Response of soil properties to rangeland use in grassland and savanna biomes of South Africa
A significant portion of grassland and savanna ecosystems is over-utilized by livestock, due to inappropriate rangeland management. South Africa´s rangelands are increasingly threatened by overgrazing, followed by altered grassland composition and loss of vegetation cover in the grassland ecosystem, and by bush encroachment in the savanna ecosystem. Although not all land is overgrazed, there are some parts where signs of degradation can be found. Overgrazing has detrimental effects on soil and vegetation, but these changes can be reversed or prevented by proper rangeland management practices. The causes of and the processes involved in these changes and human interactions with them are poorly understood. Literature has indicated that rangelands can recover if managed accordingly, however scientists still have much to learn about how grazing affects soil properties. Sustainable utilization of the rangeland ecosystem is based on the appropriate application of rangeland management principles that will safeguard long-term productivity and profitability of the production system at the lowest possible risk. The main aim of this study was to investigate how soil chemical, physical and microbiological properties responded to different management systems in a clayey grassland and sandy savanna ecosystem of South Africa. For this purpose we sampled rangeland management systems under communal (continuous grazing), commercial (rotational grazing) and land reform (mixture of grazing systems mentioned) farming. Within each of these systems a grazing gradient was identified with increasing grazing pressure, indicated by indicator grass species for the purpose of rangeland condition assessment. Different grass species exist in the clayey grassland and sandy savanna ecosystems, with Acacia shrub and tree species being dominant in the savanna ecosystem. Rangeland condition ranged from poor, moderate to good grazing conditions. The results revealed that soils in both ecosystems responded differently to increased rangeland degradation. In the grassland ecosystem bare patches and soil crusts lead to a degradation of the soils, whereas in the savanna ecosystem bush encroachment lead to a temporary improvement of the soil quality. As a consequence of management, soil degradation in the piosphere of continuous grazed rangeland of the clayey grassland ecosystem is driven by the deterioration of aggregates and associated SOM losses in the poor and moderate rangeland condition, as well as nutrient losses caused by lower plant cover and litter input in the sacrifice area of the piosphere. Rotational grazed camps, in contrast, showed little evidence of soil degradation, but they exhibited an early deterioration of the aggregate structures nearby the water points. Furthermore, aggregate fractionation is a sensitive indicator for detecting the beginning of soil degradation in this ecosystem. Soil degradation was less pronounced under rotational than under continuous grazing systems. Hence, soil analyses confirm that fences and appropriate grazing periods are needed to manage these rangelands sustainably. In the sandy savanna ecosystem, results also revealed that communal farms were affected negatively by continuous grazing, which exhausted most plant nutrients especially close to the water points, when compared to rotational grazing in commercial farms. In contrast, the communal farms had more plant nutrients than commercial farms when moving away from the water points, which coincided with an increase in Acacia species. Only near the water points, high grazing pressure had overridden the positive effects of Acacia species. Hence, and in contrast to the results from the grassland ecosystem, rangeland degradation in communal farms of the savanna ecosystem improved soil quality due to bush encroachment, but at the cost of palatable grass area. Our data also demonstrated that in both ecosystems a decrease in grazing pressure on a rangeland, such as by commercial farmers practicing rotational grazing, could stimulate microbial activity. There was a positive feedback between microbial mediated nutrient mineralization and plant growth, as all microbial biomass and activity as well as grass cover and biomass were elevated when grazing pressure changed. Results further showed that in the long-term, the sandy soils seem to be more resilient to soil degradation, indicated by less significant differences in all measured parameters between the rotational and continuous grazing systems. In the short-term, however, it were the clayey soils in the grassland ecosystem that showed evidence of resilience, as the resting times in the rotational grazing systems was obviously able to compensate or restore disturbances from high grazing pressure, which was not possible under continuous grazing management.