Assessing Water Use Efficiency, Soil Health, and Pollinators within a Transition from Irrigation to Dryland Management in the Texas High Plains

Progress report for LS20-341

Project Type: Research and Education
Funds awarded in 2020: $299,208.00
Projected End Date: 09/30/2023
Grant Recipients: Texas Tech University; United States Department of Agriculture- Agricultural Research Service (USDA-ARS)
Region: Southern
State: Texas
Principal Investigator:
Dr. Scott Longing
Texas Tech University
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Project Information


The rapid decline in water supply for irrigation in the Texas High Plains is encouraging some growers to convert their irrigated cropland to production of dryland crops and low water requiring forages. Reduced irrigation will directly depress crop yields and financial security. Research on the impacts of transitioning toward reduced water input can reveal soil and crop management practices that build up the soil’s health. Switching to more diverse cropping systems can enhance numbers and activities of introduced and native bees, which are potential pollinators. We propose to study changes in soil health indicators, ground-nesting bees (as an indicator of pollinator habitat), and water use by annual crops and perennial forages in the context of transition from high irrigation to low irrigation to dryland management. Water use efficiency measurements of crops will serve as an integrator management practices aimed at building soil water storage and effective water use in crop rotations. The forage component concentrates on alfalfa growing in monoculture (but at modest water input), in mixture with old world bluestem grass, and compared to grass alone. Studies will be done on four commercial farms in the South Plains of the Texas High Plains, and the Texas Tech University pasture research facility near New Deal, TX. Three of the growers are already part of long-term studies by a cooperating agency (USDA-ARS) for soil health; hence three more years of data will extend those trends. Crops on those farms comprise cotton, corn, sorghum, and rye cover crop. Results will reveal whether transitioning part of the producers’ field to dryland management improves habitat for soil microbes, wild bees, and water retention, in turn will inform decisions on crop type and water management.

Project Objectives:

1) Estimate the water use and water-use efficiency of annual crops and perennial forages transitioning to limited irrigation and dryland production.

2) Evaluate soil health indicators under different transitions to dryland including changing annual crops from irrigated to dryland production, and interseeding alfalfa into established perennial grassland.

3) Compare irrigated and recently transitioned dryland croplands, alfalfa, grass-alfalfa, and grass pastures on ground-nesting bee communities as indicators of insect pollinator health and abundance.


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Materials and methods:

Grower groups: Our team is cooperating with growers to conduct on-farm studies of soil health indicators, crop water use, and ground-nesting bees (pollinator indicators) in relation to water use. The on-farm studies fall into two groups. 1) Alfalfa production for hay by grower Randy McGee will be paired with alfalfa-grass and grass-alone pastures and hay fields at the TTU New Deal Research Station (8 miles apart on the same soil mapping unit, Pullman clay loam, 38% clay content) to emphasize measurements of season-long water use, but also including evaluations of soil microbial communities and ground-nesting bees. 2) Annual crops on farms near Littlefield, TX, owned and operated by Randy Gray, Brian Mueller, Nick Martin. These growers have taken part in the NRCS cost-share program to place some of their land under subsurface drip irrigation to conserve water, while still having other areas under center-pivot and dryland production. Data will be collected to emphasize soil health indicators in relation to transitions from irrigated (center pivot) cotton into subsurface drip irrigated cotton, dryland sorghum, cotton or grasses (e.g., short-term and long-term dryland rye fields). They have already allowed us access to their fields since October 2018 for soil sampling. Soils evaluated in this study represent the predominantly sandy-loam Amarillo soil west of Lubbock which contain >49% sand and <1% organic matter.

Water use and water use efficiency

Measurements will track water inputs on the Group 1 farms (Gray, Mueller, Martin) from rainfall and irrigation. A simplified WUE calculation will be performed assuming no runoff and no deep percolation. We justify that because these soils are essentially level and a nearly impermeable limestone (caliche) layer exists at around 3 feet depth which restricts water flow and root penetration (Dhakal et al., 2019b). We will use the method used by TAWC (explained the 2017 annual report at for calculating WUE.

   Water use = Effective precipitation + Irrigation – change in soil volumetric water content (VWC) between beginning and end of cropping season.

   WUE = Crop yield (lbs.) / Water use (acre-inch)

The growers will collect on-site rain gauge precipitation data. Researchers will calculate “effective precipitation” according to Brouwer and Heibloem (1986). determine changes in soil VWC by extracting cores in three sites per field down to the caliche layer, weighing the core samples, followed by drying and reweighing the cores. This gravimetric water content data will be converted to volumetric content by correcting for bulk density. Soils will be measured by sampling for bulk density at the time of core sampling. Crop yield will be reported by the grower by his harvester yield monitor.

