Abstract
Land use conversion is generally accompanied by large changes in soil organic carbon (SOC). SOC influences soil erodibility through its broad control on aggregate stability, soil structure and infiltration capacity. However, soil erodibility is also influenced by soil properties, clay mineralogy and other human activities. This study aimed to evaluate soil organic carbon as proxy of soil erosion risk in the Nyumba ya Mungu (NYM) catchment in Northern Tanzania. Soil organic carbon (SOC) was measured by an AgroCares scanner from which the soil organic matter (SOM) was derived using the conversional van Bemmelen factor of 1.72. A regression analysis performed between the measured loss on ignition (LOI) values and SOM from the AgroScanner showed a strong positive correlation in all land use classes (LOIFL R2 = 0.85, r = 0.93, p < 0.0001; LOICL R2 = 0.86, r = 0.93, p = 0.0001; LOIGL R2 = 0.68, r = 0.83, p = 0.003; LOIBS R2 = 0.88, r = 0.94, p = 0.0001; LOIBL R2 = 0.83, r = 0.91, p = 0.0002). This indicates that SOC from the soil scanner provided a good representation of the actual SOM present in soils. The study also revealed significant differences in the soil aggregate stability (WSA) and SOM stock between the different land use types in the Upper Pangani Basin. The WSA decreases approximately in the following order: grassland > forest land > bare land > cultivated > bush land. Land use change can thus potentially increase the susceptibility of soil to erosion risk when SOC is reduced. Since WSA was directly related to SOM, the study indicates that, where formal measurements are limited, this simple and inexpensive aggregate stability test can be used by farmers to monitor changes in their soils after management changes and to tentatively assess SOC and soil health.
Highlights
The changing demographics in Tanzania have created an increasing demand for land, food and water, leading to changes in land and water use
The local manifestation of urbanization includes the establishment of the Siha district and the emergence of many villages and urban suburbs along roads across the catchment [52,53]
The soil slake test method adapted from the USDA protocol was appropriately used to separate aggregates from different land use management types
Summary
The changing demographics in Tanzania have created an increasing demand for land, food and water, leading to changes in land and water use. While the land remains constant, the demand for food and water is expanding linearly while the population increase in Tanzania is growing exponentially, with an average growth rate of 3% annually [1]. Deforestation and the loss of permanent vegetation through the fast expansion of agricultural land and growing urbanization with respect to population increase has accelerated soil loss rates and downstream siltation [2,3,4,5]. The mean rate of soil loss by water in Tanzania is 4.1 and 10.1 tha−1 yr−1 for all land cover classes and per cropland, respectively [6].
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