Elephant megacarcasses increase local nutrient pools in African savanna soils and plants
Data files
Feb 05, 2025 version files 27.24 KB
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Foliar_Isotopes.csv
2.18 KB
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Foliar_Nutrients.csv
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README.md
5.88 KB
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Soil_Ions.csv
3.94 KB
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Soil_Isotopes.csv
2.19 KB
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Soil_Micronutrients.csv
4.06 KB
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Soil_Organic_Carbon.csv
1.98 KB
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Soil_Respiration.csv
2.77 KB
Abstract
African elephants (Loxodonta africana) are the largest extant terrestrial mammals, with bodies containing enormous quantities of nutrients. Yet we know little about how these nutrients move through the ecosystem after an elephant dies. Here, we investigated the initial effects (1-26 months post-death) of elephant megacarcasses on savanna soil and plant nutrient pools in Kruger National Park, South Africa. We hypothesized that: (H1) elephant megacarcass decomposition would release nutrients into soil, resulting in higher concentrations of soil nitrogen (N), phosphorus (P), and cations near the center of carcass sites; (H2) carbon (C) inputs to the soil would stimulate microbial activity, resulting in increased soil respiration potential near the center of carcass sites; and (H3) carcass-derived nutrients would move from soil into plants, resulting in higher foliar nutrient concentrations near the center of carcass sites. To test our hypotheses, we identified 10 elephant carcass sites split evenly between nutrient-poor granitic and nutrient-rich basaltic soils. At each site, we ran transects in the four cardinal directions from the center of the carcass site, collecting soil and grass (Urochloa trichopus, formerly U. mosambicensis) samples at 0, 2.5, 5, 10, and 15 m. We then analyzed samples for CNP and cation concentrations and quantified soil microbial respiration potential. We found that concentrations of soil nitrate, ammonium, δ15N, phosphate, and sodium were elevated closer to the center of carcass sites (H1). Microbial respiration potentials were positively correlated with soil organic C, and both respiration and organic C decreased with distance from the carcass (H2). Finally, we found evidence that plants were readily absorbing carcass-derived nutrients from the soil, with foliar %N, δ15N, iron, potassium, magnesium, and sodium significantly elevated closer to the center of carcass sites (H3). Together, these results indicate that elephant megacarcasses release ecologically consequential pulses of nutrients into the soil that influence soil microbial activity and are absorbed by plants into the above-ground nutrient pools. These localized nutrient pulses may drive spatiotemporal heterogeneity in plant diversity, herbivore behavior, and ecosystem processes.
README: Elephant megacarcasses increase local nutrient pools in African savanna soils and plants
https://doi.org/10.5061/dryad.wpzgmsbwm
Datasets
Dataset: Soil_Ions.csv
This dataset contains soil ion measurements for 10 elephant carcass sites at distances of 0, 2.5, 5, 10, and 15m from the center of each carcass site. The columns contain identifying information for each site and sample, including the date the soil sample was collected ("Date_Collected"), the nickname identifier for the site ("Site"), the soil type ("Soil_Type"), the distance from the center of the carcass site in meters ("Distance_m"), and the age of the carcass ("Days_Post_Death"). The measurements taken at each site include phosphate ("PO4"), nitrate ("NO3"), and ammonium ("NH3") concentrations in mg/L and mg/kg, as well as the concentration of plant-available phosphorus ("P_Bray_I") in mg/kg.
Dataset: Soil_Isotopes.csv
This dataset contains soil isotope measurements for 10 elephant carcass sites at distances of 0, 2.5, 5, 10, and 15m from the center of each carcass site. The columns contain identifying information for each site and sample, including the date the soil sample was collected ("Date_Collected"), the nickname identifier for the site ("Site"), the soil type ("Soil_Type"), the distance from the center of the carcass site in meters ("Distance_m"), and the age of the carcass ("Days_Post_Death"). The measurements taken at each site include soil percent nitrogen ("N_per") and isotopic nitrogen-15 ("N15").
