Active Root Zone Research Articles

Impact of active root zone soil potassium levels on cotton yield and fiber quality under no tillage.

Potassium deficiency significantly hinders cotton growth and development, adversely affecting yield and fiber quality. Applying potassium fertilizer is a common practice to address potassium deficiency in the soil. However, the effectiveness of potassium fertilizer application depends on the appropriate soil potassium levels in cotton fields. This study used a randomized block design with six different soil potassium levels and conducted experiments across 18 micro-zones in the field. This study aimed to investigate the response of cotton yield and quality to different soil potassium levels, to try to clarify the suitable soil potassium levels for cotton growth, so as to provide practical and effective help for determining the amount of potash fertilizer in the cotton field. The results showed that the seedcotton yield was increasing, with the soil potassium level increased under no tillage. There was no significant difference among K4, K5, and K6 on seedcotton yield, which were significantly higher than K1 and K2. As soil potassium levels increased, the proportion of autumn boll and the proportion of outer boll also increased, indicating that higher soil potassium levels support the better growth and development of cotton in the middle and late stages, leading to increased boll sets and higher yields. Additionally, the available potassium content in the 0-40-cm soil layer was significantly correlated with yield and yield parameters but not with fiber quality indices. It is concluded that K4 treatment could provide sufficient potassium to meet the growth and development needs of cotton. Potassium fertilizer application is recommended when the available potassium content in the 0-40-cm soil layer falls below 122.88 mg kg-1 in the cotton field.

Rootzone Soil Moisture Dynamics Using Terrestrial Water‐Energy Coupling

AbstractA lack of high‐density rootzone soil moisture (θRZ) observations limits the estimation of continental‐scale, space‐time contiguous θRZ dynamics. We derive a proxy of daily θRZ dynamics — active rootzone degree of saturation (SRZ) — by recursive low‐pass (LP) filtering of surface soil moisture (θS) within a terrestrial water‐energy coupling (WEC) framework. We estimate the LP filter parameters and WEC thresholds for the piecewise‐linear coupling between SRZ and evaporative fraction (EF) at remote sensing and field scale over the Contiguous U.S. We use θS from the Soil Moisture Active‐Passive (SMAP) satellite and 218 in‐situ stations, with EF from the Moderate Resolution Imaging Spectroradiometer. The estimated SRZ compares well against SMAP Level‐4 estimates and in‐situ θRZ, at the corresponding scale. The instantaneous hydrologic state (SRZ) vis‐à‐vis the WEC thresholds is proposed as a rootzone soil moisture stress index (SMSRZ) for near‐real‐time operational agricultural drought monitoring and agrees well with established drought metrics.

Soil Localized Leaching and Reclamation with Gypsum and Sulfuric Acid: A Case Study of Pistachio Orchard, Kerman-Iran

ABSTRACT Localized leaching method is a new reclamation leaching technique. Localized leaching method by using drip tapes allows water to be applied directly to the areas of salt accumulation. This study applied localized leaching method on a sandy loam soil in a pistachio orchard in Kerman, Iran to evaluate the impact of three treatments: adding gypsum soil remediation (G), soil remediation with sulfuric acid (SA) andirrigation water (IW) on soil remediation. Three treatments (IW, G and SA) in four repetitions were used to remove soluble salts from the Main Active Root Zone (MARZ) in a pistachio orchard which irrigated by a subsurface irrigation system. The localized leaching method consist of two T-tape pipelines at a distance of 40 cm located on both sides of the tree rows. Soil sampling was carried out before and after leaching from soil unit profiles with 80 cm (Length) × 120 cm (Depth) with 20 cm intervals. Concentrations of sodium (Na), magnesium (Mg), calcium (Ca) cations, and electrical conductivity (EC) were examined before and after leaching process. Results revealed that application of G-treatment compared to IW-treatment and SA-treatment led to more leaching of Na and Mg from the soil by 61% and 50%, respectively. The amount of leached Ca in both G-treatment and SA-treatments was less than that of IW-treatment. The leaching efficiency of total soluble salts for G, IW and SA treatments were 58%, 56% and 45%, respectively. Gypsum application could be recommended as a soil amendment. In addition, the localized leaching technique offers water-saving potential opposed to traditional leaching methods which applying water to the entire field.

