Agronomic options for improving rainfall-use efficiency of crops in dryland farming systems

Yields of dryland (rainfed) wheat in Australia have increased steadily over the past century despite rainfall being unchanged, indicating that the rainfall-use efficiency has increased. Analyses suggest that at least half of the increase in rainfall-use efficiency can be attributed to improved agronomic management. Various methods of analysing the factors influencing dryland yields and rainfall-use efficiency, such as simple rules and more complex models, are presented and the agronomic factors influencing water use, water-use efficiency, and harvest index of crops are discussed. The adoption of agronomic procedures such as minimum tillage, appropriate fertilizer use, improved weed/disease/insect control, timely planting, and a range of rotation options, in conjunction with new cultivars, has the potential to increase the yields and rainfall-use efficiency of dryland crops. It is concluded that most of the agronomic options for improving rainfall-use efficiency in rainfed agricultural systems decrease water losses by soil evaporation, runoff, throughflow, deep drainage, and competing weeds, thereby making more water available for increased water use by the crop.     

Analysis of water- and nitrogen-use efficiency of wheat in a Mediterranean climate

Water-use efficiency (WUE [g grain yield m^–2 mm^–1 ET]) and nitrogen-use efficiency (NUE [ g grain yield g^–1 N_applied]) are important measures that can affect the productivity of crops in different environmental systems. However, measurement and interpretation of WUE and NUE in the field are often hampered by the high degree of complexity of these systems due to season-to-season variability in rainfall, the variation in crop responses to soil types and to agronomic management. To be able to guide agronomic practice, experimentally-derived measurements of WUE and NUE need to be extrapolated across time and space through appropriate modelling. To illustrate this approach, the Agricultural Production Systems Simulator (APSIM), which has been rigorously tested for wheat (Triticum aestivum L.) in a Mediterranean environment, was used to estimate and analyse the WUE and NUE of wheat crops in the Mediterranean-climatic region of the central Western Australian agricultural zone. The APSIM model was run for three locations (average annual rainfall of 461 mm [high rainfall zone], 386 mm [medium] and 310 mm [low]) and two soil types that had contrasting plant-available water-holding capacities in the rooting zone (sand: 55 mm, clay soil: 109 mm). Simulations were carried out with historical weather records (82–87 years) assuming current crop management and cultivars. The modelling analyses highlighted the inherently high degree of seasonal variability in yield, WUE and NUE of wheat, depending on soil type, N fertiliser input, rainfall amount and, in particular, rainfall distribution. The clay soil tended to be more productive in terms of grain yield, WUE and NUE in the high and medium rainfall zones, but less productive in most years in the low rainfall zone. The sandy soil was less productive in the high rainfall zone due to the high nitrate leaching potential of this soil type, but more productive than the clay in the low rainfall zone due to poorer pre-anthesis growth and less water use, less water loss by soil evaporation and relatively more water use in the post-anthesis phase. When a wheat crop was sown early on clay soil in the low rainfall zone, it yielded as high as in the other rainfall zones in seasons when rainfall was above average or there was a good store of water in the soil prior to sowing. The simulations confirmed findings from a limited number of field experiments and extended these findings both qualitatively and quantitatively across soil types, rainfall regions and crop management options. Furthermore, by using long-term historical weather records, the simulations extended the findings across the wide range of climatic scenarios experienced in mediterranean-climatic regions.     

Water budgets and nutrients in a native Banksia woodland and an adjacent Medicago sativa pasture

In southern Australia, the spread of dryland salinity can be traced to increased leakage of water through the root zone to the groundwater, associated with clearing of perennial vegetation and its replacement with annual crops and pastures. Agricultural activity, through fertilizer addition and subsequent nutrient export, has also changed the nutrient status of the soils, often causing increases in soil acidity. In this trial, an area of native vegetation on a deep sandy soil in south-western Australia (dominated by Banksia prionotes Lindley), and an adjacent introduced perennial pasture (Medicago sativa L.), were compared in terms of their water balance and nutrient fluxes for the period between August 1998 and March 2001. Initially, the Banksia woodland vegetation maintained a drier soil profile below 2 m than the establishing lucerne pasture, and leakage beyond 4 m in 1999 was 80 mm under the Banksia woodland and 180 mm under the lucerne. However, in 2000, lucerne's rate of water use during winter was faster than any other vegetation observed on this soil type, possibly due to direct groundwater extraction, and it dried the soil to the same level as the Banksia woodland vegetation. Nutrient (nitrate, phosphate, sulphate, potassium, calcium, magnesium and sodium) fluxes under both systems were generally low, reflecting the inherently low fertility of the soil type. However, sodium and nitrate appeared to accumulate in soil at a depth of 4 m under the Banksia woodland (particularly between the Banksia canopies), but not under the lucerne, possibly due to a history of leaching under the lucerne. Whilst both vegetation types effectively controlled excess water leakage, the differences in nutrient cycling and production levels suggests that some aspects of native perennial vegetation function may not be suitable for incorporation into agricultural systems.     

