Iain Goodrick

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

RATIONALE Pyrogenic carbon (C(P)) is an important component of the global carbon budget. Accurate determination of the abundance and stable isotope composition of C(P) in soils and sediments is crucial for understanding the dynamics of the C(P) cycle and interpreting records of biomass burning, climate and vegetation change in the past. Here we test hydrogen pyrolysis (hypy) as a new technique potentially capable of eliminating labile organic carbon (C(L)) from total organic carbon (C(T)) in a range of matrices in order to enable reliable quantification of both the C(P) component of C(T) and the stable carbon isotope composition of C(P) (δ(13)C(P)). METHODS We mixed C(P) at a range of concentrations with common C(P)-free matrices (C(L) = cellulose, chitin, keratin, decomposed wood, leaf litter, grass and algae) and determined the amount of residual carbon not removed by hydrogen pyrolysis (C(R)) as a ratio of C(T) (C(R)/C(T)). Mixing C(P) with a unique δ(13)C value provided a natural abundance isotope label from which to precisely determine the ratio of C(P) to residual C(L) remaining after hypy. RESULTS All C(P)-free matrices contained trace carbon after hypy, indicating that hypy does not remove all the C(L). However, there was a strong correlation between C(R)/C(T) and C(P)/C(T), viz. C(R)/C(T)= 1.02(C(P)/C(T)) + 4.0 × 10(-3), r(2) = 0.99, p <0.001, suggesting that only a small and reasonably constant fraction of C(L) remains after hypy. Uncertainties associated with the correction for contamination of C(R) by residual C(L) are minimal allowing for reliable determinations of both C(P) and δ(13)C(P) in many cases. CONCLUSIONS Hydrogen pyrolysis appears to be a robust technique for estimating C(P) abundance and δ(13)C(P) across a range of materials. Nevertheless, caution is required in interpreting δ(13)C(P) values when C(P)/C(T) is low, with C(P)/C(T)>4% being required for the determination of the δ(13)C(P) values within an interpretable error under our experimental conditions.

Soil carbon balance following conversion of grassland to oil palm.

Oil palm (Elaeis guineensis Jacq.) crops are expanding rapidly in the tropics, with implications for the global carbon cycle. Little is currently known about soil organic carbon (SOC) dynamics following conversion to oil palm and virtually nothing for conversion of grassland. We measured changes in SOC stocks following conversion of tropical grassland to oil palm plantations in Papua New Guinea using a chronosequence of plantations planted over a 25‐year period. We further used carbon isotopes to quantify the loss of grassland‐derived and gain in oil palm‐derived SOC over this period. The grassland and oil palm soils had average SOC stocks of 10.7 and 12.0 kg m−2, respectively, across all the study sites, to a depth of 1.5 m. In the 0–0.05 m depth interval, 0.79 kg m−2 of SOC was gained from oil palm inputs over 25 years and approximately the same amount of the original grass‐derived SOC was lost. For the whole soil profile (0–1.5 m), 3.4 kg m−2 of SOC was gained from oil palm inputs with no significant losses of grass‐derived SOC. The grass‐derived SOC stocks were more resistant to decrease than SOC reported in other studies. Black carbon produced in grassfires could partially but not fully account for the persistence of the original SOC stocks. Oil palm‐derived SOC accumulated more slowly where soil nitrogen contents where high. Forest soils in the same region had smaller carbon stocks than the grasslands. In the majority of cases, conversion of grassland to oil palm plantations in this region resulted in net sequestration of soil organic carbon.

