, Mg, and exchangeable acidity) and a modified Olsen solution (K, Fe, Mn, and P). Bold numbers in boxes are fresh- dry comparisons (Ca, Mg, K, and exchangeable acidity). See text for details of methods. All values (except pH) were log transformed. Significant correlations are marked with * (19 < 0.05) and • * (p < 0.01), n = 86, NC = no comparison

Exchangeable nutrients and acidity (cmol + k g - l)

Soil Fresh Fresh Fresh Dry Dry Fresh Dry P Total Organic

property Ca Mg K Fe Mn acidity (#g g - 1) N (%) matter (%) pH

Dry Ca [0-~1 ~* 0.66 ** 0.45 ** -0 .4 * 0.23 -0 .3

DryMg 0.82 ** ~ * * 0.34" -0 .5 ** 0.39"* - 0 . 4 " *

Dry K 0.35" 0.30 10.301 -0 .2 -0.1 -0.1

Dry Fe -0 .57 "* -0.53 ** -0.15 I NC I - 0.54 *"

DryMn 0.36 ~* 0.39 ** 0.12 -0.54 ** [NC I -0.38 **

Dry acidity -0 .69 ** -0.48 ** 0.24 0.70 ** -0.50 ** 0 ~ *~

Dry P 0.10 0.17 0.31 0.34 -0.19 0.35

N 0.04 0.04 0.46 ** 0.33 -0.12 0.33

O.M. -0.02 -0.03 0.46 ** 0.44 ** -0.27 0.38 ~

pH 0.63 ** 0.56 ** 0.03 -0.60 ** 0.43 ** -0.73 **

0.06 -0.1 -0 .1 0.47 **

0.19 0.02 - 0 0.58 **

0.19 0.14 0.12 0.13

0.28 0.25 0.27 -0 .60 **

I N c l -

o.67-I, cl _ _

o . 6 7 , , _

- 0 . 3 0 - 0 . 4 5 , - - 0 . 4 7 - . l " : 1

sample size in the lower catena, or result from differences in soil characteristics between the different zones. As noted above, cation exchange in the ridge and slope sites is probably dominated by organic matter. Drying and grinding the soil releases more nutrients from the organic component (Bartlett and James, 1980), but probably has a smaller effect on mineral soil particles (Anderson and Beverly, 1985). In the poorly drained upland and riparian valleys, the lower organic matter content resulted in less of an increase in cation extractability following drying and grinding than observed in the upper catena. This, together with the spatial and temporal variability in soil saturation in the lower topographic positions (Silver, unpublished data; McDowell et al., 1992), may be responsible for the poor correspondence between extraction techniques in these zones.

Extraction techniques and nutrient availability

The similarities and differences between the extraction techniques used here raises some interesting questions for tropical ecology. Many different soil processing techniques and extracting solutions have been used on tropical soils. For many years, soils have been traditionally oven- or air-dried and ground pri- or to extraction (Anderson, 1987; Page, 1982). More recently, soils from wet tropical forests are frequently extracted under field moisture conditions (Tanner, 1977; Vitousek et al., 1988), based on the assumption that since these soils rarely reach a true air-dried state in nature, extracting soils fresh more closely mimics the environment of plant roots. In this study, soils extracted fresh generally yielded significantly lower exchangeable cation concentrations than soils that had been dried and ground prior to extraction. This was true even though NH4C1, the solution used on fresh soils, extracted significantly more Ca and Mg on test

140

samples than KC1 when sample preparation was the same (Table 3).

