DOI: 10.5061/dryad.mgqnk990w
title: Divergent patterns and spatial heterogeneity of soil nutrients in a complex and dynamic savanna landscape

description: Many grassy ecosystems around the world are experiencing woody
 encroachment. These woody encroachers often cause nutrient enrichment in
 the plant-soil environment, potentially facilitating their growth and
 reproduction. However, studies of encroachment effects on nutrient
 distributions have been confined to a few major elements (e.g., N, and P)
 and limited in spatial extent. We analyzed 19 elements in dominant plants
 and in georeferenced soils across a subtropical savanna landscape
 experiencing woody encroachment to quantify their spatial patterns and
 elucidate drivers responsible for these patterns. We found divergent
 patterns of spatial heterogeneity of these elements in surface soils
 across this complex landscape. Nutrient accumulation underneath woody
 canopies and redistribution by woody plants occurred in a subset of
 elements (i.e., N, P, S, Ca, Cu, and Sr). Though some of these elements
 are not necessarily growth-limiting, they do occur in higher
 concentrations in woody compared to herbaceous plants. Distributions of
 the other elements were closely related to spatial variation in soil pH,
 clay content, and slope rather than to woody encroachment. Our nuanced
 spatial sampling approach and analysis reveal significant variation in
 nutrient distributions in response to woody encroachment, and illustrate
 the role of landscape patterns in mediating ecosystem processes. These
 changes in the concentrations and distributions of key essential nutrients
 broaden our understanding of the biogeochemical consequences of woody
 encroachment, and provide new insights regarding the significance of
 long-term vegetation dynamics in dryland ecosystems.
description: All plant and soil samples
 were collected from Texas A&M AgriLife La Copita Research Area
 (27˚40̍ N, 98˚12̍ W) in southern Texas, USA. Briefly, on an upland portion of
 this area, a 160 m × 100 m landscape subdivided into 10 m × 10 m grid
 cells (i.e., 160 grid cells in total) was established in 2002. In July
 2014, two sampling points were randomly selected within each grid cell,
 yielding a total of 320 points. At each sampling point, two adjacent soil
 cores (2.8 cm in diameter and 5 cm in length) were collected. One soil
 core was used to estimate fine root (&lt; 2 mm) biomass by washing
 through sieves. The other core was air-dried and then passed through a 2
 mm sieve to remove coarse organic matter prior to subsequent
 analyses. In
 addition, fine roots and fully expanded new leaves were collected from 16
 dominant plant species (woody species = 5, grasses = 6, and forbs = 5)
 during the peak of the growing season. Four replicates were obtained for
 each species. In addition, leaf litter and bulk fine roots were collected
 from grasslands and woody patches. Leaf litter was collected within a 25
 cm × 25 cm frame. After removing leaf litter, soil cores (7 cm diameter ×
 5 cm length) were collected to obtain bulk fine roots. All plant samples
 were carefully washed with deionized water and then oven-dried at 65 °C
 for subsequent analyses. Soil samples were pulverized by
 mortar and pestle while plant samples were pulverized using a Retsch MM400
 mixer mill (Retsch GmbH, Haan, Germany) with a titanium grinding jar to
 avoid metal contamination. All plant and soil samples were analyzed for a
 suite of biologically essential macronutrients (N, P, K, Ca, Mg, and S),
 essential micronutrients (Fe, Mn, Cu, Zn, Ni), and nonessential
 micronutrients (Al, Ba, Co, Cr, Li, Sr, V, and Zr). Briefly, for each sample, 0.1 g
 sample, 0.5 mL nitric acid (16M), 0.5 mL hydrochloric acid (12M), and 4 mL
 deionized water were placed within a disposable borosilicate glass tube
 with a Teflon cap, and digested in a single reaction chamber microwave
 system (ultraWAVE, Milestone Inc., Milan, Italy) (U.S. EPA 2007). Digested
 solutions were diluted with deionized water to 50 mL, transferred to a
 screw-cap vial, and then analyzed with an Avio 500 inductively coupled
 plasma optical emission spectrometer (PerkinElmer, Inc., Waltham, MA, USA)
 at the Texas A&M Trace Element Research Laboratory.
 In addition, N
 concentrations for soil and plant samples were analyzed using an EA 4010
 Costech Elemental Analyzer (Costech Analytical Technologies Inc.,
 Valencia, CA, USA) at the Texas A&M Stable Isotopes for Biosphere
 Science Laboratory. Soil pH was determined on a 1: 2 (10 g soil: 20 ml,
 0.01 mol/L CaCl<sub>2</sub>) mixture using a glass electrode.
 Soil texture was analyzed using the hydrometer method.