Impact of Soil Health Practices in an Irrigated Agroecosystem

V. Crowe, J. Norton, T. V. Bush
University of Wyoming
In the arid and semi-arid west, climate change and soil degradation create the need to implement soil health management practices for resilient agroecosystems that not only tolerate disturbance but also continue to function. Soil health practices such as reducing tillage, diversifying crop rotations, keeping soil protected by planting fall cover crops, and integrating livestock show potential to improve soil in irrigated agroecosystems. Preserving topsoil, building SOM, tightening nitrogen (N) and carbon (C) cycles mediated by soil microbes, replacing fallow periods with other annual or perennial crops and integrating livestock, may increase soil’s resilience and improve profitability and sustainability. 
  • Many farms in irrigated agroecosystems that rely on a two-crop rotation of sugarbeet and barley use field preparation for furrow irrigation, and its water delivery method, degrading soil through disturbance increasing the rate of SOM loss (Hurisso et al. 2013, 2014, 2015, Liebig et al. 2004, Norton 2011).
  • Lack of rotation diversity and intensive tillage due to sugarbeet crop’s economic value creates soil loss from wind erosion decreasing soil resilience (Afshar et al. 2018).
  • Planting sugarbeets using strip tillage can protect sugarbeets from wind damage and reduce soil loss (Overstreet and Cattanach 2010).
  • Crop research has shown that increasing the time between sugarbeet rotations by adding another annual or even perennial crop and reducing tillage can improve soil health and retain SOM (Krupinsky 2006, Hurisso et al. 2015). 
Regional farming practices in cold semiarid irrigated agroecosystems with short growing seasons are shifting to reduced tillage and overhead irrigation to stay economically viable, but more research is needed on the impact of soil health practices such as crop diversification, reduced tillage, and livestock integration on soil physical and chemical properties in these systems.
A multi-year study from 2014-2020 in the Bighorn Basin of Northwest Wyoming used a three-crop rotation of edible dry beans, malt barley, and sugarbeets, along with conventional and minimum tillage, full and deficit irrigation, and cover crop treatments after barley harvest to evaluate the effects of these practices on soil health. 
In this presentation we are reporting total and mineralizable nitrogen and carbon as they represent pools that turn over on an annual basis, provide plant nutrients, and typically respond relatively rapidly to changes in management. 
Definitions for Conventional and Minimum Tillage for this experiment:

Research Questions: What are the differences in soil health indicators between the conventional (CT) and minimum (MT) tillage treatments? What are the differences in soil health indicators between the two irrigation levels (full and 75% of full)? Does the current rotation’s crop affect soil health indicators?

ApproachOn-station collection of soil and lab analysis of soil health indicators in each of the 36 plots.

Hypothesis1: We expect long term crop rotation in MT plots will result in greater soil health indicators than in CT plots. 

Hypothesis 2: We expect differences in yield between full and deficit irrigation to decrease due to improved soil health under MT.

Study Site
The study occurs at the University of Wyoming Powell Research and Extension Center (PREC), 44°45’32” N, 108°45’30” W, elevation 4376 feet above mean sea level in the Bighorn Basin of North Central Wyoming east of Yellowstone National Park (Craig 2017) (Figure 1).
Figure 1. Study site location at the University of Wyoming Powell Research and Extension Center (indicated by the red dot) in North Central Wyoming east of Yellowstone National Park. 
The site is Garland Series soil, made up of mixed alluvial material and classified as fine-loamy and fine-loamy over sandy or sandy-skeletal, mixed, superactive, mesic Typic Haplargids (Staff NRCS USDA 2019). Powell typically has about 149 frost-free growing days (US Climate Data 2019, 2019). This long-term rotation study started in 2014 and includes three subplots each of malt barley, sugarbeets, and edible dry beans, under minimum till (MT), conventional till (CT), and two irrigation levels (full and 75% of crop needs) for a total of 36 plots, 44 x 120 feet each (0.12 acres) (Figure 2).

