Plant-available Water-holding Capacity Research Articles

Predicting plant available water holding capacity of soils from crop yield

Quantification of variations in plant available water holding capacity (PAWC) of soils helps to improve yield forecast and inform spatially variable management practices in dryland agriculture systems. We developed and tested a general inverse approach to estimate PAWC from crop yield. The APSIM model was used to simulate wheat yield on synthetic soils with contrasting PAWC and climates. The simulated results were used to develop an empirical model to relate simulated yield to PAWC. The empirical model was inversely used to predict PAWC from observed crop yield. Potential prediction ability was quantified using independently simulated wheat yield on actual soils. The actual ability was assessed with measured wheat yields and PAWC. The approach had higher accuracy for sites with high rainfall or dominant summer rainfall. It could potentially provide acceptable PAWC predictions across contrasting climate regions (prediction error < 37 mm, 33.5%). The prediction error using crop yield against measured PAWC was <25 mm (26.5%). Our results demonstrate that soil PAWC can be reliably predicted from crop yield. This approach provides an alternative way to predict PAWC rather than directly measuring it via soil sampling, with profound implications for reducing labour and costs.

Machine learning approaches for spatial modeling of agricultural droughts in the south-east region of Queensland Australia

A quantitative understanding of the hydro-environmental factors that influence the occurrence of agricultural drought events would enable more strategic climate change adaptation and drought management plans. Practical drought hazard mapping remains challenging due to possible exclusion of the most pertinent drought drivers, and to the use of inadequate predictive models that cannot describe drought adequately. This research aims to develop new approaches to map agricultural drought hazard with state-of-the-art machine learning models, including classification and regression trees (CART), boosted regression trees (BRT), random forests (RF), multivariate adaptive regression splines (MARS), flexible discriminant analysis (FDA) and support vector machines (SVM). Hydro-environmental datasets were used to calculate the relative departure of soil moisture (RDSM) for eight severe droughts for drought-prone southeast Queensland, Australia, over the period 1994–2013. RDSM was then used to generate an agricultural drought inventory map. Eight hydro-environmental factors were used as potential predictors of drought. The goodness-of-fit and predictive performance of all models were evaluated using different threshold-dependent and threshold-independent methods, including the true skill statistic (TSS), Efficiency (E), F-score, and the area under the receiver operating characteristic curve (AUC-ROC). The RF model (AUC-ROC = 97.7%, TSS = 0.873, E = 0.929, F-score = 0.898) yielded the highest accuracy, while the FDA model (with AUC-ROC = 73.9%, TSS = 0.424, E = 0.719, F-score = 0.512) showed the worst performance. The plant available water holding capacity (PAWC), mean annual precipitation, and clay content were the most important variables to be used for predicting the agricultural drought. About 21.2% of the area is in high or very high drought risk classes, and therefore, warrant drought and environmental protection policies. Importantly, the models do not require data on the precipitation anomaly for any given drought year; the spatial patterns in AGH were consistent for all drought events, despite very different spatial patterns in precipitation anomaly among events. Such machine-learning approaches are able to construct an overall risk map, thus assisting in the adoption of a robust drought contingency planning measure not only for this area, but also, in other regions where drought presents a pressing challenge, including its influence on key practical dimensions of social, environmental and economic sustainability.

On the relation between soil water holding capacity and dryland crop productivity

Plant available water holding capacity (PAWC) influences the efficiency of rainfall used by crops and consequently crop productivity in dryland cropping systems. However, how soil PAWC interacts with rainfall to determine crop yield has not been quantified systematically. In this study, we used an agricultural production systems model (APSIM) and wheat as an example to quantify crop growth and yield in response to a wide range of soil PAWCs. Results show that wheat yield increased with soil PAWCs following a negative exponential curve. There is a critical PAWC above which PAWC has no impact. A summary model incorporating the impact of soil PAWC, annual rainfall and the fraction of rainfall during growing season together could explain over 95% variations in long-term averaged wheat yield. The results and the summary model provide the scientific basis to systematically explain crop yield variation in response to soil PAWC change across contrasting climates.

