Critical review of the impacts of grazing intensity on soil organic carbon storage and other soil quality indicators in extensively managed grasslands

Abstract

Livestock grazing intensity (GI) is thought to have a major impact on soil organic carbon (SOC) storage and soil quality indicators in grassland agroecosystems. To critically investigate this, we conducted a global review and meta-analysis of 83 studies of extensive grazing, covering 164 sites across different countries and climatic zones. Unlike previous published reviews we normalized the SOC and total nitrogen (TN) data to a 30 cm depth to be compatible with IPCC guidelines. We also calculated a normalized GI and divided the data into four main groups depending on the regional climate (dry warm, DW; dry cool, DC; moist warm, MW; moist cool, MC). Our results show that taken across all climatic zones and GIs, grazing (below the carrying capacity of the systems) results in a decrease in SOC storage, although its impact on SOC is climate-dependent. When assessed for different regional climates, all GI levels increased SOC stocks under the MW climate (+7.6%) whilst there were reductions under the MC climate (-19%). Under the DW and DC climates, only the low (+5.8%) and low to medium (+16.1%) grazing intensities, respectively, were associated with increased SOC stocks. High GI significantly increased SOC for C4-dominated grassland compared to C3-dominated grassland and C3-C4 mixed grasslands. It was also associated with significant increases in TN and bulk density but had no effect on soil pH. To protect grassland soils from degradation, we recommend that GI and management practices should be optimized according to climate region and grassland type (C3, C4 or C3-C4 mixed).

Keywords: Grassland; Grazing; Grazing intensity; Soil organic carbon; Total nitrogen.

Fig. 1

Map of mean Net Primary Production (NPP) in mg C ha⁻¹ y⁻¹ derived from the mean annual temperature and mean annual precipitation using the Miami model with the locations of experimental sites considered in this paper.

Fig. 2

The initial SOC (mg ha⁻¹) and NPP values (mg mg C ha⁻¹ y⁻¹) for different climatic zones (DC = dry cool, DW = dry warm, MC = moist cool, MW = moist warm), 0–30 cm depth.

Fig. 3

Comparison of published grazing intensities (high, medium and low) compared with those derived from NPP and number of animals. The symbols are showing the median (⊗) and the mean (●), with 95% confidence interval as a bar and individual site values as grey dots.

Fig. 4

Regression of un-grazed NPP (mg C ha ⁻¹ y⁻¹) to grazing intensity calculated from NPP and number of animal units (values greater than zero are overgrazed) for each climatic zone (DC = dry cool, DW = dry warm, MC = moist cool, MW = moist warm).

Fig. 5

Impacts of grazing on soil organic carbon (SOC) stocks (0–30 cm soil depth) under the different climatic zones. (DC = dry cool, DW = dry warm, MC = moist cool, MW = moist warm). Grazing intensities are described as percentage of the annual net primary production (over (grazed) ≥ 100%, high = 100–66%, medium = 66–33%, low ≤ 33%). Impact in the natural logarithm of the ratio of un-grazed SOC to grazed SOC. ⊕ is mean, box shows 95% confidence and median as a bar.

Fig. 6

Principle component analysis for four climatic zones using Ln (response ratio soil organic carbon), Initial soil organic carbon to 30 cm, grazing intensity on a scale of 0–1 and net primary productivity (NPP) as variables.

Fig. 7

Radar plot of the contribution of explanatory variables: initial soil organic carbon to 30 cm, grazing intensity on a scale of 0–1 and net primary productivity (NPP) and response variable Ln (response ratio soil organic carbon) (ln(RR)) to four principle components for four climatic zones.