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USDA-ARS, Natl. Soil Tilth Lab., 2150 Pammel Dr., Ames, Iowa 50011
Corresponding author (hatfield{at}nstl.gov)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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Abbreviations: WUE, water use efficiency
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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Tanner and Sinclair (1983) summarized the different forms of relationships that have been used to characterize WUE. Most researchers describe WUE as
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Changes in WUE can be manifested through soil management practices via the components of the surface energy balance:
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In our review, we discuss the potential implications of soil management practices on WUE in crops. Soil management in our discussion includes any practice that alters any soil component within or on the soil surface. Soil management can affect water and nutrient status within the soil, and the impact of these changes on plant response in terms of increased plant growth or yield offers opportunities to improve WUE. Earlier summaries developed by Unger and Stewart (1983) and Power (1983) provide a strong foundation for understanding the role of soil management on WUE. This report will focus on the more recent literature prepared during the last 30 yr.
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PRECIPITATION USE EFFICIENCY |
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TOPABSTRACTINTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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Much of the research that forms the foundation for understanding the relationships among precipitation, soil water, plant water use, and crop response has been conducted in semiarid regions. Good and Smika (1978) found that wheat (Triticum aestivum L.) yields increased from 1000 to 3000 kg ha-1 as available soil water at planting increased from 220 to 400 mm. In semiarid regions, fallow (no crop during the growing season) to increase soil water storage has been considered to be a viable and necessary practice. However, recent research has demonstrated that precipitation use efficiency may increase with reduced tillage systems coupled with more intensive cropping systems. Farahani et al. (1998), among others, have shown that efficiency gains are due to a reduced use of fallow seasons and using water for crop growth that otherwise is lost during fallow by soil water evaporation, runoff, or deep percolation processes. In areas where fallow is practiced, the efficiency of precipitation storage is often low (between 10 and 15%), largely because a large portion of the precipitation falls when no crop is growing and partly due to disturbance of the soil surface to control weeds (Johnson and Davis, 1971). These results would suggest that we need to examine these studies to achieve the next increment in WUE.
Musick et al. (1994), like Good and Smika (1978), found that wheat yields were positively and linearly related to soil water stored at planting and that this relationship was more significant than a relationship to seasonal water use. Their research is illustrative of the need to understand the role that soil modification plays in changing WUE. Modification of the soil surface will lead to changes in the soil water balance in terms of soil water evaporation and infiltration into the soil profile. Soil management practices will ultimately have some effect on how efficiently crops use precipitation as a water supply. There are four major influences on the evapotranspiration flux (Eq. [3]) from a surface for a given period of time. These include the availability of energy (Rn), gradients of water vapor, temperature and wind speed, amount of soil water stored in the soil profile, and the ability of the plant to extract water from the soil profile. These terms are not independent of one another, and throughout the course of this review, we will determine why these interrelationships exist and how they can be modified to improve WUE.
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SOIL SURFACE MODIFICATIONS |
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TOPABSTRACTINTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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Cresswell et al. (1993) found that the tillage of bare soils increased saturated hydraulic conductivity (rate of water movement when the soil is saturated) while soil water content before tillage had no noticeable effect. Unsaturated hydraulic conductivity (rate of water movement at water content less than field capacity) was affected by tillage sequence, and excessive tillage caused the lowest conductivities because of the increase in air-filled pores. The effect of tillage on water infiltration was still considered to be positive; however, these results suggest that excessive tillage may reduce infiltration through the effect on hydraulic conductivity. Christensen et al. (1994) found that more soil water was conserved during fallow periods with no tillage than with clean till, and in contrast to Creswell et al. (1993), the infiltration rates were larger with no tillage as evidenced by the slow rate of the advance of water down irrigation furrows. They reported that sorghum [Sorghum bicolor (L.) Moench] grain yields were higher with no tillage, but wheat yields were less with clean-till. More water was conserved during the fallow periods, and there was deeper wetting of the soil profile in no-tillage plots.
A strong relationship has not been developed among types of tillage systems and WUE. It is impossible to discuss the effects of tillage practices without also discussing the effects of mulch or crop residue management because most studies compare residue management with various tillage practices. Infiltration rates under no tillage are increased. In the northern Great Plains, Pikul and Aase (1995) found that infiltration rates were increased because of the protection of the soil surface and that infiltration over 3 h was 52 mm with conventional tillage in a wheat fallow and 69 mm for the annual cropping system with no tillage. They stated that no tillage has an advantage over tillage because surface cover is maintained, and this reduces the potential for soil crusting and erosion. Aase and Pikul (1995) found that decreasing tillage showed a trend toward improving WUE because of improved soil water availability through reduced evaporation losses.
