Environmental change and soil organic carbon storage in a dust emission hotspot, Free State, South Africa
Drylands farming areas are subject to land cover or land use change, climate change, and a variety of land management practices which may contribute to dust emissions and soil loss, a common symptom of land degradation. The goal of this study was to establish how the shift in land cover or land use change affects the occurrence of dust and determine the climate constraints and mitigations to crop production with focus on the dustiest parts of South Africa in the Free State Moreover, the study wished to establish land management strategies suitable for enhancing the storage of soil organic carbon and associated ecosystem services. To investigate how the shift in land cover or land use affects the occurrence of dust from 2006- 2016, the study firstly identified the spatial and temporal changes of land cover (LC) using the 2000 and 2013-2014 land cover maps derived from Agricultural Research Council. The results showed that grassland is still dominating in 2013-2014 but has decreased by 22 % from 2000. Cultivated land on the other hand has increased by 26 % from 2000 to 2013-2014. Some of the changes seen in recent data might also be the result of the associated data products. The change is however continuous and significant as major LC changes occurred over long-time scales. Cultivated land and grassland are dominant land cover and cultivated land is substantially being used to meet the economic demand (international maize price). Moderate Resolution Imaging Spectroradiometer Enhanced Vegetation Index (MODIS EVI) was used to assess vegetation cover for the site of the start of dust. MODIS EVI indicated that vegetation plays a significant role during the occurrence of atmospheric dust. Sparse vegetation (EVI ranging from 0 – 0.18) is directly proportional to dust production. Dust events were generally observed in cultivated land especially in 2015 and 2016. Moreover, findings showed that vegetation is not solely a factor that affects dust production; rainfall, temperature and wind speed also contribute. Crop and land cover data obtained from Department of Agriculture, Forestry and Fisheries (DAFF) identified the dominating crop and land cover around each dust source as fallow land, maize fields, and pasture. For instance, dust source August 29th 2006 had 11533 ha of fallow land and 10795 ha of maize, and dust source September 15th 2016 had 7737 ha of fallow land and 4599 ha of pasture which suggest that dust does not exclusively depend on the amount of vegetation but also on the type of vegetation. To achieve the second aim, the study used data from Climate Hazard Infrared Precipitation with Stations (CHIRPS), Agricultural Research Council (ARC) and Standardised Precipitation 2 Evapotranspiration Index (SPEI) to determine the variation of rainfall in Free State. SPEI data showed that drought is becoming prominent and severe in Free State (2005, 2012, 2014 and 2016). ARC data showed a slight increase (357-408 mm/year) from two stations and a slight decrease from the other two stations in rainfall from 2005- 2016 and CHIRPS data showed a slight decrease (458-404 mm/year) from all the stations during the same period. Both CHIRPS and ARC data showed that there is a seasonal shift on the onset of rainfall; rainfall has shifted to late summer by approximately 62 days on average from 1981-2019 in Free State. Rainfall in Free State is becoming late (e.g., in 2011 rainfall adequate to make the soil moist was received in November) and number of rain events are decreasing (e.g., September 1981-2019 had 7 rainfall events greater or equal to 15 mm/day, 19 events in October, 52 events in November and 33 in December in Bultfontein weather station). This study used temperature data from ARC weather stations to determine how temperature and frost variation affect crop production in Free State. Findings indicated that temperatures have increased by ~ 1- 2 ○C in Free State and frost has decreased, and its onset has shifted by ~ 11 days. Frost occurred in Julian day of the year (DOY) 144 in 2005 and in 2016 there was no frost for the whole year in Bultfontein. The onset of frost is shifting to May and resulting to frost period decreased over the years. Decrease in frost is an advantage for crops that are planted in late summer, however increased temperatures are likely to result in crop failure, especially if rainfall is also low. Furthermore, the relationship between wind speed and dust occurrence was determined using the ARC and ERA5 datasets. The results showed that some years were characterised by high winds and some by low winds but generally there is no constant trend. The ARC maximum wind speed was 10 - 15 m/s and ERA5 maximum wind speed was 10 m/s. Both datasets showed that high wind speed occurs from August- November and many dust events are observed in these months. Additionally, high winds are mostly observed to occur on surfaces with sparse vegetation and in the absence of wind breaks which further promotes an increase in dust. Thus, it is important for farmers to leave crop residues after harvesting and practice moderate grazing. To achieve the third aim, the study further established land management strategies suitable for enhancing the storage of soil organic carbon. Soils sampled in 2019 from different sites in Free State were used to determine the amount of carbon and nitrogen. It is apparent that grassland can store more carbon especially in the Lixisols which appeared to have stored an average carbon of 1.23 g and maximum of 1.86 g from 0-2 cm to 20-30 cm depths. Moreover, cultivated land can store less carbon especially in the Arenosols which stored an average of 0.18 g and 3 maximum of 0.22 g from 0-2 cm to 30-45 cm depths. Findings suggest that grassland can store more carbon than cultivated land due to minimal disturbance of the land when compared to cultivated sites. Moreover, the study demonstrated that soil texture and grain size distribution play an additional role in carbon storage with higher loads linked to higher silt content (Lixisols) and less carbon being stored in soils with high sand content (Arenosols). More carbon was observed to be stored in soils with moderate sand content (Lixisols = 40 % of sand) and high silt content (54 %) and less carbon was stored in soils with high sand content (Arenosols = 80 % of sand) and moderate silt content (18 %). More carbon was shown to be stored in the top layers of the soil 5-10 cm and was observed to decrease with increasing depth. DAYCENT carbon model was further used to simulate soil carbon in different land management scenarios to come up with suitable land management options for storing more carbon in future. Results illustrated that native grassland can substantially store more carbon than cultivated land, however moderate grazing should be applied. This would allow for a reduction of future dust emissions and increase ecosystem services in the cultivated lands of the Free State. Overall, the study demonstrated that combining the investigation of LCLU, climate change and land management was important to better understand land degradation and identified possible mitigation measures.
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