Remote Sensing Derived Phenological Metrics to Assess the Spatio-Temporal Growth Variability in Cropping Fields

Precision Agriculture (PA) recognizes and manages intra-field spatial variability to increase profitability and reduced environmental impact. Site Specific Crop Management (SSCM), a form of PA, subdivides a cropping field into uniformly manageable zones, based on quantitative measurement of yield limiting factors. In Mediterranean environments, the spatial and temporal yield variability of rain-fed cropping system is strongly influenced by the spatial variability of Plant Available Water-holding Capacity (PAWC) and its strong interaction with temporally variable seasonal rainfall. The successful adoption of SSCM depends on the understanding of both spatial and temporal variabilities in cropping fields. Remote sensing phenological metrics provide information about the biophysical growth conditions of crops across fields. In this paper, we examine the potential of phenological metrics to assess the spatial and temporal crop yield variability across a wheat cropping field at Minnipa, South Australia. The Minnipa field was classified into three management zones using prolonged observations including soil assessment and multiple year yield data. The main analytical steps followed in this study were: calculation of the phenological metrics using time series NDVI data from Moderate Resolution Imaging Spectroscope (MODIS) for 15 years (2001-2015); producing spatial trend and temporal variability maps of phenological metrics; and finally, assessment of association between the spatial patterns and temporal variability of the metrics with management zones of the cropping field. The spatial trend of the seasonal peak NDVI metric showed significant association with the management zone pattern. In terms of temporal variability, Time-integrated NDVI (TINDVI) showed higher variability in the “good” zone compared with the “poor” zone. This indicates that the magnitude of the seasonal peak is more sensitive to soil related factors across the field, whereas TINDVI is more sensitive to seasonal variability. The interpretation of the association between phenological metrics and the management zone site conditions was discussed in relation to soil-climate interaction. The results demonstrate the potential of the phenological metrics to assess the spatial and temporal variability across cropping fields and to understand the soil-climate interaction. The approach presented in this paper provides a pathway to utilize phenological metrics for precision agricultural management application.

Estimating the economic value of soil organic carbon for grains cropping systems in Western Australia

Soil organic carbon (SOC) has the potential to benefit soil function and fertility, and in agricultural production systems, it is considered integral to sustainable farming. We analyse the value of SOC in cropping systems of the south-west of Western Australia in terms of agronomic benefits from increasing productivity (through increased plant-available water-holding capacity) and reducing fertiliser use (due to increased mineralisation of nitrogen). We also present the potential value of SOC in terms of sequestration benefit if landholders were able to participate in a carbon-sequestration program. We estimate the marginal value of SOC (the value of a soil with more SOC, by 1 t C/ha, than a standard soil) to be AU$7.1–8.7/t C.ha.year, depending on rainfall zone and crop type. Approximately 75% of this value is the estimated sequestration value, 20% is the nitrogen-replacement value, and 5% is the estimated productivity improvement value. Over 50 years, this equates $130–160/t C.ha depending on the rainfall zone. These values are sensitive to variations in fertiliser and carbon prices. Our results imply this it is unlikely that the SOC benefits will drive practice change in the south-west of Western Australia.

P2 - Mobilization of Nitric Oxide bioactivity by Sulfide: Chemical Biology and bioactivity of s/n Hybrid molecules in the Cardiovascular System