Soil fertility changes following conversion of grassland to oil palm

Impacts of palm oil industry expansion on biodiversity and greenhouse gas emissions might be mitigated if future plantings replace grassland rather than forest. However, the trajectory of soil fertility following planting of oil palm on grasslands is unknown. We assessed the changes in fertility of sandy volcanic ash soils (0–0.15 m depth) in the first 25 years following conversion of grassland to oil palm in smallholder blocks in Papua New Guinea, using a paired-site approach (nine sites). There were significant decreases in soil pH (from pH 6.1 to 5.7) and exchangeable magnesium (Mg) content following conversion to oil palm but no significant change in soil carbon (C) contents. Analyses to 1.5 m depth at three sites indicated little change in soil properties below 0.5 m. There was considerable variability between sites, despite them being in a similar landscape and having similar profile morphology. Soil Colwell phosphorus (P) and exchangeable potassium (K) contents decreased under oil palm at sites with initially high contents of C, nitrogen, Colwell P and exchangeable cations. We also assessed differences in soil fertility between soil under oil palm (established after clearing forest) and adjacent forest at two sites. At those sites, there was significantly lower soil bulk density, cation exchange capacity and exchangeable calcium, Mg and K under oil palm, but the differences may have been due to less clayey texture at the oil palm sites than the forest sites. Cultivation of oil palm maintained soil structure and fertility in the desirable range, indicating that it is a sustainable endeavour in this environment.

Emission of CO2 from tropical riparian forest soil is controlled by soil temperature, soil water content and depth to water table

Tropical forests play a key role in the global carbon cycle. However, little is known about carbon cycling in the substantial portion of tropical forests that are low-lying, with shallow and fluctuating water tables. This study aimed to determine what factors control emissions of CO₂ from soil in a riparian rainforest in Queensland, Australia. Emissions were measured over the course of 1 year, using static chambers. Emission rates were significantly related to soil temperature (0–0.1 m depth), soil water content (0–0.12 m depth) and depth to water table. The most efficient linear model of emissions as a function of measured parameters, which also included soil pH (0–0.1 m depth), had r² = 0.355. CO₂ emissions were highest (5.2–7.5 μmol m⁻² s⁻¹) at moderate soil temperature (24-28°C), water table depth (0.2–1.5 m) and soil water-filled porosity (0.25–0.79). They were lowest (<0.5 μmol m⁻² s⁻¹) at low soil temperature (<22°C) or when the water table was within 0.15 m of the surface. An additional interaction between temperature and soil water was determined in the laboratory. Incubation of soil cores showed that temperature sensitivity of the heterotrophic component of respiration increased as the soil dried. It is clear that models of soil respiration in lowland tropical forests should take into account depth to water table, which is a key, but hitherto unreported, controller of CO₂ emissions in tropical forests.

Tree-scale spatial variability of soil carbon cycling in a mature oil palm plantation

Soil carbon fluxes are highly variable in space and time under tree crops such as oil palm, and attempts to model such fluxes must incorporate an understanding of this variability. In this work, we measured soil CO2 emission, root biomass and pruned frond deposition rates and calculated carbon fluxes into and out of the soil in a mature (20-year-old, second planting cycle) oil palm plantation in Papua New Guinea. Tree-scale spatial variability in CO2 emission and root biomass was quantified by making measurements on a 35-point trapezoid grid covering the 38.5-m2 repeating unit of the plantation (n = 4 grids). In order to obtain an overall mean soil CO2 emission rate within 5% of the most accurate estimate, ≥24 measurement points were required. Soil CO2 emissions were spatially correlated with calculated carbon inputs (r2 = 0.605, slope 1 : 1), but not with soil water content or temperature. However, outputs were higher than inputs at all locations, with a mean overall output of 7.24 µmol m–2 s–1 and input of 3.02 µmol m–2 s–1. Inputs related to fronds, roots and groundcover constituted 60%, 36% and 4% of estimated inputs, respectively. The spatial correlation of carbon inputs and outputs indicates that mineralisation rate is controlled mostly by the amount rather than the nature or input depth of the additions. The spatially uniform net carbon emission from soil may be due to inaccuracies in calculated fluxes (especially root-related inputs) or to non-biological emissions.

