Soil organic matter in rehabilitated soils as an expression of soil health

Last modified by Thomas Baumgartl on 2017/01/31 14:50

Executive summary Soil organic matter (SOM) is often used as a criteria to assess the state of a soil to sustain vegetation. SOM and its accumulation over time is a result of the productivity of a soil substrate and hence reflects directly in a feedback response on the quality of a soil for plant growth. Soil carbon originates from the decomposition of organic matter from plants and is a key parameter for soil formation. Decomposed organic carbon is incorporated and accumulated in the soil over space and time as soil organic matter. This process is controlled by factors such as soil texture, climate, type of vegetation and time. The accumulation of SOM on rehabilitated sites is highly desired as it improves the functional properties of soils such as water flow, infiltration and gas exchange, stabilises soil from a mechanical viewpoint and increases storage of nutrients and water. SOM also plays a vital role in carbon (C) storage as the largest natural C sink globally.

Background

Soils used for rehabilitation are commonly low in SOM content with contents ranging well below 0.5% Green Carbon derived SOM. An increase of SOM by 1% over a soil depth of ca. 0.3m is equivalent to the incorporation of ca. 100 t CO2 ha-1. Studies of the C-concentration distribution in Red Ferrosol Soils of southeast Queensland found C-concentrations between 2-5% within the first 0.5m under native shrubs and up to 10% C under plantations (Maraseni et al., 2008). In natural soil profiles there is a distinct spatial differentiation of SOM, with high concentrations found near the surface decreasing with depth (Figure 1). This study on carbon distribution under different land use systems in south east Queensland showed that, under mature forest conditions, OM can accumulate to concentrations of ca. 10% C close to the surface. With depth there is a significant reduction in stored SOM to levels of around 0.2 to 1% C. Cultivation is depleting organic carbon from topsoil to concentrations of around 1% C.

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Figure 1. Carbon concentration distribution in soil profiles under different land use (from: Maraseni et al, 2008)

In contrast to natural sites, recently rehabilitated sites either contain some quantity of SOM, if previously stripped topsoil is used for rehabilitation, or lack SOM, if subsoil is used. The spatial distribution of SOM within such layers is usually uniform throughout the depth of the horizon due to mixing of the disturbed soil. Over time, the distribution of SOM will re-build following the natural equilibrium of accumulation and decomposition and a gradual decrease of SOM with depth. Quantifying the magnitude and intensity of this soil-forming process can be utilised as a means to assess the success of rehabilitation. A study on the influence of management practices under semi-arid conditions on a rehabilitated site in Wyoming, USA, showed the potential of accumulation of 1 to 3 t C ha-1 year-1 (Table 1). The authors found a positive correlation of SOC (soil organic carbon content) concentration and time. The magnitude of SOC accumulation is controlled by climatic conditions and reduced under dry climatic situations. 



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Table 1: Management practices and soil organic carbon content of a rehabilitated site (from Anderson et al, 2008)

Improved soil management or return to native vegetation will increase the amount of SOM. Within the first decades after a management change, the increase will be more pronounced than at later stages, when the soils reach an equilibrium of accumulation and consumption (Figure 2). A similar pattern can be assumed for soils or soil-like substrate used for rehabilitation. The starting point for the soil organic matter balance would be the point of highest depletion of SOM. In comparison to natural soils, it can be assumed that the buiId up of SOM in the semi-arid environments of the Bowen Basin and considering the soil quality available will require more time for these rehabilitated soils. Important for the long-term C sequestration is the build-up of relatively stable organic matter. Labile organic matter is, however, considered to be the dominant part of SOM accumulation.

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Fig. 2: Changes in soil carbon content after deforestation/cultivation and reforestation (from: Hillel and Rosenzweig, 2011)

 

Objectives

Objectives of the study were to:

-        quantify SOM of rehabilitated soils from various mine sites of different age and vegetation type

-        identify a suitable sampling procedure to quantify soil organic carbon

Method/ techniques

The investigated site is located in the Bowen Basin, Central Queensland, Australia, characterised by a large number of coal mine sites. Exemplary data are presented here extracted from the ACARP report C19029 and data shown are from Curragh Mine site .

