Page 96 - FAO-IPCC Expert meeting on climate change

Appendix 01: Speakers’ summary notes
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Soil health and soil nutrient management, including soil organic carbon, erosion control and other options to raise agricultural productivity and resilience
DAN PENNOCK
PLENARY SESSION 7:
CLIMATE CHANGE ADAPTATION, RESILIENCE, AND LINKAGES TO FOOD SECURITY
Introduction
Increasing yields of food, fibre, and fodder
on current agricultural land in a sustainable manner is a perennial goal of research and extension on sustainable soil management practices. In an ideal scenario (Figure 1), changes to management practices would reinforce positive trajectories in soil functions that support increases in agricultural yields. Unfortunately the regional summaries of trajectories contained in the 2015 Status of the World’s Soil Resources (SWSR) report (FAO and ITPS 2015) indicate that significant threats to soil functions persist in most regions of the world, and that the alternatives to the ideal scenario must be explored.
Figure 1: Three trajectories for the effect of human- induced changes in soil functions on agricultural productivity
Reversible versus irreversible soil-induced declines in agricultural productivity
At the conceptual level three distinct trajectories for the effect of human actions on soil functions exist (Figure 1).
The first is the ideal trajectory that seems regrettably uncommon at present. In the second, the current trajectory is a gradual decline in many soil functions (e.g. nutrient supply to plants) due to a set of chronic disturbances (e.g. average annual levels of soil erosion due to agricultural practices).
In the third trajectory the decrease in soil functions is essentially irreversible at time scales of relevance to human society (Figure 1). The new equilibrium reached is well below the original starting point and renders the land unusable for agricultural production. This trajectory can occur either due to catastrophic event (e.g. a major wind erosion event or a contaminant spill) or to a chronic disturbance passing a threshold or “tipping point” where the trajectory can no longer be reversed. For threats to soil functions associated with soil chemistry (e.g. salinization/sodification, acidification, contamination) the thresholds where land becomes unusable for crop production are generally well established.
From a management perspective it is critical to discern between the latter two productivity trajectories. The priority
for development and implementation of management practices should be first directed at soils that are most likely
to experience irreversible loss of soil functions or where the gradual loss of productivity cannot readily be offset by technological measures. It is also critical that the relationship between these processes and climatic drivers be explored through modeling using regionally relevant climate change scenarios.
Examples of irreversible and reversible losses: Soil erosion
Overall the SWSR (FAO and ITPS 2015) found that soil erosion remains as the number one threat to agricultural productivity. Agriculturally induced sheet, wind, and tillage erosion lead to the removal of organically enriched surfaces horizons, which is the main contributor to the gradual productivity decline. If, however, there is a growth-impeding layer within the soil profile (for example, impermeable bedrock or a saline or sodic soil layer) then sheet, wind, and tillage erosion may lead to irreversible loss of productivity when the growth-limiting layer enters into the rooting zone of plants. In this situation the initial period of erosion is marked by a gradual decline in productivity until the growth- limiting layer intersects with the rooting zone, at which time the irreversible productivity decline begins. Major erosion
FAO-IPCC Expert meeting on climate change, land use and food security