Loss of organic matter in mineral soils

What is loss of organic matter in mineral soils?

Soil organic matter (SOM) can be defined as the total organic content of a soil after excluding non-decayed plant and animal remains1. Carbon is the prime element present in SOM, comprising 48%-58% of the total weight and therefore soil is the second largest pool of carbon on earth, after the oceans and twice the size of the atmospheric carbon pool. Due to the importance of the carbon element, SOM is typically quantified as soil organic carbon (SOC).

Where does it occur?

There is great uncertainty about SOM/SOC stocks and trends in Europe. There are very few long term soil monitoring networks with a sufficient number of sampling sites to detect SOC changes at a regional level and contrasting SOC trends among countries are often reported. Also existing estimates of SOC stocks based on modelling exercises contain a significant level of uncertainty, either because of the model used or due to uncertainties in the input datasets. However, it is clear, as the figures show, that there are greater quantities of soil organic carbon in northern European countries compared to the south of Europe.


Topsoil (0-30 cm) organic carbon content (%)in Europe ²		LUCAS Topsoil organic carbon content (g/kg)³

What causes it?

Soil organic carbon (SOC) stock in the top-soil layer (0–30 cm)of European agricultural soils ⁴

SOM content variations occurring over the long-term are mainly due to climatic, geological and soil forming factors, but for short-term periods, vegetation disturbances and land use changes affect SOM storage.

Climate
In natural ecosystems, climate is the main driver of change in SOM through the effects of temperature, moisture and solar radiation. SOM increases with precipitation and reduces with temperature, explaining for example the general pattern of decline from northern to southern Europe in maps.

Vegetation type
At a local scale the effect of vegetation type on SOM increases in importance. The initial decomposition rates of plant residues are negatively correlated with the substrate C: N ratio or the fraction of plant tissue that is lignin.

Soil Properties
Soil type is a major factor involved in the stabilization mechanisms of SOM by means of physical preservation.

Human activities
Management systems affect SOM mainly through: a) the input rates of organic matter and its decomposability; b) the distribution of photosynthates in roots and shoots; c) the physical protection of SOM.

The SOM cycle is also affected by other external drivers and pressures, such as government policies (e.g. agri-environment, energy), technological developments, climate change and demographic trends, etc., mainly through changes in land use and agricultural management.

a) Natural	b) Anthropogenic/human activities	c) Socio-economic-politics
Climate (precipitation, temperature, solar radiation)	Land management	Technological change/development
Topography	Grazing intensity and grass coverage	Policies (Agricultural – Environment – Energy sectors
Soil type and properties	Tillage and soil disturbance	Economic growth and cost/price squeeze
Land cover/vegetation type	Residues management/Bare fallow Crop variety and species management Intensive farming Deforestation Biomass burning Drainage of wetlands Land use change/conversion Contamination/Pollution

How can it be measured or assessed?

The table below lists key and/or proxy indicators for soil threats identified by RECARE and ENVASSO.

Soil threat	Soil threat RECARE	ENVASSO ⁶
Decline in OM in mineral soils	Total carbon stocks to 1 m depth (t ha-1) Clay/SOC TOP2 indicators by ENVASSO	Topsoil organic carbon content (%, g kg-1) Topsoil organic carbon stocks (tha-1)

The table below lists key indicators, the purpose of the indicator and methods used for measuring
soil erosion by water.

Indicators	Purpose	Methods
Clay/SOC	Describe interaction between SOM & mineral particles	Clay/SOC
Topsoil organic carbon content	Measure SOC content	Dry or wet combustions
Topsoil organic carbon stocks	Measure bulk density	Bulk density = oven-dried weight of soil/volume of soil
Topsoil organic carbon stocks	Estimate organic carbon stocks	SOC models such as CENTURY, or Roth-C

How can it be prevented or remediated?

