Soil Carbon Dynamics on the Northern Rangelands Trust Conservancies

Soil carbon may be a key mediator of soil fertility and the capture of available rainfall, but the influence of management on soil carbon is not well understood. A study conducted in the Northern Rangelands Trust Conservancies in Samburu and Isiolo Districts in northern Kenya aimed to establish a baseline for comparison of soil carbon over time and test a predictive model that estimates the accrual rate of carbon based on a few soil and vegetation characteristics. The study involved a ground survey of vegetation and soils at 86 sites across 8 different conservancies. Sites were non-randomly selected to encompass a wide range of soil types across as many conservancies as possible, and to compare areas of different livestock grazing management. Soil and vegetation were sampled together, with soil sampled to 20 cm depth and analyzed for total organic carbon, texture, and bulk density. Vegetation was sampled by clipping live, aboveground plant biomass, and analyzed for lignin and cellulose content. Current and past grazing intensity were also estimated for each site. The SNAP model, a predictive soil carbon dynamic model, was used to predict current soil organic carbon (SOC) stocks based on the estimated history of grazing. The mean predicted SOC stocks were compared with mean observed SOC stocks for each type of management, and predicted SOC stock at a particular site was compared against observed SOC at the same site. The results suggested that the model predicted mean and individual site SOC values with more than 90% accuracy. The study found that prolonged, heavy, continuous grazing in the NRT Conservancies over the past 30 years has greatly depleted SOC stocks. However, reduction in grazing intensity will lead to recovery of SOC at a potential rate of 0.3-0.5 tons C/ha/yr across a variety of soil types. The results suggest that planned grazing management beginning in the NRT Conservancies should help restore SOC and productivity in these semi-arid grasslands, and could result in an economically viable carbon offset project. The study also highlighted the importance of understanding the impact of altered grazing regimes on SOC. The SNAP model, which places grazing and the more complex responses of ecosystems to grazing as a centerpiece in the calculation of changes in SOC, performed very well in predicting current soil carbon stocks in response to long-term grazing management. The model's features, including compensatory responses of plants to increased grazing intensity and positive feedbacks between SOC and available moisture, make it more distinct in its predictions compared to other available models. The study's findings have significant implications for the management of pastoral systems in semi-arid grasslands. The results suggest that planned grazing management can help restore SOC and productivity in these areas, and could result in an economically viable carbon offset project. Further sampling is planned to validate the accuracy of these assessments across all Conservancies potentially participating in the grazing management program, and to more precisely assess the progress of specific grazing management actions within a few selected conservancies.