Soil carbon is often cited as having potential to provide both climate change mitigation and adaptation benefits. Given the extensive ecosystem service benefits of soil organic matter (SOM), including increasing N supply and plant-available water-holding capacity (PAWHC), we hypothesized that on-farm benefits provide ample justification for maintaining high levels of SOM, separate to its carbon sequestration potential. To investigate this we used whole-farm system modelling to simulate pastures with high SOM (initial carbon amount similar to long-term pasture) and low SOM (initial carbon condition similar to long-term cropping). These scenarios are deliberately confounded with land use change, as this allowed for comparison of the mineralization, PAWHC, and associated productivity benefits on high and low carbon soils with the same management. Low-carbon soils were modelled with two amounts of PAWHC to investigate the importance of this effect in isolation. These three scenarios (one high carbon and two low carbon with differing PAWHC) were run for two climatic zones each with two soil types. Across both climatic zones and soil types, soil C accumulated at a rate of 0.30 0.48 t C ha−1 year−1 (0–30 cm) over the first 20 years in soils with low initial carbon amounts. On soils in a high-rainfall climate, annual pasture production in low-SOM soil was 590–900 kg DM ha−1 less than for high-SOM soil, attributable primarily to increased N mineralization (68–77 kg N ha−1 year−1) overcoming an N limitation in spring. On low-rainfall sites, a reduction in annual pasture production of 290–810 kg DM ha−1 on low carbon soils compared to high carbon soils was attributable to reduced PAWHC. The increased pasture production associated with higher SOM was valued between AUD 26 and 95 ha−1, across the soils and sites. The entire value on low-rainfall sites (AUD 26-85) was attributable to differences in PAWHC, while on high-rainfall sites, increased pasture production was attributed to N mineralization valued from AUD 85–105 ha−1. These results indicate that soil carbon sequestration, through increased SOM, can provide substantial on-farm benefits that contribute to future productivity.

Process modelling to assess the sequestration and productivity benefits of soil carbon for pasture

Soil carbon is often cited as having potential to provide both climate change mitigation and adaptation benefits. Given the extensive ecosystem service benefits of soil organic matter (SOM), including increasing N supply and plant-available water-holding capacity (PAWHC), we hypothesized that on-farm benefits provide ample justification for maintaining high levels of SOM, separate to its carbon sequestration potential. To investigate this we used whole-farm system modelling to simulate pastures with high SOM (initial carbon amount similar to long-term pasture) and low SOM (initial carbon condition similar to long-term cropping). These scenarios are deliberately confounded with land use change, as this allowed for comparison of the mineralization, PAWHC, and associated productivity benefits on high and low carbon soils with the same management. Low-carbon soils were modelled with two amounts of PAWHC to investigate the importance of this effect in isolation. These three scenarios (one high carbon and two low carbon with differing PAWHC) were run for two climatic zones each with two soil types. Across both climatic zones and soil types, soil C accumulated at a rate of 0.300.48tCha−1year−1 (0–30cm) over the first 20 years in soils with low initial carbon amounts. On soils in a high-rainfall climate, annual pasture production in low-SOM soil was 590–900kgDMha−1 less than for high-SOM soil, attributable primarily to increased N mineralization (68–77kgNha−1year−1) overcoming an N limitation in spring. On low-rainfall sites, a reduction in annual pasture production of 290–810kgDMha−1 on low carbon soils compared to high carbon soils was attributable to reduced PAWHC. The increased pasture production associated with higher SOM was valued between AUD 26 and 95ha−1, across the soils and sites. The entire value on low-rainfall sites (AUD 26-85) was attributable to differences in PAWHC, while on high-rainfall sites, increased pasture production was attributed to N mineralization valued from AUD 85–105ha−1. These results indicate that soil carbon sequestration, through increased SOM, can provide substantial on-farm benefits that contribute to future productivity.

Water use efficiency and crop water balance of rainfed wheat in a semi-arid environment: sensitivity of future changes to projected climate changes and soil type