The management of soil through tillage changes the water storage and evaporation losses. However, maintaining the soil profile is an important factor. Tanaka (1990) concluded that management practices should be developed and practiced that would preserve topsoil depth because soil losses in the northern Great Plains decrease WUE and dry matter production. Studies such as these demonstrate the importance of understanding the role of tillage on efficient water use and crop growth.
Crop Residue Management
Soil Water Availability
Covering the surface with mulch or residue has been studied relative to changes in WUE. Greb (1966) found that residue and mulches reduce soil water evaporation by reducing soil temperature, impeding vapor diffusion, absorbing water vapor onto mulch tissue, and reducing the wind speed gradient at the soil–atmosphere interface. Sauer et al. (1996a) found that the presence of residue on the surface reduced soil water evaporation by 34 to 50% and that creating a 15-cm bare strip increased soil water evaporation by only 7% over the weathered residue cover. Deibert et al. (1986) stated that tillage effects on storage efficiency were minimal; however, they concluded that proper soil management could lead to both increases in precipitation storage efficiency and WUE. Deibert et al. (1986) found that precipitation storage efficiency in the northern Great Plains was similar among tillage systems but varied among years and locations during the nongrowing season under continuous wheat. Precipitation storage efficiency was defined as the amount of soil water stored in the upper 1.2 m relative to the precipitation during the nongrowing season. The difference among tillage systems ranged from 56% with no tillage to 47% with spring sweep operations at Williston, ND, but at Minot, ND, they ranged from 59% with no tillage to 57% with spring sweep. However, among years for a given tillage practice, precipitation storage efficiencies ranged from 20 to 98%. The authors attributed the variation in storage efficiency to variations in the total annual precipitation and variations in precipitation patterns among years. They found that yields under no tillage were lower than with spring sweep or spring plow, which caused WUE to be lower with no tillage. Yield decreases with no tillage were related to increased weed competition, foliar disease, and insect damage (Deibert et al., 1986).
Azooz and Arshad (1995) found higher soil water contents under no tillage compared with moldboard plow in British Columbia. However, Zhai et al. (1990) noted that the presence of corn (Zea mays L.) residue intercepted significant amounts of precipitation and reduced soil water evaporation under no-tillage systems in Ontario. Johnson et al. (1984) reported that more soil water was available in the upper 1 m under no tillage compared with other tillage practices in Wisconsin. In the Upper Midwest and Canada, there was generally an increase in soil water content under reduced tillage practices. This increase was caused by residue providing a barrier to soil water evaporation and by less disturbance of the soil surface via tillage operations.
The management of snow represents a significant portion of the water balance in the cropping systems of the northern Great Plains. In the northern portions of the USA, standing residue or stubble increases snow trapping. Aase and Siddoway (1990) showed that standing wheat residue increased the soil water content in spring by 10 to 30 mm. The difference between bare soil and standing residue was not as evident when rain occurred as it was with snow. The presence of crop residue on the surface influences the rates of energy exchange between the soil surface and the atmosphere due to effects on albedo, aerodynamic coefficients, and water vapor exchange rates. Sauer et al. (1996b) showed that the aerodynamic properties of corn stubble changed over the winter. They found roughness lengths and drag coefficients to be highest in the fall and lower in the spring after the residue had been weathered and compacted beneath the snow. The larger roughness lengths and drag coefficients in the fall increased the potential water vapor exchange rates; however, this was offset by the residue having a large amount of air-filled pore space. Although the exchange mechanisms were present for rapid water loss, the limiting factor was the rate of water movement through the stubble. In the spring, the roughness lengths and drag coefficients were reduced and became the limiting factors to the water vapor exchange rates. The seasonal changes in crop residue properties need to be understood to quantify the effects of changing residue management on water exchange processes.
In the southern High Plains, wheat residue is used as a barrier around cotton (Gossypium hirsutum L.) to reduce the effect of blowing sand on young cotton plants. Lascano et al. (1994) found that total evapotranspiration was similar between conventional tillage practice (305 mm) and cotton planted into killed wheat residue (304 mm). The largest difference was the partitioning of evaporation into the components. Wheat residue modified the microclimate, which increased transpiration to 69% of the total evapotranspiration compared with 50% for the conventional tillage practice. However, the WUE of cotton was not modified by the presence of wheat residue. Hatfield (1990) found an increase in water vapor content and a decrease in wind speed within wheat residue that reduced the gradient for water vapor transfer in the early season. This increased WUE by 25% in the first part of the season, but the effect did not persist for the entire season because the microclimate changes were diminished when the cotton height exceeded the wheat residue height. The presence of the wheat residue affected the humidity and wind speed around the young cotton seedling and placed the cotton plant into a microclimate that had a reduced evaporation gradient. Once the plant grew taller than the wheat residue, the effect was no longer present. These studies demonstrate that there are a number of potential options to modifying WUE in cropping systems.
