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Soil carbon is predominantly grass-derived in tropical savannas, even under woody vegetation encroachment

Citation

Zhou, Yong; Staver, Carla (2022), Soil carbon is predominantly grass-derived in tropical savannas, even under woody vegetation encroachment, Dryad, Dataset, https://doi.org/10.5061/dryad.c59zw3rbg

Abstract

Tropical and subtropical savannas have been increasingly targeted for carbon (C) sequestration from afforestation, based on projections that assume large gains in soil organic C (SOC) accompanying increasing tree cover. These assumptions may not reflect real changes in SOC under woody vegetation encroachment, because savanna SOC is derived not only from trees but also from highly productive grasses. However, the contributions of grass-derived C to SOC and the variation of SOC responses to increasing tree cover remain poorly understood. Here we utilize a case study from an intensively studied African savanna together with data synthesized from 196 soil profiles (0-100 cm) across tropical and subtropical savannas to show that grass-derived C constitutes more than half of the total savanna SOC, even in soils sampled directly under trees. The largest SOC concentrations were always associated with the largest contribution of grass-derived C (> 70%), suggesting that grassy ecosystems can have especially high soil carbon storage. Moreover, SOC concentration and grass-derived C were not predicted by tree cover regionally or more broadly across the tropics/subtropics. Increasing tree cover marginally increased SOC storage on average (by 6.54 ± 5.28 Mg C/ha, n = 45), but this sequestration potential is 90% less than optimistic projections assume. More importantly, both SOC gain and loss were commonly observed across broad gradients of rainfall and soil sand content following woody encroachment. These results underscore the continued large contribution of grasses to SOC and the considerable uncertainty inherent in SOC responses to increasing tree cover, and challenge the widespread assumption that afforestation universally enhances SOC storage across tropical and subtropical savannas.

Methods

SOC dynamics across Kruger: We extensively sampled soil cores across a semiarid savanna in Kruger National Park to explore factors that influence C4-derived C (i.e., grass-derived C) and SOC concentration at the regional scale.  Kruger covers nearly 20,000 km2 (lat. 22°20'-25°30'S, long. 31°10'-32°00'E) and consists of tropical and subtropical savannas. In order 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 (i.e., 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 of 2016-2017 and LiDAR data. During the rainy seasons of 2010 and 2011, soil samples were collected to a depth of 20 cm from 97 VCA sites that were easy to access. 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 (i.e., 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 concentration and stable carbon isotopic value 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 (δ13C) was expressed as deviations of soil samples from an international standard (i.e., Vienna Pee Dee Belemnite) in parts per thousand (‰) using delta-notation.

SOC dynamics across tropical and subtropical savannas: In order to get a full picture of grass-derived soil C throughout the whole 1-m soil profile across global savannas, we used the key words savanna AND soil AND 13C OR stable carbon isotope, 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 C4 grasses, and (2) on whether the publication included at least one soil profile with at least four soil stable carbon isotopic measurements, with one of these measurements located at a depth of > 80 cm in the soil profile. Overall, a total of 200 soil profiles were collected from 54 publications. Among these soil profiles, 40 were sampled directly underneath individual tree canopies or woody patches within savannas. For each soil profile from each publication, 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, soil C concentration (or storage), soil stable C isotope ratios (i.e., δ13C) of total SOC, and end members for calculating C3- vs. C4-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).

SOC storage following woody encroachment: In order 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 keywords woody encroachment OR woody thickening OR woody invasion AND soil profile OR soil column OR deep soil AND carbon, 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 before and after woody encroachment, including the conversion of grasslands to woodlands (hereafter encroached grasslands) and the thickening of savannas to woodlands (hereafter encroached savannas), and (2) on whether SOC storage was measured throughout the whole 1-m soil profile. We also included savanna studies that have reported woody encroachment and measured SOC storage underneath tree canopy (or woody patches) and in the herbaceous layer as encroached grasslands. Likewise, we extracted SOC storage from encroached and non-encroached sites, and all other information outlined in the previous section from each eligible publication.

Usage Notes

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.

Funding

Yale University