Wheat production is expected to be affected by climate change through changing components of the crop water balance such as rainfall, evapotranspiration (ET), runoff and drainage. We used the Agricultural Production Systems Simulator (APSIM)-wheat model to simulate the potential impact of climate change on field water balance, ET and water use efficiency (WUE) under the SRES A2 emissions scenario. We ran APSIM with daily climate data statistically downscaled from 18 Global Circulation Models (GCMs). Twelve soil types of varying plant available water holding capacity (PAWC) at six sites across semi-arid southeastern Australia were considered. Biases in the GCM-simulated climate data were bias-corrected against observations for the 1961–1999 baseline period. However, biases in the APSIM output data relative to APSIM simulations forced with climate observations remained. A secondary bias correction was therefore performed on the APSIM outputs. Bias-corrected APSIM outputs for a future period (2021–2040) were compared with APSIM outputs generated using observations for the baseline period to obtain future changes. The results show that effective rainfall was decreased over all sites due to decreased growing season rainfall. ET was decreased through reduced soil evaporation and crop transpiration. There were no significant changes in runoff at any site. The variation in deep drainage between sites was much greater than for runoff, ranging from less than a few millimetres at the drier sites to over 100 mm at the wetter. However, in general, the averaged drainage over different soil types were not significantly different between the baseline (1961–1999) and future period of 2021–2040 (P > 0.05). For the wetter sites, the variations in the future changes in drainage and runoff between the 18 GCMs were larger than those of the drier sites. At the dry sites, the variation in drainage decreased as PAWC increased. Overall, water use efficiency based on transpiration (WUE_T) and ET (WUE_ET) increased by 1.1 to 1.6 and 0.7 to 1.3 kg ha−1 mm−1, respectively, over the baseline historical climate. Significant relationships between changes in wheat yield and PAWC were only seen at three sites. At the dry sites, the impact of a future climate under a soil of high PAWC was less than that under one of low PAWC. Conversely, the opposite response was seen at two wetter sites, highlighting the importance of PAWC and rainfall in determining the interactive response of crops to primary components of the water balance.

Integrating the effects of climate and plant available soil water holding capacity on wheat yield

In the Mediterranean farming systems of the Western Australian wheatbelt, crop yields are influenced primarily by the amount and distribution of rainfall and the soil's capacity to hold moisture. The wheatbelt's growing season rainfall varies in the range of 200–400 mm (average) and the plant available water holding capacity (PAWC) of soils is generally in the 40–140 mm range. The grain yield of wheat is sensitive to this combination of small rainfall and small storage capacity. In this study, we explore the relationship between yield and PAWC using a combination of simulation modelling and analysis of field data. Crop yields and soil properties were monitored in detail at 17 locations (PAWCs 43–131 mm) across six seasons (1997–2005). Crop yields were also simulated using the APSIM crop simulator (RMSE = 311 kg/ha) to evaluate the long-term relationship between crop yield and plant available water capacity using 106 years of historical climate data. The relationship between crop yield and PAWC varied with season, and two important factors emerged: (1) for PAWC < 65 mm, there was a linear relationship with crop yields that ranged from 17 kg/ha/mm to 58 kg/ha/mm of PAWC across seasons; (2) for PAWC 65–131 mm the crop yield response to PAWC ranged from 11.5 kg/ha/mm in 45% of seasons to no response. The impact of PAWC on crop yield was reduced in seasons with late rainfall, and magnified in seasons with reduced rainfall late in the growing season. Six distinct season types with different yield–PAWC relationships are identified and season-specific management strategies that exploit within-field variation in PAWC are developed to manage the spatial variation of PAWC in a field.

Capacity of soils to buffer impact of climate variability and value of seasonal forecasts

Soil provides a buffer to store water for use by plants between rainfall (or irrigation) events. This paper presents a study on how the plant available water holding capacity (PAWC) of soil interacts with rainfall variability and nitrogen input level to determine wheat yield, water use and the potential value of seasonal forecasting in nitrogen (N) management in southeast Australia. An agricultural production systems model was used to simulate those interactions. Results show that optimising N management towards maximum economic return led to increased production risk. Wheat yield, gross margin and crop water use increased, while deep drainage decreased with increasing soil PAWC. In high rainfall areas where the risk of crop loss was generally small, increased soil PAWC reduced the variability in gross margin, leading to higher yield, less water loss by deep drainage, and reduced potential value of seasonal forecasts. In low rainfall areas, the increase in wheat yield, gross margin and crop water use with increase in soil PAWC was much smaller due to limited rainfall. In those dry regions, the potential value of seasonal climate forecasts was simulated to increase with increased soil PAWC. Soils with higher PAWC enabled more rainfall to be used by crops, but did not change the seasonal crop water use efficiency in terms of kg grain produced per unit water consumed by evapotranspiration.

