Soil sealing and crusting effects on infiltration, erosion and microbial composition under different rainfall intesities and slope conditions

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.