**5. Form and resilience of carbon stored under tropical grasses**

Many of tropical pasture species have a distinctive carbon fixing (photosynthesis) pathway and are referred to as C4 plants [43]. All plant species have the more primitive C3 pathway, described by the Calvin Cycle [85] but an additional C4 pathway evolved in species in the wet and dry tropics. C4 pastures are those that have the photosynthetic processes divided between mesophyll and bundle sheath cells that are anatomically and biochemically separate, while C3 pastures are those which use only the Calvin cycle photosynthesis pathway for fixing CO2 which takes place inside of the chloroplast in mesophyll cells [86, 87].

In terms of photosynthetic efficiency, C4 grasses are approximately 50% more efficient than C3 plants as a result of this distinctive carbon fixation mechanism [88]. Wang et al. [87], indicated that more efficient use of light and CO2 in C4 plants results in an increase in both biomass production and CO2 fixation. Hence, as a result of their high photosynthetic efficiency and productivity, tropical C4 grasses might be expected to have larger potential for SOC sequestration compared with temperate and annual pastures [55]. Most tropical pastures are important perennials and provide a permanent soil cover and thus prevent soil surface erosion [89], which is of particular importance in the prevention of SOC loss by erosion. Greenland [90] hypothesized that, with suitable management practices, tropical grasses could have a significant potential as a soil carbon sink. Our knowledge of perennial tropical species growth, interaction with the soil, potential quantities and mechanisms of carbon storage remains incomplete [50].

It has been speculated that carbon storage in sub–soils might be an important mechanism leading to increased SOC storage in soils [44, 77] and it is known that tropical grasses translocate large quantities of carbon to their root systems [44]. This suggests an effective translocation to deeper soil layers where soil carbon is typically more protected from decomposition processes [91–94]. Accumulation of carbon in deeper soil layers might therefore be an important mechanism for carbon storage under this vegetation type [59, 89, 95]. The deep rootedness of tropical pastures might, therefore, potentially play an important role in transporting carbon to deeper soil layers and therefore facilitate SOC storage. Indeed, Fisher et al. [52], estimated that the introduction of deep rooted African grass pastures in Colombia might account for the sequestration of 100–507 Mt. soil carbon year−1 if their study sites were indeed representative of similar pastures throughout South America. These studies indicate the potential benefits of introducing deep rooted tropical perennial grasses for SOC storage but also the need for further carbon inventory.

#### **6. Factors affecting SOC sequestration**

Tropical pastures grow continually year round and are adapted to a wide range of soil and climate conditions because of the close interaction between climate factors and soil properties [28, 96]. In addition to soil type, management and site history could be important factors determining the direction and magnitude of change in soil carbon stock [28]. Similarly Chan and McCoy [43], indicated the higher effectiveness of pastures in increasing SOC storage under appropriate management. Wilson and Lonergan [97], also demonstrated in Australia that native and improved pastures in this environment had the same SOC quantity and that historical and contemporary management practice is a key factor influencing net SOC. The management of tropical pastures is therefore a critical determinant of whether the soils under this land use will represent a source or a sink of atmospheric carbon [62]. Poor pasture management such as over grazing, frequent burning and conversion to cultivated agricultural land could result in degradation and low productivity which can reverse the carbon sequestration potential of tropical pastures leading to carbon loss by erosion and oxidation [98]. Hence, the effects of tropical pastures on soil carbon are likely to vary because of environmental and management factors. For example Dalal et al. [99], demonstrated historical management as a key driver of SOC stock particularly in the surface soil layers. Therefore, there is a need for controlled studies that measure soil carbon with some certainty of the effects of both environmental and management factors.

Clay soils in general play a greater role to slow the rate of decomposition, due to both physical and chemical protection of SOC and typically promote larger soil carbon concentrations compared with sandy soils due to these SOC stabilization processes [100–102]. Similarly, a water saturated soil might have lower rates of organic matter breakdown because of a lack of oxygen for soil organisms compared to soils exposed to the atmosphere. Therefore, soil improvement and adding essential inputs are important to increase the rate of organic carbon addition and pasture production [28]. In addition, McKenzie and Mason [28], indicated that deep soil profiles with fertile subsoil allow deep root penetration into subsoil which is much cooler (less likely to promote decomposition) than the topsoil. Hence, maximizing the carbon input by increasing the net primary production through nutrient addition, increased nutrient and water use efficiency and minimizing the rate of organic matter decomposition after deposition in soil are important factors which can help to increase the amount of carbon sequestered from the atmosphere [96].

Carbon accumulation in pasture lands can also be determined by the length of time the land remains under pasture [64]. Hence, regardless of technologies or mechanisms, the length of time must also be taken into account when considering long–term carbon storage. Bouman et al. [103], stated that, due to various economic and biophysical dimensions, sustainability of tropical pastures can also be affected by the pasture type, age, and management which in turn can affect the carbon accumulation. Hence, McDermot and Elavarthi [104], recommended that best management practices, site specific policies and using technological options can offer good opportunities to generate positive effects on soil carbon accumulation by using tropical grasses.

Therefore, factors such as input versus outputs, climatic conditions, soil type and properties, land use control, management practices are the factors affecting SOC storage. Whenever there is a vegetation cover change from C3 to C4 plants, the ratio of stable carbon isotopes (δ13C) can be used to track changes in SOC between the C3 and C4 plants and the quantity of "new" carbon added [99, 105, 106]. The use of stable isotopes offers a useful quantitative technique to allow the estimation of organic carbon storage and turnover in soils, even when TOC changes are of limited magnitude [107].
