Soil sealing and crusting effects on infiltration, erosion and microbial composition under different rainfall intesities and slope conditions
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Authors
Mrubata, Kaya
Issue Date
2019-07
Type
Dissertation
Language
en
Keywords
Bacteria , Clay mineralogy , Climate change , Crust strength , Erosion , Infiltration rate , Rainfall simulation , Runoff , Soil crusting , Soil organisms
Alternative Title
Abstract
Soil crusting is a major land degradation driver in arid regions because of variations in rainfall
characteristics and inherently poor soils. This study aimed to determine the effects of rainfall
intensity and slope steepness on soil sealing and crusting and the effects on infiltration rate,
runoff, erosion and microbial composition in selected soils of different texture and mineralogy.
A rotating disc rainfall simulator was used to apply rainfall varying in intensity (RI) (45 mm/h,
70 mm/h and 100 mm/h) to six soils (K1, K2, K3, S1, S2 & S3) at two slopes (5° and 8°). The
effects of these two factors on crusting (strength (CS) & thickness (CT)) and subsequent effects
on infiltration (IR), runoff (RO) and erosion (SL) were determined. The number of bacterial
communities was also measured before and after each subsequent treatment. The potential of
these microbes to solubilize phosphorus, fix nitrogen and produce indole acetic acid was
measured. The high clay smectitic soils (S1, S2 & S3) developed the strongest crusts with S2
showing significantly (p <0.05) highest CS of 18.54 Kpa at 45 mm/h intensity and 8° slope.
Soil K3 had the lowest CS (5.4 Kpa) at 100mm/h and 8°. Soils K1, K2 and K3 are non-swelling
sandy loams, with good drainage, hence low crustability. Infiltration rate generally decreased
between 45 mm/h and 70 mm/h and increased again going to 100 mm/h and the effect of slope
was soil dependent. However, the highest IR values, 33.32 mm/h and lowest 7.97 mm/h, were
obtained at 70 mm/h and 5° for soils K3 and S3, respectively. The higher infiltration rate at the highest intensity compared to the medium one can be attributed to reduced sealing due to lower
slaking forces at high energy rainfall. Runoff expectedly showed an opposite trend to that of
IR, being highest at 70 mm/h and 5°. Soil loss increased with increasing intensity and slope
for the low-medium clay kaolinitic soils with K1 being most erodible (468.2 kg/ha) at 100
mm/h and 8°. Soils S3 (1248.13 kg/ha) and S2 (1145.55 kg/ha) were statistically (p <0.05)
similar and the most erodible at 100 mm/h and 70 mm/h, respectively. Nitrogen fixing bacteria
were affected by slope gradient whilst indole acetic acid responded to rainfall intensity. Edaphic factors proved more influential when it came to phosphorus solubilization. The study
showed that high clay smectitic soils are vulnerable to crusting and that the type of clay can be
more influential than the amount. The study also found that soil conditions were the most
influential factor when it came to total number of bacteria and the numbers of phosphate
solubilizing bacteria. On the other hand, no statistically significant changes were observed for
nitrogen fixation and indole acetic acid production. The interactive nature of the factors
involved in crusting suggests that a study of other parameters could provide further
illumination.