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Article

Shade and Nitrogen Fertilizer Effects on Greenhouse Gas Emissions from Creeping Bentgrass Putting Greens

by
Katy E. Chapman
1,* and
Kristina S. Walker
2
1
Department of Math, Science, and Technology, University of Minnesota Crookston, Crookston, MN 56716, USA
2
Department of Agriculture and Natural Resources, University of Minnesota Crookston, Crookston, MN 56716, USA
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(8), 832; https://doi.org/10.3390/horticulturae10080832
Submission received: 29 June 2024 / Revised: 3 August 2024 / Accepted: 4 August 2024 / Published: 6 August 2024
(This article belongs to the Special Issue Sustainable Strategies and Practices for Soil Fertility Management)
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Abstract

:
Climate change mitigation requires creative solutions to reduce greenhouse gases (GHG). Little research has been performed on GHG emissions from shaded turfgrass systems, resulting in a lack of best management practice (BMP) development. The aim of this research was to investigate the soil flux of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) as impacted by shade [shade (98.8%) versus sun (100%)] and differing sources (fast- versus slow-release) and rates (147 versus 294 kg ha−1 yr−1) of nitrogen (N) fertilizers on creeping bentgrass putting greens. The results show that emissions of soil CO2 and soil N2O are significantly lower in shaded plots versus sunny plots. The presence of N fertilizer significantly increased soil CO2 emissions over unfertilized plots. Quick-release N fertilizer fluxed significantly more soil N2O than the slow-release N fertilizers. Turfgrass color was significantly higher on the sunny green versus the shaded green except in late summer. Turfgrass quality was significantly higher for the shaded green versus the sunny green. Milorganite improved turfgrass quality whereas urea decreased turfgrass quality due to fertilizer burn. When N is needed to improve turfgrass color and quality, the use of slow-release N sources should be a BMP for shaded greens.

