Herbage biomass and its relationship to soil carbon under long-term grazing in northern temperate grasslands

Abstract

While northern temperate grasslands are important for supporting beef production, it remains unclear how grassland above- and belowground biomass responds to long-term cattle grazing. Here, we use a comprehensive dataset from 73 grasslands distributed across a broad agro-climatic gradient to quantify grassland shoot, litter, and shallow (top 30 cm) root biomass in areas with and without grazing. Additionally, we relate biomass to soil carbon (C) concentrations. Forb biomass was greater (p < 0.05) in grazed areas, particularly those receiving more rainfall. In contrast, grass and total aboveground herbage biomass did not differ with grazing (total: 2320 kg ha−1 for grazed vs. 2210 kg ha−1 for non-grazed; p > 0.05). Forb crude protein concentrations were lower (p < 0.05) in grazed communities compared with those that were non-grazed. Grasslands subjected to grazing had 56% less litter mass. Root biomass down to 30 cm remained similar between areas with (9090 kg ha−1) and without (7130 kg ha−1) grazing (p > 0.05). Surface mineral soil C concentrations were positively related to peak grassland biomass, particularly total (above + belowground) biomass, and with increasing forb biomass in grazed areas. Finally, total aboveground shoot biomass and soil C concentrations in the top 15 cm of soil were both positively related to the proportion of introduced plant diversity in grazed and non-grazed grasslands. Overall, cattle grazing at moderate stocking rates had minimal impact on peak grassland biomass, including above- and belowground, and a positive contribution exists from introduced plant species to maintaining herbage productivity and soil C.

Grazing alters the sensitivity of plant productivity to precipitation in northern temperate grasslands

“…Moreover, it highlights the need for more in-depth studies into the specific role of plant compositional changes under grazing for their ability to regulate ecosystem stability. Findings from two studies conducted at the same sites as ours showed that species richness increased with ongoing grazing (Lyseng et al, 2018;Bork et al, 2019), but we found no evidence of association between species richness and ANPP sensitivity to precipitation. The result is unexpected given the fact that species richness is considered an important determinant of changes in ecosystem productivity and stability (Isbell et al, 2009;Hallett et al, 2014;Tilman et al, 2014).…”

“…We found that an increased contribution of forb biomass to total ANPP at the expense of grasses led to an increased ANPP sensitivity to precipitation, and this occurred specifically in grazed rather than non‐grazed grasslands (Figure 3). Previous long‐term grazing studies report that grazing has a deterministic impact on plant functional group composition (Verón & Paruelo, 2010; Gaitán et al, 2014; Irisarri et al, 2016) and their contribution to total ANPP (Bork et al, 2012; Bork et al, 2019), which includes a decline in grasses under grazing. Together these results strongly suggest that any compositional shifts induced by grazing, even among coarse plant functional groups, are likely to account for differences in ANPP sensitivity among ecosystems with comparable mean annual precipitation (Verón & Paruelo, 2010; Irisarri et al, 2016).…”

“…We compiled species richness data from grazed and non‐grazed locations at each site from two published studies that examined changes in species composition as a function of grazing. We matched the data to the sites included in this study (see Lyseng et al, 2018; Bork et al, 2019 for a description of data collection). We used average species richness to capture the long‐term cumulative effects of grazing on species richness for each plant community.…”

“…Grazing is an important disturbance in grasslands that can change vegetation structure and composition directly through impacts on plants (Milchunas & Lauenroth, 1993; Filazzola et al, 2020) and indirectly through alteration of edaphic conditions as a result of trampling and litter removal (Willms et al, 1986; Deutsch et al, 2010). In temperate grasslands, grazing increases the contribution of forb biomass to total ANPP at the expense of grasses (Bork et al, 2012; Bork et al, 2019). Such shifts in the relative contribution of functional group biomass to total ANPP can influence ANPP sensitivity to precipitation (Verón & Paruelo, 2010; Gaitán et al, 2014; Irisarri et al, 2016) as forbs and grasses exhibit differential sensitivity to water availability (Felton et al, 2019).…”

Multi‐year drought alters plant species composition more than productivity across northern temperate grasslands

“…For example, we observed the following patterns of response to drought in this experiment: immediate changes in soil moisture content, followed by shifts in species composition (though not richness or evenness), but inconsistent effects on ANPP. This may chiefly be through variation in the abundance of graminoids, which contribute most to the stability of productivity to precipitation in the region (Batbaatar et al, 2021;Bork et al, 2019). The positive link between the effects of reduced precipitation on species composition and evenness further suggests that changes in the abundance of graminoids may have led to differences in species composition between the ambient and reduced precipitation treatments.…”