Optimizing the water-ecosystem-food nexus of avocado plantations

Climate change, food and water security and ecosystem sustainable management are tightly interlinked and require holistic approaches to achieve solutions that do not impact adversely one-another. The objective of this work was to conduct studies, collect data and assess the Water-Ecosystem-Food (WEF) nexus in avocado plantations in the Mediterranean region systematically to minimize the environmental footprint while maximizing the benefits for the farmer and the environment. The study includes two distinct experiments; the first addresses the impact of soil organic amendments addition to optimize the WEF nexus and the second monitors experimentally crop water needs and thus illustrates how irrigation practices aided by technology can reduce substantially water consumption. The results showed that organic amendments addition improves fertility, nutrient sequestration and structure but only had a weak effect on biodiversity by increasing the number of unique species. For the development of an efficient irrigation system it is necessary to determine the radius around the tree, the depth of the roots and the time required for the water to reach the active root zone to determine the amount and duration of irrigation. In this way sufficient water will be added to replenish the soil moisture deficit created due to the evapotranspiration. HYDRUS-1D model was used to simulate soil moisture and the hydrologic budget of an avocado tree located in Koiliaris river basin and confirm the percolation losses to groundwater. The results of this study showed that the actual irrigation needs of avocados in the Mediterranean is less than 2,000 m3/ha which is 75% less than what is recommended and could become the primary measure for the mitigation of climate change impacts especially in semi-arid regions such as the Mediterranean.

In situ monitoring of the impacts of Melaleuca styphelioides transpiration on soil water dynamics in an urban environment

Expansive soils are susceptible to substantial cyclical moisture variations under the prevailing climate, experiencing considerable shrinkage during drought and swelling when water availability increases. These dynamic volumetric changes jeopardize the functionality of lightweight infrastructures with shallow foundations. The presence of trees may amplify seasonal soil movements, as extensive water uptake by root networks in the summer months can induce soil moisture depletion and the resulting desiccation-driven settlement around the trees. This study assessed the drying effect of mature Melaleuca styphelioides on soil movement and water dynamics at an urban experimental site in Melbourne, Australia, instrumented for in situ monitoring over 12 months. Sap flow instrumentation was used to quantify seasonal variations in tree transpiration. The monitoring results revealed that the drying influence of the tree has extended to a depth of 2.2 m. Tree root-induced soil desiccation occurred 4.0 m away from the tree, as evidenced by the seasonal soil movement profiles. Linear regression analysis showed that the soil water content explained 60% of the variability in tree transpiration. In contrast, the soil water content and suction exhibited a robust negative correlation (R2=0.96) within the active root zone.

A simple method for evaluating the relative water uptake rate of drip‐irrigated crops

AbstractDrip irrigation is widely acknowledged for its water use efficiency, yet evaluating relative water uptake rates (RWURs, ratios between the water uptake rates and the irrigation rates) remains pivotal for effective system design and management. This article presents a novel method employing straightforward measurements of wetted soil surfaces around emitters or perpendicular to driplines, both with and without water uptake, emphasizing simplicity and practicality. The proposed method offers valuable insights into agronomic water use efficiency, facilitating the optimization of drip irrigation for both annual and perennial crops. While effective for intensively irrigated crops, the method does have limitations for smaller wetted areas and longer irrigation cycles, depending also upon a reasonable determination of the active root zone depth and the soil capillary length. Despite relying on a simplified water uptake model, the accessibility and cost‐effectiveness of the method render it a valuable tool for assessing RWURs in diverse agricultural settings, contributing to the sustainable utilization of water resources in drip irrigation.

