**2. SOC storage potential of tropical grasses**

A number of studies have considered the soil carbon storage potential of tropical pastures by comparison with other management systems. An empirical, five year study of tropical ecosystems in South America by Amézquita et al. [46], demonstrated that although tropical pastures were second only to native forest in the quantity of SOC stored, organic carbon in the soils of these pasture systems represented a higher proportion (95–98%) of the total ecosystem carbon than comparable native tropical forest systems and silvo–pastoral systems. Desjardins et al. [47], reported that where tropical forest was converted to tropical pasture in Brazilian Amazonia, a slight increase in SOC content occurred in both sandy and clay soils while Post and Kwon [48] described the similarity of the average rates of SOC accumulation in forest and grasslands of 33.8 and 33.2 g C m−2 y −1, respectively through time following management although above ground carbon is lost. In Australia, Chan and McCoy [43] also identified the potential of introduced perennial pasture (Kikuyu) to store a mean of 73 Mg C ha−1 in soil which was similar to soils under native trees (77 Mg ha−1). Under some circumstances, tropical pastures have been reported to have a greater capacity to store SOC compared with trees or forest. For example,


#### **Table 1.**

*Total soil carbon stored under different tropical grasses with different soil sampling depth and age of plantation.*

Guo et al. [49], reported 15–20% larger soil C stocks under native pasture compared with a 16 year old pine plantation to 1.0 m in the soil profile. These findings seem to be convincing, although some caution must be attached to many such results given that they typically do not account for above-ground biomass and are rarely reported on an equivalent mass basis. There is nevertheless, growing evidence that tropical pastures might have the capacity to store SOC that is at least equivalent to that of forest systems in terms of rate and quantity of accumulation. However, the quantity and rate of carbon accumulation would appear to be moderated by environmental conditions and both preceding and ongoing management practices. Consideration and knowledge of the behavior and potential carbon storage of particular tropical grass species has much to add to this debate.

Some specific tropical grass species (**Table 1**) such as *Andropogon guyanus* (gamba grass, Rhodesian bluegrass, tambuki grass), Lemongrass (*Cymbopogon citratus*), Palmarosa (*Cymbopogon martinii*), Kikuyu (*Pennisetum clandestinum*); Miscanthus (*M. giganteus*), Vetiver (*Chrysopogon zizanioides*) have been highlighted for improving soil carbon storage potential even though their efficiency is determined by a range of environmental and management factors [46, 60–64]. However, Fearnside and Barbosa [62], found that management practices could on the other hand determine whether tropical pasture soils could be net sinks or sources of carbon, demonstrating in Brazilian Amazonia, that under "typical" (without inputs or other practices) and "ideal" (with variety of appropriate practices) management, tropical pasture soils were a net carbon source releasing an average of 12 Mg C ha−1 following deforestation.

### **3. Processes of SOC storage**

The process by which organic carbon stored in soils follows various pathways such as roots, root exudates and litter (both above- and below-ground). Plant litter consists of dead roots, is a primary source of soil organic matter which is the largest terrestrial pool of carbon [65]. Despite, often considered separate processes of litter decomposition and soil organic matter stabilization is an important control of carbon storage and SOC dynamics [66, 67]. Decomposition of plant litter is one of the main processes driving nutrient and carbon (C) cycling in terrestrial ecosystems [68]. The effect of litter quality on SOM stabilization is inconsistent and litter addition promotes SOC mineralization, but this promotion alters by soil moisture and litter type [69]. Hence, understanding the interactions between the initial composition and subsequent decomposition of plant litter help to understand the flow of organic matter between soil carbon pools [70]. Root exudates are also one of the various pathways through which the carbons fixed released into soils [71]. Plants release a part of their metabolome into soils and thereby provide information about the potential biological function of exudates in the rhizosphere [72].

Root biomass production is an important plant component that can contribute to soil carbon sequestration. A strong fibrous root system, penetrating deep into the soil profile and growing vertically rather than horizontally, is therefore desirable to maximize soil carbon sequestration. Hence, the large root systems of tropical grasses might potentially facilitate long term deep carbon storage and reduce the chance of decomposition and carbon loss [44]. For example, the roots of vetiver grass have been found to contribute significantly more to additional SOC storage than those of other grass species [60, 63, 73]. Although the extent of SOC sequestration potential of tropical grass species still requires further research, they would appear to have particular promise with regard to soil carbon storage compared with other species.

Due to their large biomass production and their extensive and fast growing root system, tropical perennial grasses would seem to have the capacity to rapidly *Soil Carbon Storage Potential of Tropical Grasses: A Review DOI: http://dx.doi.org/10.5772/intechopen.97835*

store or contribute large quantities of carbon in addition to their other varied uses [53, 74]. Deep rooted tropical perennial grasses have been identified as the most promising plants that could contribute to SOC storage and thus climate change mitigation [44, 75–78]. Awoke [79] highlighted further the potential of tropical grasses for both above- and below-ground C sequestration by planting strategically on appropriate lands.

Most of studies relating to tropical grasses to date have focused on the actual biomass production potential. However, there are only few studies which have considered the actual net accumulation of carbon stored in the soil under tropical grasses (**Table 1**) highlighting the need for controlled studies to determine not only biomass and inputs but also the net effect of tropical perennial grasses in terms of carbon storage and the mechanisms, stability and longevity of the carbon stored such as the rate of new carbon turnover and carbon cycling of the newly added carbon and the extent to which it is retained in the soil system.
