Bridging Ecology and Agronomy to Foster Diverse Pastures and Healthy Soils

Abstract

Renovating pastures to increase forage species diversity is a burgeoning practice among producers. Over a century of grassland and small-plot research suggests that increasing plant diversity can lead to improved pasture productivity, resilience, and soil health. However, it remains hard to decipher how these benefits translate to grazed production systems given the limited experimentation in realistic grazing systems. There is a disconnect between ecological and agronomic research regarding what qualifies as a “diverse” grassland or pasture. This review aims to examine the current state of research regarding plant diversity and its potential benefits for soil health in pasture systems, and outlines how we can improve our understanding and implementation of this practice in production systems.

1. Introduction

Globally, over three-quarters of agricultural lands are used for livestock production [1], making the management of these ecosystems critical to the health of our planet. Grazed pastures have replaced a substantial proportion of native prairies, grasslands, and forest habitats across the world, with consequences for ecosystem function and biodiversity [2]. Despite strong evidence that plant diversity in grasslands has a wide variety of benefits, most pastures are dominated by only a few species [3,4]. Much of the research to date on plant diversity and soil health has been conducted on ungrazed grasslands. This makes it difficult to leverage lessons from grassland ecology to benefit pasture soils and expand our agronomic toolbox. The aim of this review is to discuss studies of grassland and pasture diversity that measure aspects of soil health, identify key findings and knowledge gaps, and provide guidance on how to restore pasture diversity. While the relationship between plant diversity and aboveground productivity is also of interest to agronomists, this topic has been previously reviewed [5,6,7] and is beyond the scope of this article.

2. Integrating Grassland Ecology and Pasture Agronomy

Grassland ecology and ecosystem science provide a great deal of knowledge relevant to pasture agronomy, yet a schism exists between these fields. This disconnect stems from an inherent difference in the goals of applied and basic research and the metrics used to draw conclusions on the efficacy of plant diversification as a management practice.

The aim of many grassland ecology studies is to elucidate interactions among organisms and the environment by identifying the specific mechanisms through which plant diversity influences ecosystem function. Grassland ecology studies of herbaceous plant diversity have measured changes to ‘soil health’ properties including soil carbon storage, soil moisture retention, and arbuscular mycorrhizal fungi colonization. These experiments are often well-replicated (e.g., using small plots in a randomized block design) and are conducted over many years (Table 1). The largest of these experiments (the Jena Experiment) manipulated both species composition, richness, and functional diversity in a full factorial design with as many as 60 species in small, randomized plots [8]. Other similar experiments have been conducted around the world and, together, have resulted in an abundance of published research studying above- and belowground biotic and abiotic responses to plant diversity [8,9,10,11,12,13,14,15,16,17,18,19,20,21].

Governmental organizations and agricultural producers alike have been increasingly interested in the potential benefits of increasing biodiversity in modern agriculture [22]. Many of those potential benefits are drawn from conclusions of grassland diversity experiments which were extrapolated to agricultural systems, yet the efficacy of forage diversification for improving soil health under grazing is less clear. Livestock grazing significantly impacts ecosystem function, soil health, and plant species composition and diversity [23,24,25,26,27,28]. To add to the complexity, grazing management is not a monolith and varies greatly across farms. Grazing intensity (i.e., stocking rates and pasture rest periods) has the potential to influence plant diversity and soil health in complex ways [25,29,30]. Consequently, it is necessary to exercise caution when considering if and how grassland diversity research can be leveraged to improve soil health in grazed pasture systems.

Table 1. Agronomy and grassland ecology field experiments that manipulated seeded plant diversity and associated changes in soil properties. Agronomy experiments are defined by their focus on forage species and the inclusion of grazing. Soil changes in response to elevated plant diversity are indicated as follows: ↑ = increase or improvement; ↓ = decrease; - = no change; empty fields = data not collected.

	Experimental Details						Soil Parameters
Agronomy	Management	Length (Years)	Max Seeded Species	Plant Families	Sample Size	Plot Size (ha)	Carbon	Nitrogen	Microbial
Skinner & Dell 2016 [31]	Rotational Grazing	10	5	3	4	0.90	↑
Bonin et al. 2014 [32]	Rotational Grazing	5	10	3	4	0.35	-
Skinner et al. 2006 [33]	Rotational Grazing	4	11	3	2	0.40	↓	-
Grassland Ecology
Roscher et al. 2004, Weisser et al. 2017 [5,25] (JENA)	Clipped Annually	15 +	60	15	~16	0.04 *	↑	↑	↑
Tilman et al. 2001, 2006 [22,24] (Cedar Creek) 1	Burned Annually	28 +	18	4–5	25 +	0.0081	↑	↑	↑
Van Ruijven & Berendse 2003, Cong et al. 2014 [23,30] (Netherlands)	Clipped Annually	22 +	8	5	6	0.0001	↑	↑
Hector et al. 1999, Stephan et al. 2000 [24,31] (BIODEPTH)	Clipped Annually	3	14	7	2	0.0004			↑

Additional Publications: ¹ Fornara and Tilman 2008 [13], Mueller et al. 2013 [19], Steinauer et al. 2015 [15]. ⁺ Ongoing experiments. * plot size reduced to 0.0033 in 2010.

