DOI: 10.5061/dryad.c59zw3rbg
title: Soil carbon is mostly grass-derived in tropical savannas, even under woody encroachment

description: Tropical savannas have been increasingly targeted for carbon (C)
 sequestration from afforestation, assuming large gains in soil organic C
 (SOC) with increasing tree cover. Because savanna SOC is also derived from
 grasses, this assumption may not reflect real changes in SOC under
 afforestation, but grass contributions to SOC and changes in SOC with
 increasing tree cover remain poorly synthesized. Here, we combine a case
 study from Kruger National Park, South Africa, with data synthesized from
 tropical savannas globally to show that grass-derived C constitutes more
 than half of total SOC to a soil depth of 1-meter, even in soils directly
 under trees. The largest SOC concentrations were associated with the
 largest grass contributions (&gt; 70% of total SOC). Regionally and
 across the tropics, SOC concentration was not explained by tree cover.
 Both SOC gain and loss were observed following increasing tree cover, and
 on average SOC storage within 1-meter profile only increased by a
 negligible and non-significant 6% (SE = 4%, n = 44). These results
 underscore the substantial contribution of grasses to SOC and the
 considerable uncertainty in SOC responses to increasing tree cover,
 challenging the widespread assumption that afforestation universally and
 substantially enhances SOC storage across tropical savannas.
description: <strong>SOC dynamics across Kruger</strong>: We
 extensively sampled soil cores across a semiarid savanna in Kruger
 National Park to explore factors that influence
 C<sub>4</sub>-derived C (<em>i.e.</em>,
 grass-derived C) and SOC concentration at the regional scale.  Kruger
 covers nearly 20,000 km<sup>2</sup> (lat.
 22°20'-25°30'S, long. 31°10'-32°00'E) and consists of
 tropical and subtropical savannas. Mean annual rainfall increases from 350
 mm in the north to 750 mm in the south with most precipitation falling
 from November to April. Elevation ranges from 260 to 839 m above sea
 level. Based on long-term fire records, the average fire return interval
 for Kruger is <em>c</em>. 3.5 years, but fire regimes vary
 spatially across the park, ranging from one fire per year to one every 34
 years. Kruger is dominated by two underlying parent materials, granite and
 basalt, which strongly influence soil and vegetation properties. The flora
 of Kruger includes over 400 woody species and 200 herbaceous species. The
 herbaceous layer is dominated by the C4 graminoids, including
 <em>Aristida congesta</em>, <em>Digitaria
 eriantha</em>, and <em>Panicum maximum</em>. The most
 dominant woody encroachers are <em>Dichrostachys cinerea</em>
 and <em>Combretum apiculatum</em>. To
 monitor grass biomass to inform fire management, Kruger established 533
 Veld Condition Assessment (VCA) sites throughout the park in 1989.  At
 each VCA site, grass biomass was measured with a calibrated disc pasture
 meter every year in April from 1989 to 2008 within a plot of 50 m × 60 m.
 Plot-level grass biomass estimates were averaged from measurements taken
 every 2 m along four 50-m transects (i.e., 100 measurements in total),
 running at 0, 20, 40, and 60 m along the length of the plot. In 2008, a
 one-time woody plant survey was performed. Plot-level stem density and
 basal diameter were averaged from two measurements located along each
 transect (<em>i.e.</em>, 8 measurements in total).
 Measurements for the woody plant survey were located at 30 and 50 m along
 the first and third transects, and 20 and 40 m along the second and fourth
 transects. Woody cover for each VCA site was extracted from a 10-m
 resolution remote sensing product across Kruger based on Sentinel-1 time
 series for the year 2016-2017 and LiDAR data (Fig. 1). 
