The effects of land management (grazing intensity) vs. the effects of topography, soil properties, vegetation type, and climate on soil carbon concentration in Southern Patagonia
Introduction
Temperate grasslands are one of the largest biomes in the World occupying 9 million km2 which represent 8% of Earth’s terrestrial ice free surface (White et al., 2000). Disturbance to surface soils by activities such as livestock grazing can influence arid-land ecosystems in many ways including through the alteration of vegetation cover, soil physical properties, microbial communities, carbon cycling, nitrogen fixation and hydrologic properties (Schlesinger et al., 1990). In this context, rangeland livestock production using sustainable management practices is essential to support increasing human populations and lifestyles. Despite the extension and economic importance of rangelands in Santa Cruz there has been relatively little scientific focus on soil properties related to grazing in these ecosystems.
Soil carbon in grasslands and rangelands provides a range of important ecosystem services and functions, such as supporting the capacity of the land to sustain plant and animal productivity, maintaining and/or enhancing water and air quality, supporting human health and habitation (Karlen et al., 1997). The extension of rangelands and the impact of livestock grazing on ecosystem properties, and the need for sustainable grazing management to meet the demand of an increasing human population has been reported (Havstada et al., 2007, Kremen, 2005). However, this potential depends on how rangelands are managed for domestic animal grazing (Doran and Jones, 1996).
Over the last 70 years, degradation of the steppe (desertification) has occurred due mainly to an overestimation of the carrying capacity of these rangelands, inadequate distribution of animals in very large and heterogeneous paddocks, and year-long continuous grazing (Golluscio et al., 1998). Grazing impacts have led to substantial ecosystem modification, in particular an increase in bare ground and changes away from the original floristic composition (Bisigato and Bertiller, 1997, Peri et al., 2013), consistent with reviews that demonstrate that overgrazing may reduce aboveground Net Primary Production (ANPP) and change floristic composition in rangelands (Milchunas and Lauenroth, 1993, Oesterheld et al., 1999).
In response to the history of unsustainable large management in Patagonia, key stakeholders in the region have developed a certification scheme to promote sustainable land management practices (Borrelli et al., 2013). For certification and auditing purposes the standard developed a range of indicators to assess rangeland condition (see Borrelli et al., 2013 page 62). All of these indicators are qualitative and therefore open to interpretation. There is a definite need to develop quantitative measures of rangeland condition that can reflect the impacts of unsustainable grazing and land management practices in a way that is not subjective. In this context soil quality, and especially soil organic carbon (SOC), has been proposed as an integrative indicator of environmental quality, food security and economic viability (Monreal et al., 1998, Lal, 1999).
Grazing intensity on extensively managed grasslands may affect ecosystem C stocks (Piñeiro et al., 2010). Peri (2011) reported that C stocks in grasslands decreased from 130 Mg C ha−1 under low grazing intensity (0.10 ewe ha−1 yr−1) to 50 Mg C ha−1 at a heavy stocking rate (0.70 ewe ha−1 yr−1) mainly due to a decline in plant cover and loss of the organic layer of the soil and because of increased extension of bare areas and as a consequence of soil erosion by strong winds. Also, it has been documented in arid zones the importance of measuring grazing together with other environmental variables as drivers of soil C (Rabbi et al., 2015). In Patagonia, the influence of grazing on soil C interacting with environmental factors remains poorly understood, despite the vast area and the economic importance of grazing.
The objectives in this study where 1) to assess the potential for soil C concentration in topsoil to be used as an indicator of rangeland condition and therefore sustainable land management, and b) assess the extent to which environmental variables (vegetation type, topography, climate, soil chemistry) other than stocking rate influenced topsoil C concentration with the idea that any environmental variable that strongly affected soil C might be a candidate for use as a covariate that would facilitate efforts to relate soil C to rangeland condition.
Section snippets
Materials and methods
In Santa Cruz province (Argentinian Southern Patagonia), there are three main ecosystem types: Nothofagus forest, steppe and wetlands (mallines) which are found interspersed amongst the steppe vegetation. The native Nothofagus forest and woodland cover a narrow (100 km wide) but long (1000 km) strip of land. Southern beeches, lenga (Nothofagus pumilio), ñire (Nothofagus antarctica) and guindo (Nothofagus betuloides) are the dominant species covering 535,889 ha. The steppe ecosystem, mainly
Results
Our analyses indicate that SOC concentration is heavily dependent on vegetation type (Fig. 2). The partial least squares regression produced a model that was an effective predictor of soil C in the regional data set presented here (Fig. 3, R2 = 0.75). In this analysis stocking rate was only a moderately important predictor of soil C (see Row 1094 of 1150 of Appendix S1). In these Patagonian rangelands, climate (i.e. Mean Annual Climatic Water Deficit) and simple ecosystem classifications (i.e.
Discussion
In the present study soil carbon concentration was mainly a function of climate, vegetation type and soil properties. These types of data are readily available and, as we have demonstrated previously (Ladd et al., 2013), the prediction and mapping of soil carbon is possible across large geographical regions using readily available data and without the need for large amounts of field work. However contrary to our initial hypothesis the effects of land management (grazing intensity) on soil
Acknowledgements
This research was supported by ‘Operationalisation of ecosystem services and natural capital: From concepts to real-world applications’ (OpenNESS) project financed under the European Commission’s Seventh Framework Programme (Project number 308428).
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