Table 1. Agronomy and grassland ecology field experiments that manipulated seeded plant diversity and associated changes in soil properties. Agronomy experiments are defined by their focus on forage species and the inclusion of grazing. Soil changes in response to elevated plant diversity are indicated as follows: ↑ = increase or improvement; ↓ = decrease; - = no change; empty fields = data not collected.

	Experimental Details						Soil Parameters
Agronomy	Management	Length (Years)	Max Seeded Species	Plant Families	Sample Size	Plot Size (ha)	Carbon	Nitrogen	Microbial
Skinner & Dell 2016 [31]	Rotational Grazing	10	5	3	4	0.90	↑
Bonin et al. 2014 [32]	Rotational Grazing	5	10	3	4	0.35	-
Skinner et al. 2006 [33]	Rotational Grazing	4	11	3	2	0.40	↓	-
Grassland Ecology
Roscher et al. 2004, Weisser et al. 2017 [5,25] (JENA)	Clipped Annually	15 +	60	15	~16	0.04 *	↑	↑	↑
Tilman et al. 2001, 2006 [22,24] (Cedar Creek) 1	Burned Annually	28 +	18	4–5	25 +	0.0081	↑	↑	↑
Van Ruijven & Berendse 2003, Cong et al. 2014 [23,30] (Netherlands)	Clipped Annually	22 +	8	5	6	0.0001	↑	↑
Hector et al. 1999, Stephan et al. 2000 [24,31] (BIODEPTH)	Clipped Annually	3	14	7	2	0.0004			↑

Additional Publications: ¹ Fornara and Tilman 2008 [13], Mueller et al. 2013 [19], Steinauer et al. 2015 [15]. ⁺ Ongoing experiments. * plot size reduced to 0.0033 in 2010.

There are relatively few studies examining the influence of ‘diverse’ perennial forage plantings in grazed agroecosystems (Table 1). Only three forage diversity studies could be found in our literature review that include grazing by livestock and measuring soil health metrics (namely, soil carbon) [31,32,33]. Taken together, the results from these experiments are somewhat inconclusive. The complicating factors of experimental design limitations, grazing intensity, weather/climate, and land-use history convolute the limited soil health conclusions. The authors generally recommend tailored and conscientious use of the practice. They advised to not simply renovate pastures for diversity, per se, but to maximize a pasture’s flexibility by choosing each species carefully for each individual system [7,33,34]. Applied research which examines forage diversification on working farms under different management and climates is needed to confirm efficacy as a soil health practice [7,35,36]. Most agronomic research on pasture diversity does not consider soil health and focuses on fulfilling producer goals and needs, namely by measuring the biomass yield and/or nutritive factors of different species mixtures [34,37,38,39]. Whereas, many of the species in the diverse mixes of grassland ecology studies are not appropriate for animal production systems as they (1) have poor retention or regrowth following grazing, (2) are not nutritious and/or palatable to grazing animals, or (3) have seeds that are not accessible to producers [40]. These differences in research aims contribute to the disconnect between basic diversity research and the application of the methodology in production systems.

3. Assessing Biodiversity

As scientists, our understanding, conceptualization, and quantification of diversity continues to evolve over time [41]. For the purposes of this review, we defined diversity broadly as the variety of life forms present in an ecosystem. Ideally, assessments of diversity should involve quantitative measurement of at least three components: richness, evenness, and disparity [41]. Richness is the number of different kinds of individuals, usually at the level of species in pasture research. Species richness is usually measured in plant diversity research, but alone it is inadequate. Evenness is how similar individuals are with regard to their abundance in the community, and can be quantified independently (e.g., Pielou’s evenness [42,43]) or combined with richness to inform diversity metrics such as the commonly used Shannon diversity index [44,45]. There is no agreement on a universal metric for evenness, and the ideal index is dependent upon the research aims [41].