Systems analysis of wheat production on low water-holding soils in a Mediterranean-type environment: I. Yield potential and quality

Wheat grain yields, grain protein and grain size are often variable in a Mediterranean-type environment due to large rainfall variability. Grain yields are often low due to low N inputs as a result of large uncertainties of rainfall, particularly during the latter part of the season. Larger amounts of N fertiliser might increase yields and also grain protein in such environments but could have negative effects on grain size. A systems analysis approach was taken, using the APSIM-Nwheat crop model, to assess potential yield, grain protein concentration and grain size of wheat across a range of conditions. Rainfall and temperature varied with location. Soil type, different amounts of initial stored water and a range of management options, including varying sowing dates and N fertiliser applications, were also considered. In Australia, penalties for small grain size are based on ‘screenings’: the percentage of grain to pass through a 2 mm sieve. A new routine was developed for the model describing the relationship between screenings percentage and average simulated grain size. At low rainfall locations, 30 mm of plant-available soil water at seeding gave a significant increase in grain yield (>20%) above that for no soil water. At high rainfall locations there was little effect (<7%). Three typical coarse textured soils had similar yields at low N supplies but soils with higher plant-available water-holding capacity responded more to N applications and thus at high N applications achieved higher grain yields. Above average rainfall from May to September in combination with below average temperatures resulted in the highest simulated yields. However, seasons with an average amount of in-seasonal rainfall but with rainfall well distributed throughout the season also had above average yields. Applications of small amounts of N (30 kg N/ha) caused a slight reduction in grain protein concentration, but higher N applications (>30 kg N/ha) resulted in increased protein and screenings. Nevertheless, with moderate N (60 kg N/ha) applications and early sowing, the proportion of years with low grain protein (<10%) and low screenings (<2%) was still more than two-thirds. Simulated average yields of a location increased with average rainfall but declined with increasing average temperature of a location. This allowed the development of a simple rule of thumb in which average grain yields can be related to the rainfall/temperature index of a location. The simulation analysis showed that there is potential to increase current farmer yields through higher and split N applications on light soils in Mediterranean-type environments without increasing average screenings above 3%. However, grain protein concentrations are likely to be low in most seasons on low water-holding soils.

The High Water‐Holding Capacity of Petrocalcic Horizons

Petrocalcic soil horizons occur in most arid and semiarid ecosystems around the world, often within the plant rooting zone. Little is known, however, about the water-holding characteristic of soils indurated with CaCO 3 . We conducted a replicated experiment to define the soil-water release curve (SWRC) for a range ofpetrocalcic horizon materials. Samples from both plugged and laminar zones of two Stage V petrocalcic horizons in southern New Mexico were characterized. Wetter soil-water potentials were measured using a pressure plate; more negative potentials (down to less than < - 10 MPa) were measured using a chilled mirror water activity meter. Measured SWRC data were fitted to the van Genuchten equation. The SWRC methods used were found to be both reliable and repeatable. Plant-available water-holding capacity (AWHC) for desert species (with wilting point set at -4.0 MPa) ranged from 0.26 m 3 m -3 in plugged zones to 0.06 m 3 m -3 in some laminar zones in contrast to about 0.07 m 3 m -3 in the loamy sand parent material. Correlation analyses across morphologies of AWHC and soil properties resulted in significant statistical relationships only with bulk density and porosity. The AWHC and CaCO 3 content, however, were significantly negatively correlated within the laminar and positively correlated within the plugged petrocalcic horizon morphologies. Cementation by CaCO 3 dramatically alters the water-holding characteristics of soils and understanding these horizons is crucial to understand patterns of soil water in desert systems throughout the world.

