Cover crops can play a more prominent role in water management; however, the more widespread use of cover crops may be hindered by the inconsistencies of experimental data demonstrating cover crop effects on soil water retention, as well as cover crop effect inconsistencies arising from complex interactions between soil carbon, water, and land management.
1. The Challenges Facing Soil and Water Resource Management
Agriculture is in the midst of a defining era. Whereas precise estimates vary, there is near universal consensus that global food production and sustainable soil management practices will need to increase substantially in the coming decades to meet the demands of a growing population 
. These production gains must occur against the backdrop of climate change; and given the importance of climate to soil formation processes 
, climate change is likely to cause significant disruptions to global agricultural production 
. Incidences of extreme temperatures and heatwaves are expected to increase significantly 
, likely straining rain-fed cropping systems. The availability of freshwater, the lifeblood of agriculture, is also expected to change markedly in the coming decades. Warmer air temperatures are expected to cause changes to global precipitation patterns, which will increase the severity of floods in some areas and exacerbate drought in others 
. Additionally, trends in groundwater depletion, if continued, will further aggravate water stresses in regions across the globe 
The economic impacts of climate change are expected to have profound consequences for U.S. and global agricultural markets 
. Environmental stresses associated with climate change are expected to diminish U.S. agricultural production 
and by extension the export of virtual water 
. Significant reductions in the export of virtual water, which is the combined total of water needed in each step of the agricultural production process, is likely to exacerbate global food insecurity 
particularly in arid and semi-arid regions 
The challenges that climate change present are compounded by the continuance of severe and pervasive degradation of agricultural soils, which presents an extraordinary challenge to sustaining and improving upon current levels of agricultural production. Across the globe, approximately 25–30% of all agricultural lands are classified as degraded 
, and soils are being degraded at a faster rate than they can be sustained 
. Soils are sensitive to the impacts of agricultural production and can take decades, or longer, to recover pre-cultivation soil quality characteristics 
. Given the fact that soil water supports approximately 90% of global agricultural production 
, soil degradation is inescapably tied to water management. Climate change impacts in temperate cropland regions will likely result in an intensification of excessive spring precipitation events 
, particularly in the U.S. Corn Belt, one of the highest producing rain-fed agriculture regions in the world. These impacts are expected to reduce overall crop yields and exacerbate soil erosion 
. Moreover, crop yield reductions from these environmental stressors will not be fully compensated for by crop physiological benefits attributable to increased atmospheric carbon 
. The cumulative effects of climate change and natural resource depletion, along with a burgeoning global middle class and a growing world population, will require agriculture to intensify while simultaneously using fewer resources. Failure to implement viable solutions to address these momentous challenges will increase the likelihood of conflict and political instability across the globe 
2. The Case for Cover Crops
One soil resource management strategy that has been gaining increasing popularity is the use of cover crops. Although the total percentage of farmland managed with cover crops remains minute, growth rates in cover crop adoption may be signaling a shift in the role that cover crops play in modern row-crop agriculture 
. Cover crop acres have increased by an average of 50% in the U.S. between 2012 and 2017 
. Iowa, which leads the U.S. Corn Belt in cover crop gains, had approximately 3% of its farmland managed with cover crops in 2017; however, the increase in cover crop acreage was 156% from 2012 to 2017 
. Increases in cover crop adoption are due in large part to the demonstrated benefits for soil and nutrient conservation.
The importance of soil microbes in facilitating and maintaining soil biogeochemical processes cannot be overstated. In addition to mediating nutrient cycles 
, microbes are also an integral part of soil food webs, creating microhabitats that in turn foster enhanced biodiversity in the root zone 
. Soil microbes require carbon compounds to satisfy their energy demands, which make them highly dependent on plant root exudates. A temporal expansion of the rhizosphere (e.g., incorporation of winter cover crops), can improve soil quality through enhancing soil biological activity 
, which in turn promotes soil aggregation 
and soil organic matter accrual 
. Cover crops can increase soil carbon by more than double compared with treatments that added one or more crops to a rotation but which lack a cover crop 
, adding further evidence that the benefits of cover crops are due, in part, to a temporal expansion of the rhizosphere during the growing season.
Although cover crops have been shown to improve soil quality 
, relatively little is known about cover crop influences on soil water retention. Nichols et al. 
found that long-term cover management did not affect soil macroporosity or water content at saturation, although there was some increase in water content at field capacity. Undisturbed native soils often have higher water content than cultivated soils, with restored soils having intermediate water content 
3. Challenges and Opportunities for Cover Crops as a Soil Water Management Strategy
Plant available water (PAW) is defined as the amount of soil water that is held between field capacity (FC) and permanent wilting point (PWP). The standard used for FC varies across the globe, with values generally ranging from −5 to −33 kPa; however, a fixed value of −1500 kPa for PWP is more universally agreed upon 
. Although water in soils is physically present at matric tensions less than −1500 kPa (i.e., PWP), the water is held so tightly to the soil matrix that plants are typically unable to extract it at a rate sufficient to meet the transpiration demand.
