Summary Points

1. Turfgrass soils have the potential to sequester carbon and emit nitrous oxide (N₂O), both greenhouse gases.
2. Our results indicate high and low turf-management-input regimes result in similar net carbon sequestration rates in zoysiagrass golf course fairway turf, which had an average gross carbon sequestration rate of 902 pounds per acre per year.
3. The use of a controlled-release fertilizer, such as polymer-coated urea, instead of a urea, and/or less irrigation, will reduce N₂O emissions in turfgrass.
4. Golf course superintendents can utilize less irrigation and controlled-release nitrogen fertilizers to promote environmental stewardship by conserving water and reducing N₂O emissions while still promoting sequestration of atmospheric carbon.
5. In addition to a reduction in N₂O emissions and promoting carbon sequestration, these management practices can result in labor-saving benefits such as reductions in mowing requirements and fertilizer applications.

In experiment 1, the annual rate of change in soil organic carbon (SOC) – measured in kilograms of carbon per hectare per year (ΔSOC; kg C ha^‑1 yr^-1) – was measured under two turf management regime treatments.

Experiment 1 Treatments

• High-management-input regime (HMI): Irrigation replacement level of 66% reference evapotranspiration (ET_o) + included urea applied at 98 kg N ha^-1 yr^-1 (2 pounds of nitrogen per 1,000 square feet per year)
• Low-management-input regime (LMI): Irrigation replacement level of 33% ET_o + unfertilized turf

During June through August of 2014, 2015, and 2016, the two irrigation treatments were applied by hand watering individual plots twice a week using calculated amounts based on daily ET_o rates from an on-site weather station. From September through May, when the rainout shelter was not activated, all plots received adequate irrigation that was less than or equal to one-half inch of water per week and any occurring precipitation. The annual rate of change in soil organic carbon – which was the gross carbon sequestration rate – was calculated from the difference between 2013 and 2016 soil samples from a 0-12 inch depth and averaged over the 3.16-year (1,154-day) period. Energy expenditures in carbon equivalents (CE) known as “hidden carbon costs” (HCC) from turfgrass maintenance practices – i.e., irrigation, mowing, fertilizer and pesticide applications – along with CE and HCC from N₂O emissions, were measured and calculated for each plot within each management regime over the entire three-year study. These HCC (kg CE ha^-1 yr^-1) were then subtracted from the gross carbon sequestration rates to determine net soil carbon sequestration rates for each management regime.  Additional data collected included percent green turf cover of plots using digital image analysis.

In experiment 2, N₂O emissions were measured in zoysiagrass receiving two irrigation treatments and three nitrogen fertilization treatments.

Experiment 2 Treatments

• Two Irrigation Treatments
  • Irrigation replacement rate of 66% ET_o
  •  Irrigation replacement rate of 33% ET_o
• Nitrogen Fertilization Treatments
  • Urea (46-0-0) applied at total of 98 kg N ha^-1 yr^-1 (2 pounds of nitrogen per 1,000 square feet per year)
  • Polymer-coated urea (PCU) (41-0-0; 90-day release) applied at total of 98 kg N ha^-1 yr^-1 (2 pounds of nitrogen per 1,000 square feet per year)
  • Unfertilized “control” (UF) receiving no nitrogen fertilizer

Irrigation treatments were applied using the same method as experiment 1. For the nitrogen fertilization treatments, urea was applied at a rate of 1 pound of nitrogen per 1,000 square feet at the beginning of summer and again in midsummer for a total of 2 pounds of nitrogen per 1,000 square feet per year and the controlled-release, PCU was applied once at the beginning of summer for a total of 2 pounds of nitrogen per 1,000 square feet per year. Nitrous oxide emissions were measured with static chambers placed over the turf surface and then analyzed with gas chromatography.

Measurements began on October 29, 2014 [day of year (DOY) 302], and continued until October 19, 2016 (DOY 293), concluding two full years of data collection. Measurements were collected on weekly intervals from March through October, biweekly-to-monthly intervals from November through February, and more intense daily measurement intervals surrounding fertilization applications during the growing season. Additional measurements included soil moisture, soil temperature, soil nitrate, soil ammonium, mowing frequency and visual turf quality of plots.

Results

Results from experiment 1 revealed soil organic carbon increased after the study period, indicating that carbon was sequestered in the turfgrass soil. The average gross carbon sequestration rates for the two treatments were not statistically different at 1,046 kg C ha^-1 yr^-1 (933 pounds of C per acre per year) and 976 kg C ha^-1 yr^-1 (871 pounds of C per acre per year) in HMI and LMI, respectively, prior to subtracting HCC (Table 1). Overall, the zoysiagrass golf course fairway turf had an average gross carbon sequestration rate of 1,011 kg C ha^-1 yr^-1 (902 pounds of C per acre per year). Once the total estimated HCC was included, the average net carbon sequestration rate was 412 kg C ha^-1 yr^-1 (368 pounds of C per acre per year) and 616 kg C ha^-1 yr^-1 (550 pounds of C per acre per year) in HMI and LMI, respectively, with no statistical differences.