None of the indices of nutrient availability have meaning ecologically unless they can be related directly to plant uptake and growth. Yet, most would agree that it is impossible to determine the nutrient requirements of a diverse, mature tropical forest flora. In the absence of direct measurements of nutrient requirements, we are left with indirect indices that best reflect the nutrient content and use by the vegetation. Jordan (1985) compared the nutrient storage in soils and vegetation of tropical and temperate forests in an effort to better understand the relative roles of tropical vegetation and soil in nutrient cycling. In his comparison, montane tropical sites generally exhibited greater stores of labile Ca in the soil, while K was stored primarily in the vegetation (Jordan, 1985). A similar comparison in the Bisley Watersheds shows that the soil (0-60 cm) stores the majority of labile Ca, Mg, and total N in this ecosystem (Fig. 2). If we consider cations extracted from fresh soil as the most readily available nutrients to plants, pool sizes of exchangeable Ca on the ridges and exchangeable K in all topographic positions were smaller than pool sizes of the same nutrients stored in the vegetation in these zones (Fig. 2a). When soils are dried and ground prior to extraction, only the soil K pool on the ridge tops was smaller than the corresponding vegetation pool (Fig. 2b). Mineral soil pools of exchangeable Ca (fresh), K (fresh and dried/ground), and extractable P in the major rooting zone (0-35 cm) were smaller than the vegetation pools of these elements (Fig. 2a, b).

Another important source of nutrients to the vegetation is the forest floor. Total pool sizes of forest floor nutrients were small in the Bisley Watersheds when compared to other ecosystem nutrient compart- ments (Fig. 2a, b). However, the forest floor repre- sents a nutrient rich resource in comparison to the soil when examined on a concentration per unit mass basis (Fig. 3). In this study, we observed that the forest floor and surface soils in the Bisley Watersheds are heavily colonized by fungi, fine roots, and root mats (Silver and Vogt, 1993). In the Amazon basin, it has been suggested that high surface root biomass develops in response to nutrient limitations, and through mycor- rhizal associations, roots can directly take up nutrients from the litter and circumvent the soil where nutrients are likely to be lost (Cuevas and Medina, 1988; Stark and Spratt, 1977; Went and Stark, 1968). While many tropical forests exhibit similar rooting behavior, the studies supporting the direct nutrient cycling theory

were conducted on nutrient poor, heavily leached spo- dosols (Cuevas and Medina, 1988; Stark and Spratt, 1977; Went and Stark, 1968). In the montane forests of Puerto Rico such dramatic nutrient limitations appear unlikely. Here, the skewed distribution pattern of fine roots in forest floor and surface soil may represent a strategy to exploit a near-by, nutrient-rich resource which is frequently renewed through litterfall. The carbon cost of exploiting the forest floor resource is likely to be less than the cost of penetrating and searching the mineral soil, particularly in a humid environment where the chances of root death due to desiccation are reduced.

Comparisons with others studies

Nutrient cycling has been particularly well studied in tropical montane forests, and a relatively large data set on soil nutrients is available (Table 5). Cross-site comparisons of tropical soils are complicated by differences in methodology, parent material, climate, and vegetation type, so such comparisons should be made with caution. Here, we compare data from several montane forests throughout the new- and old-world tropics (Table 5). We separated studies that had extracted fresh soil from those that use dry/ground soils, but did not attempt to further subdivide the studies by other methodological differences due to great variety of techniques used. The considerable variability among sites in soil nutrient availability is striking (Table 5). Tropical montane forests are often cited as low in P (reviewed by Vitousek and Sanford, 1986), however results here indicated a wide range of concentrations of this element (Table 5). Jordan and Herrera (1981) suggested that the tabonuco forest was a particularly eutrophic tropical site, because of high Ca concentrations in the soil. They used Edmisten's (1970) values (although they probably overestimated the values by mistakenly substituting centimeters for depths measured in inches) which were between 2.4 and 8 times higher than those measured here. However, the results in Table 5 indicate that Edmisten's values, while in the upper range for exchangeable Ca concentrations, are in the lower range of total N concentrations, and mid-range for exchangeable K when compared to other montane tropical sites. The Bisley site occurs in the mid to lower range of Ca concentrations depending upon the extraction technique used.

In this study, we sampled soils much more inten- sively than is usual as an attempt to describe the spatially heterogeneity of the ecosystem. The results showed

141

l2 r Ca mg g-l

10 -

1.0 _ p w w-1

0.8 -

0.4

0.6

0.8 -

12 ’ Ridge She Valley Rip.Valiey

4

i

WI n-42 g-1

m ?

