Investigating soil carbon dynamic along land use change gradients in subtropicaland tropical grazing lands

Durcan, Rose and Rufino, Mariana and Ostle, Nick and Quinton, John (2023) Investigating soil carbon dynamic along land use change gradients in subtropicaland tropical grazing lands. PhD thesis, Lancaster University.

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Abstract

Tropical and subtropical forests play an important role in the carbon cycle. Being situated in warmer climates, these forests have the potential to sequester large amounts of soil carbon and mitigate against climate change. Despite this, due to increased demand for livestock and other agricultural products, deforestation followed by grazing in these climates have led to the emission of large amounts of greenhouse gas emissions, and degradation of soils. In order to understand how soil carbon sequestration can be maximized in these environments in the future, it is imperative to understand the impacts of land use change on soil carbon dynamics and the complex interactions between soil characteristics and vegetation, something which is understudied in particular for the tropics and sub-tropics. This thesis aimed to investigate the impacts of land use conversion for grazing on soil carbon dynamics and soil quality through the analysis of existing data, and further detailed research into a subtropical dry forest ecosystem. A meta-analysis gathered existing data to investigate the impacts of grazing and land management practices on soil organic carbon and other soil properties in subtropical and tropical climates, identifying that low intensity grazing (into grazing lands through a chronosequence design over a land use transition in the Chaco, Argentina (Forest – Young Pasture (0-5 years) – Intermediate Pasture (10-15 years) – Old Pasture (>20 years), showed that deforestation decreases soil carbon stocks (0-50 cm)in the topsoil by 16 – 53%, alongside decreases in soil nitrogen (5 – 55%), total phosphorus (6 – 68%), and root biomass (68 – 82%). Whilst age of pasture did not impact soil C or N, total P was observed to increase by 46% between intermediate and old pastures. Deforestation in this region has increased pH and decreased electrical conductivity (by 8 – 13%, and 28 – 69% respectively), and increased the salinity and sodicity of these soils, with changes in the dominance of cations in sites with the loss of native vegetation. The analysis of phospholipid-derived fatty acids (PLFAs) in soil samples – a proxy of microbial community composition showed that intermediate pastures had decreased total fungal and total gram-negative bacteria than forest soils in the topsoil, whereas in the subsoil total fungal PLFAs decreased in comparison to young pastures.