Measuring soil carbon

Soil represents one of the most important storehouses for carbon on earth. It contains more carbon than plants and the earth’s atmosphere combined (see figure). However, the importance of soil as a storehouse for carbon has been historically under-recognized in global accords and policy debates regarding climate change, as has the role of agriculture in accelerating or mitigating carbon emissions from the soil. These omissions have been largely due to difficulties measuring the levels and stability of carbon in the soil. Until recently, such measurements were only possible in a laboratory, using sophisticated and expensive equipment.

Scientists at CIP and the Brazilian Agricultural Research Corporation (EMBRAPA) worked together to validate a new affordable and reliable device for measuring soil carbon. Developed by EMBRAPA-Agricultural Instrumentation, the device employs laser-induced optical techniques to measure carbon levels and their stability in soil samples. The device is so light and convenient that it can be used directly in the field.

CIP and EMBRAPA established a 1,000 km transect from the Pacific ocean over the Andes to the jungle in Southern Peru, and then tested soil carbon levels in a variety of agricultural and land use systems. The samples represented the most common agroecosystems found in tropical areas worldwide. There were wide variations in the levels and stability of carbon stored in the soil, depending on land use, type of crop, elevation, and so on (see table).

Soil use

Carbon stocks (tons/ha-1)

Wet grasslands – high plateau

301.7

Peat lands – high plateau

228.9

Alfalfa (under irrigation)

91.9

Shaded coffee (Amazon)

91.3

Primary rainforest

75.2

Avocado (intercropping)

68.2

Grape

65.2

Potato

55.6

Maize

42.4

Olive

38.1

For example, wet grasslands and peatlands from highland plateaus contained four times the amount of carbon found in primary rainforest soil, the common standard against which levels are compared. In contrast, areas planted with potato, maize, or olive trees held only half to three-quarters of the amount of carbon stored in rainforest soil. There were also important differences in the chemical structure of carbon found in different soils: less stable carbon was more common in samples from high-altitude grassy plateaus and, thus, more likely to escape into the atmosphere if the soil is disturbed.

 

The implications for carbon emissions are great. Undisturbed soil is a natural carbon sink. However, activities such as plowing release carbon from the soil into the atmosphere as carbon dioxide (CO2), a major greenhouse gas. In fact, poor farmers in the Andes already are converting grassland to cropland at higher elevations, because warming trends are increasing the threat of crop pests and disease at lower elevations. CIP has the tools and means to help mitigate these effects and to transform an environmental risk into an opportunity for retaining soil carbon while improving the livelihoods of poor farmers.

 

CIP and partners are working on three strategies:

  • Developing more stress-resistant crops, so farmers can continue to crops in existing fields at lower elevations
  • Using crop management techniques (e.g., mulching, water management) and diversified farming systems that balance soil carbon losses with methods that capture and retain soil carbon
  • Implementing financial incentives and other techniques that reward farmers for improved stewardship of natural carbon sinks and soils
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