 During the rainy seasons of 2010 and 2011, soil samples were
 collected to a depth of 20 cm from 98 VCA sites that were easy to access
 (Fig. 1). At each site, soil samples were collected at each of four
 corners and in the middle of the 50 × 60 m plot, and those five samples
 were then homogenized and subsampled. All soil samples were dried at 60°C
 and sieved through a 2-mm sieve. A subset of each soil sample (~ 50 g) was
 used to measure soil texture using a hydrometer, with soil sand, silt, and
 clay content adding up to 100%. Another subset was ground to a fine
 powder, acid washed, and used to analyze SOC concentrations and stable
 carbon isotopic ratios using a Costech ECS 4010 Elemental Analyzer
 interfaced via a ConFlo III device with a Delta V Advantage isotope ratio
 mass spectrometer at the Yale Analytical and Stable Isotope Center. The
 stable carbon isotopic value was expressed as deviations of soil samples
 from an international standard (Vienna Pee Dee Belemnite) in parts per
 thousand (‰) using ⸹-notation (<em>i.e.</em>,
 δ<sup>13</sup>C). <strong>SOC
 dynamics across tropical and subtropical savannas</strong>: However,
 soil samples from Kruger only covered a narrow rainfall gradient (460-700
 mm) and were constrained to surface soils. To get a fuller picture of
 grass-derived SOC across a full gradient of rainfall and to a soil depth
 of 1-m across global savannas, we used the keywords
 <em>savanna</em> AND <em>soil</em> AND
 <em>13C</em> OR <em>stable carbon isotope</em>, to
 search relevant literature on Google Scholar on July 17, 2021. We used the
 Preferred Reporting Items for Systematic Reviews and Meta-Analyses
 (PRISMA) protocol to screen and identify publications to be included in
 this analysis. Briefly, the title and abstract of each publication were
 initially screened for eligibility based on whether the publication
 included soil-stable carbon isotopic measurements within savanna soils.
 Eligible publications were further filtered based (1) on whether the
 herbaceous layer was dominated by C<sub>4</sub> grasses, and
 (2) on whether the publication included at least one soil profile where
 both soil δ<sup>13</sup>C and SOC values were measured to up
 to a depth of 1-m. Overall, a total of 148 soil profiles met these
 criteria. Among these soil profiles, 33 were sampled directly underneath
 individual tree canopies or woody patches within savannas. The geographic
 locations of soil profiles are shown in Fig. 1. For
 each soil profile, we recorded locations (latitude and latitude),
 continents (Africa, Australia, North America, and South America), climatic
 variables (mean annual rainfall [MAP] and temperature [MAT]), elevation,
 soil sand content, tree cover, soil bulk density, SOC concentration (or
 storage), soil δ<sup>13</sup>C, and end members for
 calculating C<sub>3</sub>- vs.
 C<sub>4</sub>-derived carbon if reported. When results were
 presented graphically, we used WebPlotDigitizer 4.4 to digitize the data.
 If these data were not reported, we contacted the corresponding authors
 for additional information. Otherwise, MAT and MAP were extracted from the
 WorldClim database (https://www.worldclim.org/); soil sand content was extracted from SoilGrids database (https://soilgrids.org/); elevation was extracted from the Global Multi-resolution Terrain Elevation Data 2010 (https://earthexplorer.usgs.gov/); fire frequency (2000-2019) was extracted from MODIS Burned Area Product (https://lpdaac.usgs.gov/products/mcd64a1v006/); and tree cover was extracted from the Global Tree Cover 2010 (https://glad.umd.edu/dataset/global-2010-tree-cover-30-m). <strong>SOC storage following woody encroachment</strong>: To quantify changes in SOC storage following woody (or forest) encroachment throughout the whole 1-m soil profile across tropical and subtropical grassy ecosystems, we used the key words <em>woody encroachment</em> OR<em> woody thickening</em> OR <em>woody invasion</em> AND <em>soil profile</em> OR <em>soil column</em> OR <em>deep soil</em> AND <em>carbon</em>, to search the relevant literature on Google Scholar on October 18, 2021. Similarly, we used the PRISMA protocol to screen and identify publications to be included in this analysis. Eligible publications were identified based (1) on whether SOC storage was measured from woody encroached and non-encroached sites, including the conversion of grasslands to woodlands/forests (hereafter encroached grasslands) and the thickening of savannas to woodlands/forests (hereafter encroached savannas), and (2) on whether SOC storage was measured throughout the whole 1-m soil profile. Overall, 44 paired-soil profiles with and without woody encroachment have been collected across tropical savannas and grasslands (Extended Data Table 3). Likewise, we extracted SOC storage from encroached and non-encroached sites, and all other information outlined in the previous section. The geographic locations of soil profiles related to woody encroachment are shown in Fig. 1.
description: The dataset comprises a CSV file with different tabs showing soil
 organic carbon measurements from Kruger National Park and more broadly
 across tropical and subtropical savannas.