Corrigendum to: Soil water extraction and biomass production by lucerne in the south of Western Australia

To understand the factors involved in lucerne reducing drainage below the root-zone and influencing lucerne biomass production and water extraction were analysed in the south of Western Australia. The lucerne was grown for 3 years before removal. The factors investigated as part of the water extraction analysis included the rate of advance of the extraction front or extraction front velocity (EFV, mm/day), the soil plant-available water-holding capacity (PAWC, mm/m soil), and the temporal change in soil water deficit (drainage buffer, mm). The drainage buffer is related to the EFV and PAWC. A site with deep sand had the highest EFV (mean of 9.2�mm/day) but the lowest PAWC (mean of 32�mm/m soil) to a depth of 4�m. In the duplex soils the EFV was 18.34% of the deep sand EFV and the PAWC was 60.222% higher than the deep sand PAWC to a depth of 1.6.2.1�m. The EFV was reduced by the higher clay content and sodicity in the B horizon of the duplex soils. The highest drainage buffer measurements occurred in the deep sand site and the better structured duplex soils and therefore these soils will have the greater effect on reducing drainage below the root-zone. However, lucerne was able to create a drainage buffer to at least a depth of 1.5�m over 3 years and therefore contribute to a reduced drainage even on the most sodic and saline sites. Low soil pH did not affect the drainage buffer as much as soil texture and structure. Variation in biomass production of lucerne-based pastures was positively related to rainfall and water use (taking into account soil water storage and drainage losses) across sites, explaining approximately 50% of the biomass variation. Rainfall and water use could therefore be used for predicting lucerne biomass production in Western Australia. Biomass water use efficiency was highest in spring (15 kg/ha.mm) and least during autumn (4.5 kg/ha.mm).

Soil water extraction and biomass production by lucerne in the south of Western Australia

To understand the factors involved in lucerne reducing drainage below the root-zone and influencing lucerne biomass production and water extraction were analysed in the south of Western Australia. The lucerne was grown for 3 years before removal. The factors investigated as part of the water extraction analysis included the rate of advance of the extraction front or extraction front velocity (EFV, mm/day), the soil plant-available water-holding capacity (PAWC, mm/m soil), and the temporal change in soil water deficit (drainage buffer, mm). The drainage buffer is related to the EFV and PAWC. A site with deep sand had the highest EFV (mean of 9.2 mm/day) but the lowest PAWC (mean of 32 mm/m soil) to a depth of 4 m. In the duplex soils the EFV was 18–34% of the deep sand EFV and the PAWC was 60–222% higher than the deep sand PAWC to a depth of 1.6–2.1 m. The EFV was reduced by the higher clay content and sodicity in the B horizon of the duplex soils. The highest drainage buffer measurements occurred in the deep sand site and the better structured duplex soils and therefore these soils will have the greater effect on reducing drainage below the root-zone. However, lucerne was able to create a drainage buffer to at least a depth of 1.5 m over 3 years and therefore contribute to a reduced drainage even on the most sodic and saline sites. Low soil pH did not affect the drainage buffer as much as soil texture and structure. Variation in biomass production of lucerne-based pastures was positively related to rainfall and water use (taking into account soil water storage and drainage losses) across sites, explaining approximately 50% of the biomass variation. Rainfall and water use could therefore be used for predicting lucerne biomass production in Western Australia. Biomass water use efficiency was highest in spring (15 kg/ha.mm) and least during autumn (4.5 kg/ha.mm).

