No Detectable Broad-Scale Effect of Livestock Grazing on Soil Blue-Carbon Stock in Salt Marshes

Abstract

Grassland carbon capturing and storage (CCS) is thought to benefit from regulation of grazing. The impact is likely to depend on livestock density. Yet, few studies have tested this principle or evaluated the consistency of grazer-carbon relationships across multiple sites. We sampled four intertidal zones across 22 salt marshes along a 650 km stretch of coast in the UK to examine the impact of livestock density on globally important saltmarsh "blue carbon" stocks. Although there were marked impacts of grazing pressure on above ground vegetation composition, structure and biomass, there was no detectable relationship between grazing intensity and soil organic carbon, irrespective of tidal zone in the marsh or soil depth-layer analyzed. A substantial spatial variation in soil carbon was instead explained by contextual environmental variables. There was evidence that compensatory responses by vegetation, such as increased root growth, countered carbon loss from grazing impacts. Our work suggests that grazing effects on carbon stocks are minimal on broader scales in comparison with the influence of environmental context. The benefits of grazing management to carbon stores are likely to be highly context dependent.

Consumer regulation of the carbon cycle in coastal wetland ecosystems

“…In our study, herbivores reduced APC regardless of taxon and functional traits, while their effects on BPC were much more variable. Indeed, positive, negative and no effects of herbivores on belowground carbon processes in coastal wetlands have all been reported in individual site-scale studies [22,34,35]. Therefore, it is important to consider differential responses of aboveground and belowground carbon processes to food web alterations.…”

Livestock grazing promotes ecosystem multifunctionality of a coastal salt marsh

“…Grazing‐induced changes in plants and soil conditions may further impact the structure and activities of soil biota, and consequently, nutrient cycling, and hence C sequestration may be affected (Bardgett & Wardle, 2010; Teague & Barnes, 2017). In general, the effects of grazing on specific ecosystem functions (EFs) may be positive, negative or neutral (Eldridge et al., 2016; Harvey et al., 2019; Hautier et al., 2015). How grazing affects many ecosystem functions as a whole is still an open question probably because most studies emphasized certain ecosystem components or functions but ignored others (Giling et al., 2019).…”

“…Livestock grazing in coastal salt marshes began in prehistoric ages and has since been practiced around the world including China (Doody, 2008; Gedan et al., 2009; Greenberg et al., 2014). Therefore, understanding how EFs and EMF are altered in the pasture lands of salt marshes is vital for sustainable land use and ecosystem management (Harvey et al., 2019; Moore et al., 2020). Many studies have investigated the responses of plant communities, soil biota and nutrient cycling to grazing in salt marshes, and found that livestock grazing decreases the standing biomass of plants but has varying effects on biodiversity and specific EFs (Davidson et al., 2017; He & Silliman, 2016; Mueller et al., 2017; Pagès et al., 2018).…”

Trade-offs between multiple ecosystem services in UK and US salt marshes

“…The numerous studies investigating livestock grazing have not previously been quantitatively synthesised, making it difficult to identify consistent trends, or recognise whether the over-weighting of European studies is unduly influencing the general picture that is emerging of livestock impacts. As an example, mechanistic studies of grazing in European marshes indicate that grazing does not trade-off against climate regulation in salt marshes Harvey et al 2019), but this pattern may not apply in other regions. In terms of conservation grazing, there is considerable research investigating the impact of grazing on vegetation (e.g.…”

“…Where trade-offs between grazing and other ES are explicitly investigated, these generally concern trade-offs with carbon storage Harvey et al 2019). Investigations into interactions between saltmarsh grazing and more than one other ES are extremely rare (but see Ford 2012).…”