Phosphorus (P) is a finite resource essential for food production currently lost from fields at an unsustainable rate via runoff and crop harvests. These losses could be addressed by pairing peren­nial crops, which reduce runoff with their deep roots that stabilize the soil, with recovering nutri­ents from human excreta. Urine contains the majority of P and other nutrients that humans excrete and therefore has been the focus of recent nutrient reclamation efforts. Urine fertilizer has yet to be explored for perennials, however, and under­standing the biophysical effects of urine ferti­lizer on soil nutrients and biomass in perennial crops could inform the design of a more circular food system. To that end, we started the first known workplace urine nutrient reclamation pro­ject in the state of Kansas, U.S., to test the feasibil­ity of supplying available soil P from urine to alfalfa (Medicago sativa), a perennial legume forage crop. After one growing season, urine fertilizer had no effect on aboveground biomass but did increase available soil P which otherwise decreased in the control treatments. Urine also increased soil nitrate and sodium compared to the water-only controls. The field study was coupled with a survey of staff members who participated in urine collection to identify opportunities and potential barriers to urine diversion in the U.S. The survey revealed a lack of awareness of both unsustainable P manage­ment and urine recovery as a potential solution, underscoring the need for increased education. Regulatory challenges faced in the second field sea­son also highlighted the need for policy that explic­itly defines urine separately from wastewater in the U.S. We hope that results from this project will make it more feasible to conduct additional studies and circular food system community-based pro­jects on a larger scale going forward.

Biological nitrogen fixation (BNF) is the largest natural source of new nitrogen (N) that supports terrestrial productivity1,2, yet estimates of global terrestrial BNF remain highly uncertain3,4. Here we show that this uncertainty is partly because of sampling bias, as field BNF measurements in natural terrestrial ecosystems occur where N fixers are 17 times more prevalent than their mean abundances worldwide. To correct this bias, we develop new estimates of global terrestrial BNF by upscaling field BNF measurements using spatially explicit abundances of all major biogeochemical N-fixing niches. We find that natural biomes sustain lower BNF, 65 (52-77) Tg N yr-1, than previous empirical bottom-up estimates3,4, with most BNF occurring in tropical forests and drylands. We also find high agricultural BNF in croplands and cultivated pastures, 56 (54-58) Tg N yr-1. Agricultural BNF has increased terrestrial BNF by 64% and total terrestrial N inputs from all sources by 60% over pre-industrial levels. Our results indicate that BNF may impose stronger constraints on the carbon sink in natural terrestrial biomes and represent a larger source of agricultural N than is generally considered in analyses of the global N cycle5,6, with implications for proposed safe operating limits for N use7,8.

Restoring soil organic matter (SOM) in arable land is considered one of the best natural solutions to sustain food production and mitigate climate change. With typically deep, robust root systems compared to annual grains, perennial systems are likely to promote soil organic carbon (C) sequestration while offering many ecosystem co-benefits. The intermediate wheatgrass domesticated for grain production as Kernza® (Thinopyrum intermedium) is the first perennial grain available to US growers. We quantified the formation of SOM over 2 years from the roots and shoots of Kernza grown alone and in an alfalfa (Medicago sativa) intercrop using continuously 13C- and 15N-labeled plant material. We compared SOM formation of the Kernza tissues under three contrasting agronomic environments: (1) unfertilized Kernza monoculture, (2) unfertilized Kernza biculture with nitrogen (N)-fixing alfalfa, and (3) fertilized (100 kg N ha−1 year−1) Kernza monoculture. We hypothesized that the management and plant tissues with higher N would enhance mineral associated organic matter (MAOM) formation by alleviating microbial N-limitation and leading to enhanced efficiency of microbial residue transformation. Furthermore, we hypothesized that root tissues would contribute to SOM formation primarily as occluded particulate organic matter (oPOM) due to their chemistry and interface with the soil matrix. We found that overall Kernza promoted new SOM formation with 14% of roots and 8% of shoot-derived C recovered in bulk soil after 27 months compared to 5% for alfalfa roots and shoots. There were no differences between the efficiency of MAOM formation of alfalfa vs. Kernza. The intercrop sustained similar C and N stocks to the fertilized treatment, although we found little evidence that N management was a major influence on SOM formation. Of the Kernza root tissue C incorporated into SOM, we found 3.5% in MAOM and 6% in oPOM, implying that 9.5% of root tissue C inputs may be stabilized in the soil. Legume intercrops can support Kernza cropping systems with minimal synthetic inputs, although in our study, they did not lead to enhanced SOM formation even with comparable levels of productivity.