Detecting soil water redistribution in subsurface drip irrigated processing tomatoes using electrical resistivity tomography, proximal sensing and hydrological modelling

In this study, multiple soil-plant-atmosphere continuum (SPAC) monitoring methodologies, including electrical resistivity tomography (ERT), proximal thermal sensing techniques, and micrometeorological data, were combined with two-dimensional (2-D) soil hydrological modelling using HYDRUS 2-D to explore the soil water redistribution, and infer the relative crop water status in a subsurface drip irrigated (SDI) processing tomato field located in California (Yolo County, USA). Specifically, time-lapse ERT surveys were performed at two transects distributed parallel and perpendicular, respectively, to the SDI line, during an irrigation event. The ERT results were compared to HYDRUS 2-D outputs and the relative differences were explained in the form of local heterogeneities in electrical resistivity (ER) changes, as a proxy for soil water content (SWC) variations. Concurrent simultaneous soil wetting and root water uptake during the last irrigation event of the season caused negligible changes in ER in the active root zone. Slight differences in ER were observed in the top 20 cm along the dripline, confirming that the emitter spacing is small enough to create a wetted strip along the processing tomato bed. These changes were also compared to SWC values measured with time domain reflectometry soil moisture sensors. A comparison between HYDRUS 2-D and ERT confirmed negligible changes in ER during irrigation due to simultaneous wetting and root water uptake processes. In addition, a good correlation was observed between the proximal sensed and the ERT results. Finally, the findings of this study underscore the necessity of using multiple methods for improving our knowledge of the SPAC system under real field conditions.

Relative Effectiveness of a Radionuclide (210Pb), Surface Elevation Table (SET), and LiDAR At Monitoring Mangrove Forest Surface Elevation Change

Sea-level rise (SLR) is one of the greatest future threats to mangrove forests. Mangroves have kept up with or paced past SLR by maintaining their forest floor elevation relative to sea level through root growth, sedimentation, and peat development. Monitoring surface elevation change (SEC) or accretion rates allows us to understand mangrove response to SLR and prioritizes resilient ecosystems for conservation or vulnerable ecosystems for restoration. We compared three methods to measure SEC and accretion in mangrove forests: 210Pb, surface elevation tables (SETs), and a terrestrial light detection and ranging system (compact biomass LiDAR—CBL). Lead-210 accretion rates were not significantly different than SET SEC rates and differences between the two methods (− 2 to 2 mm/year) were within the error of our measurements. Lead-210 only measures accretion in the upper meter of sediment and cannot capture deeper subsurface processes (e.g., subsidence, compaction) that SETs can. The lack of differences suggests the following: (1) surface processes in the active root zone are influencing forest floor elevation more than subsurface processes, (2) subsurface processes were not large enough to effect elevation, or (3) the SETs were not installed deep enough to capture subsurface processes. CBL SEC rates did not differ significantly from SET SEC rates. The larger spatial scale of the CBL scans resulted in significantly different SEC rates from some of the plots. This was due to the CBL measuring areas missed by the SET. The greater number of points measured by CBL (~ 30,000 vs 36) increased precision and lowered standard error. The traditional SET/rSET method is currently 3–10 × cheaper than the 210Pb or CBL method, respectively, and can accurately track changes in forest floor elevation. Costs of the use of LiDAR are likely to decrease in the future with the advent of newer and more cost-effective technology.

Empirical equations for estimating field capacity in dryland cropping soils of southeastern Australia

The root zone soil water content at field capacity, θFC, is often crucial for irrigation scheduling and soil-plant-atmosphere modelling. Although θFC is traditionally determined using methodologies based on matric head or when water flux at the bottom of the root-zone equal to a prescribed negligible flux, these approaches can be problematic for some soils and applications. In this study, a novel water storage approach was used to estimate θFC for 11 soil texture classes in southeastern Australia (northwest Victoria) by employing the Soil-Water-Atmosphere-Plant (SWAP) model to simulate gravity drainage from saturation in the active root zone (top 60 cm). Field capacity was specified as the average root zone water content corresponding to a 1% relative change in daily soil water storage. We have also estimated flux-based θFC when prescribed flux at the bottom of the root-zone was 0.1 cm d-1. Three new empirical equations were developed to estimate storage-based θFC as a function of SWAP-simulated drainage flux out of the root zone, the n parameter of the van Genuchten function, and the saturated soil hydraulic conductivity, Ks, or the soil water content at − 100 cm matric head. We have evaluated these three new equations in addition to nine published equations for estimating θFC. The three new equations were found to be better predictors of θFC than most of the nine popular equations reported in the literature when compared to flux-based θFC values. Based on findings from this study the new equations are considered to be effective for irrigation scheduling and crop/climate modelling on the dryland soils of Victoria. Future studies will assess the applicability of the new equations to other parts of Australia.