In the Midwest, Sauer et al. (1998) evaluated the surface energy balance of corn residue under field conditions and found large differences in the evaporation fluxes among days. The wetness of the residue layer had a large effect on the partitioning of available energy into evaporation and sensible heat. On overcast days with a dry soil surface, between 50 and 75% of the net radiation was used in evaporation, in contrast to sunny days, when <20% of the net radiation was used in evaporation. If the soil surface was wet, there was little difference in evaporation fluxes as a function of net radiation (Sauer et al., 1998). Sauer et al. (1997) found the radiation components of residue to change over winter because albedo changed with the age of the residue and transmissivity increased as the residue weathered. Transmissivity of radiation through the residue layer was a function of the residue area index and represents a measure of how much energy penetrates to the soil surface. Crop residue is not uniformly distributed across a field, and the spatial distribution changes dramatically with decomposition and as the wind rearranges the residue after harvest. Residue characteristics affect energy balance components and have a large impact on evaporation fluxes. The changes in residue over the year demonstrate that its effectiveness on water storage and evaporation rates will vary throughout the year and spatially across a field because of the nonuniform distribution of residue.
Soil Temperature
One aspect of crop residue management is the effect of residue on soil temperatures. Soils with surface residue management are cooler than tilled soils (Allmaras et al., 1964; Anderson and Russell, 1964; Greb, 1966; Wilhelm et al., 1989). These cooler temperatures cause slower crop growth during the early season and are often the reason cited for a lack of adoption of no-tillage practices in the Upper Midwest. Hammel (1989) found that reduced tillage and no tillage in northern Idaho increased soil impedance, and in combination with the cool, wet soil conditions in the spring, limited root function and decreased crop growth potential.
Kaspar et al. (1990) showed that removing corn residue from the seedbed increased the rate of corn emergence. This was attributed to higher maximum soil temperatures due to residue removal. Hatfield and Prueger (1996) found that average soil temperatures were only slightly affected by the presence of corn residue on the surface and that the greatest effect occurred in the fall when the residue was fresh compared with the spring when it was weathered. Sauer et al. (1996a) showed that there was a different response to corn residue on a Monona silt loam (fine-loamy, mixed, mesic, Typic Hapludoll) than on a Nicollet loam (fine-loamy, mixed, mesic Aquic Hapludoll). The Monona soil was 1 to 2°C cooler than the Nicollet soil when the same levels of residue were placed on the surface. This difference can be explained by the differences in thermal conductivity between the soils and by the effect of soil water on thermal properties.
Unger (1988) found that soil surface temperatures in the High Plains of Texas were more affected by the season of the year than by residue management practices. In the summer, he found the highest soil temperatures after dryland wheat in standing residues, while in the winter, the highest temperatures were in the no-tillage treatment with shredded residue. Although there is an effect of crop residue on soil temperatures, the impact of the residue on the soil water content and the interactions with the soil thermal properties must be considered in interpreting the results of different experiments.
Crop Growth and Yields
Increasing crop residue or adopting no tillage increases soil water availability and affects crop growth and yield. In western Kansas, in a wheat–row crop–fallow rotation, the use of no tillage increased corn yields by 31% (Norwood, 1999). The row crops studied were corn, sorghum, sunflower (Helianthus annuus L.), and soybean [Glycine max (L.) Merr.]. The effect was not consistent among crops because corn yields were increased in 3 yr, sunflower and sorghum yields were increased in 2 yr, and soybean yields were increased in 1 yr. Unger (1994) addressed the effect of limited irrigation coupled with conservation tillage on wheat and sorghum yields and found that while the use of conservation tillage increased soil water use, these practices did not affect the grain yield of either crop. Similarly, Jones and Popham (1997) found that while continuous sorghum was the most efficient at using precipitation during the growing season, sorghum grain yields were not affected by residue management compared with fallow systems on the southern High Plains. Unger (1991) compared eight sorghum cultivars under no tillage and found that WUE varied among years and cultivars. The highest-yielding cultivars in this study had the highest water use.