Phosphorus (P) is a finite resource essential for food production currently lost from fields at an unsustainable rate via runoff and crop harvests. These losses could be addressed by pairing peren­nial crops, which reduce runoff with their deep roots that stabilize the soil, with recovering nutri­ents from human excreta. Urine contains the majority of P and other nutrients that humans excrete and therefore has been the focus of recent nutrient reclamation efforts. Urine fertilizer has yet to be explored for perennials, however, and under­standing the biophysical effects of urine ferti­lizer on soil nutrients and biomass in perennial crops could inform the design of a more circular food system. To that end, we started the first known workplace urine nutrient reclamation pro­ject in the state of Kansas, U.S., to test the feasibil­ity of supplying available soil P from urine to alfalfa (Medicago sativa), a perennial legume forage crop. After one growing season, urine fertilizer had no effect on aboveground biomass but did increase available soil P which otherwise decreased in the control treatments. Urine also increased soil nitrate and sodium compared to the water-only controls. The field study was coupled with a survey of staff members who participated in urine collection to identify opportunities and potential barriers to urine diversion in the U.S. The survey revealed a lack of awareness of both unsustainable P manage­ment and urine recovery as a potential solution, underscoring the need for increased education. Regulatory challenges faced in the second field sea­son also highlighted the need for policy that explic­itly defines urine separately from wastewater in the U.S. We hope that results from this project will make it more feasible to conduct additional studies and circular food system community-based pro­jects on a larger scale going forward.

We constructed a generalizable grassland model of within-plant carbon allocation strategies with the objective of furthering understanding of the evolutionary ecology of perenniality and illuminating possibilities for breeding high-yielding, long-lived crops in the service of regenerative agriculture. The grassland Perfect Plasticity Approximation model handles space-filling, recruitment, and carbon balance to determine the within-plant allocation of carbon given trade-offs involving perenniality. We used our model to conduct (1) a gametheoretic analysis of evolutionarily stable strategies to determine the effects of natural selection on perenniality and annual reproductive yield, and (2) analyses of physiological feasibility and sensitivity to determine the range of allocational strategies that might be achievable through breeding. Model results suggest that natural selection already maximizes annual reproductive yield in plants that have adapted to the evolutionarily stable strategy in the context of constraints of carbon allocation to foliage, roots, stems, and growth, leaving little room for yield improvement without also breeding a reduction in carbon allocation elsewhere. Breeding reductions in stem mass and root mass show promise for increasing annual reproductive yield. Further research is needed, however, to understand how reductions in root or stem mass would affect a perennial's provisioning of ecosystem services.

The intensive cultivation practices of annual cereal crops have been causing unprecedented degradation of natural resources. Perennial crops such as intermediate wheatgrass (IWG) could provide numerous benefits to address these issues, but there is still little comprehensive information about the establishment, fertilization needs, or range of IWG productivity on a regional basis in the first production year, which can be the highest over the lifespan of IWG’s grain production. The objective of this study was to evaluate how IWG establishment and first-year grain and forage yields varied across soil types, climate conditions, and in response ten fertilization treatments at six locations in the Midwestern USA. The 10 treatments included N fertilizer application at 5 rates; N application with or without P or K; varied timing of N application, and varied N fertilizer source. We found that fertilization influenced summer and fall forage yields but not grain yields. We also found that grain and forage yields varied greatly between locations, ranging from 556–1343 kg ha -1 for grain yields, 3732–8930 kg ha -1 for summer forage, and 927–3561 kg ha -1 for fall forage yields. Using a multiple linear regression approach, we found that a combination of local edaphic soil and climate factors explained 74%, 92%, and 69% of variance in grain, summer forage, and fall forage yields, respectively. Anomalies in expected and actual yields across locations led us to identify potential critical periods for IWG grain and forage production. We found accumulated precipitation in the 60 days before anthesis explained the most variance in grain and summer forage yields while the accumulated precipitation from May through October explained the most variance in fall forage yields. These findings are a first step toward identifying the regional expectations for IWG yields and could inform grower management and decisions regarding grain and forage harvest.

