Soil organic carbon is made up from different types of organic materials with different chemical and physical properties. Soil organic carbon is frequently divided into relatively labile and stable organic fractions (also referred to as “pools”) that vary in their vulnerability to decomposition.
Soil organic carbon in a labile fraction decomposes relatively rapidly (days to years) and is composed of pieces of plant debris, including fresh crop residues and roots that are 0.053–2 mm in size (referred to as particulate organic matter; POM), living organisms and remnants of dead organisms.
More stable fractions of soil organic carbon take longer to decompose (more than a few years to decades to centuries). This stable fraction includes finer-sized organic matter that is physically protected (e.g. clay mineral-protected or contained within soil aggregates) or chemically persistent (humus-like) in soil.
Char carbon is referred to as recalcitrant organic carbon that may persist in soil for centuries to millennia.
Labile fractions of soil organic carbon serve as a readily available energy source for soil organisms and so greatly enhance nutrient cycling in the soil. Labile organic carbon is directly linked to improvement in soil quality, and supports the formation of soil aggregates that improve soil structure for better infiltration and water holding.
Labile organic carbon is a more sensitive indicator of changes in soil health and fertility in response to changes in management than total organic carbon (TOC) in soil. Differences in the organic carbon levels in the same soil, after 19 years under different management practices, can be seen in figure 1. In the paddock where three tillage passes and stubble burning was practised there is substantially less labile organic carbon present compared with the no tillage and stubble retained paddock. The difference in the labile organic carbon between the treatments is the major factor for the lower TOC in the traditional tillage soil system (indicated by the smaller overall circle).
Figure 1: Soil organic carbon (sizes of the circles) and the three different carbon fractions (individual slices) of the same surface soil (10 cm) under two different management practices in Wagga Wagga.(Chan et al. 2010) .
Currently a physical fractionation method that isolates particulate organic carbon (POC) from soil is more commonly used in Australia to monitor changes in labile organic carbon in response to land management. This POC pool has been shown to be more sensitive to increases in organic carbon in soil from pasture phases (Skjemstad et al. 2006) than another widely used method for isolating labile soil organic carbon (potassium permanganate oxidisable carbon).
The POC method estimates the carbon available for decomposition based on particle size of organic matter, while the potassium permanganate method uses a chemical oxidation approach as a proxy for in field decomposition. There are a variety of other methods for measuring labile carbon; each with their strengths.
A soil with more labile organic carbon will usually have a larger microbial community and greater potential for nutrient release via organic matter decomposition compared to a soil with less labile organic carbon. The capacity of micro-organisms to release plant-available nitrogen is influenced by the carbon to nitrogen (C-to-N) ratio of organic matter inputs.
The C-to-N ratio of a residue decreases as decomposition progresses. Net release of nitrogen occurs when the C-to-N ratio of residues is or falls below 20:1. A labile organic carbon pool containing a high proportion of higher C-to-N ratio residues (>20–25:1) may temporarily induce a net nitrogen immobilisation, decreasing its supply for crop uptake.
The responses of labile organic carbon to management may vary depending on site specific conditions including soil texture, mineralogy and climate as well as variations in farming systems. Generally, labile organic carbon can be increased by increasing organic carbon inputs and/or reducing losses (table 1).
Table 1: Management options to increase labile organic carbon in soil long term (Hoyle 2013, Chan et al. 2010).
- Maximise crop and pasture production by managing constraints to growth, such as compaction, acidity and diseases, and optimising agronomic management such as nutrition.
- Include more pasture phases and include legumes in the mix.
- Introduce perennial pastures.
- Grow green manure crops.
- Retain crop and pasture residues on the paddock.
- Return manure and recycled organic materials to the soil.
- Irrigate to increase biomass production.
- Reduce erosion because it carries off organic matter in the topsoil.
- Minimise stubble burning because it reduces organic matter inputs and increases erosion.
- Reduce tillage because excessive tillage accelerates organic matter decomposition and encourages erosion losses.
- Minimise fallowing because it accelerates organic matter decomposition.
- Avoid overgrazing because it reduces productivity and increases erosion.
Chan KY, Oates A, Liu DL, Li GD, Prangnell R, Poile G and Conyers MK (2010) A farmers Guide to increasing soil carbon under pastures, Industry and investment NSW, Wagga Wagga. (online)
Hoyle F (2013) Managing soil organic matter: A practical guide, Grains Research and Development Corporation, Kingston,
Skjemstad JO, Swift RS, McGowan JA (2006) ‘Comparison of the particulate organic carbon and permanganate oxidation
methods for estimating labile soil organic carbon’, Soil Research, 44, 255–263.
The New South Wales Department of Primary Industries has further information on soil carbon (online)
Authors: Sally Muir, Abigail Jenkins, Stephanie Alt, Bhupinder Pal Singh & Susan Orgill(NSW Department of Primary Industries), 2013
The National Soil Quality Monitoring Program is being funded by the Grains Research and Development Corporation, as part of the second Soil Biology Initiative.
The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.