Evelyn S. Krull

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 (C₄) grasses dominate in the Burdekin catchment, and thus δ¹³C analyses of SOC fractions with different turnover times can be used to assess whether the relative proportion of trees (C₃) and grasses (C₄) had changed over time. However, a method was required to permit standardized assessment of the δ¹³C 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 (C₃) to grasses (C₄) 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.

Understanding the chemical character and turnover time of the oldest soil organic carbon (SOC) fraction is fundamental in deciphering soil carbon sequestration processes and the fate of soil‐eroded carbon in aquatic sediments. Two main processes are thought to extend the turnover time of SOC: protection by the mineral matrix and chemical recalcitrance. Various oxidation methods have been proposed to isolate the oldest and most recalcitrant SOC fraction, which is often assumed to be black carbon (BC). However, few data have been published that confirm the chemical character of the isolated fractions. Using established and newly developed methods together with ¹³C‐NMR spectroscopy and AMS dating, we show that protection by the mineral matrix prolonged the turnover time of SOC by tens of years, but long‐term (hundreds of years) stabilization was controlled by the inherent recalcitrance of SOC, determined by the type of ecosystems. In ecosystem without significant fire occurrences, the older SOC pool was comparably small and was represented by alkyl carbon. In ecosystems with high fire frequency charcoal constituted the oldest SOC pool, and constituted up to 35% of the total SOC. By applying methods with different oxidative strengths, it was possible to isolate different age groups of charcoal with different degrees of weathering. Further substantiation of this finding could provide a much greater resolution of paleo‐fire events. Our results demonstrate that fire frequency plays a dominant role in determining the chemical nature and ¹⁴C abundance of SOC and that the separation of age groups of charcoal provides a means to reconstruct detailed fire histories. Our results indicate that modeling SOC turnover, transport and sequestration for frequently burnt environments requires modification of existing models, specifying an input and decay function for the charcoal pool in different environments.

Stable carbon isotopic (d13C) analyses of soil organic matter (SOM) have been used in the past to characterise C3–C4 vegetation changes. However, the temporal and spatial resolution of these isotopic data are not well established. Here, we present data from d13C analyses of whole and size-separated SOM, which are discussed in conjunction with organic (total organic carbon (TOC) content) and inorganic (%clay) soil data. These data are put into context with the current vegetation state (assessed from tree size-class distribution) and the 50-year vegetation history (assessed from aerial photographs). By linking below- and above-ground datasets, we show that d13C analyses of SOM can accurately record vegetation-change histories over short- (10 and 50 years) and longer-term (hundreds of years) time scales. Our data also show that spatial variability was relatively small for the clay TOC content but was much larger for d13C data, indicating that the number of soil cores required for statistical significance is highly dependent on the kind of measurements intended. Finally, interpretation of d13C data from SOM to assess the history of C3–C4 vegetation change is complicated by the inherent 13C-enrichment of SOM, owing to decomposition processes, which occurs regardless of vegetation change. We suggest a method for distinguishing 13C-enrichment of SOM that is due to soil-inherent (decomposition-related) processes from 13C-enrichment that is due to increased inputs of C4 organic matter.

A subtropical peat, developed on a costal plain in southern Queensland, has been studied with respect to its morphology, radiocarbon (14C) age, total organic carbon (C) content, stable C isotopic (δ13C) values, and spectroscopic characteristics (13C-NMR and FTIR). The combination of techniques allowed for an interpretation of changes in peat development over time, an assessment of paleoclimatic changes that apparently occurred during peat growth, and a comparison with perched lake sediments on Fraser Island. Geochemical data from the peat showed a relative increase in abundance of aromatic C (which may include charcoal) from the Pleistocene to the early Holocene and much decreased abundances of aromatic C thereafter. This trend may be due to changes in fire frequency from the Pleistocene–early Holocene (high fire frequency) to the early and middle Holocene (relatively low fire frequency). This interpretation is consistent with other published data from this area, showing increased aridity, increased fire frequency and dominance of sclerophyll forest during the late Pleistocene–early Holocene. Another significant geochemical change in the acid peat occurred within the middle to late Holocene. This period is characterised by highly 13C-depleted organic matter and a comparably high alkyl C (lipid) content. These data are interpreted as indicating wetter and year-round waterlogged conditions, possibly associated with poor drainage during the Holocene sea level maximum c. 5500–3000 years BP. By comparison, abundance of alkyl C decreased and δ13C values increased in the latest Holocene. The geochemical data from the acid peat point to the occurrence of a dry phase during the late Pleistocene to early Holocene and a wet phase in the middle to late Holocene, followed by another dry phase. These data correspond well with published data of 2 phases of dune formation (dry periods) in the early to middle Holocene and in the latest Holocene for North Queensland.

This paper reviews current knowledge of soil organic carbon (SOC) dynamics with respect to physical protection, soil moisture and temperature, and recalcitrant carbon fractions (such as charcoal) in predominantly agricultural soils. These factors are discussed within the framework of current soil organic matter models. The importance of soil structure in the stabilisation of organic residues through physical protection has been documented previously in various studies. In addition, changes in soil structure associated with tillage can significantly affect soil organic matter decomposition rates. The concept of physical protection has been incorporated into several soil carbon models as a function of soil texture. While soil texture can affect the soil's capacity for aggregation and adsorption, factors such as soil moisture and temperature may further enhance or reduce the extent of physical protection. While adsorption and aggregation can slow decomposition processes, it is unlikely that these processes are solely responsible for the high mean residence times measured in biologically active surface soils. Accordingly, chemical recalcitrance appears to be the only mechanism by which soil organic carbon can be protected for long periods of time.

Soil organic matter (SOM), leaf litter, and root material of an Ultisol from the tropical rainforest of Kakamega, Kenya, were analyzed for stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic values as well as total organic carbon (TOC) and total nitrogen (TN) contents in order to determine trends in SOM decomposition within a very well-developed soil under tropical conditions. In addition, we quantified mineralogy and chemistry of the inorganic soil fraction. Clay mineralogical variation with depth was small and the abundance of kaolin indicates intense weathering and pedoturbation under humid tropical conditions. The soil chemistry was dominated by silica, aluminium, and iron with calcium, potassium, and magnesium as minor constituents. The relative depletion of base cations compared with silica and aluminium is an indicator for intense weathering and leaching conditions over long periods of time. Depth profiles of δ¹³C and δ¹⁵N showed a distinct enrichment trend down profile with a large (average ¹³δC = 5.0 and average ¹⁵δN= 6.3) and abrupt offset within the uppermost 10–20 cm of the soil. Isotopic enrichment with depth is commonly observed in soil profiles and has been attributed to fractionation during decomposition. However, isotopic offsets within soil profiles that exceed 3 are usually interpreted as a recent change from C₄ to C₃ dominated vegetation. We argue that the observed isotopic depth profiles along with data from mineralogy and chemistry of the inorganic fraction from the Kakamega Forest soil are a result of intense weathering and high organic matter turnover rates under humid tropical conditions.