Biological nitrogen fixation (BNF) is the main natural source of new nitrogen inputs in terrestrial ecosystems, supporting terrestrial productivity, carbon uptake, and other Earth system processes. We assembled a comprehensive global dataset of field measurements of BNF in all major N-fixing niches across natural terrestrial biomes derived from the analysis of 376 BNF studies. The dataset comprises 32 variables, including site location, biome type, N-fixing niche, sampling year, quantification method, BNF rate (kg N ha−1 y−1), the percentage of nitrogen derived from the atmosphere (%Ndfa), N fixer or N-fixing substrate abundance, BNF rate per unit of N fixer abundance, and species identity. Overall, the dataset combines 1,207 BNF rates for trees, shrubs, herbs, soil, leaf litter, woody litter, dead wood, mosses, lichens, and biocrusts, 152 herb %Ndfa values, 1,005 measurements of N fixer or N-fixing substrate abundance, and 762 BNF rates per unit of N fixer abundance for a total of 424 species across 66 countries. This dataset facilitates synthesis, meta-analysis, upscaling, and model benchmarking of BNF fluxes at multiple spatial scales.

Societal Impact StatementAgricultural practices have had a negative impact on the physical, chemical, and biological components of soil. Perennial cropping systems that facilitate positive soil microbial interactions could not only rebuild soils but also sustain productivity through expected variations in environmental conditions. Here, we show the presence of arbuscular mycorrhizal (AM) fungi, soil symbionts that can improve host performance and soil health, increased the growth of intermediate wheatgrass, a novel perennial grain crop, in populations that have been increasingly bred for desirable agricultural characteristics. The right pairing of intermediate wheatgrass and a beneficial AM fungal community could lead to more sustainable agroecosystems.Summary Intermediate wheatgrass (IWG) is a novel perennial grain that can provide many soil health benefits in agroecosystems; however, little is known about how selection for agronomic traits has impacted interactions with soil biota. Here, we assess how the selection for agronomic traits in IWG has impacted its relationship with arbuscular mycorrhizal (AM) fungi. First, growth response to AM fungi was compared across five generations of IWG with varying degrees of selection. Second, variation in AM fungal responsiveness was compared among genets of IWG individuals within a more advanced generation. Finally, a meta‐analysis was performed on all published studies exploring AM fungal inocula effects on IWG performance to increase understanding of selection effects. AM fungal responsiveness increased with selection for agronomic traits, responsiveness varied among genets in the advanced generation, and a majority of genets performed better in the presence of AM fungi. The meta‐analysis supported the findings that AM fungal responsiveness has increased with selection in IWG. Further studies are needed to realize the combined potential soil health and sustainability benefits of IWG and AM fungi, including assessment of symbiotic benefits beyond biomass production, identification of IWG traits correlated with responsiveness, and characterization of AM fungal community response to IWG.

First paragraphs: A great deal of attention is currently focused on how agriculture in highly industrialized coun­tries contributes to greenhouse gas (GHG) emis­sions, and how certain farming innovations might curb the emissions of nitrous oxide and methane and draw down carbon dioxide from the atmos­phere. What is not being discussed is how agricul­ture in general, and grain agriculture in particular, will need to change as society phases out its dependency on fossil fuels in order to achieve carbon (C) neutrality. Over the last century in the U.S., the number of farmers on the land declined by about 66%, in close proportion to the increase in average farm size (U.S. Department of Agriculture Economic Research Service [USDA ERS], n.d.). Integral to these trends has been the simplification of farming systems in which practices like fertility-generating rotations have been replaced with lower-diversity monocultures maintained by applications of fossil fuel–based fertilizers and pesticides (Crews & Peoples, 2004). Between fossil fuel–powered mechanization and fossil fuel–based input intensi­fication, the energy used by farmers to grow maize in the state of Nebraska is 99.7% from fossil fuels and 0.3% human labor (Grassini & Cassman, 2012; Pimentel & Pimentel, 2008). Even organic farming systems often require prodigious fossil fuel inputs with intensive tractor tillage, manure hauling, and mechanical harvesting (Smith et al., 2015). In con­trast to our modern grain-producing agroeecosys­tems, ancestral agroecosystems and natural ecosys­tems of all types—forests, grasslands, deserts, tundra—have remained productive for millennia with no fossil fuel inputs. In this commentary I explore the dependency of grain agriculture on fossil fuel use in the U.S. set in a global context, and approaches for reducing this dependency, including a shift to perennial polycultures that rely on ecological intensification in place of energy-intensive inputs. . . .

