Russell, Kerri
Nitrogen and Water Interactions in Drought Stressed Kentucky Bluegrass Conserving our Vital Resource
Faculty Mentor: Bryan G. Hopkins, Plant and Wildlife Sciences
Introduction
Turfgrass is the irrigated crop of greatest acreage in the United States. As urban
and suburban developments are growing at an unprecedented rate, the demand for turfgrass is in
high demand. Its ecosystem services are valuable and include groundwater protection, erosion
control, soil health, and air purification. It also provides high utility to residential and public
lands. Despite its environmental benefits, turfgrass has been brought under scrutiny due to
natural resource consumption and pollution, particularly in arid and semi-arid regions of the
western US. Turfgrass requires a significant amount of irrigation water, a resource that is scarce
in the arid and semi-arid regions of the west. Along with high water demand, healthy turfgrass
requires nutrient application. Over fertilization leads to an increase in water use, which may lead
to runoff resulting in pollution of the atmosphere and hydrosphere and can permanently damage
rare and fragile ecosystems surrounding turfgrass. Increased nutrients, especially nitrogen (N)
and phosphorous, in waterways leads to algal blooms and speeds up the natural eutrophication
process. An increase in algal blooms may result in injury or death to aquatic life and negatively
impact organisms’ drinking water. Just recently in July 2016, Utah Lake was closed down due to
a harmful algal bloom, leading to damage to the local ecosystem and surrounding areas that rely
on water from Utah Lake for irrigation. Over fertilization and irrigation are partially to blame.
Methodology
A preliminary study was conducted, followed by a follow up trial. The final study
was conducted from April 28, 2016 through August 4, 2016 in Provo, UT. Established Kentucky
bluegrass (Poa pratensis L.) sod was transplanted and then grown in 0.015 m diameter by 0.012
m height pots in an environmentally controlled growth chamber. The grass was trimmed weekly
to an approximate 0.06 m height.
The study was organized in a randomized complete block design with four replications of all
combinations of two irrigation levels [sufficient at 100% and drought at 60% of
evapotranspiration (ET)], and three N levels (deficient at 49.0 kg ha-1, optimum at 147 kg ha-1,
and excessive at 440 kg ha-1). N was applied in the form of a traditional rapid release urea
(micro-prills SGN: 250).
N treatments were applied on June 3, 2016 and the turf was watered daily, the same amount
across N treatments until July 6, 2016 when N stress became evident. Field capacity was then
determined by saturating pots and weighing after 24 h. Once field capacity was determined,
treatments received 60% or 100% of daily ET. Visual observations and NDVI readings were
taken weekly. Biomass was collected bi-weekly. The study was ended when ET values leveled
off on August 4, 2016. Final results are not available at this time, but Statistical analysis will be
performed by Analysis of Variance with mean separation by the Tukey-Kramer method.
Results and Discussion
– Results for the final study have not yet been analyzed, but they appear to be similar to that of the preliminary study. As shown in Fig. 1, increased N rates lead to
increased evapotranspiration. When turfgrass was over fertilized, it required more water for
irrigation. Biomass also increased with increased N rates. This becomes a problem with
maintenance because it results in increased need for mowing, resulting in increased fossil fuel
consumption and exhaust and landfill wastes. There was a significant difference in N
concentration found in the leaf clippings for optimum and excess N for both irrigation
treatments. At a deficient N rate there was not a difference in N concentration between irrigation
treatments. NDVI, a measure of plant health, indicates a higher N rate leads to a healthier
plant—even when deficit irrigation is applied. At times of drought, plant health can be conserved
with a relatively higher N rate for a time without increasing irrigation rate. Canopy temperature
indicates that deficit irrigation and deficient N result in a significantly higher canopy temperature
shown in Fig. 2. However, with a higher N rate the canopy temperature was similar for 60 and
100% irrigation.
Results from this study suggest that N and water resources can be conserved with proper
management. Increasing N leads to over irrigation causing nutrient pollution and runoff. An
increased consumption of fossil fuels also occurs with increased N. However, in drought
conditions, applying a higher amount of N, and only irrigating 60% of the daily ET, water can be
conserved while maintaining an acceptable NDVI plant health rating. Improper management of
N, water, or both leads to an excess consumption of these valuable resources and can cause
environmental damage—of which cannot always be reversed.
Conclusion
– N has an impact on plant health and productivity and interacts with water. Although we have gained understanding of the interaction between N and water, further studies will need
to be done in order to determine best management practices. The N interaction with irrigation is
complicated but it will be necessary to gain an understanding as the turfgrass demand continues
to grow.
