Abstract
AbstractShrub encroachment is a well‐documented phenomenon affecting many of the world's drylands. The alteration of vegetation structure and species composition can lead to changes in local microclimate and soil properties which in turn affect carbon cycling. The objectives of this paper were to quantify differences in air temperatures, soil carbon, nitrogen, and CO2 efflux under trees (Vachellia erioloba), shrubs (Grewia flava), and annual and perennial grasses (Schmidtia kalahariensis and Eragrostis lehmanniana) collected over three seasons at a site in Kgalagadi District, Botswana, in order to determine the vegetation‐soil feedback mechanism affecting the carbon cycle. Air temperatures were logged continuously, and soil CO2 efflux was determined throughout the day and evening using closed respiration chambers and an infrared gas analyser. There were significant differences in soil carbon, total nitrogen, CO2 efflux, light, and temperatures beneath the canopies of trees, shrubs, and grasses. Daytime air temperatures beneath shrubs and trees were cooler compared with grass sites, particularly in summer months. Night‐time air temperatures under shrubs and trees were, however, warmer than at the grass sites. There was also significantly more soil carbon, nitrogen, and CO2 efflux under shrubs and trees compared with grasses. Although the differences observed in soils and microclimate may reinforce the competitive dominance of shrubs and present challenges to strategies designed to manage encroachment, they should not be viewed as entirely negative. Our findings highlight some of the dichotomies and challenges to be addressed before interventions aiming to bring about more sustainable land management can be implemented.
Highlights
The impact of livestock on soils and vegetation has been well documented in the Kalahari (e.g., Bhattachan, D'Odorico, Dintwe, Okin, & Collins, 2014) and elsewhere (e.g., Linstädter et al, 2014)
Air temperatures were logged continuously, and soil CO2 efflux was determined throughout the day and evening using closed respiration chambers and an infrared gas analyser
Our results demonstrate that shrub encroachment is another driver that will lead to significantly greater CO2 efflux from dryland sand soils
Summary
The impact of livestock on soils and vegetation has been well documented in the Kalahari (e.g., Bhattachan, D'Odorico, Dintwe, Okin, & Collins, 2014) and elsewhere (e.g., Linstädter et al, 2014). Biocrust microorganisms have been identified as common rhizosphere and endophyte species in overlying grasses and shrubs (Elliott, Thomas, Hoon, & Sen, 2014; Steven, Gallegos‐Graves, & Kuske, 2014) suggesting functional interdependence and an important beneficial microbial refugia role for biocrusts. They are, damaged by livestock trampling, and a reduction in the area covered by biocrusts is an immediate consequence of grazing disturbance (Dougill & Thomas, 2004). The negative impacts of short periods of intense grazing are temporary, and damaged biocrusts can recover after a period of respite (Thomas, Elliott, Griffith, & Mairs, 2015)
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