Influence of peatmoss on hydraulic properties and strength of compacted soils

Due to its high organic matter content, peatmoss can be highly beneficial to agricultural soil. In this research, the impact of varying organic matter contents at different compaction efforts on water retention, saturated hydraulic conductivity, and modulus of rupture of three soils (sandy loam, clay loam, and clay) has been investigated under laboratory conditions. Compaction changed the ability of the soils to hold water, increased modulus of rupture, and decreased the plant-available water-holding capacity of the soils. On the other hand, organic matter generally increased the ability of the soils to hold water, expanded the available water capacity, and decreased the modulus of rupture of compacted soils. Key words: Peatmoss, water retention, saturated hydraulic conductivity, modulus of rupture

Double‐Crop Soybean Leaf Area and Yield Responses to Mid‐Atlantic Soils and Cropping Systems

Double‐crop soybean (Glycine max L.) often yields less than full‐season soybean, due in part to decreased leaf area development. Information is lacking on the effect of cropping system and plant‐available water holding capacity (PAWHC) as affected by soil type on LAI in field‐scale environments. The objectives of this research were to (i) determine the effect of three cropping systems and three soil types that vary in PAWHC on double‐crop soybean leaf area and yield; and (ii) validate the LAI–yield relationship in a field‐scale experiment. Soybean LAI and yield were measured at random geographically located positions during the 1999 to 2001 cropping seasons. During years of early season or little drought stress, LAI and yield were 1.1 to 1.2 units and 480 to 558 kg ha−1 less on the lowest PAWHC soil compared with the soil with the highest PAWHC. Soybean LAI and yield were reduced 0.4 units and 301 kg ha−1 when soybean experience early season drought stress and was not rotated. Under conditions of late‐season drought stress, LAI and yield of rotated soybean were 1.6 units and 1350 kg ha−1 less on the lowest PAWHC soil. When soybean was not rotated and under conditions of late‐season drought stress, LAI and yield were reduced by 2.9 units and 1770 kg ha−1 for the soil with the lowest PAWHC, and 1.6 units and 760 kg ha−1 for the soil with intermediate PAWHC. Soybean LAI was linearly related to yield only on the lowest PAWHC soil where LAI was low.

Double-Crop Soybean Leaf Area and Yield Responses to Mid-Atlantic Soils and Cropping Systems

Double-crop soybean (Glycine max L.) often yields less than full-season soybean, due in part to decreased leaf area development. Information is lacking on the effect of cropping system and plant-available water holding capacity (PAWHC) as affected by soil type on LAI in field-scale environments. The objectives of this research were to (i) determine the effect of three cropping systems and three soil types that vary in PAWHC on double-crop soybean leaf area and yield; and (ii) validate the LAI–yield relationship in a field-scale experiment. Soybean LAI and yield were measured at random geographically located positions during the 1999 to 2001 cropping seasons. During years of early season or little drought stress, LAI and yield were 1.1 to 1.2 units and 480 to 558 kg ha−1 less on the lowest PAWHC soil compared with the soil with the highest PAWHC. Soybean LAI and yield were reduced 0.4 units and 301 kg ha−1 when soybean experience early season drought stress and was not rotated. Under conditions of late-season drought stress, LAI and yield of rotated soybean were 1.6 units and 1350 kg ha−1 less on the lowest PAWHC soil. When soybean was not rotated and under conditions of late-season drought stress, LAI and yield were reduced by 2.9 units and 1770 kg ha−1 for the soil with the lowest PAWHC, and 1.6 units and 760 kg ha−1 for the soil with intermediate PAWHC. Soybean LAI was linearly related to yield only on the lowest PAWHC soil where LAI was low.

Physical properties of a Chernozemic clay loam soil under long-term conventional tillage and no-till

