Climate-smart grazing to restore tropical soils

Problem statement
The restoration of degraded landscapes has increasingly been promoted as a sustainable investment, sparking large-scale global commitments to restore soil functions in ecosystems. A key premise is that optimisation of grazing can lead to net carbon (C) sequestration in grasslands, sufficient to offset livestock-mediated methane (CH4) emissions. Management of grasslands with perennial vegetation and 'optimal' stocking rates is promoted as climate-smart management of soils1. However, there is much uncertainty as to the net effects on greenhouse gas exchange of different grassland and livestock management approaches, particularly in tropical regions. Current estimates of net C uptake in soils may be overestimated by 100%2. Interacting factors determine whether grasslands become sinks or sources of C, among which grazing pressure, soil type and rainfall play fundamental roles3. Livestock species and grazing management control soil C and macronutrient inputs, via selective grazing, excreta deposition, and effects on soil exposure and erosion. Models can be applied to understand overall impacts of different livestock management on soil net greenhouse gas exchange4, but require data to reduce uncertainties for intensive grasslands systems. The Centre for International Tropical Agriculture (CIAT) have recently compiled soil and plant biomass data from grassland surveys and experiments carried out since the 1980s, and have requested research collaboration to develop simple models that represent key controls on net emission coefficients. The dataset includes a wide range of soil textures, climatic zones and grazing practices. The project aims to use this resource, together with a systematic review and some empirical work, to develop simple statistical and process models that represent where and how restoration of tropical grasslands can provide net C sequestration. This will contribute to integrated landscape planning.
Project description
This project will collate empirical evidence from tropical grasslands of Latin America that identifies under which conditions grasslands become C sinks, and therefore potentially suitable for landscape restoration. The research will combine field level, and mechanistic research with laboratory manipulations to parameterise and test existing soil models. The research will be organised in four phases:
1. Literature review and model testing to estimate the size of the effects of soils and grazing management on soil C. The student will conduct a systematic literature review on tropical grasslands, and access an extensive database of experiments at CIAT in Colombia. Results will be used to set up simple ecosystem models such as N14CP5, withholding some datasets for model testing. Statistical analyses will be used to assess major factors and interactions.
2. Field work. The student will develop experimental and/or survey designs to fill knowledge gaps that become evident after assessing the existing database. CIAT can offer field sites in the Llanos of Colombia with contrasts in soil texture, depth and organic matter contents, and with seasonal, permanent, and rotational grazing management. The supervision team will assist with experimental design and methods for assessing carbon stocks (to 1 meter depth) and fluxes in vegetation (CO2), and soil (CO2 and CH4).
3. Manipulation experiments. Based on outcomes of the data review and modelling study, the student will select key factors controlling C decomposition such as soil texture, macronutrient inputs (mainly N and P) and water potential, and conduct laboratory incubations aimed at closing remaining knowledge gaps.
4. Scenario analyses. The models and relationships developed will be used to assess where net C sequestration can be achieved in degraded land, subject to soil and management constraints.