WHAT IS THE POSSIBLE MARGIN FOR CARBON STORAGE IN SOILS OF DRYLANDS?

The majority of dryland soils in Africa are low in SOC and are even experiencing an increasing loss of OM (Lai 2004; FAO 2004, 2005). This situation correspond to “trend a” (Figure 5.3), where the residual SOC is often stabilizing at a critical minimum (C_(b)). Assuming that a recovery of SOC is

FIGURE 5.3 Idealistic and feasible options for soil organic carbon recovery and storage under dryland conditions.

C_(r)= carbon level from which recovery is triggered

C_0(nl = ('(climate, soil properties, ideally all management practices adopted)

C_(b) = ffclimate, soil properties, management practices business as usual)

C_(f) = ('(climate, soil properties, feasible management practices adopted)

C_(ia) = ('(climate, soil properties, innovative management actions and practices adopted)

triggered from a given point on the curve of depletion, the increase of SOC (rate and maximum soil carbon) would depend on the various factors discussed earlier. This recovery is a major challenge that would require sustained efforts in terms of overcoming factors of disturbance and adopting appropriate management practices, as well as operating significant changes on farmers’ attitudes and behaviors. The recovery trend would follow a first-order kinetics and would require several years. The reconstitution half-life of the lost C would be in general slow, from 5 to 10 years depending on the local context.

Ultimately, if all conditions are favorable, the recovery of SOC would rise back toward the “idealistic” natural initial storage capacity C_(n) (trend b). However, this trend seems challenging, knowing that in most cases, it would be difficult to guarantee all the necessary conditions for a positive and increasing annual balance of SOC. For instance, a soil that has undergone significant degradation by water erosion as a result of land cover degradation and subsequent organic matter loss would be unlikely to restore to its initial situation despite rigorous efforts.

The alternative trend for SOC sequestration when adopting a set of “feasible” agricultural practices that favor SOC storage would be a new state of equilibrium C_0(f) that depends on the local context (trend c), and that would be less than C_0(n). Therefore, it is emphasized here that reversing the trends of C storage in dryland soils would depend greatly on the local conditions (climate, soil properties, and management practices). Farmers can move forward to only a set of feasible management practices according to their local context (farm size, cropping systems, level of input, productivity, income, advisory, government support, degree of adoption of innovations, etc.). The SOC would be eventually enhanced from the “business as usual” level (C_(b)) to the “maximum feasible level” (C_(f)), but would not reach the idealistic maximum level (C_0(n)).

Under special conditions relying on forced action measures, it is possible to increase SOC to a level (C_(ia)) beyond the idealistic level. This situation is optimistically achievable, but would require the introduction of innovative management practices and assumes no limiting factors to their implementation (adaptation, sufficient long-term funding, acceptance by farmers, capacity building, advisory, policy, etc.).