   For Group 2 farms (McGee and Texas Tech research station), weekly recordings of soil volumetric water content will be done with a portable Delta-T PR2/6 Profile Probe and local precipitation collection. Three access tubes will be installed in McGee’s field (they are already installed in the Texas Tech pastures). The probe records VWC at depths of 4, 8, 12, 16, 24, and 48 inches, and the probe is accurately calibrated according to Dhakal et al. (2019a). Water use will be calculated according to Dhakal et al. (2019b) and WUE according to the equation used for Group 1 above.

Soil health assessment  

Soil sampling

Soil sampling at a 0-15 cm depth will be conducted in summer (June) and fall (November) of 2020, 2021 and 2022 in order to sample different years and seasons that would likely have different soil moisture and temperature conditions and plant growth activities. Soil samples will be collected from three growers’ irrigated annual crop land (cotton and corn) and annual dryland crops (cotton, corn, and sorghum), and perennial dryland grass fields in Littlefield Texas near Texas Tech University with nearly level soil (0–1% slope) of an Amarillo fine sandy-loam. Similarly, soil samples will be collected from old world bluestem (OWB) and OWB-alfalfa pastures at TTU New Deal research farm in a Pullman clay-loam soil. Grower’s (Mr. Randy McGee) alfalfa monoculture field near to New Deal research farm will be used.

Analyses of soil microbial community size, structure, and enzymatic activities as indicators of soil health

Soil MBC and microbial biomass nitrogen (MBN) will be determined by the chloroform-fumigation extraction procedure according to Brookes et al. (1985) and Vance et al. (1987) on 15 g of field-moist sample with a CN analyzer (Shimadzu Model TOCV/CPH-TN, Shimadzu Corp., Kyoto, Japan). In brief, organic C and N from fumigated (24 h) and non-fumigated (control) soil samples will be extracted with 0.5 M K2SO4 and then quantified with a CN analyzer. The values obtained from non-fumigated soil samples will be subtracted from those from fumigated soil samples. A kEC factor of 0.45 as described by Wu et al. (1990) and kEN factor of 0.54 as described by Jenkinson (1988) will be used to calculate the MBC and MBN, respectively. For each sample, we will perform duplicate analyses and express the results on a dry-weight basis. Gravimetric soil water content will be determined by drying samples at 105°C for 48 h and adjust for bulk density. Soil microbial community size and structure will be evaluated according to the ester-linked FAMEs (EL-FAMEs) method (Schutter and Dick, 2000) to define the relative abundance of broad taxonomic microbial groups.

    Soil organic matter and nutrient cycling and availability will be evaluated according to SOC, total N (TN), POXC (Weil et al., 2003; Culman et al. 2012), and different enzyme activities involved in biogeochemical cycling.  SOC and TN will be determined by automated dry combustion (LECO TruSpec CN; Joseph, MI) of air-dried samples by a commercial laboratory (Ward Laboratories, Inc., Kearney, NE, USA). Soil pH will be determined in the same laboratory. Four different enzyme activities will be determined for mineralization of C (β-glucosidase), C and N (β-glucosaminidase), S (arylsulfatase), and P (acid phosphomonoesterase) using common methods (Tabatabai, 1994; Parham and Deng, 2000). The activities of these four different enzymes will be determined simultaneously using a combined assay developed by one of our team researchers (Acosta-Martínez et al. 2019).  The substrates for the enzymes will be, respectively, p-Nitrophenyl-β-D-glucopyranoside (0.05 M), p-Nitrophenyl-N-acetyl-β-D-glucosaminide (0.01 M), p-Nitrophenyl phosphate (0.05 M), and p-Nitrophenyl sulfate (0.05 M). The four different substrates will be added to each soil sample and their combined product, p-nitrophenol, will be determined colorimetrically. This combined activity represents an index of biogeochemical cycling potential in soil. In brief, only 0.5 g air-dried soil is needed for this combined assay (1.5g for two reps and a control) which saves time and resources.  The soil is incubated for 1 h at 37 ˚C with 0.5 mL of acetate buffer (pH 5.8) and a 2 mL solution with the four substrates of these enzymes. Following incubation, 0.5 mL of 1 M CaCl2 will be added to each sample, and the reaction will be terminated by adding 2 mL of 0.1 M THAM pH 12.0. Subsequently, the total final volume (5 mL) of soil suspension will be filtered through a Whatman No.2v filter, and the combined product released (p-nitrophenol; PNP) will be determined colorimetrically at 400 nm.