Intermediate wheatgrass [IWG, Thinopyrum intermedium (Host) Barkworth & D.R. Dewey, trade name Kernza] is a widely adapted, cool-season forage grass, actively bred for perennial grain production. Most of IWG's net primary productivity is directed to nonreproductive structures, so dual-use strategies to harvest both grain and forage represent a potentially viable pathway to increase its productivity and profitability. We conducted a 3-year trial at nine diverse environments across North America to evaluate grain and forage yields and forage nutritive value of an early IWG breeding line under contrasting forage harvest managements. These included control (no forage harvest), summer forage harvest immediately after grain harvest, and summer forage harvest with spring or fall forage harvests. Across all sites, IWG grain yields averaged 745, 296, and 221 kg ha(-1) for the first, second, and third years, respectively. Grain yields were influenced more by stand age than site. Summer forage mass after grain harvest averaged 6.0, 4.5, and 5.7 Mg ha(-1) respectively for the first 3 years. Forage mass was less influenced by stand age, and more by site and forage harvest frequency. Fall forage harvest increased grain yields while spring forage harvests decreased grain yields and both treatments increased total relative feed nutritive values. Collectively, our results demonstrate that harvesting forage can improve both grain yield and forage nutritive values. Farmers growing IWG as a perennial grain can benefit from dual-use management by harvesting both grain and forage.

Abstract Non-technical summary Agriculture has been dominated by annual plants, such as all cereals and oilseeds, since the very beginning of civilization over 10,000 years ago. Annual plants are planted and uprooted every year which results in severe disturbance of the soil and disrupts ecosystem services. Science has shown that it is possible to domesticate completely new perennial grain crops, i.e. planted once and harvested year after year. Such crops would solve many of the problems of agriculture, but their development and uptake would be at odds with the current agricultural technology industry. Technical summary Agriculture is arguably the most environmentally destructive innovation in human history. A root cause is the reliance on annual crops requiring uprooting and restarting every season. Most environmental predicaments of agriculture can be attributed to the use of annuals, as well as many social, political, and economic ones. Advances in domestication and breeding of novel perennial grain crops have demonstrated the possibility of a future agricultural shift from annual to perennial crops. Such a change could have many advantages over the current agricultural systems which are to over 80% based on annual crops mainly grown in monocultures. We analyze and review the prospects for such scientific advances to be adopted and scaled to a level where it is pertinent to talk about a perennial revolution. We follow the logic of E.O. Wright's approach of Envisioning Real Utopias by discussing the desirability, viability, and achievability of such a transition. Proceeding from Lakatos' theory of science and Lukes' three dimensions of power, we discuss the obstacles to such a transition. We apply a transition theory lens to formulate four reasons of optimism that a perennial revolution could be imminent within 3–5 decades and conclude with an invitation for research.

Purpose Perennial crops have been suggested as a more sustainable alternative to the currently most common cropping systems. Compared with annual plants, perennial plants produce more biomass and have deeper roots, and are expected to lead to higher soil organic carbon (SOC). This hypothesis, however, has not been well tested for grain crops. Methods Using perennial intermediate wheatgrass (IWG, Thinopyrum intermedium ) and annual winter wheat ( Triticum aestivum ) as focal species, and native grassland as reference, we quantified the SOC accumulation via a process-based model, describing water and heat exchanges and carbon-nitrogen cycling in the canopy and soil to a depth of 2 m. The model includes C fixation via photosynthesis, plant biomass growth and litter production, physical protection of SOC, depolymerisation, C mineralisation, nitrification, denitrification, microbial growth, and necromass turnover in the soil. While of general applicability, we considered a sandy loam under warm-summer humid continental climate. Results Following a conversion from native grassland, IWG reduced SOC losses by at least 38%, especially in the particulate organic carbon (POC) pool, within the top 2 m of soil, compared with annual wheat. Soil microbial biomass and soil respiration were higher in IWG than annual wheat. Shifting from annual wheat to high photosynthetic capacity IWG increased SOC by about 33 g C m −2 y −1 (averaged over a 4-year continuous IWG cropping), with a large fraction of SOC gain stemming from restoring POC. Conclusion Compared with annual grains, perennial grains can increase soil carbon sequestration and maintain SOC at levels nearer to that of native grasslands.