Transpiration efficiency of three Mediterranean annual pasture species and wheat

Attempts to improve water use efficiency in regions with Mediterranean climates generally focus on increasing plant transpiration relative to evaporation from the soil and increasing transpiration efficiency. Our aim was to determine if transpiration efficiency differs among key species occurring in annual pastures in southern Australia. Two glasshouse experiments were conducted with three key pasture species, subterranean clover (Trifolium subterraneum L.), capeweed [Arctotheca calendula (L.) Levyns] and annual ryegrass (Lolium rigidum Gaudin), and wheat (Triticum aestivum L.). Transpiration efficiency was assessed at the levels of␣whole-plant biomass and water use (W), leaf gas exchange measurements of the ratio of CO₂ assimilation to leaf conductance to water vapour (A/g), and carbon isotope discrimination (Δ) in leaf tissue. In addition, Δ was measured on shoots of the three pasture species growing together in the field. In the glasshouse studies, annual ryegrass had a consistently higher transpiration efficiency than subterranean clover or capeweed by all methods of measurement. Subterranean clover and capeweed had similar transpiration efficiencies by all three methods of measurement. Wheat had W values similar to ryegrass but A/g and Δ values similar to subterranean clover or capeweed. The high W of annual ryegrass seems to be related to a conservative leaf gas exchange behaviour, with lower assimilation and conductance but higher A/g than for the other species. In contrast to the glasshouse results, the three pasture species had similar Δ values when growing together in mixed-species swards in the field. Reasons for these differing responses between glasshouse and field-grown plants are discussed in terms of the implications for improving the transpiration efficiency of mixed-species annual pasture communities in the field. Received: 6 March 1997 / Accepted: 23 December 1997     

A plant functional approach to the prediction of changes in Australian rangeland vegetation under grazing and fire

Abstract. Grazing by domestic livestock and changed fire regimes by humans have caused major changes in the productivity and composition of rangelands in Australia and other continents. Of particular concern are the commonly observed loss of perennial forage species and the increasing abundance of woody plants. Grazing and fire‐induced changes are difficult to predict from current process knowledge and are often too costly or time‐consuming to investigate experimentally. We describe the development and use of ARENA, a new simulation model. A plant functional approach is used in which the relative growth rate, competitive ability and life cycle of the plant types are mainly defined by the plant's morphology and allocation pattern, plus its water‐use efficiency and nitrogen concentration. The soil and plant types can be parameterized to a large extent with information from the literature, facilitating application in a broad range of dryland environments. The model has been tested for two soils and pasture communities in the seasonally dry tropics of the Victoria River District, N Australia. Predictions of pasture production and perennial grass fraction under undisturbed conditions agreed with observations in field exclosures. Predictions of maximum tree density also coincided with observations along a rainfall gradient. Simulation experiments were conducted to explore the effect of different stocking levels and fire management regimes on pasture productivity and composition. Responses of pastures on red loam and grey clay soils were generally consistent with regional field experience, but the model did not reproduce the expected changes in the abundance of woody plants.     

Seasonal changes in characteristics of cattle dung as a resource for an insect in southwestern Australia

The value of cattle dung as a food resource for the bush fly Musca vetustissima (Walker) in the winter rainfall agricultural region of southwestern Australia was assessed by bioassay in the laboratory. The size (headwidth) of adult females was measured from flies reared on different samples of dung. Variation in size correlated with seasonal patterns of pasture growth, larger flies being produced during the growing season from autumn to spring. Size declined with senescence of annual pastures in late spring and early summer, occurring later in southern areas where the growing season was longer. After pasture senescence, dung from shorter growing season areas usually produced larger flies, apparently a result of the inverse relationship between digestibility of feed and length of growing season. Dung from irrigated perennial pastures never produced flies as large as that from annual pastures but generally high values were sustained during summer. Grazing of cereal stubble and feeding of hay in annual pasture areas during summer usually caused some increase in fly size. A spontaneous resurgence in the size of flies often occurred several weeks after pasture senescence and was attributed to more thorough digestion as a result of reduced intake of less palatable dry pasture.     