Preferential production and transport of grass-derived pyrogenic carbon in NE-Australian savanna ecosystems

Understanding the main factors driving fire regimes in grasslands and savannas is critical to better manage their biodiversity and functions. Moreover, improving our knowledge on pyrogenic carbon (PyC) dynamics, including formation, transport and deposition, is fundamental to better understand a significant slow-cycling component of the global carbon cycle, particularly as these ecosystems account for a substantial proportion of the area globally burnt. However, a thorough assessment of past fire regimes in grass-dominated ecosystems is problematic due to challenges in interpreting the charcoal record of sediments. It is therefore critical to adopt appropriate sampling and analytical methods to allow the acquisition of reliable data and information on savanna fire dynamics. This study uses hydrogen pyrolysis (HyPy) to quantify PyC abundance and stable isotope composition (δ13C) in recent sediments across 38 micro-catchments covering a wide range of mixed C3/C4 vegetation in north Queensland, Australia. We exploited the contrasting δ13C values of grasses (i.e., C4; δ13C > −15‰) and woody vegetation (i.e., C3; δ13C < −24‰) to assess the preferential production and transport of grass-derived PyC in savanna ecosystems. Analyses were conducted on bulk and size-fractionated samples to determine the fractions into which PyC preferentially accumulates. Our data show that the δ13C value of PyC in the sediments is decoupled from the δ13C value of total organic carbon, which suggests that a significant component of PyC may be derived from incomplete grass combustion, even when the proportion of C4 grass biomass in the catchment was relatively small. Furthermore, we conducted 16 experimental burns that indicate that there is a comminution of PyC produced in-situ to smaller particles, which facilitates the transport of this material, potentially affecting its preservation potential. Savanna fires preferentially burn the grass understory rather than large trees, leading to a bias toward the finer C4-derived PyC in the sedimentary record. This in turn, provides further evidence for the preferential production and transport of C4-derived PyC in mixed ecosystems where grass and woody vegetation coexist. Moreover, our isotopic approach provides independent validation of findings derived from conventional charcoal counting techniques concerning the appropriateness of adopting a relatively small particle size threshold (i.e., ~50 μm) to reconstruct savanna fire regimes using sedimentary records. This work allows for a more nuanced understanding of the savanna isotope disequilibrium effect, which has significant implications for global 13C isotopic disequilibria calculations and for the interpretation of δ13C values of PyC preserved in sedimentary records.

Mineralisation of soil organic carbon in two Andisols under oil palm: an incubation study into controlling factors

Understanding the factors controlling stability against mineralisation of soil organic matter is important for predicting changes in carbon stocks under changed environment or management. Soil carbon dynamics in oil palm plantations are little studied and have some characteristics that are unusual compared with other agricultural soils, such as high management-induced spatial variability and warm moist conditions. The aim of this work was to determine the factors controlling the mineralisability of the intermediate-stability carbon fraction of volcanic ash surface soils (0–5 and 15–20 cm depth) from oil palm plantations in Papua New Guinea. Soils with carbon contents of 2.2–35.2%, from areas with low and high organic matter inputs, were incubated for up to 812 days and soil respiration was measured periodically. Mean carbon turnover rates were 0.18–1.58, 0.07–0.23 and 0.03–0.07 a–1 on Days 54, 379 and 812 respectively. Turnover rate was initially (Day 54) correlated with pre-incubation total carbon content (r = 0.88), the ratio of permanganate-oxidisable carbon to total carbon (r = 0.62) and the ratio of oxalate-extractable Al and Fe to total carbon (r = –0.51 and –0.54 respectively), but the correlations decreased with time, being insignificant on Day 812. In the soils that had changed from C4 grassland 25 years previously, turnover rate was negatively correlated with δ13C, which increased with depth, but δ13C did not change significantly over the course of the incubation. Temperature sensitivity of mineralisation varied little, despite large differences in soil properties and changes in mineralisation rate. This suggested that turnover rates were affected to similar extents by biochemical recalcitrance and physical protection, as these two factors influence temperature sensitivity in opposing directions. Physico-chemical protection of organic matter appeared largely related to interaction with poorly crystalline Al and Fe oxides.