Spatial Distribution of Soil Properties (1)

(1)

Spatial Distribution of Soil Properties and Soil Physical-Chemical Diversity in

the Greater Everglades Ecosystem

1

**_{University of Florida, IFAS, Soil and Water Science Department, Gainesville, FL 32611. *Contact Info: Phone (352) 392-1951 ext. 210, Email: gbruland@ifas.ufl.edu}**

G.L. Bruland

1*

_{, S. Grunwald}

1 _{, T.Z. Osborne}

1 _{, K.R. Reddy}

1 _{, and S. Newman}

2

Abstract

The spatial distributions of soil properties across wetland landscapes represent the combined effects of various biotic and abiotic factors. Conse-quently, existing landscape patterns contain information about the processes that generated these patterns. In the Florida Everglades, the spatial distribution of soil nutrients can be used to assess long-term impacts to this system. To this end, over 1,300 soil samples were collected by helicopter from the upper 0-10 cm of the Greater Everglades Ecosystem (GEE) in 2003-04. The object-ives of this study were to: (i) characterize the spatial distributions of soil prop-erties such as bulk density (BD), soil organic matter (by loss on ignition), and total calcium (TCa) across the GEE, and (ii) to map the distribution of multi-variate metrics of soil physical-chemical diversity across the GEE. Interpolated maps of the individual soil properties revealed that BD was highest in the Big Cypress National Preserve (BCNP), southeastern Everglades National Park (ENP), Modellands, and northwestern Water Conservation Area (WCA) 3A. LOI was highest in WCA-1 and southern WCA-3A. Total calcium displayed high variability across the GEE with the highest values in Modellands, ENP, and BCNP. Soil physical-chemical diversity, or pedodiversity, as expressed with two metrics, a relativized variance (RV) and a modified Shannon Index (SI), displayed an integrative spatial pattern that was a composite of the distri-butions of the individual soil properties. Pedodiversity appeared to be influen-ced by a combination of factors including underlying geology, hydrologic flow patterns, vegetative communities, nutrient loading, and drainage. The indices showed similar spatial patterns in some areas and divergent patterns in others.

Sampling Design

Cores were collected by the Wetland Biogeochemistry Laboratory (WBL) and the South Florida Water Management District from >1,300 sites in the GEE between 4/03-8/04. A stratified random sampling design was used to predict soil properties at unsampled locations based on geostatistics.

Unit Boundary Definitions

A total of 13 hydrological units (HUs) were defined. For geostatistical purposes, some of these HUs were divided into subsections based on physical boundaries that affect hydrology such as highways and canals.

Laboratory Analysis

Soils were analyzed by the WBL for bulk density (BD), loss on ignition (LOI), and total P, inorganic P, N, C, Ca, Mg, Al, and Fe by standard methods.

Geostatistics: Interpolation with Kriging and Splines

Where sample size > 90, ordinary kriging was used to interpolate soil properties across the HUs. Semivariance values were fitted with spherical and exponential semivariogram models. When sample size < 90, or when data exhibited no spatial autocorrelation, a regularized spline function was used to interploate.

Pedodiversity Indices: Relativized Variance and Modified Shannon Index

Pedodiversity was first estimated with the relativized variance (RV). The RV was calculated by dividing each soil property value at each site by the maximum measured value for that property. This scaled values from 0-1, and the RV was then estimated by calculating the variance of all the scaled values at each site. Pedodiversity was also estimated using a modified version of the Shannon diversity index (Shannon and Weaver, 1949). The raw soil data were converted to 10 evenly-spaced percentiles (data shifted from continuous to categorical).

2

_{South Florida Water Management District, Everglades Division, West Palm Beach, FL}

LOI values ranged from < 10 to > 90 % and generally exhibited an inverse trend to BD. Note the high and homogeneous LOI values in WCA-1, the area with the deepest peat. 2A, WCA-3AS, and the Shark River Slough area of the ENP also exhibited high LOI values.

Calcium exhibited high variability across the GEE with values ranging from < 15,000 to > 300,000 mg/kg. The highest values were found in Modelland, ENP, and BCNP, while the WCAs generally had lower and more homogeneous calcium concentrations. These patterns reflect-ed differences in peat depth and substrate geo-logy that occur across the GEE.

Modified Shannon Index Continued

The percentiles were used to calculate Shannon’s Index (SI) as follows:

where spi= percentiles for individual soil properties

at each site with i= 1,2,…10, and S= sum of all percentile values for each site. Sites with higher RVs or SIs have higher pedodiversity, and vice versa.

∑

− =

10 ln

sp sp

i i

i S

sp S sp SI

Similar to RV, the map of modified SI revealed that BCNP, southeastern ENP, and Modellands had lower pedodiversity, while Rotenberger, Holeyland, WCA-2, WCA-3, and Shark River Slough had higher pedodiversity. Unlike the RV map, the SI map indicated that WCA-1 had low pedodiversity.

Bulk density values ranged from < 0.05 to >

1.25 g cm-3_{. The highest interpolated values}

were observed in the marl and mixed marl areas of BCNP, southeastern ENP, and Model-lands. Western WCA-3AN and Rotenberger also had relatively high BD possibly due to a combination of factors including peat oxidation and fire.

According to the RV map, the areas with the highest pedodiversity were the WCAs, especially WCA-1, and Shark River Slough. The areas with the lowest pedodiversity included BCNP, southeastern ENP, and Modellands. This suggested that changes in hydrology and nutrient loading have increased edaphic heterogeneity in the GEE.

Methods

Study Area and Maps

1

2 4

7 10

12 13 8 6 5

3 9

11

1 = WCA-1

2 = WCA-2A

3 = WCA-2B

4 = Rotenberger

5 = Holeyland

6 = WCA-3AN

7 = WCA-3AS

8 = WCA-3B

9 = BCN

10 = BCC

11 = BCS

12 = ENP

13 = Modellands

Hydrologic Units Locations

0 20 40 60 80

0.000.020.040.050.070.080.100.120.130.150.160.18More Relativized Variance

# o

f Ca

ses

0 20 40 60 80 100 120 140

1.731.781.831.871.921.972.012.062.112.162.202.25More

Modified Shannon Index

# o

f Ca

ses

Frequency distributions for the RVand SIvalues

calculated for each sampling point show the RVto

be bimodal and the SIto be skewed and

trun-cated. Further examination indicates that the SI

overestimates diversity when concentrations are

high, while the RVis unbiased, more effectively

captures the expected data structure, and is a better index of pedodiversity for this system.

Frequency

Distributions

100

Discussion

•Mapping the spatial distributions BD, LOI, TCa, the RV, and the modified SI provide a new perspective on pedodiversity in the GEE.

•Pedodiversity is influenced by both the natural variability of geology, hydrologic flow patterns, and vegetation as well as anthropogenically-influenced variability related to nutrient loading and altered hydrology.

•Maps of RV and SI were similar in some areas and diverged elsewhere. The RV appears to be a more unbiased metric of pedodiversity than SI. The categorization of the continuous soil data required for the SI also may result in a loss of information. •More research is need to develop these and other multivariate metrics of pedodiversity.

Acknowledgements

Funding was provided by the South Florida Water Management District. We would like to thank Y. Wang of the WBL for her work with the laboratory analysis, and R.G. Rivero and R. Corstanje for their support of this research.

Everglades Soil Mapping Project (PI: K.R. Reddy; co-PI: S. Grunwald)