P $2 2

2 l-

O-

l-

$2 t

3 t

4' I Ridge

lj4 0 3- b %J 2- :: Ql-

O-

l-

77 2 -

h

Slope Vallev Rip.Valley

1.0 ’ Ridge Valley

2o r Total N mg g-1

15 -

10 -

5-

o-

5 -

10

15 t

711 u---l-.-

Rip Valley

Ridge Slope Valley FQValley

Cl Live Vegetation n Forest Floor E¶ O-10 cm

q lo-35 cm q 35-60 cm

5’ Ridge Slope Valley Rip.Valley

Fig. 3. The distribution of nutrients (mg g- ’ ) on a mass-weighted basis in the vegetation and forest floor compared with exchangeable nutrients in soils extracted dried and ground with KC1 and a modified Olsen solution, and total nitrogen in the Bisley Research Watersheds, Luquillo Experimental Forest, P.R. Nutrients in the vegetation are taken from Scatena et al., (1993).

that soil chemical and physical properties in a montane wet tropical forest are complex. The degree of variability in the Bisley soils is perhaps best expressed in

Figure 4, where we compare the internal heterogeneity of the Bisley soils with the mean values for the studies listed in Table 5. The ranges of values for Ca,

30

&

0

g c

I,.U

200

10

20

0

L

142

13.

0

150

100

50

2.0

+ I

8 0

" 7 " '

T

W

25

20

15

10

5

0

T

1.5

z 1.0

0.5

0.0

t

© I I

I

__+ o

LLI o

o ,

{ 5 t Fig. 4. Box plots showing the range of values of nutrient concentrations in surface soils (0-10 cm) of the Bisley Research Watersheds, Luquillo Experimental Forest, P.R. and other tropical montane forest (TMF) sites combined. See Table 5 for details of the TMF sites. Data sets using fresh soils (Bisley Fresh, TMF Fresh) and dry/ground soils (Bisley Dry, TMF Dry) are compared. Horizontal bars dissecting the boxes represent the median between the whiskers (the range of values minus outliers), boxes enclose the interquartile range, stars represent outliers, and dots are far outside values.

143

Table 5. Concen t r a t i ons o f e x c h a n g e a b l e nutr ients , total N, and p H in the su r f ace soi ls o f s eve r a l m o n t a n e t rop ica l

fores t s . D a s h e d l ines a re g i v e n w h e r e da ta w e r e unava i lab le . S tud ies are d iv ide d by ca t ion ex t rac t ion t e c hn ique s (i.e.

dr ied , g r o u n d soi l and f r e sh soil). D a t a f r o m d i f f e ren t e l eva t ions or s i tes f r o m the s a m e s tudy a re l i s ted i f t h e y w e r e

l i s ted that w a y in the o r ig ina l pape r and i f t hey cons i s t ed o f mul t ip le s a m p l e s

Site Elevation Depth Ca Mg K P N pH Reference

(cmol + k g - 1 ) (/~g g - l ) (%)

