Kruger National Park EBP carbon
Data files
Sep 08, 2023 version files 31.19 KB
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Carbon_corr3.csv
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README.md
Abstract
- Woody plant encroachment (WPE) is a global trend occurring in many biomes including savannas and accelerates with fire suppression. Since WPE can result in increased storage of soil organic carbon, fire management which may include fire suppression can enhance ecosystem carbon sequestration in savannas.
- At our study site in Kruger National Park, South Africa, we used a long-term (~70-year) fire experiment to study the drivers and consequences of changes in woody (trees and shrubs) cover and soil organic carbon (C) sequestration. We surveyed four fire manipulation treatments replicated at eight locations within the park, annual high-intensity burn, triennial high (dry season) and low-intensity (wet season) burns and fire exclusion, to capture the range of fire management scenarios under consideration. Changes in woody cover were calculated over a period similar to the experiment’s duration (~80 years) using aerial photographs (1944–2018). Soils were analysed for soil organic C (SOC) and d13C to 30 cm, under and away from tree canopies to isolate local- and landscape-level effects of WPE on SOC.
- The largest increases in woody cover occurred with fire exclusion. We found that plots with greater increases in woody cover also had higher SOC. Yet, trees were not the only contributor to SOC gains, such that sustained high inputs of C4-derived carbon (from grasses), even under canopies in fire suppression plots, contributed significantly to SOC. We observed little difference in SOC sequestration between the cooler triennial (wet season) burns and fire suppression.
- Synthesis: Grass inputs to SOC remained high across the full range of woody cover created by varying burning regimes. Total SOC stocks stored from tree inputs only matched grass-derived SOC stocks after almost 70 years of fire exclusion. Our results point to C4 grasses as a resilient contributor to SOC under altered fire regimes and further challenge the assumption that increasing tree cover, either through afforestation schemes or fire suppression, will result in large gains in C sequestration in savanna soils, even after 70 years.
README: Kruger National Park EBP carbon
Soil carbon for four treatments (annual Aug, triennial Aug, triennial Feb and no burn) at Pretoriuskop and Skukuza experimental burn plots in the Kruger National Park.
Description of the data and file structure
- EBP location (Pretoriuskop or Skukuza)
- String (this is the repeat within the EBP location)
- Treatment is fire treatment applied (aAug_1 is the annual burn in August, bAug_3 is triennial burn in August (which is the end of the dry season), dnb stands for no burn and
- cFeb_3 is a triennial burn every February, which is height of growing season)
- Tag is a unique qualifier that combined EBP, string and treatment
- D13C: is the soil delta 13C value
- Fire: is the fire received per period (so a measure of how often fire returns, e.g., 0.1 will be once in ten years)
- Fire_int: I used the above Fire received to arbitrarily divided the interval into categories of return, either low or high, high was 0.8 or above
- Heights: should be read with columns J to N, it refers to the heights of the woody vegetation, so if lower than 5 m, then you can see how much biomass for instance in that height class, vs total
- Grass_biomass: was measured with a disk pasture meter before each burn, therefore "no burn" had no values The next four columns, kg_ha, stems_ha, kg_stem and ba_m2_ha were computed from actual tree surveys conducted on the ground
- kg_ha: biomass worked out per ha using allometric relationships to convert stem diameter
- stems_ha: computed average stem per ha from survey data
- kg_stem: divided biomass by number of stems to get weight per stem in kg
- ba_m2_ha: basal diameter was measured for each stem, so this is summing basal diameters to get basal area (m2) per ha
- mg_ha: Kg_ha was transformed to mg_ha, which is equivalent to ton per ha
- Veg: the amounts under trees vs grasses, with total=the sum of these two
- TC_ha: used bulk density to work out total soil C per ha, ie converts percentage to per ha.