In this context, disparity refers to how distinct plant species are relative to one another with respect to their physiology, morphology, and role in the ecosystem. Disparity is closely related to concepts of functional diversity [46]. It is pertinent to consider how individual plants vary in their response to environmental forces (e.g., drought and cold tolerance) and function in the ecosystem (e.g., symbiotic nitrogen fixation, rooting depth). Only considering richness and evenness implicitly assumes that different species of plants have nothing in common and are completely distinct from one another [41]. Disparity is an important quality of diverse production ecosystems that is challenging to appropriately quantify [47]. Taxonomy does not necessarily correlate with plant functional traits, but it is an accessible metric. Indeed, many farms that practice annual crop rotation, do so by rotating individual production fields between three or more plant families to “reduce the incidence of foliar… and soil-borne diseases” [48]. Even individuals within the same species can vary dramatically in function and form, and that variation can increase the functional diversity of plant communities and enhance productivity [49]. Both grassland ecology and pasture agronomy experiments, have aimed to manipulate functional diversity by grouping species into a few functional groups or guilds [8,50,51]. In grasslands and pastures, plant species are often categorized simply into grasses, legumes, and non-leguminous forbs. However, an analysis of the ecological and morphological traits of prairie species suggests eight functional guilds: warm-season grasses (C4 photosynthesis), cool-season grasses and sedges (C3 photosynthesis), legumes, annual and biennial forbs, ephemeral spring forbs, summer/fall forbs, and woody shrubs [52].

How biodiversity is measured, manipulated, and discussed contributes to the division between ecological and agronomic diversity research. For instance, there is a substantial difference in what qualifies as “high” diversity (Table 1). Agronomy studies tend to have a lower number of species from a few plant families. In contrast, grassland diversity experiments often contain mixes with dozens of species with a wide variety of functional traits from many plant families [8,21]. Most naturalized pastures in the Northeast United States have a high species richness (around 30; most are sparsely found weeds and ephemerals), low evenness (grass dominated), and low functional disparity (small contribution of forage species outside of cool-season Poaceae and Fabaceae) [4]. A random sampling of 30 individuals within a typical pasture may be like Figure 1a. Ideally, diverse pastures would have high richness, evenness, and functional disparity, more similar to Figure 1b.

4. The Problem: Low Biodiversity in Pastures

Biodiversity enhances ecosystem productivity in agricultural and natural systems through beneficial species interactions, stand resilience, and reduced disease; these benefits above ground are correlated with enhanced soil health and carbon storage [53,54,55,56]. The most common example of beneficial species interactions in pastures is the grass–legume dynamic, where the legumes fix nitrogen (N) from the air that grasses can then utilize. Pasture systems have experienced reductions in biodiversity due to several factors [6]. Early agronomic research on the topic, primarily in Britain during the 19th century, examined and proposed relatively diverse species mixtures but the majority of farmers did not have the means or a strong desire to undergo such extensive pasture renovations [57]. Over time, the field shifted toward identifying, seeding, and managing 1–4 ‘optimal’ species (usually grasses and legumes) that were productive under grazing, easily established, and adaptable to many environments [38,57]. Thus, forage seed mixtures became significantly less diverse [38,52]. These ‘optimal’ pasture grasses often included highly competitive cool season grasses that were introduced and naturalized in temperate regions around the world such as tall fescue (Festuca arundinacea), orchardgrass (Dactylis glomerata), and ryegrass (Lolium spp.). These adaptive species can outcompete most native grasses and plants, leading to diversity losses [58,59].

Agronomic management also contributes to diversity loss in pastures. High stocking rates and continuous grazing can lead to reductions in species [26,27,28]. Poor management can cause local extinction of more palatable species or those with a comparatively low tolerance to defoliation. Accordingly, overgrazed grasslands are less productive and exhibit reductions in soil organic carbon [23,24,25]. Less productive pastures may require additional soil inputs and alternative feed sources for cattle that increase the cost of production [5]. In recent decades, education and extension efforts have led an increasing number of producers to implement conservation practices such as rotational grazing [60]. Rotational grazing can reduce soil bulk density and increase soil organic carbon [61]. Well-managed rotational grazing regimes can also increase or maintain plant diversity over time [29,30]. Even so, additional efforts may be necessary to restore the diversity and soil health of pastures that have been historically overgrazed.

Managed pastures have less woody species and shrubs because they are often unpalatable or even toxic to livestock and can encroach on grazing lands [62,63]. Woody shrubs and trees make up a small but important percentage of species found in native grasslands [52]. Woody species make significant contributions to soil health and productivity by increasing structural diversity, creating microhabitats, and translocating nutrients and water [64]. Occasionally, trees are left in pastures to offer valuable shade to livestock [65]. Efforts are underway to identify woody species that are also palatable and nutritious to livestock, such that it may be possible to introduce some desirable woody shrubs into pastures [36,66,67].