Estimations of PAW are dependent on accurate measurements of FC. Field capacity is the water content at which free drainage (i.e., drainage due to gravity) of a previously saturated soil has become negligible. The period between wetting and negligible drainage is usually 48–72 h 
. Although −33 kPa is the generally accepted value for FC, there can be substantial variability across soils and landscapes 
One of the challenges that complicates the study of soil-water properties is an accurate approximation of the relative importance of soil organic matter (SOM) across varying matric potentials. Foundational research by Hudson 
and Emerson 
, as well as more recent analyses such as Basche et al. 
and Nichols et al. 
, reported increases in soil water retention resulting from organic carbon additions are more pronounced at FC than at PWP. Therefore, soil water retention characteristics could potentially be influenced by soil organic inputs derived from management practices that augment plant root biomass, such as cover crops. There remains, however, uncertainty of the degree to which cover crop management can alter soil water retention characteristics. Whereas cover crops can significantly increase soil water at FC 
, there can be substantial variation in the extent to which cover crops augment soil water in rain-fed systems 
Another factor that merits serious consideration is the soil structure changes caused by enhanced root zone activity. Evidence suggests that cover crop roots can alter the soil structure significantly 
. These changes can be achieved, in part, through increasing the number of soil macropores 
, which in turn increases water infiltration 
. Water flow and retention can also be influenced by hydrophobic compounds produced by the fungal hyphae 
One of the factors that both complicates and speaks to the importance of soil carbon–water research is anthropogenic impacts on soil. Soil erosion rates, although improving in many areas, remain alarmingly high 
and are occurring at a pace that is unsustainable for long-term productivity 
. Soil erosion has altered landscapes across the globe, resulting in loss of topsoil and the concomitant loss of soil organic matter. Tillage, which is a major contributor to soil erosion, can alter the soil’s structure-dependent properties as a function of both depth and time 
. Increased soil erosion will most certainly strain sustained crop productivity, as topsoil contains an exceptionally large volume fraction of the total organic matter in the solum.
Iowa, for example, has a well-documented history of anthropogenic soil formation and transformation processes over the past 50+ years 
and has documented soil erosion rates that exceed soil formation by at least an order of magnitude 
. The continued rate of unsustainable soil losses will undoubtedly alter soil hydrology, highlighting the urgency for implementation of management practices that effectively combat soil erosion.
Another major anthropogenic factor that may influence soil erosion rates, and concomitantly soil hydrology, is climate change. Climate models predict that the U.S. Corn Belt will experience changes in annual precipitation patterns that will likely result in a greater number of high-intensity rainfall events 
, increasing the risk of summer flooding. Improved flood and drought management strategies will be required to mitigate crop yield losses amidst a changing climate. The poorly drained soils that comprise significant areas of agricultural land in the U.S. Corn Belt make flooding concerns in the region acutely relevant to global food security.
Agricultural management practices that improve soil water retention may offer a means to blunt the severity of flooding. A foundational study by Emerson 
found that increases in soil carbon are positively correlated with increases in soil water retention, with these increases more pronounced at field capacity than at permanent wilting point.
4. The Role of Soil Texture in Water Retention
One of the basic soil properties that can help elucidate the impacts of cover crops on soil water retention is texture. Soil texture is a dominant factor in determining the kinetics of soil water transport and retention processes 
. It is well established that coarser-textured soils have larger pore spaces and increased hydraulic conductivity relative to finer-textured soils; however, the relationship between soil texture and soil water retention is complicated by climate and soil organic carbon (SOC) interactions. Climate is the sine qua non of soil formation, especially given its interdependence with other key factors such as biologic and physical weathering processes. Soil textural differences account for most of the soil-water variability under wet conditions, and SOC is responsible for most of the water variability under dry conditions, with the sand fraction and SOC fraction being the best predictors of soil water content across a wide range of soils 
The impacts of SOC seem to be most pronounced in coarse-textured soils, as evidenced by significant SOM alterations to soil matric potential in sandy soils 
. Plant available water enhancements attributable to SOC inputs are also more pronounced in coarse-textured soils when compared with other texture classes 
, even when SOC is added to coarse-textured soils with relatively high antecedent SOC 
Clay and silt content are significant factors in determining how SOC inputs respond to management changes, with clay–silt soils storing increased amounts of particulate organic matter (POM) after they have become carbon saturated 
. Higher clay content generally results in reduced SOC losses in cultivated soils, and although SOC increases with precipitation in uncultivated soils, the response is the opposite in cultivated soils 
5. The Role of Soil Organic Matter in Water Retention
Soil physical properties are significantly influenced by organic matter; however, soil organic matter is a broadly defined term; it technically encompasses everything from living organisms to decomposing plant tissues and humic substances. Soil organic matter pools vary significantly depending upon multiple factors, including age and chemical composition of the original organic substrate, soil hydrology, and climate 
. Soil organic matter is often categorized according to relative chemical reactivity. These categories include labile, slow, and recalcitrant organic matter pools.