Improving water use in crop production

Globally, agriculture accounts for 80-90% of all freshwater used by humans, and most of that is in crop production. In many areas, this water use is unsustainable; water supplies are also under pressure from other users and are being affected by climate change. Much effort is being made to reduce water use by crops and produce 'more crop per drop'. This paper examines water use by crops, taking particularly a physiological viewpoint, examining the underlying relationships between carbon uptake, growth and water loss. Key examples of recent progress in both assessing and improving crop water productivity are described. It is clear that improvements in both agronomic and physiological understanding have led to recent increases in water productivity in some crops. We believe that there is substantial potential for further improvements owing to the progress in understanding the physiological responses of plants to water supply, and there is considerable promise within the latest molecular genetic approaches, if linked to the appropriate environmental physiology. We conclude that the interactions between plant and environment require a team approach looking across the disciplines from genes to plants to crops in their particular environments to deliver improved water productivity and contribute to sustainability.     

Factors regulating the contributions of fixed nitrogen by pasture and crop legumes to different farming systems of eastern Australia

On-farm and experimental measures of the proportion (%Ndfa) and amounts of N₂ fixed were undertaken for 158 pastures either based on annual legume species (annual medics, clovers or vetch), or lucerne (alfalfa), and 170 winter pulse crops (chickpea, faba bean, field pea, lentil, lupin) over a 1200 km north-south transect of eastern Australia. The average annual amounts of N₂ fixed ranged from 30 to 160 kg shoot N fixed ha^–1 yr^–1 for annual pasture species, 37–128 kg N ha^–1 yr^–1 for lucerne, and 14 to 160 kg N ha^–1 yr^–1 by pulses. These data have provided new insights into differences in factors controlling N₂ fixation in the main agricultural systems. Mean levels of %Ndfa were uniformly high (65–94%) for legumes growing at different locations under dryland (rainfed) conditions in the winter-dominant rainfall areas of the cereal-livestock belt of Victoria and southern New South Wales, and under irrigation in the main cotton-growing areas of northern New South Wales. Consequently N₂ fixation was primarily regulated by biomass production in these areas and both pasture and crop legumes fixed between 20 and 25 kg shoot N for every tonne of shoot dry matter (DM) produced. Nitrogen fixation by legumes in the dryland systems of the summer-dominant rainfall regions of central and northern New South Wales on the other hand was greatly influenced by large variations in %Ndfa (0–81%) caused by yearly fluctuations in growing season (April–October) rainfall and common farmer practice which resulted in a build up of soil mineral-N prior to sowing. The net result was a lower average reliance of legumes upon N₂ fixation for growth (19–74%) and more variable relationships between N₂ fixation and DM accumulation (9–16 kg shoot N fixed/t legume DM). Although pulses often fixed more N than pastures, legume-dominant pastures provided greater net inputs of fixed N, since a much larger fraction of the total plant N was removed when pulses were harvested for grain than was estimated to be removed or lost from grazed pastures. Conclusions about the relative size of the contributions of fixed N to the N-economies of the different farming systems depended upon the inclusion or omission of an estimate of fixed N associated with the nodulated roots. The net amounts of fixed N remaining after each year of either legume-based pasture or pulse crop were calculated to be sufficient to balance the N removed by at least one subsequent non-legume crop only when below-ground N components were included. This has important implications for the interpretation of the results of previous N₂ fixation studies undertaken in Australia and elsewhere in the world, which have either ignored or underestimated the N present in the nodulated root when evaluating the contributions of fixed N to rotations.     

