The Kyoto Protocol imposed limits to industrial emission of six greenhouse gases (GHGs), mostly CO₂, by the years 2008-2012. Concomitantly, it recognized three mitigation actions, all associated with land-use, and known as the land-use, land-use change, and forestry actions. Within the land-use category, tillage, crop residue, and nitrogen management effects on carbon sequestration have been studied and evaluated (e.g. Schlesinger, 2000; Allmaras et al., 2004). Similarly, the capacity of global forests to sequester CO₂ has also been assessed but, due to variability in tree cover and biotic conditions, the current estimates are at best qualitative. No attempt has been made so far to evaluate the impact of plant nutrition on CO₂ sequestration, although this approach is embodied in the land-use mitigation category. Potassium is of particular interest since crops are very responsive to it. Additionally, it requires considerably less energy than nitrogen for fertilizer manufacture and at an adequate level in plants can enhance N-use efficiency (NUE). This improving effect of potassium on NUE thus optimizes N management, while contributing to a lowering in emission of CO₂ and other GHGs.

In 2001, the Soil Science Society of America drafted a position paper that included the following statement: "Worldwide, SOC (soil organic carbon) in the top 1 meter of soil comprises about 3/4 of the earth's terrestrial carbon; nevertheless, there is tremendous potential to sequester additional carbon in soil".

In order to explore this potential further, a group of soil scientists working together with the International Potash Institute (IPI) are currently studying the contribution of potassium fertilization to sequester additional SOC. The study comprises mathematical modeling and comparison of the results from this modeling with experimental findings. In addition, the interpretation of this information is expressed in terms of the contribution of potassium to carbon sequestration.

The IPI position
The unique IPI approach links two systems: (i) a soil-crop-atmosphere model, and (ii) the modeling of soil processes involving carbon sequestration. The integration of these two systems will be of particular value in assessing the beneficial effect of raising the K nutritional status of crops to increase carbon sequestration in the soil. The IPI approach is the attainment of increased crop production for the benefits of economic productivity, as well as for increased carbon sequestration at a global level. The IPI position can be realized by applying currently recognized best nutrient management practices based on a combination of modeling and scientific experience.

How potassium affects C sequestration
Raising the potassium status of the plant to an adequate level affects CO₂ sequestration in two ways, namely by: (i) stimulating photosynthetic activity thereby increasing dry matter (DM) production of all plant organs, including roots; (ii) increasing the root:shoot ratio as a consequence of activating shootroot transport of photosynthates and thus also C allocation to the roots. Carbon released from roots into the soil as occurs, for example, when roots are sloughed off and decomposed to SOM is, to a large extent, sequestered in the soil. The processes of carbon mineralization and transport of soil CO₂ are well understood (Simunek and Suarez, 1993; Pumpanen et al., 2003) and are included in the comprehensive model which is described below.

A model for assessing the role of K in C sequestration
The modeling approach is based on the measured effects of K-fertilizer on dry matter yields of tops and roots of major crops across the world. The model is modular in that it is based on a progressive build up of CO₂ sequestration. It starts from calculating the gross potential carbon sequestration (GPCS). This is the total carbon present in the DM production i.e. the sum of roots and tops. It then determines the role of potassium fertilization and the potential carbon sequestration (PCS), which is based on the calculated DM accumulation in the roots. The actual carbon sequestration (ACS) is then further calculated using more detailed data pertaining to the specific soil conditions as they affect mineralization and other soil-related processes. It should be mentioned here that the model (as any model) is incomplete because it accounts only for the three major elements (N, P and K) and neglects crop response to other nutrients. The basis of the approach is that when K supply is inadequate, the root:shoot ratio will be low because the consequent low concentration of leaf K will impair photosynthate loading into the phloem and translocation to the roots. Raising the K supply will overcome this effect providing that other nutrients are not growth limiting. Moreover, the higher supply of K will not only increase the root:shoot ratio but also increase the absolute root weights and thus CO₂ sequestration. Magnesium is also particularly important as adequate amounts must be present because, in common with K, it is required both in photosynthesis and in the loading of photosynthates into the phloem and hence their translocation to the root (Cakmak and Kirkby, 2008).