Understanding the difference between extensive as compared to intensive land management is central to managing soil fertility on grasslands for habitat and biodiversity (see Sidebar).
Agricultural intensification over the 20th century, in conjunction with atmospheric nitrogen (N) and phosphorous (P) deposition has led to grassland soil nutrient enrichment. The impact on grassland species richness (SR) has generally been negative (Stevens et al. 2010, Lyons et al. 2023, Soons et al. 2017). This is compounded by species loss due to landscape fragmentation, decrease in seed distribution associated with livestock transhumance (and thus population isolation), and competition with fast-growing introduced species (Walker et al. 2004, Soons et al. 2017), among other factors.
How nutrient enrichment influences plant species dynamics
The general observation regarding soil fertility for species-rich grasslands is that high nutrient levels preference generalist (typically “increaser”) over specialist (typically “decreaser”) species because generalists respond most rapidly to elevated nutrients, a degree of disturbance, and resource abundance (McKinney and Lockwood 1999). Common Eurasian grasses introduced to Puget Prairies including Kentucky bluegrass (Poa pratensis), Colonial bentgrass (Agrostis stolonifera), Rattail fescue (Vulpia myuros), Quackgrass (Elymus repens), and Perennial ryegrass (Lolium perenne) fall into this category. In particular, species richness can be supported by avoiding simultaneous addition or enrichment of multiple nutrients, such as N + P, or N + P + potassium (K), as multiple elevated nutrients negatively affect native plant biodiversity the most (Scotton et al. 2024).
Sidebar. Extensive versus Intensive Grazing Systems
Many of the world’s grasslands evolved to some degree in conjunction with extensive grazing, along with other disturbances such as fire. In extensive grazing systems feed for livestock is sourced from natural grasslands, shrublands and woodlands, in contrast to intensive systems where food is sourced from seeded and fertilized pastures (FAO 1991).
Sidebar. The Role of Soil Biological Communities in Grassland Restoration
Soil biological communities and dynamics are also impacted by intensive versus extensive management, with higher fungi:bacteria ratios in the latter. Arbuscular mycorrhizal (AM) fungi may be a critical “missing link” in successful establishment of higher successional grassland species (Kozial et al 201&). Grazing land managers can consider options to culture and reintroduce AM from sites references grassland sites known to extant AM populations and a higher fungi:bacteria ratio (Kozial et al. 2022). Robust AM fungal populations are likely essential to efficient below-ground distribution of essential nutrients (e.g. N, P, and water), with more efficient transfers supporting late-successional “stress tolerator” species; whereas nutrient enrichment depresses growth of often rare stress tolerator species and preferences introduced generalist and colonizer species (including fast-growing perennials and weedy plants; Bardgett et al. 2007). Improved outcomes for increasing plant biodiversity in grazed systems thus calls for lower nutrient levels (in particular soluble N). Resulting enhanced mycorrhizal networks have been shown to support ‘herb’ or flowering plant (forb) species and suppress fast-growing grasses (Grime et al. 1987, Van der Heijden et al. 2004, Chomel et al. 2022).