Sub sections:
Fazle Rabbi - Stabilization of organic carbon in soil aggregates under contrasting land uses
Micro-aggregates (<250μm) play a key role in protecting soil organic carbon (SOC) from microbial decomposition. The concept of physically protected organic matter is widespread in the literature, but relatively little is known about how organic matter interacts with soil surfaces in three dimensions (3D). Rabbi is studying the effect of land use change on SOC storage in micro-aggregates and the role of physical and physico-chemical protection mechanisms on SOC stabilization. He has selected contrasting land uses [native pasture (NP); crop/pasture rotation (CP); woodland (WL)] on a Dermosol on the Northern Tablelands of NSW, Australia.
Soil aggregate stability was determined with low energy shaking prior to wet sieving. Total SOC (TOC) and SOC pools associated with 4000-250 μm, 250-53 μm and <53 μm aggregates were determined. Light fraction carbon (<1.6 g cm-3) (LF-C) was determined by density fractionation by NaI. Intra-particulate organic carbon (iPOC) and mineral associated carbon (mSOC) of heavy fraction (>1.6 g cm-3) was determined by complete dispersion with 0.5% sodium hexametaphosphate.
Carbon analyses were undertaken with a LECO carbon analyser (TruSpec CN) and Carlo Erba NA1500 micro-analyser depending on the amount of organic carbon recovered. The basal respiration rates of the separated aggregates were also determined to assess the decomposition rates of SOC associated with each aggregate size groups. Soil aggregates of 4000-250 μm, 250-53 μm and <53 μm obtained by wet sieving were incubated with KOH trap at 250C for 30 days at moisture content of 70% of field capacity. The amount of CO2-C produced during incubation was then determined by TIM 850 Titration Manager.
Pore size distribution and pore connectivity of aggregates were determined using microfocus X-ray computed tomography (μCT) (v|tome|xs 240, Phoenix|X-ray). Aggregate stability decreased from NP (0.81±0.06 mm)>WL(0.6±0.054 mm)>CP(0.39±0.02 mm). In NP and WL 63.21% and 53.87% of water stable aggregates respectively, were in the 4000 to 250 μm size range, whereas in CP 46.28% of aggregates were in 4000-250 μm size range. The percentages of aggregates in the 250-53 μm size range were 23.45%, 15.96% and 21.5% in CP, NP and WL, respectively. For NP, CP and WL the 4000-250 μm aggregates contained higher TOC than aggregate fractions of 250-53μm and <53μm. TOC in 4000-250 μm and <53 μm aggregates decreased in the order of WL>CP>NP while TOC in 250-53 μm aggregates was in the order NP>WL>CP.
These data indicate rapid turnover of SOC in 250-53 μm and <53 μm aggregates under crop/pasture rotation. The μCT analysis revealed that pore connectivity was significantly higher in >500 μm aggregates. We believe that the presence of small pores in micro-aggregates drive the considerably higher respiration rates of smaller aggregates but more detailed work is needed to confirm this conclusion.
Rabbi will study stabilisation of soil organic carbon in soil micro-aggregates under different organic inputs, tillage and cropping patterns. Soil organic matter is one of the major pools of the global carbon cycle. Soil micro-aggregates (<250 μm) have unique capacity to protect organic matter from decomposition. The overall objective of the research is to evaluate the effects of organic inputs and agricultural land use under conventional and no-tillage on the amount and nature of carbon associated with soil micro-aggregates in Vertosol, Chromosol and Ferrosol soils.
The aggregate stability and mineralization rate of organic carbon associated with aggregate size fractions will be determined. The dispersion ratio and pore geometry of soil aggregates will be determined to relate these parameters with the amount of organic matter in micro-aggregates. Pore geometry will be determined by computerized micro-tomography. The light fraction, particulate and stable organic matter associated with each aggregate size class especially with micro-aggregate will be determined by density fractionation and chemical methods. The chemical nature of micro-aggregate organic carbon will be determined by solid state 13C NMR. A theoretical study based on linear regression modelling will be conducted to evaluate the capacity of soil to protect organic matter.