1. Introduction

Combating climate change has become a global imperative, prompting the search for solutions to reduce greenhouse gas (GHG) emissions. Carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) contribute to the greenhouse effect, trapping heat and driving up global temperatures [1], leading to disruptions in ecosystems, agricultural patterns, and biodiversity [2]. Greenhouse gas emissions from agriculture, crop, and livestock production account for 9.4% of the total US GHG emissions [3]. Mitigating climate change will require all sectors of society to sharply reduce greenhouse gas emissions [4] specifically, adjusting management practices for maintaining crops and land.
Agricultural management practices, particularly fertilizer application, play a significant role in CO2, CH4, and N2O emissions. Nitrogen-based fertilizers can stimulate microbial activity in the soil, leading to the release of N2O through incomplete denitrification [5]. Human activities have generally increased the availability and mobility of N over large regions of the Earth, leading to environmental consequences including increased concentration of the potent greenhouse gas, N2O [6]. Different sources of nitrogen can alter nutrient dynamics in soil as they move through the nitrogen cycle. Moreover, fertilizers can affect microbial respiration by providing carbon and nutrients, thereby influencing nitrogen availability for plant uptake [7]. By impacting soil respiration, fertilizers also impact the release of soil CO2 through microbial respiration. Many studies have observed higher soil organic carbon associated with turfgrass and are beginning to see urban soils as providing important ecosystem services such as carbon storage [8,9,10]. Some of the organic carbon stored in urban soils is added through organic compost and turf-sand mixtures [11,12,13] and the stability of that stored carbon is important to understand if we are to use carbon storage in urban turfgrass as a climate mitigation technique [14,15,16]. Soil CH4 fluxes are generally associated with anoxic environments [17]; however, microorganisms in terrestrial soils can oxidize CH4 to produce CO2 [18], thus soils can serve both as a source and as a sink of soil CH4 depending on the conditions [19,20]. Understanding the biogeochemical cycling leading to the flux of these gases is important to consider as we look for climate mitigation strategies. Intensity of management as well as the presence of trees in urban landscapes has been shown to impact soil CO2 flux and soil N2O flux [21]; however, further investigations into the impact of urban landscapes on the flux of greenhouse gases are needed. The impact of urban soils on soil greenhouse gas fluxes is important to understand as well as strategies that can reduce fluxes of greenhouse gases from these soils.
Turfgrass makes up 40 million acres (1.9% of the land) across the United States [22] and 3% of the Earth’s land surface [23] worldwide. Its widespread presence on golf courses, athletic fields, sod farms, lawns, and various grounds and landscapes [24,25] makes it an ideal candidate for the adoption of recommended best management practices (BMPs) to address climate change [23,26,27]. Fertilizer usage is prevalent in areas with turfgrass, often necessitated by aesthetic preferences such as turfgrass color and quality. Turfgrass can influence GHG emissions through fertilizer application, maintenance practices, and carbon sequestration. Healthy turfgrass serves as a carbon sink, storing carbon in plants and organic matter, thereby mitigating emissions [28].
One major environmental factor affecting overall turfgrass health (turfgrass color and quality) and, consequently, GHG emissions, is sunlight exposure. Trees and shrubs provide shade, affecting microbial activity, soil moisture, and temperature within turfgrass areas [29,30]. Increased sunlight can elevate soil temperature, stimulating microbial activity but potentially reducing root length and increasing mortality, ultimately impacting turfgrass quality [31]. Trees are desirable on golf courses for the overall aesthetics and golfer challenge; however, shade caused by trees is a major cause of turfgrass failure, especially on putting greens [32]. It is estimated that 20 to 25% of all turfgrasses are maintained under some degree of shade, whether from buildings or trees [33]. Most golf courses have at least one putting green where shade stress causes the turf to thin. Other problems that can develop on shaded greens are increased susceptibility to diseases, weed invasion, susceptibility to traffic damage, bare spots, and wet spots that affect playability, turfgrass quality, and overall plant health [34]. Shade also alters the turf’s microclimate: reduced light intensity, compensation and saturation points, reduced light quality (color or wavelength), higher humidity, restricted air movement, moderated temperature extremes, and increased CO2 levels [33]. Trees not only cast shade but may also affect grass growth through competition for light, water, and nutrients.
Maintaining acceptable turfgrass color and quality on shaded golf course greens is difficult for superintendents due to a low ratio of photosynthesis:respiration, which reduces turfgrass growth [35]. Adjusting management techniques is the main approach a superintendent may employ to maintain turf in reduced light conditions. Reducing N fertility on shaded turf is a commonly recommended cultural management practice for maintaining turf in reduced light conditions. Goss et al. [32] found that using lower N rates (150–185 kg ha−1 yr−1) resulted in better turfgrass quality than higher N rates (212–235 kg ha−1 yr−1) when using a liquid N source (urea). Steinke and Stier [35] found that liquid N (urea) improved the turfgrass quality of creeping bentgrass but reduced the quality of Kentucky bluegrass when compared with a granular urea under 80% shade. Although little research has been conducted on N sources under shaded environmental conditions, it is generally thought that N source impacts the success of turfgrass grown under shade [34]. All the previous research on shade compared liquid versus granular urea, which is a fast-release source of N. Research regarding the use of slow-release N sources on shaded turfgrasses is lacking even though slow-release fertilizers are more environmentally friendly [36].
Little research has been performed on GHG emissions from shade [37], other than one paper on lawns and urban green spaces [21]. Further research is needed as the management of crops, such as turfgrass, could become part of the solution as our society addresses climate change [38]. Solutions are needed where management strategies are refined to account for GHG emissions in shade [39]. The purpose of the current research was to evaluate the soil flux of CO2, CH4, and N2O as impacted by shade (shade vs. full sun) and differing sources and rates of N (fast- and slow-release) fertilizers on creeping bentgrass putting greens. While evaluating the environmental impacts of shade and N source, we also evaluated turfgrass health to ensure environmental benefits did not compromise the overall color and quality of the turfgrass. Ultimately, the goal was to provide BMPs to superintendents fertilizing turfgrass greens in shade to reduce GHG emissions.