 Pollinator community

Pan Pan trap “bee bowls” will be used to trap bees and other foragers (mostly pollinators) that are attracted to preferred colors (i.e. fluorescent blue, yellow, and white). Bee bowls (3.3 oz., New Horizons, Upper Marlboro, MD, USA) also trap other foragers that are attracted to bright colors (Shapiro et al., 2014). Bees and other pollinators will be collected in summer of 2020, 2021, and 2022 as described by Bhandari et al. (2018b), except that bee bowls will be elevated slightly above ground level and according to the maximum height of the canopy in a particular crop. In each field, along three separate transects, 10 bee bowls will be set with 16-feet distance between adjacent bowls. In each field treatment, 30 bee bowls will be set and constitute the bee bowl sample for that field. Bees and other insects contained in all 30 bee bowls from three transects within each replicate field will be collected after 24 hours and transferred into labeled specimen jars containing 70% ethanol for preservation. In addition to bee bowl sampling, during the period of flowering for each visited crop, insect foragers visiting flowers will be hand-collected along the transects using 5-minute sweep sampling along transects. The composition of each sampled community from each field, including taxonomic composition, and measures of community structure including diversity and functional-group indices, will be compared across fields and transitional production types. An established reference collection developed by our team will aid in identification of pollinators, with a focus on bees occurring across the different agroecosystems.

 Data Analysis

The R-Program (R Core Team, 2017) will be used to evaluate the impacts of transitioning to dryland on the different soil microbial characteristics and establish linkages of changes in microbes and the functions impacted. To illustrate the microbial community composition, unconstrained principal coordinates analysis (PCoA) will be performed for FAME markers using the Vegan package in R. Analysis of variance will be used to analyze the data in a randomized complete block design which consisted of bluestem-alone, bluestem-alfalfa, and alfalfa treatments. Data will be analyzed using Proc Mixed in SAS 9.4 (Littell et al., 2006). For analyses of insect community differences across treatments and fields, a non-metric multidimensional scaling (NMS) ordination method will be used for comparison of communities based on taxonomic compositions, while indicator species analysis will be used to determine bee taxa with affinities for particular crops (PC-ORD).

Participation Summary

Project Outcomes

Project outcomes:

Soil Sampling and Analyses

Within our first year of this project, we are making significant progress in the evaluation of the soil biological health component within the dryland transition experienced (by producers) in the Southern High Plains. Our first soil sampling was conducted in July and the samples are being processed for different soil biological indicators of the soil health. We only need to conduct the second soil sampling of this year, which will be done in October. The sampling in October will complete 4 years of fall samplings allowing a long-term evaluation of the transitions that producers are making. Having two samplings in the same year, possible through this grant, will allow our evaluation of any changes in the soil microbial component due to seasonal fluctuations for different water management agroecosystems.  Specific soil health indicators linked to water conservation we identify in this research could help guide sustainable management for water-limited environments. Optimization in the detection of early changes in soil health and their linkages with soil functions, specifically related to soil water conservation, will reduce crop yield declines, improve farm incomes, and nurture rural livelihoods in the long run.

Pollinator Sampling 

Pollinator sampling was initiated at two fields in 2020, yet because of restrictions related to COVID-19 much of the field work in 2020 was not accomplished. In 2021, pollinator sampling has been initiated at the proposed fields, and we are continuing to sample pollinator communities through cotton flowering in August and September. Concurrent and related activities involving pollinators in agroecosystems was a project conducted by two undergraduate students in barley and wheat crops beginning in March 2021. These students sampled pollinators using bee bowls at different fields containing either wheat or barley crops on five different dates through Spring 2021. A primary objective of this study was for the students to initiate learning of native bee taxonomy with a focus on early-season bees foraging in cover crops. Samples from these studies are currently being processed in the lab, while bee identification and community analysis will take place in fall-winter and combined with pollinator data acquired form the same fields in the 2022 growing season.  

Concurrent with pollinator sampling in 2021 has been the development of a brief publication on pollinators in farm lands in the Southern High Plains region, emphasizing native bees, their habitats and conservation in working farms. This document is supported by a recent checklist of native bees (Discua and Longing, in manuscript) and highlights conservation practices and other farming practices in support of native bees.   

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    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.