5. Pasture Diversity, Soil Health, and Microbes

Plant species diversity improves soil health in grasslands without livestock. Both controlled experiments and large-scale observational studies confirmed that plant species richness increases soil carbon storage [55,68,69]. In the Jena Experiment (Table 1), plant species richness was the best predictor of changes in soil organic carbon storage. Specifically, soil in diverse plots with 8, 16, or 60 species accumulated over 20% more soil organic carbon than the plots with low diversity (1, 2, and 4 species) [69]. In a similar experiment (Cedar Creek, Table 1), high-diversity mixtures of herbaceous plants increased soil carbon by ~500% relative to monocultures over twelve years [13]. Legumes and C4 grasses are particularly beneficial for increasing root biomass production and soil carbon accumulation [13]. In natural ecosystems, an observational study of over 6000 sites found a positive relationship between plant species diversity and soil organic carbon across biomes. The influence of species richness was particularly strong in grasslands where diverse communities were associated with greater belowground (root) biomass and soil organic carbon [55].

Considerably less direct evidence for the benefits of plant species diversity on soil health can be found in the grazed pasture agronomy literature as the experiments generally involve fewer species and families, are shorter term, and are not as heavily replicated as the grassland ecology experiments (sources in Table 1). However, the experiment conducted over the longest period, lasting nine years [31] observed a significant increase in soil organic carbon in grazed pastures with higher diversity, while low-diversity pastures underwent reductions in soil carbon. The experiments of shorter duration found either no change [32] or a decrease in soil carbon [33] with increasing diversity. Taken together, plant diversity manipulations have displayed inconsistent effects of plant diversity on soil health in grazed pasture systems (Table 1). More experimental evidence is needed to draw robust conclusions. However, positive associations between pasture diversity and soil health in grazed systems are evident in research on the effects of grazing. Multiple meta-analyses have revealed that high grazing intensities reduce plant species diversity [29,70]. In contrast, low to moderate intensity grazing can actually increase plant species diversity (richness and evenness) relative to ungrazed grasslands [29,71]. The effects of grazing intensity on soil carbon mirror the impacts on plant diversity, with reductions under high grazing intensity and maintenance or even slight increases in soil carbon under low to moderate intensity grazing [61,72]. This covariation suggests that the connection between plant species diversity and soil health may persist in grazed ecosystems. A recent meta-analysis of 90 studies suggests that maintaining plant biodiversity is critical to preventing declines in ecosystem function and soil health in grazed pasture systems [73]. In summary, the positive impact of plant species diversity on soil health appears to persist under grazing, but additional field experiments manipulating plant species diversity in grazed systems are needed to confirm this connection.

The benefits of plant species on soil health and carbon storage are driven, at least in part, by microbial communities (Figure 2). Diverse plant communities increase soil carbon through enhanced plant productivity and belowground biomass [16]. While complementarity between plant species certainly plays a role in enhancing productivity [13,74], more recent work suggests that soil microbes also contribute to the diversity-productivity relationship [20,75]. In an experiment with sterilized soil, there was no increase in productivity with increases in diversity, while field soil (unsterilized) demonstrated the classic pattern. Diseases were more prevalent in the low-diversity treatments, suggesting that density-dependent disease burden may play an important role in the diversity–productivity relationship [75]. Diverse plant communities are also associated with greater levels of fungi, including mycorrhiza [76], which can enhance soil structure [55]. Fungal mycelium surrounds and cross-links microaggregates to form macroaggregates [77]. Fungi produce proteins that enhance aggregation and soil carbon sequestration [78,79].

Plant diversity increases bacterial and fungal diversity, biomass, and activity, leading to increased accumulation of recently fixed carbon in the soil [55,69]. Plant species richness is a particularly strong predictor of microbial biomass across grasslands globally [55]. Microbial biomass is increasingly recognized as a key progenitor of stable soil carbon [81,82]. Microbes live in close association with soil minerals and inside soil aggregates; thus, microbial residues are often protected from decomposition by sorption onto soil particles or physical disconnection from enzymes and decomposers [83]. Indeed, microbial necromass was recently estimated to account for an average of 61% of the soil organic carbon in grasslands [80]. Compelling results from the Jena Experiment suggest the following mechanism for plant diversity-mediated increases in soil carbon storage [69]. Diversity leads to increases in soil carbon by increasing carbon inputs into the soil (especially via enhanced root biomass). This, in turn, increases microbial biomass and the production of microbial necromass-derived stable (slow cycling) soil organic matter. The direct accumulation of plant detritus also contributes to the soil organic matter pools, but does not appear to be the primary mechanism (Figure 2).