Dry/Ground soil Puerto Rico 350 0-10 3.3 3.3 0.3 26 0,3 4.8 This study

Puerto Rico a 300 0-12 8.0 5.6 0.3 0.4 4.9 Edmisten, 1970

Puerto Rico 350 0-10 5.1 17.4 3.8 6 0.2 5.3 Cuevas et al., 1991

Costa Rica b 100 0-15 1.1 0.3 0.2 22 0.4 3.7 Heaney and Proctor, 1989

Costa Rica b 1000 0-15 0.2 0.3 0.3 1 1.4 4.1 Heaney and Proctor, 1989

Costa Rica b 2000 0-15 0.4 0.3 0.1 5 2 3.8 Heaney and Proctor, 1989

Costa Rica ~ 2600 0-15 2.3 1.2 0.3 - 1.9 3.7 Heaney and Proctor, 1989

Colombia 600 0-20 3.4 2.8 0.7 3 0.2 5.4 Cavelier, 1988

Colombia 600 0-12 6.7 3.7 0.6 6 0.4 55 Cavelier, 1988



Venezuela c 1300 0-10 3.1 2.4 0.2 7 0.3 5.4 Zink, 1986

New Guinea 2500 0-12 25.5 7.5 1.1 24 1.5 6.1 Edwards and Grubb, 1982

Sabah, Malaysia 280 0-15 7.7 24.6 0.1 4 5.7 Proctor et al., 1988


Sabah, Malaysia 480 0-15 4.2 11.5 0.2 5 - 6.1 Proctor et al., 1988

Sabah, Malaysia 610 0-15 12.4 10.6 0.4 7 6.0 Proctor et aL, 1988



Sarawak, Malaysia 500 0-20 0.1 0.l 0.1 - 0.3 4.1 Proctor et al., 1983

Sarawak, Malaysia d 1310 0-15 0.1 0.3 0.5 - 0 8 3.9 Tie et al., 1979

Sarawak, Malaysia d 1860 0-20 0.1 2.1 1.5 - 1.8 3.4 Tie et aL, 1979

Sarawak, Malaysia d 1930 0-15 0.1 0.7 0.6 - 1.8 3.4 Tie et al., 1979

Fresh soil Puerto Rico 350 0-10 1.0 1.5 0.1 - 4,8 This study

Jamaica 1550 0-10 0.4 l l .3 0.9 - 1.6 3.0 Tanner, 1977

Jamaica 1550 0-10 0,1 6.1 1.4 - 1.7 3.7 Tanner, 1977

Jamaica 1550 0-10 6.2 3.9 0.8 - 0.4 4.0 Tanner, 1977

Jamaica 1550 0-10 1.6 0.4 0.3 - 0.5 4.4 Tanner, 1977

Jamaica 1550 0-10 12.8 3.4 0.8 - 1.1 4.7 Tanner, 1977

Hawaii e 760 - 8.4 5.3 2.2 23 0.01 - Vitousek et al., 1988



Hawaii f 760 - 22.0 5.4 2.3 45 0.01 - Vitousek et al., 1988



a N depth unknown.

b Summarized in Heaney and Proctor, 1989.

P value omitted (2600 m site) because it was exceedingly high and unexplained.

c p and N depth are 0-25 cm.

d Cited in Proctor et al., 1983.

e Sites on young lava flow.

Sites on old lava flow.

Mg, and K are very similar for dry/ground soils. The range of values for P was actually greater in the Bisley watersheds than in the other sites combined (Fig. 4). The range of values for total N was greater in the combined tropical montane forests, although the median

was similar to the Bisley soils. The Bisley soils which were extracted fresh had a smaller range of values than montane tropical forests combined, however, relatively few data points were available using fresh soils (Table 5, Fig. 4).

144

In summary, our analyses indicate that soil chemical and physical properties vary along a catena from ridge tops to riparian valleys in a wet tropical montane forest in Puerto Rico. Drainage, downslope transport of nutrients, soil organic matter content, and the oxidation state of Fe are probably the dominant processes determining soil nutrient availability. The spatial distribution of soil properties corresponds to changes in plant species composition and basal area, presenting some intriguing questions regarding plant/soil interactions. Finally, the soils of the Bisley Watersheds are generally intermediate in terms of nutrient availability compared to other tropical montane forests, but exhibit a wide range of internal heterogeneity.

Acknowledgements

We wish to thank A E Lugo, P Sollins, and two anonymous reviewers for useful comments on the manuscript. Funding for this research was generously provided by two grants to A H Johnson from the A W Mellon Foundation. Additional infrastructural support was provided by a National Science Foundation grant (BSR-8702336) to the International Institute of Trop- ical Forestry and the Center for Energy and Environ- mental Research as part of the Long Term Ecological Research Program.

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