Soil organic matter stability decreases with increasing urbanization in highly weathered rainfed tropical arable soils

Urbanization is a major land use change factor affecting soils. There is little understanding of how expansions of tropical West African cities have affected soil organic matter (SOM) composition and dynamics. In such cities, urban agriculture is common, playing an essential role in food security and urban sustainability. However, tropical soils tend to have low nutrient contents and cation exchange capacity. Thus, management strategies that enhance soil fertility and carbon (C) sequestration are needed. Developing such strategies requires a thorough understanding of how SOM dynamics alter in response to urban growth. Here, our objective was to assess how urbanization has affected the relatively stable mineral-associated-SOM (MAOM) and the labile particulate-SOM (POM) fractions in rainfed urban arable maize fields of Kumasi, a typical expanding city in Ghana (West Africa).

Using a grid-based satellite approach, and keeping other factors constant (including climate, topography, parent material and soil type), we took topsoil samples (0–10 cm) along an urban-intensity (UI) gradient, distinguishing: (i) low UI, located >400 m away from any primary road and having been under urbanization for <30 years, (ii) mid-low UI, located ≤400 m from a primary road and having been under urbanization for <30 years, (iii) mid-high UI, located >400 m from primary road and having been under urbanization for ≥30 years, (iv) high UI, located within ≤400 m from a primary road and having been under urbanization for ≥30 years. SOM fractions were isolated from the soils using a size separation approach, whereby the sand-sized fraction (0.063 - 2 mm) was regarded as POM and the clay- and silt-sized fraction (<0.063 mm) as MAOM. Prepared samples were ultimately analyzed for SOC using a Leco temperature ramp C analyzer, where a temperature threshold of 600 ºC was used to separate organic from inorganic C.

We found that mean SOC contents of the POM fraction increased markedly from 7.7 g kg^-1 in the low UI topsoils to 13 g kg^-1 in their high UI counterparts, suggesting an increase in POM with increasing urbanization. This trend was not observed for the MAOM that showed SOC contents of 4.5, 4.1, 4.9, and 4.1 g kg^-1 for the low, mid-low, mid-high, and high UI topsoils, respectively. Moreover, the share of SOC contents of POM in the bulk SOC increased from 51% in the low UI topsoils to 64% in the high UI topsoils, whereas that of MOAM decreased by 6% from 31% to 25%, respectively. These findings suggest that while there is evidence of strong anthropogenic contributions of SOM to urban arable soils, urbanization does not seem to promote SOC storage in the relatively stable MAOM fraction. Consequently, rainfed urban arable soils in Kumasi will need management interventions for keeping appropriate long-term SOM levels to maintain soil functions.