Soil organic carbon as an indicator of soil quality
Per Schjønning and Lars J. Munkholm, Danish Institute of Agricultural Sciences, Research Centre Foulum
Agricultural use of soil influences the content of soil organic carbon (SOC). Arable farming will reduce SOC compared with permanent grass. The crop rotation has a major influence on SOC, a high frequency of annual cash crops giving rise to a reduction compared with forage cropping including grass leys.
There are no internationally agreed standards on threshold values of SOC. However, SOC influences a range of soil quality aspects of crucial importance to soil functions. The identification of a lower threshold of SOC is hence a key issue in the search for sustainable management.
The Danish Research Centre for Organic Farming (DARCOF) has in recent years completed two projects addressing the influence of soil management on soil tilth. Here, we summarize our results in the context of SOC as an indicator of soil quality.
Long-term effects of fertilization
The classical fertilization trials at Askov Experimental Station within DIAS include plots that for more than a century have been dressed with distinctly different amounts and types of fertilizers. The plots receiving no nutrients at all (labelled UNF) display SOC contents as low as ca. 1%. Synthetic fertilizers (labelled NPK) have given rise to SOC contents in the range of 1.15 to 1.25% (dependent on the amount of nutrients applied), while plots receiving animal manure (AM) have achieved SOC contents in the range of 1.30 to 1.35%.
Our investigations revealed that the soil in the UNF treatment had become very dense and difficult to till. In wet conditions, it had a high clay dispersibility. A very high tensile strength of dry aggregates for this treatment could be seen partly as an effect of cementation of dispersed clay particles during drying. In contrast, the AM soil was stable in wet conditions and friable in dry conditions. This means that it fragments to a desirable tilth during tillage and that roots may easily penetrate the soil.
Our measurements also showed that the range of water content allowing tillage was narrower for the UNF than for the AM treatment (Munkholm et al., 2002).
A general conclusion from these studies is that the SOC of the UNF treated soil clearly had decreased to a critical level for sustainable farming.
Crop rotation effects
The Askov plots dressed with mineral fertilizers (NPK treatment) generally displayed soil tilth characteristics intermediate to those of the UNF and the AM treatments. However, for most properties investigated, the NPK treatment was rather close to the AM treatment. Generally, the AM was superior to the NPK treatment, but the question is whether the difference is important. The problem is that no internationally agreed standards have been identified for the soil quality indicators studied. How much is enough and what is the critical level?
One reason that the soil dressed only with mineral fertilizers had a satisfactory tilth may be the beneficial effect of a relatively versatile crop rotation in the Askov trials. The four-year crop rotation included:
- barley undersown with a grass clover ley
- grass clover ley
- winter wheat
- fodder beets.
A case study performed in 1998 at two neighbouring farms on Zealand showed very significant management effects (Munkholm et al., 2001, Schjønning et al., 2002ab).
One of these soils had been grown organically for more than 40 years. This included a forage crop rotation with frequent grass leys and with the application of animal manure (labelled DFG for 'Dairy Farm with Grass ley' in the following).
The other neighbouring soil had for at least 20 years been grown with continuous annual cash crops (CCC system) and with no application of animal manure.
The SOC content of the DFG soil was ca. 2%, while that of the CCC soil was 1.45%. The CCC soil in many ways resembled the UNF treatment of the Askov trial. It was dense and with mechanical properties yielding a poor workability and with a high demand of energy input in tillage (rotary cultivator).
Crop rotation more important than application of manure
In 1996, DARCOF initiated trials with different crop rotations, application of animal manure, the use of catch crops, etc. One of these trials was established on a field belonging to the DIAS Research Centre Flakkebjerg. This field had for decades been grown with annual cash crops and the SOC content was below 1%. In 2002 and again in 2003, the DARCOF project ROMAPAC studied selected crop rotations in the trial:
- a small grain cereal crop rotation excluding catch crops and the use of animal manure
- the same crop rotation as 1 but with the application of animal manure
- a crop rotation combining small grain cereals and grass clover leys and catch crops (but excluding animal manure).
Although our investigations were carried out only 6-7 years after the start of the experiment, a significant increase in SOC was measured in rotations 2 and 3 compared with rotation 1. The effect of crop rotation was more pronounced than that of animal manure.
Generally, the soil of this field is very difficult to till. The soil has a tendency to break up into clods, which necessitates a high energy input in tillage when preparing the seedbed. The tensile strengths of dry aggregates measured for this soil were considerably higher than experienced for any other Danish soils studied.
A higher porosity was detected for rotation 2 and 3 compared with rotation 1. Again, the crop rotation effect was stronger than the effect of animal manure. There was a tendency for an increased friability for rotation 3, but this effect was not statistically significant. Our studies clearly show that this soil has a very poor tilth and that there is an urgent need for increasing the content of SOC. The trends observed indicate that continued application of a versatile crop rotation and to a minor extent the application of animal manure may improve the tilth characteristics.
Threshold levels of soil quality indicators
Environmental regulations often include specific and precise limits for selected indicators of environmental quality in evaluations of sustainability. This is the case when dealing with water and air quality. The results presented in this report indicate that the indicator approach is less straightforward when speaking of soil.
From the Askov and Flakkebjerg trials it could be concluded that a SOC content of 1% (or more correctly somewhat higher) is a potential 'universal' lower limit or threshold for sustainable farming. However, the case study revealed critical mechanical properties for the CCC soil despite it having a SOC content of 1.45%, which is higher than the level found in the AM treatment at Askov.
It seems that soil type differences and perhaps differences in climatic conditions may influence the beneficial effect of SOC on soil functions and that SOC cannot be used as a universal soil quality indicator independent of soil type.
This conclusion has a major impact on how to develop sustainable management systems. Rather than regulation based on soil quality indicators, the development of organic farming has to be based on knowledge of the soil system and the effect of management for specific soil types. It is thus a great challenge for organic farmers to implement new achievements and scientific understanding.
It seems that as has been the case for the development of Danish agriculture through more than a century a close link between farmers, extension officers and scientists is crucial for the success in this endeavour (Schjønning et al., 2004). One specific input in this process, which may be derived from the present results is that crop rotation may be expected to have a more pronounced influence on SOC than the application of animal manure.
Munkholm, L.J., Schjønning, P. & Petersen, C.T. 2001. Soil mechanical behaviour of sandy loams in a temperate climate: case-studies on long-term effects of fertilization and crop rotation. Soil Use and Management 17, 269-277.
Munkholm, L.J., Schjønning, P., Debosz, K., Jensen, H.E. & Christensen, B.T. 2002. Aggregate strength and mechanical behaviour of a sandy loam under long-term fertilization treatments. European Journal of Soil Science 53, 129-137.
Schjønning, P., Elmholt, S., Munkholm, L.J. & Debosz, K. 2002. Soil quality aspects of humid sandy loams as influenced by organic and conventional long-term management. Agriculture, Ecosystems & Environment 88, 195-214.
Schjønning, P., Munkholm, L.J., Moldrup, P. & Jacobsen, O.H. 2002. Modelling soil pore characteristics from measurements of air exchange: the long-term effects of fertilization and crop rotation. European Journal of Soil Science 53, 331-339.
Schjønning, P., Elmholt, S. & Christensen, B.T. 2004. Soil Quality Management Synthesis. In: Schjønning, P., Elmholt, S. & Christensen, B.T. (Eds.) Managing Soil Quality: Challenges in Modern Agriculture. CABI Publishing, Wallingford, UK, pp. 315-334.