Dataset: Soil_Micronutrients.csv
This dataset contains soil anion measurements for 10 elephant carcass sites at distances of 0, 2.5, 5, 10, and 15m from the center of each carcass site. The columns contain identifying information for each site and sample, including the date the soil sample was collected ("Date_Collected"), the nickname identifier for the site ("Site"), the soil type ("Soil_Type"), the distance from the center of the carcass site in meters ("Distance_m"), and the age of the carcass ("Days_Post_Death"). The anion measurements at each site include sodium ("Na_23"), magnesium ("Mg_24"), potassium ("K_39"), calcium ("Ca_43"), and iron ("Fe_57") concentrations in mg/kg.
Dataset: Soil_Organic_Carbon.csv
This dataset contains soil ion measurements for 10 elephant carcass sites at distances of 0, 2.5, 5, 10, and 15m from the center of each carcass site. The columns contain identifying information for each site and sample, including the date the soil sample was collected ("Date_Collected"), the nickname identifier for the site ("Site"), the soil type ("Soil_Type"), the distance from the center of the carcass site in meters ("Distance_m"), and the age of the carcass ("Days_Post_Death"). The measurements taken at each site include phosphate ("PO4"), nitrate ("NO3"), and ammonium ("NH3") concentrations in mg/L and mg/kg, as well as the concentration of plant-available phosphorus ("P_Bray_I") in mg/kg.
Dataset: Soil_Respiration.csv
This dataset contains soil respiration potential measurements for 10 elephant carcass sites at distances of 0, 2.5, 5, 10, and 15m from the center of each carcass site. The columns contain identifying information for each site and sample, including the date the soil sample was collected ("Date_Collected"), the nickname identifier for the site ("Site"), the soil type ("Soil_Type"), the distance from the center of the carcass site in meters ("Distance_m"), and the age of the carcass ("Days_Post_Death"). The measurements taken at each site include soil water content ("H2O_mmolpmol_obs") and soil respiration potential ("ugCO2_hr").
Dataset: Foliar_Isotopes.csv
This dataset contains foliar (leaves from Urochloa mosambicensis) isotope measurements for 10 elephant carcass sites at distances of 0, 2.5, 5, 10, and 15m from the center of each carcass site. The columns contain identifying information for each site and sample, including the date the soil sample was collected ("Date_Collected"), the nickname identifier for the site ("Site"), the soil type ("Soil_Type"), the distance from the center of the carcass site in meters ("Distance_m"), and the age of the carcass ("Days_Post_Death"). The measurements taken at each site include leaf percent nitrogen ("N_per") and isotopic nitrogen-15 ("N15"). Three sites did not have sufficient grass for analysis at the distance of 0m, so the "N15" and "N_per" measurements are listed as "NA".
Dataset: Foliar_Nutrients.csv
This dataset contains foliar (leaves from Urochloa trichopus) isotope measurements for 10 elephant carcass sites at distances of 0, 2.5, 5, 10, and 15m from the center of each carcass site. The columns contain identifying information for each site and sample, including the date the soil sample was collected ("Date_Collected"), the nickname identifier for the site ("Site"), the soil type ("Soil_Type"), the distance from the center of the carcass site in meters ("Distance_m"), and the age of the carcass ("Days_Post_Death"). The micronutrient measurements at each site include sodium, magnesium, potassium, calcium, iron, and phosphorus in both percentages and mg/kg. Three sites did not have sufficient grass for analysis at the distance of 0m, so micronutrient measurements are listed as "NA".
Note: all dates are recorded as M(M)/(D)D/YY (e.g., 3/15/23)
Code
Script: Megacarcass_Nutrients_Manuscript_Code.Rmd
This is an annotated R Markdown file that can be used to generate the analyses and figures in the elephant megacarcass manuscript using the datasets listed above. The user needs to download necessary packages (listed in the markdown file) and set the file pathways for the above datasets. The script provides examples of each major analysis, which the use can then modify for any variable in the data. The code was originally run in R version 4.3.1.