In Australia, Gibson et al. (1992) found that retaining sorghum stubble on the soil increased the sorghum yield by 393 kg ha-1 due to increased WUE because of a greater amount of water stored in and extracted from the soil profile compared with conventional tillage. They also found that decreasing tillage frequency increased soil water extraction; however, no tillage did not result in the optimum yield or WUE. In the southern High Plains of the USA, WUE for irrigated wheat was 8 kg ha-1 mm-1 compared with 4 kg ha-1 mm-1 for dryland wheat (Musick et al., 1994). In this study, there was nearly 100% variation in WUEs for a given amount of crop water use. Increasing the soil water availability to the crop in the absence of any other yield-limiting factors can lead to increased WUE. Differences in WUE among years for the same set of practices within a location create a dilemma in determining how much of an increase in WUE is feasible under a given management practice.
Tompkins et al. (1991) found that no-tillage winter wheat yields in Saskatchewan increased with an increased seeding rate and decreased row spacing. Water use efficiency increased when row spacing was decreased from 36 to 9 cm and the seeding rate was increased from 35 to 140 kg ha-1. Grain yield increased from 1.49 to 1.68 kg m-2 while WUE increased from 9.4 to 10.3 kg ha-1 mm-1. The total water use increased with narrow row spacing and high populations, but increased yield was the primary factor associated with the increased WUE (Tompkins et al., 1991). A similar result was found for wheat in India; however, WUE was optimum at the 75 kg ha-1 seeding rate (Srivastava and Sidique, 1978). For grain sorghum, WUE was not affected by within-row density, but it decreased in 1 of 3 yr with the use of narrow rows (Jones and Johnson, 1991). Water use efficiency varied among the years of the study by 75% across the row width and plant density treatments.
Barley (Hordeum vulgare L.) and canola (Brassica campestris L.) had a different response to tillage and residue management in British Columbia (Azooz and Arshad, 1998). Azooz and Arshad (1998) found differences among years when they compared the effects of no tillage and a 75-mm strip till with conventional tillage on the water use and yield of barley and canola on a silt loam and a sandy loam soil. In a dry year, there was an increase in yield with no tillage and strip till; however, in a wet year, yield was higher with conventional tillage. Water use efficiency for barley was increased in the dry year by 21% with no tillage and by 18% with strip till in the silt loam; it was increased by 19% with no tillage and by 10% with modified no tillage in the sandy loam compared with conventional tillage. In wet years, WUE was highest with conventional tillage. Water use efficiency by canola was higher under conventional tillage in the wet year, but data were not available for the dry year. Liang et al. (1991) found that corn yield increased with higher plant populations and higher fertilizer rates in response to increased temperatures (heat units) and water inputs during the growing season. Their study showed a WUE of 10.2 kg ha-1 mm-1 for these conditions and only a small variation in yield for a given water use. The interaction between heat units and water use on corn yields suggests that early season crop growth affects WUE. Zhang and Qweis (1999) found similar responses for wheat in the Mediterranean region where WUE was increased by agronomic factors that lead to high yields.
Differences in WUE among growing seasons are often observed. Chan and Heenan (1996) showed that for a wheat–lupin (Trifolium subterraneum L.) rotation, early season growth of wheat caused differences in crop water use among years. Early season growth affected the ability of the wheat crop to effectively use soil water. There was no effect of soil water differences on the lupin crop. Dao and Nguyen (1989) showed that there were large differences in the growth response of wheat cultivars to different tillage practices at El Reno, OK, but they concluded that it would not be necessary to develop cultivars for specific tillage methods. They also found that no-tillage management showed the greatest response in growth and yield under unfavorable growing conditions.
Eck and Winter (1992) evaluated the effect of modifying the soil profile on sugarbeet (Beta vulgaris L.) and corn WUE and found that although water was extracted from deeper depths of the modified soil profile, there was not a consistent increase in yield. They only found an effect on WUE in one year of the study and concluded that modifying the soil profile to increase water use was not warranted because of the limited effect on yield. Under sodic soils in New South Wales, WUE was higher for digitaria (Digitaria eriantha spp. Eriantha) than for lucerne (Medicago sativa L.) (Tow, 1993). Across the three seasons of the study, WUE varied by 110% in the digitaria, 84% in the lucerne, and 72% in the mixture of both. Variation among seasons is a problem in WUE studies, and the role of the soil is not clearly understood.
In comparing different corn hybrids, Howell et al. (1998) found that WUEs for grain yield and biomass were the same for both short-season and full-season hybrids. Soil water extraction patterns during the growing season will vary with hybrid maturity. Tolk et al. (1998) found a soil type effect on water use and corn yield. Much of the effect on yield was due to the water extraction pattern during the season from the different soil profiles. In comparing results shown in Table 1, it is necessary to define the soil profile characteristics and the maturity class of the crop.
