In an effort to regain some of the valuable ecosystem services provided by natural ecosystems, some land managers have begun taking an ecological approach to agricultural practices. One such method is the incorporation of perennial plants and polyculture plantings into cereal and oil seed agroecosystems. The benefit of these perennial polycultures is that they mirror some of the ecological benefits once provided by native perennial ecosystems, such as a prairie grassland. Prairies are diverse assemblages of grasses, legumes, composites, and other major plant groups that remain productive each year with little to no inputs. Prairies created the highly fertile Mollisol soils of the Midwest that have high carbon sequestration [1
], soil aggregate stability [2
], and provide essential habitats for migratory birds, insects, and microbial populations [4
]. Intercropped systems have been shown to have better weed control [5
] and increased yields and nitrogen uptake [6
], perhaps resulting from plant complementarity [7
]. In mimicking some of characteristics of prairies, such as improved plant diversity and minimal soil disturbances, taking a perennial polyculture approach to agriculture will also likely improve soil and ecosystem health and function and lead to more sustainable cropping practices.
Many plant species that are candidates for perennial agroecosystems are derived from or closely related to native grassland species. Many of these prairie grassland species are known to be highly dependent on arbuscular mycorrhizal (AM) fungi [8
]. Therefore, it is likely that many perennial crops benefit from mycorrhizal fungi as well. AM fungi are known to improve plant growth by acquiring soil nutrients which are difficult for plants to acquire, such as inorganic phosphorus. AM fungi can also provide non-nutritional benefits to their plant host through alleviation of environmental stressors such as drought [9
] as well as providing resistance to pathogens [11
] and herbivory [12
]. AM fungi contribute to other valuable ecosystem services, such as improving soil health and mitigating rising CO2
levels because AM fungi act as carbon sinks [13
] and can decrease erosion by producing soil-binding proteins that increase soil aggregate stability [14
]. AM fungal abundance is also tightly correlated with nitrogen and carbon sequestration [15
]. Due to their many ecosystem contributions, AM fungal communities may be key components utilized in sustainable cropping systems agricultural practices that rely on organic, biologic, or low-input agricultural practices.
Although AM fungi are commonly present in soils, AM fungi in agricultural environments may be limited. The site histories of many agricultural systems include land manipulations known to disrupt fungal communities. For instance, processes such as tilling [16
], the use of soluble fertilizers and biocides [18
], and the planting of crop monocultures [19
] can lead to reduced AM fungal abundance, infectivity, and diversity. Reintroducing native AM fungal communities into sites with disturbed soil communities has been shown to benefit grassland plantings by improving plant survival, growth, and fecundity [20
], and soil aggregate stability [3
]. Fungal inoculations seem to be particularly useful when the fungi were isolated from local soils [21
], likely because fungi are adapted to local nutrient and water conditions [23
]. However, it remains to be seen whether reintroducing native fungal communities back into disturbed agricultural environments improves the productivity and resilience of perennial polyculture agroecosystems.
While evidence from grassland plantings suggest that AM fungal amendments may also benefit the perennial plants used in polyculture agroecosystems, the recommended application rates and methods provided by commercial producers of AM fungi are highly variable. Inoculation methods commonly include planting pre-inoculated seedlings or broadcasting inocula [25
]. Generally, the inoculation rates recommended by commercial producers of inocula range from around 2 kg to 120 kg of inocula per hectare. This volume tends to be significantly lower than what has been reported to be effective application rates from within the scientific literature, which have ranged from an estimated 700–75,000 kg per hectare [20
]. Although lower inoculation rates have been reported to be effective for certain crop species [30
], others have found commercial mycorrhizal application to be ineffective [21
]. It remains to be seen whether the ineffectiveness of commercial mycorrhizal products stems from too low of recommended application rates, or from being non-beneficial for other reasons, such as non-native isolates of commercial fungi being maladapted to specific soil or water regimes in which they are applied. Thus, a more thorough assessment of the whether inoculation can be useful in agroecosystems, and at what density, is needed.
In these studies, we test whether mycorrhizal inoculation can improve the establishment and productivity of perennial crop species. First, we conducted a greenhouse study, where we tested the overall mycorrhizal responsiveness of 19 plant species with a locally derived mycorrhizal inoculum. In a paired field study, we designed an inoculation density gradient using the same inocula that covers the range of application rates suggested by commercial producers of mycorrhizal inocula as well as several higher rates corresponding to effective inoculation rates reported in the scientific literature. We followed the establishment and growth response of seven perennial crop candidates selectively bred by The Land Institute (Salina, KS, USA) species that have cereal, oilseed, and legume production potential.
5. Conclusions and Implications to Sustainable Cropping Systems
Incorporating perennial plants and polyculture plantings into agroecosystems has been suggested as a way to mirror the benefits of native perennial ecosystems including increased carbon and nitrogen storage, more stable soils, and reduced anthropogenic input [8
]. This body of work demonstrates the importance of ecological approach in sustainable cropping practices by highlighting the importance of the plant microbiome in perennial agricultural systems. Many perennial plant species are strongly dependent on their soil microbes including Rhizobia
], arbuscular mycorrhizal fungi [42
], earthworms [43
], and a suite of other macro- and microorganisms. Perennial crop species being bred by The Land Institute are newly perennial and generation times are subject to quick turnover. Given that work in grasslands indicate that the beneficial effects of microbial amendments including beneficial mycorrhizal inoculation can increase over time [44
], future work should assess if the benefits of amending perennial croplands with symbiotic organisms can outweigh the potential costs of harmful components of the perennial microbiome including pathogen accumulation that can occur in long-living plant species [45
]. The plant microbiome may have a particularity important role in polyculture cropping systems. Not only might polycultures prevent the rapid accumulation of soil pathogens, but many choice companion plants are legume species. Many legumes, including several used in this study, are highly dependent on soil mycorrhizae, and may directly or indirectly use them to transfer soil nutrients to companion plants [46
]. Taken together, amending new perennial plantings with beneficial microbiome components has the potential to improve plant productivity and establishment. Future work should assess which key soil biotic components to amend with and the optimum planting design for sustainable cropping systems agricultural practices that rely on organic, biologic, or low-input agricultural practices.