Different measures are available to prevent or remediate soil erosion by water. The most appropriate measure to use is dependent on the local situation. High SOM accumulation is favoured by management systems, which add high amounts of biomass to soil, cause minimal soil disturbance, improve soil structure, enhance species diversity and strengthen mechanisms of element cycling⁵.

Practices that increase organic matter include: growing green manure crops or catch crops, perennial forage crops and cover crops; applying animal manure or compost; leaving crop residues in the field; applying reduced or conservation (minimum) or no tillage to minimize disruption of the soil’s structure, composition and natural biodiversity and crop rotations with high residue plants with large amounts of roots and residue.

Measures
Apply animal manures, compound fertiliser, trash, recycled waste	Inter-planting
Green manure crops	Reduce period of bare fallow
Cover crops with plant-based materials	Crop rotation
Retain crop residues	Retain crop residues

Case Study Experiments

Olden Eibergen, The Netherlands

Increasing or maintaining soil organic matter

Veneto region, Italy

Increasing organic matter with crops and conservation agriculture

How does it interact with other soil threats?

SOM decline has a strong impact on other soil threats and, in particular, soil erosion by water and wind, compaction, biodiversity and desertification. SOM plays a pivotal role in soil aggregate stability and cohesion, which in turn affects water erosion. SOM also exerts an important control on soil wind erodibility, by influencing the detachment and transport of soil particles. SOM reduces soil compaction, as it improves the soil structure in terms of total porosity as well as pore size distribution. There are strong links between SOM and soil biodiversity, as SOM is the main source of energy for the decomposer organisms and an important pool of macronutrients. SOM exerts a significant control on desertification since, inter alia, it increases the water retention capacity and improves the soil structure.

How does it affect soil functions?

Biomass production - SOM depletion in mineral soils negatively affects the soil function of food and other biomass production. Direct effects are a reduction in the pool of nutrients, in ion exchange capacity, and in water and nutrient use efficiency. SOM depletion also negatively influences biological activity and its complex biogeochemical mechanisms related to biomass production.
Storing/filtering/transforming – SOM stock depletion reduces the soil’s storage capacity for energy and nutrients. Also, SOM depletion can indirectly reduce soil hydraulic properties and ultimately the water cycle.
Gene pool - SOM depletion is usually associated with a lower biological activity and diversity

Fact Sheet

References

¹ SSSA (Soil Science Society of America), 1987. Glossary of soil science terms. SSSA, Madison, WI.

² Jones, R.J.A.,, Hiederer, R.., Rusco, E.,1 Montanarella., L., 2005. Estimating organic carbon in the soils of Europe for policy support. European Journal of Soil Science, 56, 655-671.

³ Tóth, G., Jones, A., Montanarella L., 2013a. The LUCAS topsoil database and derived information on the regional variability of cropland topsoil properties in the European Union. Environmental Monitoring and Assessment, 185, 7409-7425.

⁴ Lugato E., Panagos P., Bampa, F., Jones A., Montanarella L., 2014. A new baseline of organic carbon stock in European agricultural soils using a modelling approach. Global change biology. 20, 313-326.

⁵ Lal, R., 2004. Soil carbon sequestration impacts on global climate change and food security. Science 304, 1623-1627.

⁶ Huber, S., Prokop, G., Arrouays, D., Banko, G., Bispo, A., Jones, R., Kibblewhite, M., Lexer, W.,Möller, A., Rickson, J., Shishkov, T., Stephens, M., Van den Akker, J., Varallyay, G., Verheijen, F., 2008. Indicators and Criteria report. ENVASSO Project (Contract 022713) coordinated by Cranfield University, UK, for Scientific Support to Policy, European Commission 6th Framework Research Programme.

Useful links

FAO UN - The importance of soil organic matter: Key to drought-resistant soil and sustained food production (2005)
European Communities - Organic matter decline (2009)
University of Minnesota Extension - Organic matter management
US Department of agriculture: Soils and Men, Yearbook of Agriculture; Loss of Soil Organic Matter and Its Restoration By William A. Albrecht (1938)