Assessing in-field soil moisture variability in the active root zone using granular matrix sensors

Assessing in-field soil moisture variability in the active root zone using granular matrix sensors

Methods for downhole soil water sensor calibration—Complications of bulk density and water content variations

AbstractDownhole soil volumetric water content (VWC) sensors are used in access tubes to assess the soil water content at multiple depths. If sensor readings are spaced closely enough vertically and are accurate enough, then accurate soil profile water content storage and change in storage can be determined over the depth range of readings, leading to accurate estimates of evapotranspiration (ET) if readings extend to well below the root zone. Even if sensing only covers the active root zone, soil water depletion may be determined well enough to inform irrigation scheduling. While sensor accuracy is dependent on many factors, including the sensor's physical principle of operation, soil‐specific calibration is typically required for good accuracy. In soils with multiple horizons (layers) of different texture, bulk density (BD), or chemical composition, horizon‐specific calibrations may be necessary. We describe methods and equipment used for downhole sensor calibration to typical accuracy of <0.01 m3 m−3 with specific reference to calibration of 10 neutron scattering meters in a soil that required three different horizon‐specific calibrations. Our results contrast with the factory calibration, which would result in a 38‐mm error in water stored in a 1.5‐m deep profile of our soil. We describe variability of measured VWC and BD with depth, distance, and water content and the errors that result from using BD to convert mass basis (g g−1) water content data to VWC data, which can be as much as 35 mm (7.26% underestimation) for soil water storage in a 1.5‐m deep profile of our soil.

Spatiotemporal variation of soil water potential and its significance to water balance for a desert shrub area

In dry sandy soil, soil water is naturally redistributed by forces owing to matric and gravitational potential, and temperature gradients. Occurring shrub vegetation strongly influences these patterns by hydraulic redistribution within the soil. This means that plants can move water from moist to drier soil areas to extract nutrients from the drier soil and enhance nutrient uptake. This incorporates both upward and downward transport of water through the roots. Diel soil water potential (ψs) and soil temperature fluctuation were investigated at 30-min intervals for a respectively vegetated desert shrub area and non-vegetated bare area. Soil water potential fluctuations at the vegetated area reflected daytime depletion and nocturnal re-supply of water due to hydraulic redistribution while corresponding fluctuations at the non-vegetated area were mainly due to variation in soil temperature. Thus, the greatest diel ψs variability occurred in the vegetated area that averaged −0.401 MPa with a smaller variation in the non-vegetated area with an average of −0.009 MPa. At the shrub scale, ψs was strongly correlated with root distribution pattern that corroborates that diel variation in the active root zone of the shrub was mainly attributed to hydraulic redistribution. In the non-vegetated area, ψs responded more strongly to temperature gradients. The total of hydraulic redistribution of soil water storage increment accounted for 27 % of the total rainfall through the growing season from April to September. The study shows that it is important to consider biotic-factors such as hydraulic redistribution when analyzing water balance changes and hydrological niche segregation for desert ecosystems.

Spatial and Temporal Nutrient Dynamics and Water Management of Huanglongbing-Affected Mature Citrus Trees on Florida Sandy Soils