2. Materials and Methods

2.1. Site Description

This two-year field study was conducted from May 2016 through October 2017 at Lincoln Golf Course in Grand Forks, ND, USA. The golf course was built in 1909 and is located on the west bank of the Red River of the North. Two putting greens were selected based on the amount of shade (98.8%) and sun (100%) they received (Figure 1) as determined by quantifying the area of shade vs. sun at the time-of-day sampling occurred. The shade green was surrounded by 4 trees: Colorado Blue Spruce (Picea pungens), Japanese Lilac (Syringa reticulata), and 2 American Elms (Ulmus americana). The Colorado Spruce was 31 m in height and the Japanese Lilac and American Elms were approximately 50–60 m in height. The trees were planted when the golf course was built, thus at the time of the study, the trees were approximately 100 years old. The area of the shade green was 277 m2 and the sun green was 428 m2. Both greens consisted of 80% ‘Penncross’ Creeping bentgrass (Agrostis stolonifera L.) and 20% annual bluegrass (Poa annua L.) grown on a sand-based root zone (90:10 sand:organic matter). The putting greens were constructed as native soil-based push-up greens. They have been modified over the years with the addition of topdressing (90:10 sand:organic matter). The putting green profile consists of a 1.91 cm organic layer over 10.2 cm of sand (90:10 sand:organic matter). The underlying native soil is a fine-silty mixed, superactive frigid Cumulic Hapludoll.
Fifteen soil samples (10.2 cm depth) were randomly taken per location to make one composite sample at the beginning of the study and sent to AgVise (Harwood, ND, USA) for analysis. The shaded putting green had a pH of 7.8, 39 kg ha−1 P, 267 kg ha−1 K, and 37 g kg−1 organic matter. The putting green located in the sun had a pH of 7.5, 45 kg ha−1 P, 215 kg ha−1 K, and 55 g kg−1 organic matter.

2.2. Fertilizers Evaluated

Turfgrass plots were fertilized May through October with an annual N rate of 0 to 294 kg N ha−1 yr−1. Three sources of granular fertilizers were used to supply N: untreated control (UNT), urea (URE) (46-0-0), and Milorganite (MIL) (5-2-0). Urea is a fast-release N source whereas MIL (natural organic) is a slow-release N source. There were five fertilizer treatments: untreated control (UNT, 0 kg N ha−1 yr−1), urea low (UREL, 147 kg N ha−1 yr−1), urea high (UREH, 294 kg N ha−1 yr−1), Milorganite low (MILL, 147 kg N ha−1 yr−1), and Milorganite high (MILH, 294 kg N ha−1 yr−1). For the low fertilizer treatments (UREL and MILL, 147 kg N ha−1 yr−1), a rate of 24.5 kg N ha−1 was applied to each plot in May, June, July, August, September, and October. For the high fertilizer treatments (UREH and MILH, 294 kg N ha−1 yr−1), a rate of 49 kg N ha−1 was applied to each plot in May, June, July, August, September, and October. Monthly applications were applied the first week of each month throughout the growing season and prior to data collection.

2.3. Experimental Design

Canopy coverage (shade vs. sun) was treated as a block (1.2 m × 6.1 m). Each of the five fertilizer treatments was replicated four times and plots were arranged in a randomized, complete-block design. The plot size was 0.61 m × 0.61 m for a total of twenty plots per canopy coverage and a total of forty plots in the experiment.

2.4. Greenhouse Gas Analysis

Gas samples were taken weekly during the growing season (May–October) at midday following the protocols of the US Department of Agriculture-Agriculture Research Service Greenhouse Gas Reduction through Agricultural Carbon Enhancement Network [40,41]. Briefly, a polyvinyl chloride pipe (0.152 m diameter × 0.114 m height) was tamped into the ground until it was flush with the soil surface. The bases remained in the soil for the entirety of the study. Gas samples were taken by tamping a vented close gas chamber covered in reflective tape (no light penetration) over the base in the ground for the sampling period. Gas samples were taken at chamber closure; and 20- and 40-min post chamber closure. The samples were placed into gas tight vials using a 10 mL syringe.
These samples were analyzed using a gas chromatograph to determine the concentration of CO2, CH4, and N2O in each sample. The gas chromatograph used was a Varian 350 equipped with a thermal conductivity detector for CH4, an electron capture detector for N2O, and a flame ionization detector for CO2. Concentrations were determined by interpolation using gas standards obtained from Scott Specialty Gases (Air Liquide). Standard curves were used if they had an r2 value of 0.99 or greater. The concentrations of the samples collected were then used to determine a change in concentration (flux rate) during the 40 min sampling period using linear regression.