The eco-hydrology of partly cleared,native ecosystems in southern Australia: a review

Water use by the native vegetation that existed in southern Australia prior to European settlement was largely in balance with rainfall. European settlers altered the landscape by clearing land to grow agricultural crops and pastures, and with the introduction of livestock to graze the partly cleared, native ecosystems. The aim of this review is to contrast the hydrology of grazed, partly cleared ecosystems, intact indigenous ecosystems, and entirely cleared agricultural systems in the intensive land-use zone (350–1000 mm annual rainfall zone) of southern Australia. Since European settlement, the areas of forests and woodlands in the Murray–Darling Basin have declined by approximately 64% to make way for agricultural enterprises. Modern-day vegetation surveys estimate between 52 and 58% of the intensive land-use zone of the Murray–Darling Basin has been entirely cleared, while about 40% is in the partly cleared state (disturbed ecosystems with canopy cover exceeding 5%). The replacement of native vegetation by agricultural crops and pastures has disturbed the water cycle that existed prior to European settlement, and has markedly elevated the amount of water leaking beyond the root zone of introduced species, and contributing to groundwater systems. Estimates of annual leakage beneath the root zone of annual crops range from 0 to 63 mm per annum; however, no estimates of leakage for partly cleared woodlands exist. Yet, because the groundwater beneath partly cleared woodlands rises considerably more slowly than under entirely cleared landscapes, it is likely that less water leaks beneath the roots of grazed woody ecosystem. However, aging of these systems by livestock grazing will reduce the numbers of woody individuals and will impact on groundwater recharge.     

Changes in soil organic carbon under perennial crops

This study evaluates the dynamics of soil organic carbon (SOC) under perennial crops across the globe. It quantifies the effect of change from annual to perennial crops and the subsequent temporal changes in SOC stocks during the perennial crop cycle. It also presents an empirical model to estimate changes in the SOC content under crops as a function of time, land use, and site characteristics. We used a harmonized global dataset containing paired‐comparison empirical values of SOC and different types of perennial crops (perennial grasses, palms, and woody plants) with different end uses: bioenergy, food, other bio‐products, and short rotation coppice. Salient outcomes include: a 20‐year period encompassing a change from annual to perennial crops led to an average 20% increase in SOC at 0–30 cm (6.0 ± 4.6 Mg/ha gain) and a total 10% increase over the 0–100 cm soil profile (5.7 ± 10.9 Mg/ha). A change from natural pasture to perennial crop decreased SOC stocks by 1% over 0–30 cm (?2.5 ± 4.2 Mg/ha) and 10% over 0–100 cm (?13.6 ± 8.9 Mg/ha). The effect of a land use change from forest to perennial crops did not show significant impacts, probably due to the limited number of plots; but the data indicated that while a 2% increase in SOC was observed at 0–30 cm (16.81 ± 55.1 Mg/ha), a decrease in 24% was observed at 30–100 cm (?40.1 ± 16.8 Mg/ha). Perennial crops generally accumulate SOC through time, especially woody crops; and temperature was the main driver explaining differences in SOC dynamics, followed by crop age, soil bulk density, clay content, and depth. We present empirical evidence showing that the FAO perennialization strategy is reasonable, underscoring the role of perennial crops as a useful component of climate change mitigation strategies.     

Water relations in competitive interactions of Mediterranean grasses and shrubs

In Mediterranean ecosystems, competition between opportunistic grasses and slower-growing woody species may affect the speed and path of ecosystem recovery and the success of restoration plantings after natural or human-induced disturbance. In this experiment, competitive interactions between Mediterranean annual and perennial grass species (Avena fatua and Brachypodium retusum, respectively) and an important Mediterranean shrub (Rosmarinus offlcinalis) were examined under semi-controlled conditions simulating wet and dry Mediterranean rainfall regimes. The identity of the grass competitor and the level of water availability in the plots interacted to produce differing rates of R. offlcinalis growth but similar levels of mortality. In particular, competition with the perennial grass resulted in very low rates of R. offlcinalis growth at both irrigation levels. Measurements of soil water content showed that both grasses reduced soil moisture to low levels, though this effect was temporary in the case of the winter annual grass. Resistance to hydraulic flow in roots was highest in the perennial grass, smaller but of similar magnitude in the shrub, and much lower in the annual grass. Transpirational response to decreasing leaf water potential was a quick, sharp drop in conductance in R. offlcinalis, in contrast to a moderated decline from much lower initial transpiration rates in B. retusum. The annual grass largely maintained both leaf water potential and transpiration through leaf-tip senescence and death. Quantification of the rate of hydric recuperation of leaves after irrigation of drought-stressed plants showed that the perennial grass recovered at a rate four times that of R. offlcinalis, suggesting a strategy for making quick use of rare summer rains that may contribute to its competitive success. The appropriateness of planting or suppressing grasses in restoration of disturbed sites in Mediterranean Spain is discussed.     