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SOIL NUTRIENT STATUS |
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TOPABSTRACTINTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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Nitrogen dynamics and availability vary across the landscape. Wood et al. (1991) showed that slope position had little effect on plant N uptake or soil N dynamics. They did find that aboveground biomass and plant residue production increased downslope due to increased soil water availability. Halvorson et al. (1999) showed that N in dryland cropping systems had a positive impact on the amount of residue returned to the soil and to the belowground residue C. Increasing N rates increased soil organic C and total N. Earlier they had found that the increased cropping intensity, as suggested by Farahani et al. (1998), would lead to changes in N management practices because of the low mineralization potential of dryland soils (Halvorson and Reule, 1994). Nitrogen management is linked to water use rates in cropping systems. Maskina et al. (1993) found that growth and N uptake by corn increased as residue amounts from previous crop production increased. They found that N uptake was not affected by tillage in this study. Changes in crop residue management used to increase WUE may be linked to N dynamics in the soil.
In a study in Kentucky, Corak et al. (1991) found that WUE increased with the addition of N fertilizer and hairy vetch (Vicia villosa Roth.) residue. Water use efficiency increased from 6.1 to 8.5 kg ha-1 mm-1 in 1986 and from 9.1 to 16.6 kg ha-1 mm-1 in 1987 with the addition of 255 kg ha-1 N. These are large variations between the 2 yr. The addition of hairy vetch residue reduced the effect of N fertilizer on WUE. Varvel (1995) found that adding N fertilizer increased WUE in grain sorghum; Smika et al. (1965) found a similar response for native grasses as did Campbell et al. (1992) for wheat and Varvel (1994) for corn. For these studies, N additions increased WUE through increases in biomass production.
Nitrogen management and its effect on WUE can be different in poorly drained soils. Stout and Schnabel (1997) showed that for perennial ryegrass (Lolium perenne L.), WUE decreased with poor drainage because of the loss of available N through denitrification, resulting in poor plant growth. Water use efficiency declined 26% in the spring and 20% in the summer due to decreased biomass production. Water use efficiencies for perennial ryegrass ranged from 2.2 to 7.7 kg ha-1 mm-1 as N application increased from 0 to 126 kg ha-1. Water use efficiencies were 20.2 kg ha-1 mm-1 for orchardgrass (Dactylis glomerta L.) and 22.7 kg ha-1 mm-1 for tall fescue (Festuca arundinacea Schreb.) (Stout, 1992). These WUEs are much higher than those for perennial ryegrass. Nitrogen dynamics within the soil may have a large impact on WUE.
Phosphorus
The addition of P to soils and it effect on WUE has been documented on a limited number of crops. Singh and Bhushan (1980) found that addition of P to chickpea (Cicer arietinum L.) increased yield, water use, and WUE. The increase in WUE was from 8.5 to 12.2 kg ha-1 mm-1 at 0 and 100 kg P ha-1. This gain was due to a greater depletion of soil water with fertilizer and a yield increase.
Payne et al. (1992)(1995) found that in low-P soils, the addition of P fertilizer increased the dry matter yield and WUE of pearl millet. The soil nutrient status affects the growth efficiency of crops, which leads to improved dry matter production relative to a given amount of water used by the crop. These changes increase the WUE.
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SOIL MANAGEMENT PRACTICES |
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TOPABSTRACTINTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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If we examine the results shown in Tables 1 and 2, there are a number of features that begin to emerge. First, there is a large amount of variation among studies on WUE. Second, there is large degree of variation among years within locations. These patterns begin to reveal some characteristics about WUE relative to soil management. Variation among years is related more to water use rates caused by changes in precipitation and net radiation. Variation within a location can be attributed to any soil management practice that affects biomass production or the interception of radiation for plant growth. This is illustrated in Fig. 1 where we plotted a distribution of water use relative to biomass production. The distribution of water use rates and the biomass production reveal information about the potential of implementing soil management practices that would have a positive impact on WUE.
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CHALLENGES AND IMPLICATIONS |
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TOPABSTRACTINTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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The challenge before us is to continue to develop soil management practices with the goal of increasing WUE. Challenges for research are to understand the water x nutrient interactions for a range of cropping systems and to incorporate this information into tools that can assist producers in making management decisions that will lead to increased WUE and nutrient use efficiency. Variation among genetic material exists for their photosynthetic response, and incorporating this knowledge with light interception patterns during the growing season will reveal how management practices affect biomass and grain yields.
Received for publication January 31, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONPRECIPITATION USE EFFICIENCYSOIL SURFACE MODIFICATIONSSOIL NUTRIENT STATUSSOIL MANAGEMENT PRACTICESCHALLENGES AND IMPLICATIONSREFERENCES |
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