- Soil C change: gives the difference between the carbon for Aug3 and the other treatments, we assumed that Aug3 was closest to the natural background fire return
- tN_ha, th same as for C but total nitrogen per ha
- Sum_grass and sum_tree is how much of this is derived from grass vs trees. Can be read with column O (Veg), then you can see how much carbon under trees come from trees vs grass
- Under sum_grass, total is the total of the tree + grass values
- Tree_cover and grass_cover (%) were obtained via aerial photos, and adds to a 100%.
- Tree_change gives the change in tree cover over 80 years, as obtained from aerial photos.
- Enrich_C: the differences between the Aug3 and the other treatments in delta 13C.
- %N: percentage N in soil
- P: percentage P in soil
- K: percentage K in soil
- Ca: percentage Ca in soil
- Sand: percentage sand present in soil
- Clay_sand: clay:sand ratio
- Claysilt_sand: clay+silt:sand ratio
Sharing/Access information
This is a section for linking to other ways to access the data, and for linking to sources the data is derived from, if any.
Links to other publicly accessible locations of the data:
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Data was derived from the following sources:
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Methods
Soil samples were obtained under and away from the canopies of five large (> 6 m) Sclerocarya birrea trees on each plot in 2016. Under-canopy samples were taken halfway between the tree trunk and the edge of the canopy. Away from canopy samples were taken away (> 5 m) from tree canopies. After litter was scraped away, a trowel was used to sample to 5 cm and, below this depth, a 4 cm-diameter soil auger was used to sample from 5-30 cm (at increments of 5–10 cm, 10–20 cm, and 20–30 cm) and soils kept separate for each depth. The soil was dry sieved through a 2 mm sieve to remove all roots after which a bulk sample was taken to make one sample per canopy and one away from canopy per plot per depth (two vegetation types ´ 4 replicates (strings) ´ 4 fire treatments ´ canopy/away from canopy ´ 4 depths = 256 samples). After soil sampling, soil texture was analysed at Elsenburg Laboratory (Western Cape, South Africa) following the hydrometer method for soil particle analysis (Committee, 1990).
Soil carbon and 13C/12C ratios of the soil carbon were determined using a Thermo Finnigan Delta plus XP mass spectrometer coupled with a conflo III device to a Thermo Finnigan Flash EA1112 Elemental Analyser with automatic sampler (Thermo Electron, Bremen, Germany). Although we did not expect to find carbonates in our study soils based on the pH values of the soils, we treated a subset (about 25%) of the soil samples with HCl to remove carbonates and reran these for variation in the C content, which confirmed the absence of inorganic carbon. These results were calibrated relative to Pee-Dee Belemnite as well as to correct for drift in the reference gas. The results are expressed as parts per thousand (‰) and relative to the Pee-Dee Belemnite standard are denoted by the term δ, with precision of duplicate analysis 0.1‰ (February et al., 2011). Based on the δ13C values of the soil and end member (mean) δ13C values of the grasses (-13.17‰) and trees (-27.61‰) at our study site (February & Higgins, 2010), a standard end-member mixing model was used to determine the relative proportion of C3 (trees)- and C4 (grass)-derived carbon in the soil. This mixing model was only applied to the 0–30 cm soil horizon (surface soils) because of unrelated fractionation processes at deeper depths causing enrichment of soil δ13C unrelated to the inputs from C3 or C4 derived carbon (Balesdent & Mariotti, 1996; Nel et al., 2018).
We calculated bulk density of the soil (Wigley et al., 2013) for each depth category and used this value to convert C concentrations to total C per volume of soil (i.e., stocks). Stocks of C per m2 for each depth was calculated before summing these values and converting to total soil C per ha for soils from 0-30 cm deep (Mg ha-1). We then incorporated the impact of localised enrichment of tree canopies on SOC by weighting our calculation of C stocks by the relative tree cover using the following equation:
plot total C (Mg ha-1) = ((proportion woody cover ´ soil Ctree) + (proportion grass cover ´ soil Cgrass)),
where grass cover equals 1 - woody cover.