Nutrients are vital for plant growth, development, and aid in disease control because nutrients affect host plant and pathogen interactions. Once a citrus tree is infected with the phloem-limited, Gram-negative bacteria, Candidatus Liberibacter asiaticus (CLas), huanglongbing (HLB; citrus greening), it would fall under threat of survival as the disease has no known control mechanism discovered thus far. The objective of this study was to determine if split soil applications of essential nutrients improve the availability and accumulation, reduce leaching of these nutrients beyond the root zone, and promote root growth and water dynamics of HLB-affected citrus trees in the soil–plant–atmosphere continuum. Split soil applications of three N rates (168, 224, and 280 kg ha−1 year−1) were the main blocks. Micronutrients were randomly applied to the sub-blocks assigned in a split-split plot design, applied in three splits annually. The micronutrients were applied to foliage and soil as follows: foliar only 1× (1×), foliar 1× and soil-applied 1× (2×), and foliar 1× and soil-applied 2× (3×)× (1× = 9 kg ha−1 year−1 of Mn and Zn to each foliar and soil along with 2.3 kg ha−1 year−1 of B). Significant soil NH4-N and NO3-N, Zn, and Mn were retained within the active soil root zone (0–30 cm). Higher soil acidity was detected when trees received the highest micronutrient rate in the upper soil layers (0–15 cm) as compared with the middle (15–30 cm) and the lowest (30–45 cm) soil layers. Fine root length density (FRLD) was significantly lower at the highest micronutrient rates, manifesting root growth negatively associated with high Mn and low soil pH. Invariably, the water dynamics: stem water potential (ψstem), stomata conductance (gs), and sap flow were also negatively affected when trees received foliar 1× and soil 2× (3×) treatment as compared with the other treatments. Split application of nutrients had a significant effect on FRLD growth, retaining soil-applied nutrients within the active root zone, and improved water use efficiency.

Carbon storage in coastal wetlands is related to elevation and how it changes over time

National and global scale initiatives to reduce loss and promote restoration of coastal ecosystems have leveraged the capacity of mangrove and saltmarsh to contribute to climate change mitigation through carbon sequestration. The success of these programs is predicated on reliable estimates of carbon storage and how this changes over time. Efforts to describe spatial variation in below-ground carbon storage have largely focussed on surface sediments, with few studies able to characterise carbon at greater soil depths. This study demonstrates that landscape position occupied by wetland vegetation influences both carbon storage and sources, and that understanding evolutionary infill of estuaries is crucial for characterising spatial variation in carbon storage. We focussed on coastal wetlands in southeast Australia where sea level has a long history of relative stability over the past few millennia. Under these conditions, we show that carbon storage varies across three depth zones in substrate: the active root zone (associated with distribution of contemporary vegetation), inactive root zone (associated with past environmental conditions) and subtidal zone (beyond the contemporary intertidal zone). This conceptual approach relates spatial variation in carbon storage to key processes influencing carbon addition and decomposition, and can be applied elsewhere depending on the sea-level history at the specific site. We demonstrate that models that define carbon storage in the context of variation in landscape position of vegetation in the tidal frame provide improved confidence required for blue carbon assessments.

Combining Models of Root-Zone Hydrology and Geoelectrical Measurements: Recent Advances and Future Prospects

Recent advances in measuring and modeling root water uptake along with refined electrical petrophysical models may help fill the existing gap in hydrological root model parametrization. In this paper, we discuss the choices to be made to combine root-zone hydrology and geoelectrical data with the aim of characterizing the active root zone. For each model and observation type we discuss sources of uncertainty and how they are commonly addressed in a stochastic inversion framework. We point out different degrees of integration in the existing hydrogeophysical approaches to parametrize models of root-zone hydrology. This paper aims at giving emphasis to stochastic approaches, in particular to Data Assimilation (DA) schemes, that are generally identified as the best way to combine geoelectrical data with Root Water Uptake (RWU) models. In addition, the study points out a more suitable objective function taken from the optimal transport theory that better captures complex geometry of root systems. Another pathway for improvement of geoelectrical data integration into RWU models using DA relies on the use of stem based methods as a leverage to introduce more extensive root knowledge into RWU macroscopic hydrological models.

