What are perennial grain crops, and what are their benefits?

The introduction of agricultural practices with initial domestication of seed-bearing crops almost 10,000 years ago caused a global ecological conversion from natural perennial plant communities to cultivation of annual crops basically to satisfy the increasing demand for food. Evidence indicates that anthropogenic selection pressure on wild annuals resulted in domesticated plants with more desirable traits than their wild relatives. This led to the preferential cultivation of annual crops with their general ability for rapid production of abundant seeds in resource-rich environments. Since then, the global area of annual crops increased dramatically and is expected to increase further to meet increasing global food and energy demands. This includes dramatic adverse effects on essential ecosystem services such as water shortage and pollution, carbon and nutrient losses, contamination with agrochemicals, biodiversity loss and climate-relevant greenhouse gas emissions.

To mitigate these ecological and economic constraints of annual crop production, it will be the global challenge of modern agroecological science and breeding to develop environmentally sustainable and simultaneously high-yielding agroecosystems. Whereas agricultural biotechnology is widely considered as the key component of global food security in future, innovative agroecological schemes toward sustainability, potentially integrating novel crops, are hitherto neglected. A concept gaining increasing attention in the area of sustainable agroecosystems is the reversed shift from predominant annual to perennial grain crop production. Perennial plants grow over a longer season than do annuals, providing permanent soil cover, deeper root systems, and larger water retention potential.Therefore they are more effective to intecept sunlight, utilize rainwater, and absorb nutrients during parts of the year when annual cropland is absent or unproductive.

There are two possible approaches to breeding perennial grains: (1) direct domestication, based on wild perennial species with high seed production and other traits suitable for use as grain crops. Further selection may improve characteristics such as synchronous flowering and maturity and non-shattering seeds. The most prominent example is intermediate wheatgrass. (2) The second approach to perennial grain breeding is hybridization of existing annual grain crops with perennial rwild relatives. Examples of such crops include wheat, barley, maize, sorghum and sunflower, all of them with one or more perennial wild relatives.

With respect to sustainability and environmental health, perennial crops contribute to the improvement of various shortcomings of modern annual crop production:

Soil protection

Annual grain cropping may generate serious erosion to the soil system, caused by water and wind and also promoted by tillage. This soil loss is associated with a decrease in the organic matter content, nutrient depletion, alteration and destruction of the soil structure. The conversion of natural ecosystems into annual cropland results in substantial declines of soil organic matter stocks. Perennial crops, in contrast, support soil formation, as they protect against erosion throughout the year, and allocate a considerable amount of carbon to roots, which extend much deeper into the soil profile and have greater biomass than their annual relatives This subsequently contributes to the formation of soil organic matter.

Intermediate wheatgrass field after harvest in autumn in France (left), and ploughed annual cropfield in autumn in Germany (right)

Water consumption

When natural ecosystems are converted to agriculture, the resulting soil disturbance can drastically alter the hydrological flows in the system. Depending on the type of soil and the mechanical pressure by heavy machine use, tillage can destroy soil aggregates and macropores and cause soil compaction. This reduces the infiltration rate and water runs off, carrying dissolved nutrients and particulate matter. In addition, lack of plant cover causes greater evaporative losses than under perennial cover. Because perennial crops have significantly longer lifespans and deeper roots, they have consistent access to rainwater and deep-soil water reserves. Furthermore, perennial cropping systems rarely leave the soil exposed, and the vegetation is able to more efficiently absorb water and therefore reduces runoff.

Nutrient loss

Significant amounts of Nitrogen fertilizer in annual cropping systems are not taken up by the plant, but lost due to leaching, runoff, soil erosion, denitrification, and volatilization. Nitrogen in the form of nitrate in water that leaches through soils ultimately flowing into rivers and lakes, potentially causing eutrophication and other environmental problems. Nitrogen losses under perennial crops are less than in annual systems due to the extensive root system that can capture and mobilise nutrients from greater depths and for a longer period. Additionally, the higher organic matter soil contents of perennial cropping systems compared to annual systems potentially increase biomass and effectiveness of rhizospheric microorganisms which improve nutrient uptake. Hence, much lower fertilizer inputs are required in perenninal compared to annual cropping systems.

Pest and disease management

Pest pressure and damage are generally favored by monocrop cultivation, and there seems to be no indication for general differences between annual and perennial grain cropping systems. However, the potential for biological control measures is highly enhanced in perennial cropping systems due to the improved habitat conditions for soil-dwelling antagonists, which include predators of arthropod pests (e.g., ground beetles, rove beetles, spiders), as well as feeders of pathogenic soilborne fungi (e.g., Collembola, Oribatida, Nematodes). Furthermore, increased disease suppressiveness against various crop pathogens was found to be related to higher microbial biomass and/or diversity in the soil, indicating an improved disease biocontrol potential in perennial grain cropping systems. Overall, there is strong indication that below- and above-ground soil faunal organisms contribute to plant pest and disease suppression with higher effectiveness in perennial compared to rotation cropping systems. The use of agrochemicals for pest, weed, and disease control counteracts the principle functions of the system and would strongly degrade the potential of biotic benefits.

Biodiversity

Soil macro- and microbiota diversity is closely linked to field management intensity, with evidence from many studies. Due to frequent disturbance, inputs of agrochemicals, and the absence of a permanent soil cover, annual crop fields represent the least suitable habitats for various non-pest arthropods in a landscape, compared to e.g. grasslands. Perennial grain crops support highly structured and complex food webs and, hence, functional biodiversity relative to annual cropping systems. Their functioning and ecosystem services depend much more on biological processes and species interactions than in annual cropping systems. Reduced soil cover disturbance by less intense agricultural practices promotes strongly functional biodiversity by increasing the activities of soil organisms and beneficial plant-microbe interactions, as well as pest control.

Climate change effects

Conversion of natural ecosystems into annual cropping systems is accompagnied by declines in soil organic matter and carbon, respectively, in a magnitude of around 50%. Tillage breaks open soil aggregates which become exposed to the surface, and much of this soil organic matter is released to the atmosphere as carbon dioxide (CO2) by soil microbial degradation and mineralization. The cultivation of perennial (grain) crops is an effective method to increase soil organic matter due to plant carbon input (roots, harvest residues) and minimal soil disturbance. Besides carbon sequestration in soil, perennial crops can sequester huge amounts of carbon in the belowground organs. Storing carbon in a deeper soil layer, as soil organic matter or in the belowground biomass, will stock carbon in the long term, because deeper soil layers are less favorable conditions for carbon degradation. In addition, perennial grain cropping systems have the potential to reduce emissions of the potent greenhouse gas nitrous oxide (N2O) from agricultural soils, and to reduce carbon dioxide emissions from farm equipment operations and the synthesis of inputs, especially nitrogen fertilizers.