Austin Hopkins and Faculty Mentor Dr. Neil C. Hansen, Plant and Wildlife Sciences
This project identifies turfgrass practices to conserve water by optimizing nitrogen (N) fertilization and irrigation strategies. Turfgrass is the most irrigated crop by area (163,800 km2), accounting for 1.9% of the continental U.S. This causes negative environmental impacts and puts the future of irrigated turfgrass in question due to increasing water scarcity, especially in the western U.S. More information is needed on how to irrigate in droughts as turf provides significant benefits in our environment such as aesthetics, cooling effects, carbon sequestration, and preventing soil erosion.
N is essential for healthy turfgrass. Unfortunately, like water, it is often over applied and affects turf water use. In drought conditions, over-applying N could negatively impact turf root systems, cause stress on turf health, and cause buildup of salts in the soil profile. A better understanding of N x H2O interactions allows us to cut turfgrass water needs by cutting N fertilizer and thus conserve scarce resources and limit a potential source of pollution.
Perennial ryegrass (Lolium perenne L.) was evaluated on two irrigation and three N rates in a glasshouse for 42 days. Treatments included a full evapotranspiration (ET) replacement, and a drought simulation treatment with 60% of full ET. N rates were 10, 50, and 250 kg ha-1 as low, optimum, and high treatments. N was split-applied weekly as liquid ammonium nitrate and adding any captured leachate back to each pot to eliminate N loss. The turf was otherwise grown with best management practices. We measured water usage, canopy height, temperature, shoot biomass, root biomass, NDVI, and verdure.
High N turf shoots grew rapidly at first, but later growth was stunted vs optimum. Inversely, High N treatment root biomass was stunted vs optimal and low N treatments. High N treatments have surplus resources, therefore they are not stressed. This resource abundance causes stunted root systems. Turf with stunted root systems will have a difficult time staying healthy under times of plant stress. The results for high N growth rates and root biomass correlate well.
Elevated canopy temperature indicates plant stress. High N 100% water treatments showed the highest collective canopy temperatures: 87 degrees F. The lowest was the optimum N 60% water treatment: 79 degrees F. Both medium N treatments were significantly lower vs low and high N.
Normalized Difference Vegetation Index (NDVI) is an index of plant “greenness,” or photosynthetic activity, frequently used for plant health. The NDVI data showed both optimum N treatments had significantly higher NDVI values vs low and high N treatments. There was no significant difference between optimum N 100% water vs optimum N 60% water.
Generally, plants appeared healthier and had higher NDVI with full irrigation and optimum N rate. However an almost identical level of health was achieved by applying 40% less water than normally applied. Excessive N reduced root growth and shoot growth and resulted in significant water stress, indicated by elevated canopy temperature, regardless of irrigation treatment. Root growth under excessive N was less than half that in the optimum N treatments. NDVI showed that plants tolerated drought conditions well with optimum N, but did not tolerate drought with excess or deficient N levels. These results indicate that N management influences ET in perennial ryegrass. Water conservation will be achieved by optimizing the N rate and avoiding excessive or deficient N.
Optimal N levels, roughly 50 kg ha-1, and conserving water usage improves turf health in drought sustainably. Also, applying optimal N vs low N under drought stressed conditions assists the turf’s water efficiency. However, applying higher N causes stunting in the turf, first in the root systems, then followed by the shoots in times of plant stress. Reducing water application by 40%, an almost identical level of turfgrass health can be achieved. However, further research on the application of remote sensing using NDVI and canopy temperatures are need to improve water use efficiency in the average irrigation system.