**1. Introduction**

Environmental degradation and climate change are key current threats to world agriculture and food security [1–5]. Human–induced changes to land cover have been significant driving forces of this global environmental change, of which, soil degradation resulting from land conversion, agricultural intensification, soil disturbance and increased erosion have been key factors [6–9]. An important component of this land degradation globally has been a diminished SOC stock with concomitant loss of soil condition and function, compromising food production and agricultural sustainability [10–12]. Land and soil management to increase soil organic matter content, soil condition and productivity is therefore a key need globally to safeguard agricultural production, food supply and environmental quality.

Organic carbon in soils globally is estimated to be between 1500 and 1600 Gt [13, 14] to 1.0 m depth which represents a significant component of the global carbon cycle, storing more carbon than is contained in vegetation and the atmosphere combined [15–17]. It has been estimated that, worldwide, soils have lost between 42 and 78 Gt of their original SOC as a result of management pressures [18]. With this carbon depletion, however, comes a significant opportunity, since soils are believed to have the capacity to store an additional 0.4–1.2 Gt C year−1 with the introduction of more judicious land management practices [3, 7, 19–22]. As such, soils globally have considerable potential to offset greenhouse gas (GHG) emissions and SOC storage has been widely promoted as an important strategy to help meet national and international emissions reduction targets [23]. Additional SOC storage might therefore have the dual benefit of contributing to our response to climate change globally whilst helping to restore soil condition and function to promote sustainable land management, improved production and productivity [3, 7, 19, 20, 24].

Methodologies and management practices that reduce SOC loss or promote the storage of additional soil carbon are being actively investigated globally. It has been widely reported that cultivation accelerates organic matter decomposition by exposing sites within soil aggregates that were previously protected [25–29] while soil erosion, vegetation clearing and removal of crop residue are also known to result in long–term soil carbon loss [30, 31]. However, there are management practices which seem to either arrest SOC loss (e.*G. minimum* tillage) or to promote carbon storage such as afforestation, pasture conversion, grazing management, cover crops, water harvesting, erosion control and the use of soil amendments including biochar [32]. Not all of these are practical in production landscapes globally and not all will be equally effective in the management of SOC. The effectiveness of various management practices is therefore being explored to facilitate optimum carbon storage that can be integrated with agricultural production systems.

An approach that has attracted particular attention is the use of perennial grass species within the production system, which appear to significantly increase SOC across a range of environments and this is particularly true where these perennial grasses replace cropping systems [33–36]. Pastures are varied in terms of their geographical distribution and species composition comprising native and exotic, annual and perennial grasses, legumes, herbs and shrubs [37]. They are the primary resource for many farm industries and are the basis for the production of meat, wool, milk and fodder. Schuman et al. [38] estimated the SOC under grazing lands of the world to be 10–30% of the total global SOC stock, while Janssens et al. [39] estimated the overall C sink in grassland soils of most European countries to average approximately 60 g C m−2 year−1.

Tropical perennial grass species have been particularly promoted due to the high biomass and carbon accumulation resulting from their excellent photosynthetic efficiency, rapid establishment, fast growth, deep root systems and potential annual harvest [28, 40, 41] and Parton et al. [42] suggested that tropical grasses have significant potential as a carbon sink. However, there is a research need to fully quantify their capacity to store additional soil carbon relative to other management systems and hence, their potential for greenhouse gas (GHG) abatement and soil condition recovery [43–45].

Here we aimed to review current knowledge with regard to the SOC storage potential of tropical grasses worldwide given their wide distribution and extensive use, where current agricultural policy environments have identified land management innovations as key entry point to achieve co-benefits of resilient agriculture, poverty alleviation, and climate change mitigation. Hence, we identify knowledge gaps and current research needs to fully explore the potential of tropical grass species for SOC change.