Methods
We performed this research in the southern part of the Kruger National Park (KNP), South Africa (24.996 S, 31.592 E, ~275m elevation). During the wet season in March 2023, we identified ten elephant carcass sites (1-26 months post-death), five on relatively nutrient-rich basaltic soil and five on nutrient-poor granitic soil. KNP section rangers provided precise GPS locations of where elephant carcasses had been found. These sites were recognizable in situ by a persistent bonefield, undigested gut contents, and an absence of herbaceous vegetation. At each site, we hammered a rebar post into the center of the megacarcass disturbance and ran 15 m transects out from the post in each of the four cardinal directions. We collected green leaf material from Urochloa trichopus (formerly U. mosambicensis), a common and abundant palatable grass species, and used an auger to collect soil samples to a depth of 10 cm at five points along each transect (0.5, 2.5, 5, 10, and 15 m). We pooled and homogenized the samples to yield one composite leaf and one composite soil sample per sampling distance from each carcass site. Soil samples were sieved in a 5-mm metal sieve which was cleaned in between samples with 70% ethanol. On the day of collection, we used 5 g of each soil sample for soil respiration measurements (described below). The rest of each sample was stored plastic bags in a -20°C freezer until nutrient analyses. Leaf samples were stored in paper bags at room temperature until dried for analyses (see below). Soil samples were stored in a cooler during fieldwork. On the day they were collected, we used 5 g of each soil sample for soil respiration measurements (described below). The rest of each sample was stored in plastic bags in a -20°C freezer until nutrient analyses; they were stored in coolers with ice blocks during the transition from the freezer at the field site to the freezers at the labs.
We tested our first hypothesis that elephant carcass decomposition would release nutrients into the soil by performing soil nutrient analyses. We sent 250 g of each soil sample to Eco-Analytica laboratory at the North-West University in Potchefstroom, South Africa for measurements of soil concentrations of ammonium [NH4]+, nitrate [NO3]-, phosphate [PO4]3-, and plant-available P. Samples were air-dried and sieved through < 2mm mesh prior to chemical analysis. Plant-available P was extracted from 4 g of soil and 30 ml extraction fluid (1:7.5 ratio) using an acid–fluoride solution (P Bray-1), measured colorimetrically using a Systea EasyChem200 analyser, and expressed as mg/kg. The detection limit was 0.5 mg/kg, and plant available P measurements <0.5 mg/kg were replaced with half the detection limit (0.25 mg/kg) (Croghan & Egeghy, 2003; Keenan & Beeler, 2023). Water-soluble nitrate and phosphate anions were extracted from volume on volume 100 ml soil and 200 ml deionized water, analyzed by ion chromatography on a Metrohm 930 Compact Flex System, and measured as mg/L. Ammonium (also 1:2 water extract) was analyzed colorimetrically using a Systea EasyChem200 analyzer and measured as mg/L. Detection limits for soil ions were 0.01 mg/L, and soil ion concentrations measured as <0.01 mg/L were replaced with half the detection limit (0.005 mg/L). To convert the nitrate, ammonium, and phosphate units from mg/L to mg/kg, we multiplied by 2, based on the 1:2 soil to water extraction ratio.
To determine whether soil anions were distinct and elevated at the center of carcass sites relative to soil further from the center, concentrations of sodium (Na), magnesium (Mg), iron (Fe), calcium (Ca), potassium (K), and phosphorus (P) cations were measured using microwave-assisted digestion. Air-dried and sieved (>2 mm) soil samples, weighed to 0.2 g, were microwaved in 9 ml 65% nitric acid (HNO3) and 3 ml 32% hydrochloric acid (HCl) according to EPA 3051b in a Milestone, Ethos microwave digester with UP, Maxi 44 rotor. A period of 20 minutes allowed the system to reach 1800 MW at a temperature of 200 °C which was maintained for 15 minutes. After cooling, the samples were brought up to a final volume of 50 ml and analyzed on an Agilent 7500 CE ICP-MS fitted with CRC (Collision Reaction Cell) technology for interference removal. The instrument is optimized using a solution containing Li, Y, Ce, and Tl (1 ppb) for standard low-oxide/low interference levels (£ 1.5%) while maintaining high sensitivity across the mass range. The instrument was calibrated using ULTRASPEC® certified custom mixed multi-element stock standard solutions containing all the elements of interest (De Bruyn Spectroscopic Solutions, South Africa). Calibrations spanned the range of 0 – 30 ppm for the mineral elements Ca, Mg, Na, and K and 0 – 0.3 ppm for the rest of the trace elements. Elemental concentrations were expressed as mg/kg.