The labile SOM pool has a decomposition rate ranging from several months to several years and consists largely of microbes and microbial-derived substances 
. One category of microbial-derived substances germane to soil carbon–water research is extracellular polymeric substances (EPS). These carbon-rich compounds are secreted by soil microbiota and are a widespread characteristic of soil microbes across multiple phylogenic lines 
. Although EPS constitute a relatively small percentage of SOM, they have been shown to promote soil aggregation and ameliorate the impacts of rapid changes in soil water potential 
. Extracellular polymeric substances have chemical properties that endow them with significant water storage capacity.
Measurements of EPS could potentially serve as a proxy for comparisons of water retention across various soils of similar textural classes, as small changes to EPS have been shown to result in significant changes in soil water retention. Labile carbon mineralization shows a strong relationship with soil microbial biomass carbon 
and could serve as a proxy for changes in soil extracellular polymeric substances. Permanganate-oxidizable carbon (POXC) may also serve a similar function; however, evidence of its utility remains inconclusive.
The recalcitrant SOM pool has a decomposition rate of 400–2000 years and consists of refractory compounds including humic acids and biochar 
. Recalcitrant SOM is generally deemed important for cation exchange capacity, especially in coarse and medium-textured soils 
; however, the effects can be inconsistent 
. Combined biochar and manure applications may actually serve to immobilize nutrients in medium-textured soils 
. Recalcitrant SOM can also alter soil matric potential for given gravimetric water contents 
, particularly in sandy soils 
Despite studies demonstrating the positive impacts of SOM on soil water retention, there remains a lack of consensus on the extent to which SOM alters water retention characteristics across the plant available water range. Although soil texture is the dominant factor in determining the kinetics of soil water transport and retention processes, SOM affects the shape and position of the soil water retention curve, resulting in higher water content across the PAW range 
The extensive studies by Minasny & McBratney 
and Libohova et al. 
, although insightful, do not resolve the uncertainty surrounding the relationship between SOM and plant available water. The meta-analytic methods in both studies fail to distinguish organic matter additions due to exogenous inputs (e.g., sludge or manure) from organic matter added by living plants and plant residues.
6. Interactions between Texture, Organic Matter, Structure, and Water Retention
The physical and chemical diversity that exists among SOM pools renders general statements about the relationships between SOM, texture, soil structure, and water retention overly broad with respect to advancing our understanding of soil carbon dynamics across landscapes.
Although texture is the dominant factor influencing soil-water properties, the interactions between texture, SOM, soil structure, and land management practices can significantly affect, and be affected by, soil hydrological properties 
. The importance of these interactions is also apparent in processes governing soil aggregation. A bimodal distribution of soil pore spaces is common, regardless of texture, with the sole exception being pure sands 
; soil water retention characteristics, in general, are best described using bimodal functions 
. An increase in the relative percentage of soil macroaggregates can therefore act to increase the relative abundance of macropore spaces, with concomitant increases in capillary water storage. Living plant roots act to promote soil aggregation 
and create macropore spaces 
Landscape management plays a consequential role shaping the soil physical environment. Mollic soils intensely managed under perenniality and ruminant animal agriculture can have substantially more SOC accrual than systems that rely solely on crop diversity and conservation tillage 
, suggesting that grazing can play a significant role in SOC accrual.
No-tillage systems generally have increased aggregation and SOM in the root zone relative to conventional tillage systems. Tillage is a major cause of reduced stability and number of soil aggregates when native ecosystems are converted to agriculture 
. Soil aggregates act to physically protect SOM from decomposition, as evidenced by a spike in SOM mineralization after aggregate disruption 
. Six et al. 
present a theoretical model to explain the process of soil aggregation whereby: (i) Fresh plant residues become intra-aggregate POM, which promotes microbial activity by serving as a carbon energy source. This in turn promotes the production of microbe-derived binding agents (e.g., extracellular polymeric substances) that hold soil particles together. (ii) Intra-aggregate POM transitions from coarse to finer particles as it decomposes. (iii) Microaggregates begin to form within macroaggregates. Eventually the binding agents that hold macroaggregates together weaken, releasing the microaggregates that are held inside. (iv) The released microaggregates serve as the foundation on which subsequent macroaggregates are built.