In total 15 soil profile have been sampled with a focus on the top soil horizon (0-20cm) overlying a hard and dense spoil layer. The sampling depth for each profile was 0-2, 2-5, 5-10 and 10-20cm. At natural sites the top 10 cm was collected. Sites were vegetated with trees, bushland communities (Senna grass) or grass (dominated by Buffel grass). Total organic carbon (TOC) of samples was determined following the ‘Heanes’ wet oxidation method.

Results/

observations

The largest increase of total organic carbon (TOC) in the soil profile is from a depth of 5cm to the surface. The uniformity of the depth distribution of TOC amongst the soil profiles below a depth of 5cm was considered characteristic and defined as the baseline for the spoil/soil used and applied for rehabilitation (Fig. 3).

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Fig. 3: Depth distribution of TOC for sampling locations (average value and standard deviation marked in blue; unmined soils marked in orange)

The concentration of carbon increases in almost all investigated soil profiles from approximately 5 cm depth towards the surface by a factor of around 2 (i.e. compared to depths of 0.5 or 1 cm from the surface. Given the repeatability of this behaviour across sites, age of rehabilitation and vegetation type, it can be assumed that the increase of carbon close to the surface is mainly attributable to the incorporation of green carbon as a result of litter decomposition and transport into the soil.

When normalising the accumulated amounts of carbon additionally stored in the soil profile since rehabilitation it becomes apparent that the rate of TOC increase is higher in the early years since rehabilitation, but decreases over time.

The amount of carbon added to the profile within the first 3-5 cm is relatively small, with amounts less than 15 Mg/ha. Nevertheless, there is a trend of increase of TOC within the soil profile with time (Fig. 4).

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Figure 4: Accumulation of TOC in [t/ha] from the surface to a depth of 5cm over time since rehabilitation (red circled data points = natural site)

Outcomes

SOM increases via incorporation of green carbon as a result of decomposition of litter. The incorporation through leaching and bioturbation occurs only close to the surface. Hence, for appropriate and accurate determination of changes in space and time of soil carbon there is a requirement of high spatial sampling resolution in soil profile, at least for semi-arid environments.

For the investigated mine soils (in total 4 mines sites) a minimum and more or less constant TOC concentration was determined below a depth of 5cm, independent of type of vegetation. The sampling strategy should take into account that the largest changes are close to the surface and the sampling depth should be chosen accordingly.

Total carbon stock increases with time, but carbon stocks are below those of natural sites.

Carbon stocks of the investigated rehabilitated sites are below values reported in literature for sites with similar climatic conditions 

 

Photo(s) /Video(s)

 

References(s)

Anderson, J.D., Ingram, L.J., Stahl, P.D., 2008. Influence of reclamation management practices on microbial biomass carbon and soil organic carbon accumulation in semiarid mined lands of Wyoming. Applied Soil Ecology 40, 387–397. doi:10.1016/j.apsoil.2008.06.008

Baumgartl T, P Erskine, J Chan, V Glenn, 2014. Soil Organic Matter and Green Carbon in Rehabilitation: Their Role in the Carbon Balance. ACARP report; Project C19029. Brisbane, Australia

Hillel D and C Rosenzweig, 2011. Handbook of climate change and agroecosystems. Impacts, adaptation, and mitigation. ICP series on Climate Change Impacts, Adaptation, and Mitigation – Vol.1. Imperial College Press,  London. 439pp.

Maraseni, T., Mathers, N., Harms, B., Cockfield, G., Apan, A., Maroulis, J., 2008. Comparing and predicting soil carbon quantities under different land use systems on the Red Ferrosol soils of southeast Queensland. Journal of Soil and Water Conservation 63, 250–256.

Date

Sampling and analysis of data between 2010 and 2013. Data summarised in Final Project of ACARP C19029: T Baumgartl, P Erskine, J Chan, V Glenn (2014). Soil Organic Matter and Green Carbon in Rehabilitation: Their Role in the Carbon Balance. Brisbane, Australia.

 

Location(s)Blackwater; Curragh; German Creek; Norwich Park
KeywordsSoil; soil organic matter; soil health; carbon
Created by Thomas Baumgartl on 2016/05/02 12:15
    
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