The HMI had 76% more HCC than the LMI due to nitrogen fertilization and higher irrigation amounts, which led to greater N₂O emissions and more mowing events. Therefore, results indicate high- and low-management-input regimes result in similar net carbon sequestration rates in zoysiagrass golf course fairway turf. In addition, both management regimes maintained acceptable turf quality and at least 75% green turf cover during both summers, which included unfertilized zoysiagrass plots receiving only 33% replacement of ET_o twice a week with all rainfall excluded for 92 days during the summer (Braun and Bremer, 2019).

Results from experiment 2 revealed that N₂O emissions decreased with less irrigation; specifically, N₂O emissions during two summers were reduced by 6% with 33% ET_o replacement (2.71 kg ha^‑1) versus 66% ET_o replacement (2.88 kg ha^-1) (P < 0.001) (Table 2). Overall, the majority of the differences in daily N₂O fluxes and cumulative N₂O emissions were due to the nitrogen inputs.

In both years, cumulative annual emissions of N₂O were significantly greatest in urea-treated plots and least in unfertilized zoysiagrass among the treatments (Table 2 and Figure 1). Cumulative emissions of N₂O-N for the entire two-year period were 4.06 kg ha^-1 in unfertilized plots and 4.5 kg ha^-1 in PCU, which represent reductions of 28% and 20%, respectively, from urea-treated turf (5.62 kg ha^-1) (P < 0.01). Compared with unfertilized turf, total cumulative emissions of N₂O were increased by 38% with urea fertilization and 11% with PCU fertilization. The range of annual N₂O emissions (1.82 to 2.85 kg N₂O-N ha^-1 yr^-1) in this study is comparable to the average rate of 2.7 kg N₂O-N ha^-1 yr^-1 reported from other turfgrass systems, which is also similar to rates in agricultural soils (Braun and Bremer, 2018b).

Over the two-year study, the percentage of applied nitrogen fertilizer emitted as N₂O in this study was 2.3% from PCU and 2.9% from urea fertilizer. The highest fluxes and majority of emissions occurred in the summer because of the fertilization events and, presumably, higher soil temperatures (Braun and Bremer, 2018a). There were spikes after applications of urea, but increases were much smaller after application of PCU (Braun and Bremer, 2018a). Results from this study indicate that use of controlled-release fertilizers, such as PCU, instead of a quick-release fertilizer and/or less irrigation will reduce N₂O emissions in turfgrass. Notably, all fertilizer and irrigation treatments maintained acceptable turf quality and high levels of percent green cover (Figure 2; see also Figure 1 in Braun and Bremer, 2019); however, PCU resulted in more consistent turf quality and green cover compared to urea-treated and unfertilized plots.

Currently, USGA-funded research at Kansas State University is building upon data from our research results presented above to develop simulation models of N₂O emissions and carbon sequestration in zoysiagrass fairway turf using the DAYCENT model. Research objectives are to calibrate the DAYCENT model to predict and estimate the long-term impacts of turf management practices, as well as other maintenance expenses, on N₂O emissions and carbon sequestration in a zoysiagrass golf course fairway.

Discussion

Results from these USGA-funded experiments indicate the development and application of management practices, such as less irrigation and the use of controlled-release fertilizers, may reduce N₂O emissions while promoting sequestration of atmospheric carbon at similar rates to high-management-input regimes. Such practices could also save time and labor for golf course superintendents through reduced irrigation, fewer fertilizer applications, and a potential reduction in mowing requirements. Results also indicate the importance of factoring the hidden carbon costs of turf maintenance practices into net carbon sequestration rates. The use of these specific management practices could improve a golf course's efforts toward environmental stewardship. .

Turfgrass provides significant economic, environmental, and human benefits but is sometimes criticized as being environmentally unfriendly, for example because of its use of water and fertilizers. This research revealed specific management practices that can help minimize the use of water and nitrogen fertilization in turfgrasses and reduce the amount of greenhouse gases (CO₂ and N₂O) in the atmosphere, via increased carbon sequestration and reduced N₂O emissions. For example, less irrigation not only reduced N₂O emissions but also conserved water while still promoting sequestration of atmospheric carbon. Also, fertilizing with a controlled-release nitrogen fertilizer – polymer-coated urea – not only reduced N₂O emissions, but also consistently maintained higher turfgrass visual quality and green turf cover than urea. Interestingly, at the low irrigation level, N₂O emissions were as low in zoysiagrass fertilized with a polymer-coated nitrogen source as they were in unfertilized turf. Finally, other potential benefits of fertilizing with controlled-release nitrogen include increased nitrogen availability to the turf by releasing nitrogen as the plant needs it while losing less nitrogen to the environment through volatilization or leaching.

References

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