2.5. Environmental Conditions

Total rainfall (mm) and mean air temperatures (°C) were recorded during the growing season (May–October) of 2016–2017 (Figure 2). Weather data were collected by the Grand Forks International Airport (Grand Forks, ND, USA). Canopy temperature, soil temperature, and soil moisture were recorded weekly synchronously with greenhouse gas collection during the growing season using an infrared temperature meter (Spectrum Technologies, Inc., Aurora, IL, USA), a HM digital TM-1 industrial grade digital thermometer, and a Dynamax TH300 TDR soil moisture probe, which takes the average soil moisture in the top 60 mm of soil.

2.6. Turfgrass Color and Quality

Turfgrass appearance was evaluated by quantifying canopy greenness and using visual turfgrass quality ratings. Turfgrass greenness was determined weekly using a chlorophyll meter that measured the normalized difference vegetation index (NDVI) of the turfgrass stand (FieldScout CM 1000 NDVI from Spectrum Technologies, Inc., Aurora, IL, USA). Three measurements were taken from approximately 90 cm above the turfgrass canopy using a diagonal grid pattern, which measured the back, center, and front of each plot. The three measurements were averaged to produce a single plot rating and are reported as NDVI (−1 to 1). Turfgrass quality was visually rated (per plot) weekly throughout the growing season using a 1 to 9 scale, where 1 = completely brown dead turf, 6 = minimally acceptable turf, and 9 = optimum uniformity, density, and greenness [42]. Although a number of studies have correlated NDVI and turfgrass visual quality [43,44,45,46,47,48], Leinauer et al. [49] found it valuable to include both visual turfgrass quality and turfgrass color (NDVI scale) to characterize the aesthetic appeal of turfgrass accurately.

2.7. General Plot Maintenance

The putting greens were mowed at 0.36 cm. The putting greens did not receive any additional fertilization other than the fertilizer treatments during this two-year study. In the absence of significant rainfall, irrigation was supplementally applied each week (no more than 0.38 cm) to the putting greens to promote growth and maintain the soil at or near field capacity during the growing season (April–October). The research plots were not aerated for the duration of the study due to ring location in the soil. Four applications of topdressing [United States Golf Association (USGA) rootzone mixture, 0.65 cm/application] were applied to the green during the growing period to maintain putting green uniformity. Herbicides and fungicides were not applied over the research plots for the duration of this study.

2.8. Statistical Analysis

Statistical analysis was conducted using the general linear model (GLM) for GHG data and turfgrass data using Statistical Analysis Software [50]. Data points more than two standard deviations from the mean were identified as outliers and removed from the dataset. The assumptions of these models were checked, and appropriate transformations were applied as needed. The CO2 data were transformed using a λ of 0.5, CH4 was transformed using a λ of 1.45, and N2O data were transformed using a λ of −0.5. No transformation was necessary for the turfgrass data. Treatment means were separated using Fisher’s protected least significant difference (LSD) and a significance level of α = 0.05 was established a priori. Figures and tables represent back-transformed means.