Estimates of seasonal nitrogen fixation of annual subterranean clover-based pastures using the15N natural abundance technique

Annual pasture legumes play a key role in ley farming systems of southern Australia, providing biologically fixed nitrogen (N) to drive the production of the pastures as well as subsequent crops grown in rotation. Seasonal inputs of biologically fixed N in shoot biomass of the subterranean clover (Trifolium subterraneum) component of grazed annual pastures were assessed using the¹⁵N natural abundance technique and appropriately timed sampling of herbage dry matter (DM) for N accumulation. At three study sites spanning a gradient across the Western Australian wheatbelt from 300 to 600 mm annual rainfall the performance of the clover and non-legume herbs and grasses was examined as paired comparisons involving two management treatments expected to give contrasting effects on pasture productivity, botanical composition and N₂ fixation. The proportion of clover N derived from atmospheric N₂ fixation (%Ndfa) ranged from 65 to 95% across sites, treatments and sampling times. Amounts of fixed N accumulated in clover shoot biomass ranged from 50 to 125 kg ha⁻¹, and paralleled trends in clover production. Substantial increases in pasture production in high yielding treatments generally occurred without decrease in %Ndfa, suggesting that N₂ fixation was essentially non-limiting to performance of the clover component. Seasonal profiles for accumulation of fixed N were skewed towards the late winter and spring period, particularly in low plant density pastures following a cereal crop. There were seasonal, site and treatment-specific effects on the proportion of clover and non-legume pasture components and consequently clover yield and N₂ fixation were variably affected by competition from non-legume species.     

The transpiration response by four topographically distributed Eucalyptus species, to rainfall occurring during drought in south eastern Australia

Vegetation covers a substantial proportion of the terrestrial Earth surface and where removed from human influence has evolved influenced mainly by climatic and edaphic constraints. Over the past 200 years, substantial tracts of the wheat-sheep belt of southern Australia has been cleared of native vegetation and replaced by annual crop and pasture species. As a consequence, there has been a change in the water balance of many catchments as now more water leaks beneath the roots of introduced plants, contributing to groundwater rise and expansion in the amount of land affected by salinization. In an attempt to arrest root zone leakage, scientists and managers acknowledge the need for new agricultural practices which mimic the ecohydrological behaviour patterns of remnant vegetation.
In this paper, we examine the water use characteristics of four woodland Eucalyptus species growing in different topographic and edaphic environments in south eastern Australia. Eucalyptus sideroxylon and E. rossii were the sub- and dominant species inhabiting the stony ridges, while in the adjacent valleys E. macrorhyncha and E. albens were the sub- and dominant species, respectively. At the two locations, sub-dominant species were highly responsive to episodic rainfall events both during and following drought, and exhibited distinct seasonality in daily transpiration rate; while the dominant species at each location was less responsive to rainfall and water use appeared to be less seasonally dependent. Analysis of the water use response indicated that the two sub-dominant species had shallower roots while the major of roots of the dominant trees were likely to be located deeper in the substratum. This suggested that the stony ridges may store water deeper in the substratum than previously thought, to sustain some remnant vegetation over the dry summers, and ultimately, contribute less recharge to groundwater.     

Benefits of increasing transpiration efficiency in wheat under elevated CO2 for rainfed regions

Higher transpiration efficiency (TE) has been proposed as a mechanism to increase crop yields in dry environments where water availability usually limits yield. The application of a coupled radiation and TE simulation model shows wheat yield advantage of a high‐TE cultivar (cv. Drysdale) over its almost identical low‐TE parent line (Hartog), from about ?7 to 558 kg/ha (mean 187 kg/ha) over the rainfed cropping region in Australia (221–1,351 mm annual rainfall), under the present‐day climate. The smallest absolute yield response occurred in the more extreme drier and wetter areas of the wheat belt. However, under elevated CO₂ conditions, the response of Drysdale was much greater overall, ranging from 51 to 886 kg/ha (mean 284 kg/ha) with the greatest response in the higher rainfall areas. Changes in simulated TE under elevated CO₂ conditions are seen across Australia with notable increased areas of higher TE under a drier climate in Western Australia, Queensland and parts of New South Wales and Victoria. This improved efficiency is subtly deceptive, with highest yields not necessarily directly correlated with highest TE. Nevertheless, the advantage of Drysdale over Hartog is clear with the benefit of the trait advantage attributed to TE ranging from 102% to 118% (mean 109%). The potential annual cost‐benefits of this increased genetic TE trait across the wheat growing areas of Australia (5 year average of area planted to wheat) totaled AUD 631 MIL (5‐year average wheat price of AUD/260 t) with an average of 187 kg/ha under the present climate. The benefit to an individual farmer will depend on location but elevated CO₂ raises this nation‐wide benefit to AUD 796 MIL in a 2°C warmer climate, slightly lower (AUD 715 MIL) if rainfall is also reduced by 20%.     