Figure 2. Layout of the experiment showing the three-crop rotation in two tillage systems minimum till (MT) and conventional till (CT), two irrigation levels, full and 75% of full irrigation, and rotation schedule for each set of MT and CT plots where SB = sugarbeets, EDB = edible dry beans, and BAR = barley. 
While the replications are all aligned and not fully randomized over the experiment, this design facilitates equipment available at PREC and increases the likelihood of long-term proper management. The >0.1-acre plots in this research framework are large enough to split for evaluation of different fertility, weed control, variety, cover crop, or other practices in the context of the two tillage systems.
Soil Sampling and Analysis
Zero- to 15-cm depth composite soil samples were created by collecting, mixing, and subsampling four soil cores across each of the 36 plots. For the plots under barley, MT subplots under volunteer barley treatment were used to represent the typical MT practice of no fall tillage. Samples were placed on ice and transported to the lab within 24 hours.
Basic soil properties analysis for this presentation include: particle-size distribution by the hydrometer method (Gee and Bauder, 1986), pH and electrical conductivity (EC) by electrode (Thomas 1996), inorganic C by modified pressure-calcimeter (Sherrod et al. 2002). Total organic C is determined by subtracting inorganic C from total C. To quantify available and readily mineralizable C and N, 10-g subsamples were extracted with 0.5M K2SO4. Potentially mineralizable N by 14-day anaerobic incubation (Hart et al., 1994).
Basic Soil Properties

Basic soil properties for the site analyzed after soil collection in August 2020 include pH, EC, soil texture, clay content, bulk density, calcium carbonate, and soil classification presented as site characteristics (Table 1.) At the time of soil collection, barley was in the dry-down phase ready for harvest, while sugarbeets and edible dry bean plots were still receiving irrigation. Results are consistent with values from previous years.

Table 1. Basic soil properties for PREC sampled August 2020. 
Total Nitrogen and Mineralizable Carbon

Results from the study looking at long term effects of soil health practices across two tillage treatments, two irrigation levels, and three crop rotations suggests a 33% increase in SOM over a six-year period (two full rotations) based on an increase in total nitrogen in MT compared to CT practices (Figure 1). We expect organic carbon will trend with nitrogen and represent 1/3 more SOM in MT plots, but machine analysis issues affected results and samples could not be rerun before this presentation. Irrigation is the primary effect on microbial mineralization of carbon for this irrigated system (Figure 2). Higher amounts in mineralizable carbon results suggest that in deficit irrigation plots less moisture inhibited mineralization. In Figure 3, although not significant, reduced tillage is trending towards more mineralizable carbon. Mineralizable nitrogen was not significant but the data show a similar pattern to mineralizable carbon. These initial results indicate that some soil health parameters are improving with long-term implementation of minimum tillage practices and crop rotations as we hypothesized. Subsequent yield data is still being compiled.  

Figure 1. Total nitrogen percent by tillage level indicates a significant effect p < alpha = 0.05 of minimum tillage compared to conventional tillage practices. 

Figure 2. Potential mineralizable carbon by irrigation level indicates there is a significant effect p-value < alpha = 0.05 of deficit irrigation (75% of full) vs. full irrigation as a primary effect. 
Figure 3. Mineralizable carbon increase is trending towards impact from both reduced tillage and irrigation treatments, however only irrigation was statistically significant at p < alpha = 0.1. 

Additional data including yield analysis for cash and cover crops along with other soil data is being compiled from August through October 2020 sampling. However, our preliminary findings suggest diversification in cash crop rotations and adding annual fall cover crops after barley harvest is improving soil health parameters in the short term. Additionally, these practices create forage opportunities for livestock integration into arid irrigated agroecosystems, and soil health management practices may affect dry bean variety choice when expanding to a third cash crop in a sugarbeet-barley rotation.


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