6. Practical Considerations for Pasture Diversification

As discussed previously, high-intensity grazing must be avoided to maintain pasture diversity [26,27,28]. However, species lost from a long history of overgrazing may not return even when grazing intensity is reduced, causing forage species diversity to remain low [70]. Under these circumstances, reseeding may be needed to improve plant species diversity and restore ecosystem services [84,85]. Producers have several reasons to renovate their pastures. The growing season can be extended by using both warm and cool season forages, livestock gains can be maximized through a diverse diet, and soil health can be improved if using no-till methods [36].

Achieving and maintaining high levels of diversity, found to benefit soil health in grassland ecology studies, may be a challenge in grazed pasture systems. The first challenge is to design a diverse forage seed mixture to fulfill distinct niches relevant to the needs of the system: seasonality, light requirements, drought tolerance, root structure, plant height and form, life strategy, nutritional qualities, and disease susceptibility [7]. Consideration of these niches will result in a selection of species and varieties with high functional disparity, promoting productivity, climatic resilience, and nutrition [36]. The selected species must be compatible with soil and climate conditions and be at least mildly palatable for livestock. The USDA PLANTS database provides specific information for most plant species including seasonality, growth form and rate, after harvest regrowth (i.e., grazing tolerance), height, lifespan, pH range, drought tolerance, fertility requirements, root depth, palatability for livestock, and more (https://plants.usda.gov/ [accessed on 10 July 2022]) [40]. To achieve a higher evenness, forage species should be seeded at similar proportions. It is also advisable to under-seed or omit overly aggressive species, such as orchardgrass in the northeast United States, from diverse seed mixes.

In most pasture and grassland experiments, the seeded species are roughly subdivided into categories: grasses, legumes, and non-leguminous forbs (aka herbs). Publications from the National Resource Conservation Service (NRCS) recommend pastures be 30 to 40% forage legumes, 40–50% grasses, and a smaller percentage (<20%) of grazeable broadleaf forbs such as chicory (Cichorium intybus) [86,87]. With regard to grasses, including cool- (C3) and warm-season (C4) grasses in pastures can increase soil carbon and stabilize pasture growth over the grazing season [32,88]. Having a variety of legume species with variable drought tolerance and growth forms will optimize ground cover and improve stand persistence [8,89]. Very few forbs beyond chicory and plantain (Plantago lanceolata and Plantago major) have been properly evaluated for perennial forage use, and commercially available seed mixtures rarely contain forbs [38]. Forbs can require strict management to persist on fertile soil dominated by the typical grasses and legumes [31,90]. However, forbs are responsible for over 90% of species richness in naturalized pastures in the northeast United States [4]. They are contributors to taxonomic, phylogenetic, and functional disparity in grasslands [52,91]. Accordingly, increasing the diversity of forage forbs in seed mixtures and pastures should be a high priority [36].

7. Future Directions

This review outlines the need for additional plant diversity experiments in grazed pastures to bridge the gaps between grassland ecology and pasture agronomy research to evaluate the potential of pasture diversification to improve soil health. Future experiments in grazed pasture systems would benefit from higher levels of forage diversity, longer durations, and more replicates than the agronomy studies presented in Table 1. The statistical aspects should be carefully considered when designing on-farm experiments [92,93]. Including even a small proportion of desirable woody species when designing functionally diverse pastures could significantly improve ecosystem multifunctionality and productivity [36,67]. Future experiments could aim to increase forage plant richness, evenness, and functional disparity closer to the levels attained in grassland ecology literature while considering productivity and forage nutritional quality. Soil health benefits can take several years to observe and can be reactive to land-use transition, sampling methodology, seed bed preparation methods, and initial soil properties rather than the treatment effect itself [14,94]. Consequently, long-term experiments, lasting several years if not decades, would be ideal to detect improvements in soil health. The Soil Health Institute recommends measuring total carbon, active carbon, total nitrogen, water-holding capacity, and soil aggregate stability to best quantify improvements in soil health [95]. Perhaps most importantly, pasture diversity experiments should leverage realistic grazing practices used on working farms that maintain or encourage plant diversity, as described early in this review [29,96]. Plant diversity research that incorporates grazing is necessary to assess the applicability of benefits observed in grassland ecology studies for production systems. Such research will also be useful to evaluate the compatibility of combinations of forage species with soil, climate, and grazing conditions. Increasing forage diversity may improve soil health, enhance soil carbon sequestration, and augment agroecosystem sustainability in grazed pastures around the globe.