Finally, to determine whether elevated N levels in soils were derived from the carcass, we sent 10 g of each sample to the BIOGRIP laboratory within the Central Analytical Facility at Stellenbosch University for measurements of soil %N and δ15N, obtained using a Vario Isotope Select Elemental Analyzer connected to a thermal conductivity detector and an Isoprime precisions isotope ratio mass spectrometer (IRMS). Samples were oven-dried at 60°C for 48 hours and milled to a fine powder using a Retsch MM400 mill (Germany). The powdered samples were weighed (2 – 60 mg) prior to combustion at 950°C. The gasses were reduced to N2 (undiluted) in the reduction column, which was held at 600°C. A high organic carbon (HOC) soil standard (0.52 ± 0.02 %N), along with two international reference standards (USGS40 (δ15N -4.52% AIR) and USGS41 (δ15N +47.57% AIR)) were used for calibration. The N elemental content was expressed relative to atmospheric N as N2 δ15NAIR (‰). The quantification limit for δ15N on the IRMS is 1 nA (nanoAmp), and the quantification limit for %N is 0.06%. The precision for %N was 0.02% and for δ15N is ±0.11%, determined using the HOC standard, which was run multiple times throughout the analysis.
To test our second hypothesis that nutrient inputs to the soil would stimulate microbial activity, we measured soil organic C, water content, and microbial respiration potential. We sent 10 g of each sample to the BIOGRIP laboratory for measurements of soil organic C using a Vario TOC Cube (Elementar, Germany). Samples (dried and milled as above) were weighed (10 – 60 mg), acidified using 10% HCl to remove the total inorganic C (carbonates), and dried overnight at 60°C. All samples were analyzed through combustion at 950°C. The released CO2 was measured by a non-dispersive infrared (NDIR) sensor. A high organic C (7.45 ± 0.14 %C) soil standard from Elemental Microanalysis Ltd (UK) was included during the analysis. The quantification limit for %C is 0.14%. The precision for the %C was 0.09% and was determined using the low organic C (LOC) standard (1.86 ± 0.14 %C), which was run multiple times throughout the analysis.
To quantify soil respiration and water content, we used an incubation method (Lemoine et al. 2023) in which 5 g (± 0.2 g) of each sample was placed into a 100 ml clear glass bottle, sealed, and flushed with CO2-free air. Following flushing, we incubated the bottles for one hour at 25°C. We then recorded CO2 concentrations using an LI-850 CO2/H2O infrared gas analyzer. After soil respiration measurements, we determined sample dry weight by drying each sample at 60°C for 24-48 hours until stable mass was achieved. We subtracted dry weight from starting weight to obtain soil water content. Finally, we used the dry weights and the Ideal Gas Law to standardize all respiration measurements to CO2 μg h-1g dry soil-1.
To test our third hypothesis that carcass-derived nutrients would be incorporated by plants, we measured foliar nutrient concentrations in U. trichopus. Two grams of each dried leaf sample was sent to the BIOGRIP laboratory for preparation and measurements of %N and δ15N via stable isotope analysis as described above. A Sorghum flour standard (1.47 ± 0.25 %N) from Elemental Microanalysis Ltd (UK) was used for calibration, along with two international reference standards (USGS40 and USGS41). The quantification limit for δ15N on the IRMS is 1 nA, and the quantification limit for %N is 1.3%. The precision for the %N was 0.02% and for δ15N is ±0.08‰. Limits were determined using the sorghum flour standard, which was run multiple times throughout the analysis. Additionally, we sent 5 g per sample to Cedara Analytical Services Laboratory to quantify micronutrients in grass tissue (P, Na, Mg, K, Ca, and Fe) using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES 5800, Agilent, USA). Samples were dried (110°C overnight) and milled to a fine powder. Subsamples (0.5 g) were ashed at 450°C for 4 hours, and the ash was re-wet using 2 mL conc. HCl (32%). Samples were evaporated to dryness then re-suspended in 25 mL 1M HCl before filtering. Lastly, the filtrate was diluted with de-ionized water in a ratio of 5:20 filtrate to water. To calibrate the ICP-OES, solutions containing known amounts of each element were measured (10-20 ppm for Na and C, 200-1500 ppm for Fe, 0.5-3.75% for K, and 0.125-0.5% for P), prepared from 1000 ppm primary single standards. At three of the ten sites, we did not find sufficient plant material at the central point for analysis, resulting in a sample size of N = 7 for the center (distance = 0.5m) measurement for leaf nutrient analyses.