Deep-rooted perennial crops. Are they capable of taking up water from deep soil layers? Could they be interesting for agriculture?

Agricultural systems need to evolve to meet the increasing food demand while reducing their environmental impact and their vulnerability to climate variability. In the future, water and rainfall shortage, will increasingly constrain food production. In addition, soil erosion, nitrate leaching and loss of soil fertility are some of the major issues associated with current agricultural practices. These issues are generally due to high soil disturbance, low plant diversity and the lack of perenniality in current cropping rotations. Therefore, there is urgent need to investigate alternative crops and farming concepts that could address both production and environmental challenges. While many of the agronomic studies focus on optimizing the use of soil resources and minimizing soil disturbance (i.e. tillage), the cultivation of deep-rooted plants and/or perennial crops offer interesting alternatives that are still under-studied.

Indeed, deep rooting is a trait that could confer enhance water and nutrient absorption from deep soil layers. In addition, perennial crops, that are often deep rooted, confer external ecosystem services such as permanent ground cover, high soil carbon sequestration and organic matter deposition, due to their longer growing season and reduced soil management requirement. However, understanding the agricultural context necessary for the successful development of deep-rooted and perennial crops is a major challenge for agronomists because of the multiple factors that determine the productivity of agro-ecosystems (climate, soil, crop specie and management) and that act in synergy. Therefore, understanding these factors should be at the center of any agricultural decision.

Driver behind deep water uptake

Water moves passively through roots and plants from region of high water potential (i.e. close to zero in the soil), to region of low water potential (i.e. air outside the leaves) along a pressure gradient set up mostly by transpiration. Transpiration refers to the process of evaporation of water molecule from the leaf into the air via pores called stomata (Fig.1). Before reaching the atmosphere, water has to flow within the soil to the vicinity of the root, then through the root to the leaves and then evaporate into the atmosphere. Therefore a lots of hydraulic resistances occur in the soil and the plant and the distribution of those resistances within the soil-plant-atmosphere continuum regulates the uptake of water from the soil. For example, under drying, the soil hydraulic resistances increases in the soil and water uptake from that soil layer will decrease. 

At the root level, the more xylem vessels a root has, the more easily water will flow. Within the root system of alfalfa and intermediate wheatgrass, resistances to water flow were found to increase with increasing soil depth due to the decrease in xylem vessels number in deeper roots. Intermediate wheatgrass roots had fewer xylem vessels at depth and therefore greater resistances to water flow than alfalfa. Alfalfa also had greater root length at depth and higher stomatal conductance which partly explain why alfalfa took up more than twice as much water from deep soil layers (<1 m) than intermediate wheatgrass. In both crops, deep water uptake (i.e. 1.5 and 2.0 m soil depth) increased under drought and support more than 35% of the plant transpiration.

More crop per drop

When focusing on water use, crop performance depends on the water availability and on how the plant partition the use of that water over the season. In particular, the flowering and grain filling stages are the most sensitive stages to water deficit for most crops. Therefore, deep water uptake that occurs preferentially late in the season during sensitive stages when the topsoil is dry, is highly valuable for grain production. However, deep water uptake is usually associated to extensive use of the soil water throughout the season which could in turn restrict water availability at the end of the crop cycle. Indeed, if the plant uses water too quickly, there will be no water left to complete its cycle and fill the grains. Therefore, optimal water use is usually defined as a way that maximize soil water extraction while ensuring sufficient water for the flowering and grain-filling period. As a simplification, crops could be divided in two categories based on their water usage/strategy, (1) crops with relatively low transpiration rate and a more water saving strategy and (2) crops with a high transpiration rate and a more water spending strategy. Under limited water conditions, growing crops with a water saving strategy is advantageous to save water during the early vegetative stage of the crop for later use in the season, during grain production. When water stored at depth is available, growing crops with water spending strategy is advantageous to favor deep root growth and water uptake. In this study, both crops were found to uptake large amount of water from the 0 to 1.0 m of soil layer. Alfalfa took up more water from subsoil layers and thus has a high potential in regions with high soil water availability and groundwater recharge. Even, if intermediate wheatgrass took up less water at depth such crop was found to present interesting potential in exploiting deep soil water compared to annual cereals.

Long-term productivity of agricultural systems ; the perennial response

      While previous section focused mostly on water use, a greater challenge in agricultural production is to reason in a more holistic/global manner. In addition to water, another important challenge in modern agricultural system is to improve nutrient cycling at the field level and thus reduce the amount of fertilizer inputs and losses. In that regards, major advances have come from practices that increased the number of species cultivated per land area, such as intercropping, mixing crop varieties and cover cropping and practices that reduced the soil management such as direct sowing. Overall, these results remind us of the functioning of natural ecosystems where productivity is maintained from a high level of nutrient cycling associated with a low level of nutrient loss, the most impressive example being the tropical rainforest (Fig. 2). When pushing such reasoning forward emerged the concept of perennial agriculture. Drawing example from perennial grassland, improvement in terms of nutrient cycling and soil organic matter content are possible however sound results in a proper farming rotation are lacking. To date, perennial grain crops are drawing more attention to replace current high yielding annual crops. However, perennial grain crops yield are much lower.

The challenge for successful perennial agriculture is to maintained productivity overtime while still harvesting products and therefore removing nutrients from the system. Sustainability of the system is supposed to come from (1) improved synergy between nutrient demand and supply, (2) addition of nitrogen-fixating crops to crop rotations and (3) targeting areas with high nutrient release from the soil due to weathering. While such reasoning is valid, empirical evidences are lacking. Our study showed that deep-rooted perennial such as alfalfa and intermediate wheatgrass are capable of taking up water from deep soil layers that could be advantageous under drought. However, long-term profitability of perennial cropping system require particular attention. Rising grain yield of perennial grain crops is a first challenging step that would bring farmers to incorporate perennial crops in their rotation and might provide interesting results. Long-term field study are badly needed.