Effects of staggered planting on cover crop mixture evenness and multifunctionality

Abstract

Cover crops, or non-harvested crops planted in windows between primary crops in rotations, can increase on-farm diversity, ecosystem functioning, and soil health, especially when planted in mixtures containing diverse plant functional groups with complementary traits (Blesh, 2018; Finney & Kaye, 2017; Isbell et al., 2017; Schipanski et al., 2014; Snapp et al., 2005; Storkey et al., 2015; Weidlich et al., 2017). Plants of the same functional group share functional traits, or characteristics that determine how they affect the surrounding environment (Díaz & Cabido, 2001). For example, brassicas have large tap roots that can reduce soil compaction and allelochemicals that reduce pest pressure; grasses have fibrous roots that are adept at retaining nutrients; and legumes can fix nitrogen (N) gas from the atmosphere through biological N fixation (BNF) (CTIC, SARE & ASTA, 2020; Jacobs, 2012; Snapp et al., 2005) and have low carbon-to-nitrogen ratio (C:N) litter, both of which increase N availability in the soil (Blesh, 2018; Schipanski et al., 2012). The varying impacts of plant species on ecosystem processes both across and within functional groups can be predicted using their functional traits, such as specific leaf area (SLA), height, leaf %N, and C:N (Garnier & Navas, 2012; Wood et al, 2015). Cover crop mixtures that include species with diverse sets of functional traits can increase the provisioning of multiple ecosystem functions (i.e. multifunctionality) (Blesh, 2018; Davis et al. 2012; Finney et al. 2017; Hector & Bagchi, 2007). For example, diverse agroecosystems tend to be more resilient to extreme weather events and pest outbreaks, especially when functions such as improved N supply, water and nutrient retention, and weed management allow for reduced use of external inputs (Finney & Kaye, 2017; Kremen & Miles, 2012; Snapp et al. 2005). Yet, limited diversity-associated increases in multifunctionality may be achieved if one functional group outcompetes another, because abundance is a key predictor of contributions of species to the functioning of ecosystems (Grime, 2002). Ensuring that cover crop species representing distinct functional groups have adequate representation within a mixture poses a challenge given that growth rates and competitive potential vary across species such that some functional groups tend to consistently competitively exclude others. While competitive exclusion presents a challenge to building and maintaining diverse cover crop species mixtures, priority effects present a possible solution. Priority effects refer to the variation in effects that a species has on its environment resulting from its order of establishment within a community. The order of establishment of species within a community can influence the overall species richness, composition, and biomass of the community, which in turn can affect ecosystem functioning (Fukami, 2015). Priority effects are caused by two mechanisms: niche preemption and niche modification. Niche preemption occurs when two species require the same resources and an early-arriving species reduces the resources available to a later-arriving species, thus influencing subsequent community composition and function (Fukami, 2015). Niche modification occurs when an early-arriving species enhances or degrades the resources required by later-arriving species, thereby affecting their establishment, which also influences subsequent community composition and function (Fukami, 2015). While niche preemption may be expected to consistently reduce diversity in cover crop mixtures, niche modification could either increase or decrease the diversity of cover crop communities. For example, beneficial niche modification may occur when an early-establishing 1 legume supplies N to soil, which could benefit later-establishing species and increase the productivity of the community, especially within the first growing season as found by Weidlich et al. (2017). In contrast, early-establishing grasses may reduce the growth of later-establishing species by taking up space and resources with dense roots. In agroecosystems, priority effects can potentially be managed to support evenness of cover crop mixtures by staggering the planting of each species to control order of establishment. Biotic interactions, and their impacts on co-existence and subsequently the diversity of a community, are influenced by abiotic factors. Soil fertility is particularly important in influencing species diversity and evenness in plant mixtures, as high-fertility soils tend to result in lower species richness due to strong competitors taking advantage of the abundance of nutrients (i.e., selection effect) (Buckland & Grime, 2000; Ejrnæs et al., 2006; Kardol et al., 2013; Weidlich et al., 2017). These impacts of fertility may be sensitive to the order of arrival of the different species into the community. In a microcosm experiment, increasing the time between each species’ establishment resulted in stronger priority effects that led to greater differences in community composition, and this effect was more pronounced in more fertile soil (Kardol et al., 2013). Conversely, in grasslands, there was no effect of soil fertility on the impact of order of arrival of distinct functional groups on the resulting diversity of the system (Weidlich et al., 2017). To our knowledge, no studies have tested whether priority effects impact evenness in cover crop mixtures and subsequent multifunctionality in agroecosystems, and how these effects are influenced by soil fertility levels. To understand how mixture evenness and ecosystem functions provided by cover crop mixtures are impacted by priority effects and soil fertility, we established five planting treatments in a field experiment using one brassica, one grass, and two legume species, which were planted in low- and high-soil fertility treatments in a fully factorial design. We measured the following ecosystem functions to determine multifunctionality: above- and belowground N retention, carbon (C) and N mineralization, BNF, and weed suppression. We also measured four plant functional traits: maximum plant height, specific leaf area, leaf %N, and shoot C:N. We hypothesized that the legume-first, low soil fertility treatment would lead to the highest mixture evenness, given that prior research has found high fertility treatments to become dominated by the strongest competitor. Although legumes may have a competitive advantage in low-fertility soils due to their ability to host N-fixing bacteria, we did not expect this advantage to supersede the relatively stronger competitive abilities of grasses and brassicas when co-planted. The earlier planting date for legumes may improve their establishment by reducing competition, and may also benefit subsequent species by supplying N through BNF. Second, we hypothesized more even mixtures to have higher levels of multifunctionality given that there would be adequate representation of all three functional groups. And, third, we expected to find significant differences in plant functional trait expression for each species between the staggered planting and soil fertility treatments. For instance, we expected to find higher leaf %N and lower C:N in the legume-first treatments due to legumes’ ability to fix N, perhaps facilitating other species’ abilities to acquire N.