3. Results and Discussion

3.1. Canopy Temperature, Soil Temperature, and Soil Moisture

When averaged across the growing year, canopy temperature was affected (p < 0.001) in both 2016 and 2017 (Table 1). The putting green located in full sun had a higher canopy temperature than the putting green located under the shade. Nitrogen fertilizer treatment did not have a statistically significant effect on canopy temperature for either 2016 or 2017. In 2016, the shade vs. sun treatment had a statistically significant effect (p < 0.001) 19 out of the 20 sampling dates (95%) for canopy temperature. The putting green located in full sun had a higher canopy temperature than the putting green located in the shade for all dates in 2016 except August 19. In 2017, the shade vs. sun treatment was statistically significant (p < 0.001) for 16 out of 20 dates (80%) for canopy temperature. The putting green located in full sun had a higher canopy temperature across the growing season than the putting green located in the shade. This trend occurred for all dates in 2017 except for June 22. In both 2016 and 2017, soil temperature was affected (p < 0.001) more on the sun plots than the shade plots (Table 1). For soil moisture, the shaded plots were significantly higher (p < 0.001) than the sun plots in both 2016 and 2017 (Table 1). These results are consistent with the published literature showing that trees increase shade and this shade decreases soil temperatures [21,51,52,53,54]. The presence of the trees located around the putting greens is not negatively affecting soil moisture as expected when both trees and turfgrass are competing for water.

3.2. Greenhouse Gas Flux

3.2.1. CO2 Flux

In both years of the study, soil CO2 flux from the sunny plots was significantly (p < 0.001) higher than the soil CO2 from the shaded plots (Figure 3, Table 2). The sunny plots also show this same difference on 10 of the 12 significant sampling dates in 2016 (Figure 3a) and 12 of the 13 significant sampling dates in 2017 (Figure 3b). Several studies have observed decreases in soil respiration associated with decreases in soil temperature [21,54] with shading by tree canopies decreasing soil respiration by 34% due to the decrease in soil temperature [21].
When analyzed across the entire year (2016 and 2017), the presence of fertilizer increased the soil CO2 flux (Table 2). In 2016, urea fluxed significantly more soil CO2 than the unfertilized control and in 2017, Milorganite fluxed significantly more soil CO2 than the urea and the unfertilized control (Table 2). In 2016, there were only two dates with a significant difference in soil CO2 flux by fertilizer source and on both of those dates, the urea fluxed more than the Milorganite and the unfertilized control (Figure 4a). In 2017, there were four dates where a significant difference in soil CO2 flux was observed by fertilizer source. On three of those dates, the Milorganite fluxed significantly more than the urea and the unfertilized control (Figure 4b). Thus, for soil CO2 flux, the data suggest that fertilizing the turfgrass increases soil CO2 flux and there is no consistent trend in terms of which fertilizer increases the flux more than the other. The rate of fertilizer application did not show significantly (p > 0.05) different soil CO2 flux across the study years. The addition of nitrogen into the soil environment through both mineral and organic sources increases microbial growth rates and cellular respiration [55,56,57], and this is evidenced by increased fluxes of soil CO2 when nitrogen fertilizers are used. Additionally, many others have found similar results as were outlined in this study, with inorganic fertilizers showing significantly higher soil CO2 flux as compared to organic fertilizers [58]. The regression analysis shows that soil temperature and soil moisture are significant predictors of soil CO2 flux, indicating they are correlated with soil respiration. Soil temperature was affected (p < 0.001) in both 2016 and 2017 and was positively associated with soil respiration with a parameter estimate of 15.04 (2016) and 5.15 (2017), whereas in 2016, soil moisture was also significantly (p < 0.05) correlated with soil respiration with a negative parameter estimate (−2.3) in 2016 and was not significant in 2017. This analysis supports the idea that trees decreasing the soil temperature was the primary driver of the decreased soil respiration observed in this study.

3.2.2. CH4 Flux

The soil fluxes of methane were very small and generally did not show any significant differences between the sun vs. shade or with the different fertilizer sources across both years of the study. However, in 2016, a significant difference was observed in the sun vs. shade across the entire growing season, with the shade showing significantly higher flux than the sun (Table 2). In 2016, the shade served as a net source for soil CH4 flux, while the sun served as a net sink for soil CH4. This is likely a result of the shade increasing the water retention in the shaded plots. The moisture content of the shade plots (30.1%) was significantly higher (p < 0.001) than the sunny plots (21.7%) (Table 1). It is well documented that wetter conditions result in higher emissions of CH4 [14,59].

