| Abstract |
Organic matter is of great importance in soil, because it impacts on the physical, chemical and biological properties of soils. Physically, it promotes aggregate stability and therefore water infiltration, percolation and retention. Biologically, it stimulates the activity and diversity of organisms in soil. Decomposing organic matter releases nutrients, such as nitrogen (N), phosphorus (P), sulfur (S) and potassium (K), essential for plant and microbial growth. Sustainable land management practices enhance carbon sequestration and sustain agricultural productivity, thus mitigating against climate change. In Western Africa, there is a rapid decline of soil organic carbon (SOC) levels with continuous cultivation. For the sandy soils, average annual losses may be as high as 4.7%, whereas with sandy loam soils, losses are lower, with an average of 2%. In the equatorial forest zone with higher rainfall, abundant moisture favours high biomass production, which in turn brings about higher SOC (similar to 24.5 g kg(-1) organic C) and nitrogen contents. In the Sudan savannah, organic carbon (similar to 3.3-6.8 g kg(-1)) and total nitrogen are very low, because of low biomass production and high rates of decomposition. Estimates of SOC stocks and changes made for Kenya using the Global Environmental Facility Soil Organic Carbon (GEFSOC) Modelling System indicated soil C stocks of 1.4-2.0 Pg (0-20 cm), which compared well with a soil and terrain (SOTER)-based approach that estimated similar to 1.8-2.0 Pg (0-30 cm) of soil C between 2000 and 2030 in Kenya. Direct field sampling and laboratory measurements of soil carbon in Kenya has been going on for over half a century, and the data exist in the form of numerous technical and research reports, theses, journal papers and workshop proceedings, Kenya Agricultural Research Institute annual reports and geographic information system (GIS) databases. A combination of biomass measurements and empirical equations has also been employed in Kenya to measure organic carbon stocks. Stratified random sampling of herbaceous standing crop has been carried out at Nairobi National Park to estimate primary production of the grassland savannah. The Carbon Benefits Project, developed between 2009 and 2012 by Colorado State University (USA) in collaboration with Kenya, Nigeria, Niger and China, is able to estimate carbon stocks and greenhouse gas (GHG) emissions. Increasing soil organic matter content can both improve soil fertility and reduce the impact of drought, improving adaptive capacity, making agriculture less vulnerable to climate change, while also sequestering carbon. Agronomic practices in western Kenya include using improved crop varieties, extending crop rotations, notably those with perennial crops that allocate more C below ground, and avoiding or reducing the use of bare unplanted fallow among others. Increasing the soil carbon in farms improves soil fertility, hence improves food security, increases economic returns from carbon revenues and creates business development opportunities for farmers to diversify income-generating activities. Population pressures, declining plot sizes and resource constraints in Africa have led to agricultural intensification and continuous cropping with insufficient inputs, leading to rapid decline in SOC stocks. Sustainable management of organic resources will require interventions by regional governments to help farmers access inputs cheaply, as well as educating farmers on sustainable land management practices. |