Spatio-temporal effects of urbanization on soil organic carbon sources, distribution, and composition in tropical agricultural soils

• Applicant: Dr. Stephen Boahen Asabere
• Subject Area: Soil Sciences
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 467340364

In recent years, expansion of cities into formerly rural areas has accelerated, yet its effect on soil carbon (SOC) dynamics is understudied. However, SOC is essential for soil fertility and carbon (C) sequestration, especially in tropical regions where soils have low nutrient contents and cation exchange capacity. To understand SOC dynamics, build up and preserve SOC stocks, it is crucial to identify SOC sources, characterize SOC compounds, and understand SOC stabilization mechanisms. In a previous study in Kumasi, a typical expanding city in Ghana (West Africa), we found that in the course of urbanization, household and construction waste accumulated in urban agricultural soils, increasing the stocks of total C (TC) in these soils. Visual inspection of soil samples, and observation of the surroundings suggested that the composition of TC differs from that of natural soils. This finding pointed to the need to characterize C compounds and identify contributions of various anthropogenic C compounds (ACC), and to understand their implication for SOC preservation and soil functions. Therefore, our aim in this project is to identify sources and quantify the contribution of ACC to soil TC, and ascertain their effects on SOC stocks and composition in agricultural soils of Kumasi, Ghana. Thus, the key question in this project is, in which ways the stability of SOC in agricultural fields is altered when they become part of a rapidly expanding city. To answer this question, we will quantify the contributions of ACC, analyze SOC composition, and examine how the various SOC fractions are related to soil fertility. We hypothesize that soils that have been in an urban environment for a long time (≥30 years) have increased contents of ACC, and of relatively stable SOC pools, compared to soils that have been in an urban environment for a shorter time (<30 years). We will utilize 636 top-soil (0-10 cm) samples from agricultural fields in Kumasi that have been collected from long-term and short-term urban soils. Near-infrared spectroscopy and elemental C analysis will allow for distinguishing contents of plastics, biochar, textile, natural SOC and carbonates, thereby characterizing ACC contributions. Densimetric fractionation will allow for differentiating SOC contents into particulate and mineral-associated fractions, thereby assessing labile and relatively stable SOC pools. Based on statistical and spatial modelling of the results, we will develop a mechanistic framework that can explain the spatial and temporal patterns of SOC composition and sources in so far understudied tropical urban agricultural soils. Thus, with this project we will contribute to improving soil C modelling, which has practical implications for regulating soil fertility and climate change.

• DFG Programme: Research Grants