A modelling approach and module for salinity management in drip irrigation

The article describes a new modelling approach and module for soil salinity management. The two major objectives of the module are to simulate the seasonal course of the salinity of the soil solution in the active root zone of drip-irrigated crops to assist irrigators to optimise irrigation and salt leaching, and to help evaluate the salt fluxes in the deep percolating water for assessing the longer-term salt load of the irrigated crops on the regional ground and surface water resources. The module is based on the solutions of the DIDAS program for quasi-steady water flow and uptake for various drip irrigation scenarios, and on differentiating the applied water to evaporation from the soil surface, uptake by the plant roots, deep percolation through the root zone that leaches salt out of it, and deep percolation outside of the root zone that does not leach salts. After laying out the rationale behind the suggested approach and outlining the expressions used in the module for evaluating the salinity of the root zone, several sample computations, in silico experiments, are performed to illustrate the module functionality and its potential application, and to serve for sensitivity analysis of the system parameters. Additional investigation is needed into the adequacy of the suggested modelling approach and the developed module to simulate soil salinity in the active root zone. • The module evaluates seasonal courses of root zone salinity in drip-irrigated fields. • It simulates root system development but does not predict plant growth and yields. • It is based on modelling quasi-steady water flow and uptake and salt mass balance. • The deep percolating water are partitioned into two fractions. • One that flows through the root zone and leaches salt out of it, and one that flows around it.

Tidal responses of groundwater level and salinity in a silty mangrove swamp of different topographic characteristics

Tides are one of the main factors affecting coastal aquifers, and thus they are important in supporting material transport and carbon circulation. In coastal mangrove swamps, the groundwater responses to tidal variations become more complicated owing to the complex topography, soil types, and the interactions of groundwater with fauna and flora. Perhaps because of this, quantitative relationships between the groundwater dynamics and the tidal actions of coastal mangrove swamps remain unclear. There is still a lack of studies that consider the integrated tidal effects on subsurface aquifers of different soil zones and topographic types. From January to December 2018, we simultaneously monitored the fluctuations of the piezometric head and salinity at the active root zone (shallow zone) and the low-permeability silty zone (middle zone) in the riverine forest and fringe forest and on the mudflat of a coastal mangrove swamp. Tidal signals were extracted from high-resolution time-series groundwater data using a Bayesian statistics modeling method-based code (Baytap08). Different time lags between tidal elevation fluctuations and the fluctuations of the piezometric head and salinity were detected. At the inland riverine forest and seaward mudflat, the upper aquifer quickly dried at low tides, whereas that of the fringe forest in between remained wet all the time. Although the groundwater fluctuations, especially the salinity fluctuations, were highly attenuated owing to capillary effects in the middle zone, the shallow zone showed high dependence on the tidal variations. Most previous studies have focused either on the macropore function (animal burrows and decayed roots) of mangrove swamps or on the capillary attenuation of coastal aquifers. Our results indicate that we need to consider both when analyzing the tidal responses of groundwater in mangrove swamps. The insights obtained from this study may help track the movement of water and materials in the coastal aquifers of mangrove swamps and provide references for better management of these valuable ecosystems and other similar coastal aquifers with complicated structures.

Modeling the Role of Root Exudation in Critical Zone Nutrient Dynamics

AbstractThe multifaceted interface of plant roots, microbes, water, and soil can be considered a critical zone within the Critical Zone as it is host to many important dynamically linked processes, including the promotion of nutrient cycling through absorption and rhizodeposition, interaction and feedbacks with microorganisms and fungi, root‐facilitated hydraulic redistribution, and soil carbon dynamics. Such important processes in the Critical Zone have not been fully characterized and modeled in an ecohydrologic framework linking above‐ground natural and/or anthropogenic processes to below‐ground biogeochemical cycling. Specifically, the relation between root exudates and nutrient cycling remains an open challenge. Here we present the model REWT (Root Exudation in Watershed‐scale Transport) to demonstrate the systematic modeling of root exudation in an interconnected ecohydrologic framework. REWT incorporates an explicit dynamic root exudation transport model, nutrient absorption, and coupled microbial processes within the framework of a validated ecohydrologic model. Model simulations demonstrate the influence of root exudation of glucose, a polysaccharide that serves as fuel for microbes, and flavonoids, which can act as a biological nitrification inhibitor on microbial processes linked to soil carbon and nitrogen cycling. To demonstrate the capabilities of this theoretical framework, we parameterize REWT for corn and soybean crops in the Midwestern United States, and simulations indicate that rates of nitrification and respiration were substantially altered compared to model simulations in which root exudation was not explicitly included. This work demonstrates the importance of systematically incorporating root exudates into hydrobiogeochemical models and can serve to inform experimental design for active root zone processes.