3.2.3. N2O Flux

In 2016, when analyzed across the entire growing season, neither shade nor the interaction of shade and fertilizer showed a significant difference in soil N2O flux (Table 2, p > 0.05). However, in 2017, the plots in the sun fluxed significantly (p < 0.05) more soil N2O than did the shade plots (Table 2). This followed the trend observed in soil CO2 flux in both years of the study. Although 2016 did not show any significant differences in soil N2O flux, when analyzed by date, four dates showed a significant difference and on three of those dates, the sun plots fluxed significantly more soil N2O than did the shade plots (Figure 5a). In 2017, there were seven dates that showed a significant difference and six of those dates showed the sun plots fluxing significantly more soil N2O than the shade plots (Figure 5b). Previous studies have found fertilization associated with irrigation is the primary practice promoting N2O fluxes in urban laws [14,60,61], supporting the idea that N availability in solution is the primary mechanism by which N2O flux occurs in soils. Finding ways to reduce N availability in the soils, such as having high soil organic carbon [62], using controlled release fertilizers [60], and reducing irrigation [61], have been shown to reduce soil N2O flux. Our study shows that shade increased the soil water content, but also reduced the soil N2O flux from a highly managed turfgrass. Thus, in our context, the temperature benefits provided by the trees outweigh the additional moisture content and result in reduced soil N2O flux in the shaded plots. Kunnemann et al. [21] examined the impact of shade on soil N2O fluxes but did not observe a significant difference; however, the sampling frequency (monthly) was low and they did not specifically look at fertilized turf.
In 2016, when analyzed across the entire growing season, fertilizer induced significant differences (Table 2). The urea fluxed significantly more than the Milorganite and both fertilizer treatments (urea and Milorganite) fluxed significantly more soil N2O than the unfertilized control. This trend is consistent across the entire 2016 growing season (Figure 6a). In 2017, when analyzed across the entire growing season, no significant differences were observed. However, nine dates demonstrated significant differences between fertilizer treatments (Figure 6b). On five of those nine dates, urea fluxed significantly (p < 0.05) more soil N2O, on three of those dates Milorganite fluxed significantly (p < 0.05) more soil N2O, and on the final date, both fertilizers fluxed significantly (p < 0.05) more soil N2O than the unfertilized control. The rate of fertilizer application did not show significantly (p > 0.05) different soil N2O flux across the study years. The trends observed in our study are consistent with those observed in other studies in that the quick-release nitrogen sources fluxed significantly more soil N2O than the slow-release fertilizers [14,60,61]. In the case of Milorganite, which is an organic slow-release fertilizer, organic carbon is added which has also been reported to limit N mineral availability and soil N2O flux [62]. Across all three greenhouse gases, CO2 is the primary driver of the global warming potential, ranging from 97–100% for canopy coverage or fertilizer treatment (Table 3).

