Carbon loss by water erosion in drylands: implications from a study of vegetation change in the south‐west USA

Abstract

Soil organic carbon (SOC) is an important component of the global carbon cycle, yet is rarely quantified adequately in terms of its spatial variability resulting from losses of SOC due to erosion by water. Furthermore, in drylands, little is known about the effect of widespread vegetation change on changes in SOC stores and the potential for water erosion to redistribute SOC around the landscape especially during high-magnitude runoff events (flash floods). This study assesses the change in SOC stores across a shrub-encroachment gradient in the Chihuahuan Desert of the south west USA. A robust estimate of SOC storage in surface soils is presented, indicating that more SOC is stored beneath vegetation than in bare soil areas. In addition, the change in SOC storage over a shrub-encroachment gradient is shown to be nonlinear and highly variable within each vegetation type. Over the gradient of vegetation change, heterogeneity of SOC increases and newer carbon from C 3 plants becomes dominant. This increase in heterogeneity of SOC is related to an increase in water erosion and SOC loss from intershrub areas, which is self-reinforcing. Shrub-dominated drylands lose more than three times as much SOC as their grass counterparts. The implications of this study are twofold: 1. Quantifying the effects of vegetation change on carbon loss via water erosion and the highly variable effects of land degradation on soil carbon stocks is critical. 2. If landscape scale understanding of carbon loss by water erosion in drylands is required, studies must characterise heterogeneity of ecosystem structure and its effects on ecosystem function across ecotones subject to vegetation change.

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…Greater variation in SOC contents at the woody sites (Table 2) supports the theory that the alteration of hydrological functions associated with woody encroachment results in the occurrence of positive feedback mechanisms [Peters and Herrick, 2001] reinforcing the heterogeneous woody landscape structure into islands of fertility [Schlesinger et al, 1990]. The underlying mechanism is illustrated by event organic C yields showing that woody encroachment results in increased event losses of organic C via fluvial erosion [Barger et al, 2011;Brazier et al, 2013]. These changes may be irreversible and become more widespread unless such dryland landscapes are actively managed to prevent woody species encroachment into grasslands.…”

Dynamics of organic carbon losses by water erosion after biocrust removal

“…Dissolved and sedimentbound OC removed by runoff from the crust patches is delivered to shrub patches, making this a critical process in maintaining plant productivity (Ludwig et al, 1997;Puigdefábregas, 2005). As pointed out by Brazier et al (2014), "bare" intershrub areas act as suppliers of OC to shrub patches through runoff generation and erosion, and play vital roles in soil OC redistribution around the landscape and in increasing soil OC heterogeneity and ecosystem functioning . Hence OC losses presented here on a microplot scale would probably decrease significantly at large hillslope or catchment scales, as a consequence of OC redistribution.…”

Fallow Reduces Soil Losses and Increases Carbon Stock in Caatinga

“…According to the statistical analysis (ρ < 0.10, Freitas et al (2013) and Brazier et al (2014) point to land cover as the main factor to reduce R and ΔS. In line with these results, Trabaquini et al (2015) showed that land degradation in Brazilian savannah (Cerrado) has altered the soil's physical attributes, increasing runoff; bulk density and penetration resistance, whereas air permeability and porosity decreased.…”

“…One evidence of the erosion in H D is the texture of the surface horizon that has 40 g kg -1 more clay than H F. This results from the partial removal of the A horizon in the overgrazed area causing the natric B horizon (richer in clay) to appear at depths of less than 20 cm (Sousa et al, 2012). Furthermore, the smaller area of covered ground (Figure 1) provides less protection against soil erosion (Brazier et al, 2014;El Kateb et al, 2013;Freitas et al, 2013;Fernandes et al, 2015).…”

“…In arid and semi-arid ecosystems, desertification is often associated with overgrazing (Li et al, 2006;Navarro-Hevia et al, 2014;Kalema et al, 2015) because this process affects not only physical, chemical and biological properties of the soil, but also the vegetation composition and coverage (Kurz et al, 2005;Brazier et al, 2014;Palacio et al, 2014). Environmental degradation of dry areas directly affects the social development of the surrounding region due to reduced water availability as well as food and energy production, leading to more severe social vulnerability and socioeconomic conflicts (Döll et al, 2015).…”