Conservation tillage practices such as no-till (NT) and conventional tillage (CT) with a heavy-duty cultivator can influence the physical properties of soils. This study was conducted to determine the effect of 24 yr of NT versus CT on the physical properties of a clay loam soil in southern Alberta. Physical properties quantified were bulk density (BD), mean weight diameter (MWD), plant-available water-holding capacity (PAWHC), saturated hydraulic conductivity (Ksat), soil water characteristic [θ(ψ)] and unsaturated hydraulic conductivity [K(ψ)] relationships, and pore-size distribution (PSD). Bulk soil samples and small soil cores (5-cm depth increments to 20 cm) were taken from CT and NT fields in 1992, and tension infiltrometer measurements were made in 1994. The results from this study are reported as general trends for the tillage fields. Statistical probability levels are not reported because of the unreplicated nature of the experiment, the limited number of sampling locations within each tillage field, and to a lesser extent, the different sampling times for CT and NT in 1992. Plant-available water-holding capacity was higher for the CT field (14.3%) than the NT field (10.8%), and a greater amount of water was held at a given water potential (−1500 to −1.5 kPa) for the former, indicating a higher potential for soil water conservation under conventional tillage. Geometric mean Ksat values (small soil cores) were higher for the NT field (18.20 × 10−6 m s−1) than the CT field (1.74 × 10−6 m s−1). The K(ψ) values (small soil cores) between −10 and −2 kPa were higher for the CT field than the NT field at the 0- to 5-cm, 10- to 15-cm and 15- to 20-cm depths, but values were higher for the NT field at the 5- to 10-cm depth. Near-saturated K(ψ) values (−1.5 to −0.3 kPa) of the surface soil, as derived from tension infiltration measurements in 1994, were higher for the CT field (2.43 × 10−7 m s−1) than for the NT field (6.09 × 10−8 m s−1). There was a greater percentage volume of larger pores (30–40, 40–67, 67–200, &gt;200 µm) for the NT field than for the CT field, and there was a lower percentage volume of smaller pores (0.2–0.6, 0.6–4 µm) for the CT field than for the NT field. Differences in certain soil physical properties between CT and NT fields may be related to the lag time between the most recent tillage event and sampling for the CT field. Key words: Conservation tillage, heavy-duty cultivator, physical attributes, soil water, hydraulic conductivity, porosity

Sensitivity of greenhouse summer dryness to changes in plant rooting characteristics

A possible consequence of increased concentrations of greenhouse gases in Earth's atmosphere is “summer dryness,” a decrease of summer plant‐available soil water in middle latitudes, caused by increased availability of energy to drive evapotranspiration. Results from a numerical climate model indicate that summer dryness and related changes of land‐surface water balances are highly sensitive to possible concomitant changes of plant‐available water‐holding capacity of soil, which depends on plant rooting depth and density. The model suggests that a 14% decrease of the soil volume whose water is accessible to plant roots would generate the same summer dryness, by one measure, as an equilibrium doubling of atmospheric carbon dioxide. Conversely, a 14% increase of that soil volume would be sufficient to offset the summer dryness associated with carbon‐dioxide doubling. Global and regional changes in rooting depth and density may result from (1) plant and plant‐community responses to greenhouse warming, to carbon‐dioxide fertilization, and to associated changes in the water balance and (2) anthropogenic deforestation and desertification. Given their apparently critical role, heretofore ignored, in global hydroclimatic change, such changes of rooting characteristics should be carefully evaluated using ecosystem observations, theory, and models.

Climate, interseasonal storage of soil water, and the annual water balance

The effects of annual totals and seasonal variations of precipitation and potential evaporation on the annual water balance are explored. It is assumed that the only other factor of significance to annual water balance is a simple process of water storage, and that the relevant storage capacity is the plant-available water-holding capacity of the soil. Under the assumption that precipitation and potential evaporation vary sinusoidally through the year, it is possible to derive an analytic solution of the storage problem, and this yields an expression for the fraction of precipitation that evaporates (and the fraction that runs off) as a function of three dimensionless numbers: the ratio of annual potential evaporation to annual precipitation (index of dryness); an index of the seasonality of the difference between precipitation and potential evaporation; and the ratio of plant-available water-holding capacity to annual precipitation. The solution is applied to the area of the United States east of 105°W, using published information on precipitation, potential evaporation, and plant-available water-holding capacity as inputs, and using an independent analysis of observed river runoff for model evaluation. The model generates an areal mean annual runoff of only 187 mm, which is about 30% less than the observed runoff (263 mm). The discrepancy is suggestive of the importance of runoff-generating mechanisms neglected in the model. These include intraseasonal variability (storminess) of precipitation, spatial variability of storage capacity, and finite infiltration capacity of land.