Development of a stable isotope index to assess decadal‐scale vegetation change and application to woodlands of the Burdekin catchment, Australia

Abstract

Forty-four study sites were established in remnant woodland in the Burdekin River catchment in tropical north-east Queensland, Australia, to assess recent (decadal) vegetation change. The aim of this study was further to evaluate whether wide-scale vegetation 'thickening' (proliferation of woody plants in formerly more open woodlands) had occurred during the last century, coinciding with significant changes in land management. Soil samples from several depth intervals were size separated into different soil organic carbon (SOC) fractions, which differed from one another by chemical composition and turnover times. Tropical (C 4 ) grasses dominate in the Burdekin catchment, and thus d 13 C analyses of SOC fractions with different turnover times can be used to assess whether the relative proportion of trees (C 3 ) and grasses (C 4 ) had changed over time. However, a method was required to permit standardized assessment of the d 13 C data for the individual sites within the 13 Mha catchment, which varied in soil and vegetation characteristics. Thus, an index was developed using data from three detailed study sites and global literature to standardize individual isotopic data from different soil depths and SOC fractions to reflect only the changed proportion of trees (C 3 ) to grasses (C 4 ) over decadal timescales. When applied to the 44 individual sites distributed throughout the Burdekin catchment, 64% of the sites were shown to have experienced decadal vegetation thickening, while 29% had remained stable and the remaining 7% had thinned. Thus, the development of this index enabled regional scale assessment and comparison of decadal vegetation patterns without having to rely on prior knowledge of vegetation changes or aerial photography.

Quantifying the abundance and stable isotope composition of pyrogenic carbon using hydrogen pyrolysis

“…However, all the residues from C P ‐free materials changed their δ 13 C values after hypy, from –0.7 to +2.1‰, relative to the δ 13 C value of the initial material. This is probably attributable to a trace amount of pyrolysis of these materials during hypy . For the C P plus C L mixtures, the δ 13 C R values tended toward the pyrogenic end‐member as more C P was added to the initial mix, but the δ 13 C R values were in no case the same as the known initial δ 13 C value of the C P in the sample after hypy (Table ).…”

“…In addition to better quantification, identifying and/or partitioning of the source(s) of C P in soils and sediments should enable elucidation of the mechanisms of C P cycling and the impacts of fire on ecosystems . In tropical regions, stable isotope values of carbon differ between grasses, most of which utilise the C 4 photosynthetic pathway ( δ 13 C ~ −12‰), and trees and shrubs that employ C 3 photosynthesis ( δ 13 C ~ −27‰) .…”

The carbon isotope composition of semi-labile and stable pyrogenic carbon in a thermosequence of C3 and C4 derived char

“…One component of PyC is stable polycyclic aromatic carbon (SPAC), which is highly refractory and plays a key role in slow-cycling C pools, potentially providing important long term feedbacks in the global C cycle (Masiello, 2004;Bird and Ascough, 2012). Identifying and/or partitioning of the source(s) of bulk PyC and SPAC in soil and sediments assists in resolving the impact of fire on ecosystems and on C cycling more generally (Glaser and Amelung, 2003;Krull et al, 2007;Rodionov et al, 2010).…”

The influence of C<sub>3</sub> and C<sub>4</sub> vegetation on soil organic matter dynamics in contrasting semi-natural tropical ecosystems

“…On the other hand, the arid Sahelian ecosystems studied (HOM sites) may have experienced a relatively recent reduction in woody cover as evidenced from the large enrichment in 13 C towards the surface. There may be several reasons behind this potential thinning of woody biomass at the driest sites, with a combination of overgrazing, fuel harvesting, fires, and above all the severe droughts suffered over the past few decades, being the most likely causes (Krull et al, 2007). However, Sahelian ecosystems are known to have a large resilience to drought and grazing, to the extent that woody plant population dynamics can largely vary between nearby sites as a result of contrasting substrates, grazing intensities, land use history, and species composition (Hiernaux et al, 2009a;Mougin et al, 2009).…”

“…The use of the carbon isotope composition of SOM has proven to be a useful tool for investigating the influence of C 3 and C 4 vegetation on SOM dynamics (Wynn and Bird, 2007), and for identifying recent (∼ 100 years) vegetation change patterns that in the past could only be assessed by interpreting historical aerial photography or satellite imagery (Krull et al, 2005). This approach relies on the distinct carbon isotopic (δ 13 C) values of tropical grasses, employing the C 4 photosynthetic pathway (−14 ‰) compared to the δ 13 C value of trees and shrubs that utilize the C 3 photosynthetic pathway (−27 ‰; average values from Krull et al, 2007). It is feasible to obtain an integrated measure of the carbon isotopic composition of regional vegetation using SOM, given that this pool integrates the isotopic signature of the precursor biomass, thus serving as a record of changes in the ratio of tree and grass-derived organic matter input to the soil over different spatiotemporal scales (Bird et al, 2004;Lloyd et al, 2008).…”