Plants for amelioration of subsoil constraints and hydrological control: the primer-plant concept

In this review, we propose the use of suitable plant species, termed primer-plants, for the primary purpose of preparing soil conditions for the benefit of following crops. Such plants may be used in the temperate agricultural belts of southern Australia, where dryland salinity is a major environmental and agricultural problem that threatens the viability of many crop production enterprises. It is recognised that growing plants that have deeper roots and use more water than the current shallow-rooted annual crops provide a long-term solution for managing the dryland salinity problem. Increased plant water-use is expected to mitigate the rising watertable that transfers salt to the root-zone of crop plants. On medium to heavy textured soils, common in this region, impermeability of the subsoil to roots and water movement is another major impediment to high water-use and productivity by plants, which may lead to other adverse hydrological events in the soil such as water-logging and excessive run-off. Plants that possess the ability to penetrate the dense subsoil and make it porous, in addition to having the capacity for using soil water at high rates, should be effective in combating dryland salinity. These plants normally should have thick roots that grow deep in the soil and are able to modify or withstand the adverse chemistry of the often-saturated subsoil, so that upon the death and decay of their roots, channels or biopores are created. These {biopores} have greater vertical and lateral continuity and last longer than porosity created through mechanical tillage. In this paper, we argue that potential exists for inclusion of short to medium-term phases of primer-plants in farming systems as a mimic of pre-existing perennial vegetation. We propose that ideal plants for combating dryland salinity should have high water-use and capacity to also improve soil structure and, possibly, nutrition. Examples are presented of soil amelioration that generally supports the viability of primer-plant concept, including the limited work undertaken in south-eastern Australia. We identified key knowledge-gaps, such as lack of well-defined agronomic packages for growing short-phases of Australian native species in mixtures, and our limited understanding of their root dynamics, which need to be addressed before effective implementation of the primer-plant concept.     

Daily, seasonal and annual patterns of transpiration from a stand of remnant vegetation dominated by a coniferous Callitris species and a broad-leaved Eucalyptus species

Quantifying water use of native vegetation is an important contribution to understanding landscape ecohydrology. Few studies provide long-term (more than one growing season) estimates of water use and even fewer quantify interseasonal and interannual variation in transpiration. Globally, changes in land use are significantly altering landscape ecohydrology, resulting in problems such as dryland salinity and excessive groundwater recharge. Estimating stand water use is complex in multispecies forests, due to the differences in relationships among sapwood area, basal area and tree size for co-occurring species. In this article, we examine seasonal and interannual variation in transpiration rate of the tree canopy of two co-occurring species (a conifer Callitris glaucophylla J. Thompson & L.A.S. Johnson and a broad-leaved Eucalyptus crebra F. Muell.) in an open woodland in eastern Australia. Evapotranspiration of understorey species was measured using an open-top chamber, and tree water use was measured using heat-pulse sap flow sensors. Annual stand transpiration was 309 mm in 2003, a year of below average rainfall, and 629 mm in 2004, a year with higher-than-average rainfall. Despite an almost doubling (522 vs. 1062 mm) of annual rainfall between 2003 and 2004, annual tree water use remained a constant fraction (59%) of rainfall, indicative of compensatory mechanisms linking annual rainfall, leaf area index and tree water use. Deep drainage was estimated to be 4% of rainfall (20.8 mm) in 2003 and 2% (21.2 mm) in 2004, indicating that this native woodland was able to minimize deep drainage despite large interannual variability in rainfall.     