Perennial grain crops are promoted as an alternative to annual staple crops to reduce negative environmental effects of agriculture and support a variety of ecosystem services. While perennial grains have undergone extensive testing, their vulnerability to projected future warmer and drier growing conditions remains unclear. To fill this gap, we compared leaf temperature and gas exchange rates of annual wheat and different perennial wheat ideotypes using a multi-layer process-based eco-hydrological model. The model combines leaf-level gas exchange, optimality principles regulating stomatal conductance, energy balance, radiative and momentum transfer inside the canopy, as well as soil water balance. Wheat ideotypes are parameterized based on an extensive review of field data. When compared with annual wheat, perennial wheat ideotypes with high leaf area index had between 12% and 39% higher canopy transpiration and net CO2 assimilation, depending on their photosynthetic capacity and water status. Differences in leaf temperature and instantaneous water use efficiency between annual wheat and the perennial ideotypes were moderate (-0.5 to +0.4 & DEG;C and -6 to +2%, respectively). Low soil water availability did not alter the ranking of ideotypes in terms of canopy temperature and gas exchanges. During a prolonged dry down, cumulated water use was higher and canopy temperature lower in perennial than annual ideotypes, thanks to the deeper roots, whereas cumulated net CO2 fixation depended on the specific traits and air temperature. Leaf-specific and whole plant characteristics interacted with hydro-meteorological conditions in defining the perennial's vulnerability envelopes to potential heat and water stress. These findings underline the importance of plant characteristics, and particularly leaf area and rooting depth, in defining the suitability of perennial grain crops under future climates.

Perennial grains have potential to contribute to ecological intensification of food production by enabling the direct harvest of human-edible crops without requiring annual cycles of disturbance and replanting. Studies of prototype perennial grains and other herbaceous perennials point to the ability of agroecosystems including these crops to protect water quality, enhance wildlife habitat, build soil quality, and sequester soil carbon. However, genetic improvement of perennial grain candidates has been hindered by limited investment due to uncertainty about whether the approach is viable. As efforts to develop perennial grain crops have expanded in past decades, critiques of the approach have arisen. With a recent report of perennial rice producing yields equivalent to those of annual rice over eight consecutive harvests, many theoretical concerns have been alleviated. Some valid questions remain over the timeline for new crop development, but we argue these may be mitigated by implementation of recent technological advances in crop breeding and genetics such as low-cost genotyping, genomic selection, and genome editing. With aggressive research investment in the development of new perennial grain crops, they can be developed and deployed to provide atmospheric greenhouse gas reductions.

For millennia, agriculture has been shaping landscapes on Earth. Technological change has increased agricultural productivity dramatically, especially in the past six decades, but also resulted in trade-offs such as land and soil degradation, emission of greenhouse gases (GHGs), and spreading of toxic substances. In this article we review the impacts of agriculture on the world's arable land. We start by synthesizing information on the extent of arable land and associated agricultural practices, followed by a review of the state of the art of soil health and soil carbon. We review processes of land degradation, emission of GHGs, and threats to biodiversity. To conclude, we review key social and economic aspects of arable land and identify some important concerns for the future. The article ends on a positive note describing a potential new pathway for agriculture—to gradually adopt polycultures of novel perennial grain crops.

David Pimentel was trained as an entomologist, but he was widely recognized for investigating and revealing uncomfortable knowledge on the state of global agriculture, covering topics of energy, soil erosion, biodiversity loss, pesticide contamination and energy use. While outspoken in his bold assessments of agriculture’s environmental and energetic shortcomings, he was less forthcoming with proposals for equally bold solutions. Yet one highly transformative idea that he raised repeatedly in his career after co-authoring a seminal paper in 1986 with researchers at The Land Institute was the breeding of perennial grain crops to replace annual grains on the landscape. In this paper, we look holistically at the work of David Pimentel to interpret his views on the prospects for plant breeders to develop perennial grains and the challenges that perennial grains could help address. As society continues to grapple with profound agricultural challenges, it is relevant that one of the last century’s most prominent and comprehensive scholars of agriculture honed in on perennial grains as the bold solution that would simultaneously address multiple complex environmental challenges while reducing human labor and fossil fuel dependency.

Restoration quality of native prairie can be improved by reintroducing key organisms from the native plant microbiome such as arbuscular mycorrhizal (AM) fungi. Here, we assess whether the positive effects of a native mycorrhizal inoculation observed during the first growing season remained at the end of the fourth growing season. In 2016, an experiment was initiated that assessed the response of a restored tallgrass prairie to an inoculation density gradient of native mycorrhizal fungi ranging from 0 to 8,192 kg/ha. First year results indicated that native plant establishment benefited from high but not low densities of native mycorrhizal inocula, resulting in improvements in native plant abundance, richness, and diversity. To assess whether these effects persist in later growing seasons, we resampled the prairie restoration in 2020 and analyzed the data similarly. Results from the fourth growing season indicated that the pattern of responses had persisted; the positive effects of inoculation observed during the first growing season remained after four growing seasons as demonstrated by improvements in total and native plant diversity and reduced non‐native abundance. Additionally, the low densities of mycorrhizal amendment that were not initially effective were found to reduce non‐native abundance in the fourth growing season, suggesting that low densities of mycorrhizal amendment can be amplified via positive plant‐AM fungal feedback to suppress weeds following the introduction of lesser amounts of AM fungi.

