Grassland soil organic carbon stocks: Status, opportunities, vulnerability

Abstract

Grasslands, including rangelands, shrublands, pastureland, and cropland sown with pasture and fodder crops, covered approximately 3.5 billion ha in 2000. This represented 26% of the global ice-free land area and 70% of the agricultural area, and contained about 20% of the world’s soil organic carbon (C) stocks. People rely heavily upon grasslands for food and forage production. Around 20% of the world’s native grasslands have been converted to cultivated crops, and significant portions of milk and beef production occurs on grasslands managed solely for those purposes. The livestock industry – largely based on grasslands – provides livelihoods for about one billion of the world’s poorest people and provides one-third of global protein intake. Much of the world’s grasslands are under pressure to produce more livestock by grazing more intensively, particularly in Africa’s rangelands, which are vulnerable to climate change and expected to satisfy most of the beef and milk demand in Africa. As a result of past practices, between 5% and 10% of the world’s grasslands have been degraded by overgrazing. Portions of the grasslands on every continent have been degraded due to human activities with about 7.5% of grassland having been degraded because of overgrazing. More recently, the Land Degradation Assessment in Drylands (LADA) concluded that about 16% of rangelands are currently undergoing degradation and that rangelands comprise 20–25% of total land area currently being degraded affecting the livelihood of over 1.5 billion people. Disturbance is an integral part of traditional grassland management systems to sustain yields of forage. However, disturbance through overgrazing, fire, and invasive species can also deplete grassland systems of C stocks. Harvesting a large proportion of plant biomass enhances yields of useful material (e.g., for forage or fuel), but decreases C inputs to the soil. Many management techniques intended to increase livestock forage production have the potential to increase soil C stocks by sequestering atmospheric carbon dioxide(CO2) – as much as 0.2–0.8 Pg CO2 year-1. Methods of improved management include fertilization, irrigation, intensive grazing management, and sowing of more favorable forage grasses and legumes. Grassland management to enhance production (i.e., through sowing improved species, irrigation or fertilization), to minimize negative impacts of grazing, or to rehabilitate degraded lands can each lead to C sequestration. Improved grazing management (i.e., management that increases production) can also lead to an increase of soil C stocks as can rehabilitation of degraded lands.