Evapotranspiration of annual and perennial biofuel crops in a variable climate

Eddy covariance measurements were made in seven fields in the Midwest USA over 4 years (including the 2012 drought year) to estimate evapotranspiration (ET) of newly established rain‐fed cellulosic and grain biofuel crops. Four of the converted fields had been managed as grasslands under the USDA's Conservation Reserve Program (CRP) for 22 years, and three had been in conventional agriculture (AGR) soybean/corn rotation prior to conversion. In 2009, all sites were planted to no‐till soybean except one CRP grassland that was left unchanged as a reference site; in 2010, three of the former CRP sites and the three former AGR sites were planted to annual (corn) and perennial (switchgrass and mixed‐prairie) grasslands. The annual ET over the 4 years ranged from 45% to 77% (mean = 60%) of the annual precipitation (848–1063 mm; November–October), with the unconverted CRP grassland having the highest ET (622–706 mm). In the fields converted to annual and perennial crops, the annual ET ranged between 480 and 639 mm despite the large variations in growing‐season precipitation and in soil water contents, which had strong effects on regional crop yields. Results suggest that in this humid temperate climate, which represents the US Corn Belt, water use by annual and perennial crops is not greatly different across years with highly variable precipitation and soil water availability. Therefore, large‐scale conversion of row crops to perennial biofuel cropping systems may not strongly alter terrestrial water balances.     

植被覆盖度的时间变化及其防风蚀效应

 在防治风蚀过程中过去人们只关注植被覆盖度的空间特性,但对其随时间变化的特性未引起足够的重视。该文着重强调了植被覆盖度随时间变化的特性,并对不同类型植物覆盖度的动态变化特征进行了研究。通过调查研究与理论分析,在土壤风蚀量与植被覆盖度及风蚀气候侵蚀因子三者之间建立了随时间变化的定量关系,并利用该公式计算和比较了不同类型植物防风治沙性能的动态差异、总植被覆盖度及相应的总土壤风蚀量的动态变化。结果表明在防风蚀的作用效应中灌木＞多年生牧草＞林木＞作物＞一年生牧草;总时空植被覆盖度与总土壤风蚀量呈“反相位”的动态变化;风蚀季节总植被覆盖度较低,介于0.11～0.14之间,低于20%的临界覆盖度,这也是该地区风蚀危害严重的一个重要原因所在。     

Pest-Suppression Potential of Midwestern Landscapes under Contrasting Bioenergy Scenarios

Biomass crops grown on marginal soils are expected to fuel an emerging bioenergy industry in the United States. Bioenergy crop choice and position in the landscape could have important impacts on a range of ecosystem services, including natural pest-suppression (biocontrol services) provided by predatory arthropods. In this study we use predation rates of three sentinel crop pests to develop a biocontrol index (BCI) summarizing pest-suppression potential in corn and perennial grass-based bioenergy crops in southern Wisconsin, lower Michigan, and northern Illinois. We show that BCI is higher in perennial grasslands than in corn, and increases with the amount of perennial grassland in the surrounding landscape. We develop an empirical model for predicting BCI from information on energy crop and landscape characteristics, and use the model in a qualitative assessment of changes in biocontrol services for annual croplands on prime agricultural soils under two contrasting bioenergy scenarios. Our analysis suggests that the expansion of annual energy crops onto 1.2 million ha of existing perennial grasslands on marginal soils could reduce BCI between -10 and -64% for nearly half of the annual cropland in the region. In contrast, replacement of the 1.1 million ha of existing annual crops on marginal land with perennial energy crops could increase BCI by 13 to 205% on over half of the annual cropland in the region. Through comparisons with other independent studies, we find that our biocontrol index is negatively related to insecticide use across the Midwest, suggesting that strategically positioned, perennial bioenergy crops could reduce insect damage and insecticide use on neighboring food and forage crops. We suggest that properly validated environmental indices can be used in decision support systems to facilitate integrated assessments of the environmental and economic impacts of different bioenergy policies.     

黄土丘陵区旱地作物水分生态适应性系统评价

根据调查资料和田间试验结果,系统地分析了旱地不同作物生长发育与降水分布的时序关系、旱地作物水分潜在利用率和旱地主要作物水分供需平衡与错位特征,并利用水分生态适应性数学模型,对宁南黄土丘陵区主要作物的水分生态适应性进行了定量评价．结果表明,秋熟作物生长发育与降水分布耦合性较好,夏熟作物生长发育与降水分布耦合性较差．不同作物的降水潜在利用率存在差异。其基本规律是：多年生牧草>薯类作物>谷类作物;秋熟作物>夏熟作物．作物的水分满足率和生态适应性,秋熟作物优于夏熟作物,丰水年份优于干旱年份．旱地6种主要作物的水分生态适应性指数排序依次为：谷子>马铃薯>糜子>胡麻>豌豆>春小麦．