Perennial crops can improve the ecological and economic sustainability of agroecosystems because of their potential to provide diverse ecosystem services including carbon storage. Intermediate wheatgrass (IWG; Thinopyrum intermedium) is a stress-tolerant grain and forage species that can be grown in bicultures with legumes for symbiotic nitrogen fixation that provide additional ecosystem services but also compete with IWG for resources and may diminish field-scale carbon uptake. An eddy covariance (EC) tower was installed in December 2018 in an IWG field in Wisconsin - in which half of the field was frost seeded with red clover (Trifolium pratense L.) - to investigate how perennial grain bicultures and monocultures differ in carbon accumulation compared to monocultures. Using a combination of spatially-partitioned carbon and energy fluxes, collected biomass samples, and harvest and manure estimates, we found that IWG monocultures were larger carbon sinks (-538 to -580 g C m- 2 yr- 1) compared to bicultures (-458 to -520 g C m- 2 yr- 1), due to greater photosynthetic activity during the growing season and following harvests in August each year. In contrast, evapotranspiration rates were greater in bicultures compared to monocultures (by 0.3-1.4 kg H2O m- 2 day- 1), specifically during summer. Grain and forage harvest resulted in carbon loss which was not recovered until 30 days post-harvest. Carbon loss was greater for bicultures (by 20 g C m- 2 month-1). Net ecosystem carbon balance (NECB) estimates suggested that the IWG monoculture accumulated more carbon (306 +/- 88 C m- 2), whereas the biculture was on average carbon neutral (7 g +/- 131 C m- 2), when biomass removal and manure additions were accounted for. Our study demonstrates the complexities of quantifying carbon budgets in dynamic agricultural systems over short time scales, and the importance of assessing crop multifunctionality within a site's ecological and economic context.

Biological nitrogen fixation represents the largest natural flux of new nitrogen (N) into terrestrial ecosystems, providing a critical N source to support net primary productivity of both natural and agricultural systems. When they are common, symbiotic associations between plants and bacteria can add more than 100 kg N ha−1 y−1 to ecosystems. Yet, these associations are uncommon in many terrestrial ecosystems. In most cases, N inputs derive from more cryptic sources, including mutualistic and/or free-living microorganisms in soil, plant litter, decomposing roots and wood, lichens, insects, and mosses, among others. Unfortunately, large gaps remain in the understanding of cryptic N fixation. We conducted a literature review to explore rates, patterns, and controls of cryptic N fixation in both unmanaged and agricultural ecosystems. Our analysis indicates that, as is common with N fixation, rates are highly variable across most cryptic niches, with N inputs in any particular cryptic niche ranging from near zero to more than 20 kg ha−1 y−1. Such large variation underscores the need for more comprehensive measurements of N fixation by organisms not in symbiotic relationships with vascular plants in terrestrial ecosystems, as well as identifying the factors that govern cryptic N fixation rates. We highlight several challenges, opportunities, and priorities in this important research area, and we propose a conceptual model that posits an interacting hierarchy of biophysical and biogeochemical controls over N fixation that should generate valuable new hypotheses and research.

In the semi-arid grassland ecosystems of the North American Great Plains, diverse mixtures of perennial plants co-evolved with humans and other animals through fire and grazing. These prairies held and built soil, persisted through climatic variability, and were tended to provide for Indigenous cultures. Yet due to Euro-American colonization and widespread tillage into annual grain systems dependent on fossil-fueled inputs, grassland landscapes are precarious in the 21st century. Communities and agricultures are vulnerable to food insecurity and injustice due to climate change, soil erosion and degradation, and biodiversity loss. A resilient future requires re-perennialization and diversification of landscapes, through grassland restoration and conservation, as well as a reinvention of agriculture that includes key features of natural systems such as perenniality and diversity. By engaging communities and researchers, it may be possible to build the scientific knowledge and social movement necessary to accomplish a social perennial vision for the region.