Optimizing soil nitrogen balance in a potato cropping system through legume intercropping

Negative nitrogen balance represents a major factor causing low potato yield in potato growing areas of Kenya while its excessive surplus poses a significant environmental concern. In order to synchronize this tradeoff, a field trial integrating potato (Solanum tuberosum L.) with lima bean (Phaseolus lunatus L.) and dolichos (Lablab purpureus L.) in intercropping system was conducted in the upper midland [1552 m above sea level (masl), lower highland (1894 masl) and upper highland (2552 masl)] agro-ecological zones of Kenya. Nitrogen gains from mineralization, fertilization, biological fixation, and outputs from biomass accumulation, leaching, volatilization and soil erosion were quantified using standard procedures. Soil N balance ranged from − 10.7 to − 18.1 kg N ha−1 for sole potato, 4.1 to 6.6 kg N ha−1 for intercropping and 2.9 to 22.3 kg N ha−1 for sole legumes. The intermediate range of polyphenol and lignin contents in intercropping enhanced N mineralization with peak N release of 8 to 9 kg N ha−1 matching with peak N uptake by potato (19.9 to 31.2 kg N ha−1). Nitrate was leached below the active root zone in sole potato cropping (4.2 to 46.6 kg N ha−1), a process that was diverged by the deep root systems of legume intercrops. These results suggest that legume intercropping can provide a means of balancing the nitrogen retained in the biomass and soil, thus offering a mechanism for optimizing the soil N balance in smallholder potato farming systems.

Impact of Deficit Irrigation on Shallow Saline Groundwater Contribution and Sunflower Productivity in the Imperial Valley, California

Yield and production functions of sunflower (Helianthus annuus) were evaluated under full and deficit irrigation practices with the presence of shallow saline groundwater in a semi-arid region in the Imperial Valley of southern California, USA. A growing degree day (GDD) model was utilized to estimate the various growth stages and schedule irrigation events throughout the growing season. The crop was germinated and established using overhead irrigation prior to the use of a subsurface drip irrigation (SDI) system for the remainder of the growing season. Four irrigation treatments were implemented: full irrigation (100% full sunflower crop evapotranspiration, ETC), two reduced irrigation scenarios (95% ETC and 80% ETC), and a deficit irrigation scenario (65% ETC). The salinity of the irrigation water (EC) (Colorado River water) was nearly constant at 1.13 dS·m−1 during the growing season. The depth to groundwater and groundwater salinity (ECGW) were continuously monitored in five 3 m deep observation wells. Depth to groundwater fluctuated slightly under the full and reduced irrigation treatments, but drastically increased under deficit irrigation, particularly toward the end of the growing season. Estimates of ECGW ranged from 7.34 to 12.62 dS·m−1. The distribution of soil electrical conductivity (ECS) and soil matric potential were monitored within the active root zone (120 cm) at selected locations in each of the four treatments. By the end of the experiment, soil salinity (ECS) across soil depths ranged from 1.80 to 6.18 dS·m−1. The estimated groundwater contribution to crop evapotranspiration was 9.03 cm or approximately 16.3% of the ETC of the fully irrigated crop. The relative yields were 91.8%, 82.4%, and 83.5% for the reduced (95% and 80% ETC) and deficit (65% ETC) treatments, respectively, while the production function using applied irrigation water (IW) was: yield = 0.0188 × (IW)2 − 15.504 × IW + 4856.8. Yield reduction in response to water stress was attributed to a significant reduction in both seed weight and the number of seed produced resulting in overall average yields of 2048.9, 1879.9, 1688.1, and 1710.3 kg·ha−1 for the full, both reduced, and deficit treatments, respectively. The yield response factor, ky, was 0.63 with R2 = 0.745 and the irrigation water use efficiencies (IWUE) were 3.70, 3.57, 3.81, and 4.75 kg·ha−1·mm−1 for the full, reduced, and deficit treatments, respectively. Our results indicate that sunflowers can sustain the implemented 35% deficit irrigation with root water uptake from shallow groundwater in arid regions with a less than 20% reduction in yield.