3.3. Turfgrass Color and Quality

Turfgrass color was significantly (p < 0.001) higher for the putting green located in full sun than in shade when averaged across the growing season in 2016, whereas in 2017, there was no significant difference between the green located in full sun or shade (Table 4). Canopy coverage (shade vs. sun) was significant (p < 0.05) on 8 out of 18 dates (44%) in 2016 and 11 out of 20 dates (55%) in 2017 (Figure 7). The green in full sun had significantly (p < 0.001) higher turfgrass color than the shaded green. Although in late summer (August–September), the shaded green had higher turfgrass color than the green in full sun. Fertilizer treatment was significant (p < 0.001) when averaged across the growing season in 2016 and 2017 (Table 4). The urea treatments produced the greenest turf in 2016 (Figure 8a), whereas in 2017 (Figure 8b) both urea and Milorganite (high rate) produced the greenest turf compared to the untreated control.
Turfgrass color was higher on the putting green located in full sun than the shaded putting green (Figure 7). These results are consistent with previous shade studies on creeping bentgrass. Bell and Danneberger [63] found that monthly color ratings for ‘Penncross’ creeping bentgrass in perpetual shade were consistently lower than those in full sun. Most turfgrasses are adapted to and perform best in full sun [34]. Shade from deciduous plants reduces both the quantity and quality of sunlight reaching the turfgrass stand [51]. In response, shade causes morphological (decrease in leaf thickness, leaf width, stem diameter, dry weight, and shoot density, and rhizome and stolon growth [64]) and physiological changes to the turfgrass plant (decreases in photosynthesis, respiration rate, compensation point, carbohydrate reserve, C/N ratio, transpiration rate, and osmotic pressure; increases in tissue moisture, chlorophyll, and lignin content [34]). Bell and Danneberger [63] found that creeping bentgrass has moderate shade tolerance; however, when closely mown daily on a putting green (removal of leaf tissue), photosynthesis and recuperative potential is reduced. They also found that creeping bentgrass grown in shade has a lower chlorophyll content than growing in full sun.
During the late summer (August–early September), the shaded green had better turfgrass color than the putting green located in full sun for both years (Figure 7). This was due to the high air and canopy temperatures (Figure 2 and Table 1) commonly observed during the late summer. The shade provided from the trees protected the putting green surface during times of extreme temperatures. Canopy temperatures are moderated, with cooler temperatures during the day and warmer temperatures at night [65]. The turfgrass canopy is about 3 °C cooler than air temperature in the morning and 6 °C cooler than air temperature in the afternoon [63].
Nitrogen fertilizer (urea in 2016 and both urea and Milorganite high in 2017) improved turfgrass color compared to the unfertilized control (Figure 8). Nangle et al. [66] found that repeated applications of urea improved turfgrass color on creeping bentgrass. Nitrogen is the most important essential nutrient for turfgrasses because it improves color, density, recuperative capability, and overall plant health when applied appropriately [67]. Judicious N fertilization is recommended for turfgrasses grown in shade because excessive amounts of N fertilizer have been shown to increase shade-avoidant responses such as increased shoot growth (at the expense of lateral and root growth), carbohydrate depletion, reduced N assimilation, and increased disease incidence [34]. Goss et al. [32] found that higher levels of N adversely affected turfgrass performance under shade. In the current study, this was observed in the quality of the turf, not on the turf color.
Turfgrass quality was significantly (p < 0.01) higher for the shaded vs. the sun putting green in both 2016 and 2017, respectively (Table 4). Canopy coverage (sun vs. shade) was significant (p < 0.05) on 6 out of 18 sampling dates (33%) in 2016 and 12 out of 20 sampling dates (60%) in 2017, where the shaded green had the best turfgrass quality on all significant dates in 2016–2017 (Figure 9). When averaged across years, fertilizer treatment was significant (p < 0.001) in 2017 where turfgrass quality was the highest for the Milorganite fertilizer treatment (Table 4). This trend is consistent throughout the growing season (Figure 10). Turfgrass quality was significant (p < 0.05) on 12 out of 20 sampling dates (60%) in 2017 where the Milorganite fertilizer (high treatment) produced the greatest turfgrass quality throughout the growing season (Figure 10b). Urea fertilizer (high treatment) consistently produced the poorest turfgrass quality in 2017 (Figure 10b). Turfgrass quality ratings did fall below the acceptable level (<6.0) for the urea (high treatment) on three dates in 2017 (26 July, 31 August, and 12 October).
Tree canopies reduce photosynthetically active radiation (irradiance between 400 and 700 nm) around golf course putting greens, limiting turfgrass quality [68], which can lead to a weakened turfgrass stand. Reduced turfgrass quality from shade was not observed in this study (Table 3 and Figure 8). Lowering N fertility (as well as increased mowing height, PGR use, increased fungicide applications, and decreased irrigation) has been shown to enhance a turfgrass stand when light interception is limited [32,34]. Bell and Danneberger [63] found that creeping bentgrass that is under shade for more than half the day requires less than half the N of creeping bentgrass in full sun. Fertilizing shade greens at the same rate as full-sun greens is detrimental to plant growth and development. Increasing N rates were linearly correlated with decreasing turfgrass quality when grown in shade [32]. Baldwin et al. [67] found that applying 147 kg N ha−1 yr−1 for sun-grown ultradwarf bermudagrass putting greens significantly improved turfgrass quality under reduced light compared to higher N rates. Pease and Stier [69] found that the 196 kg N ha−1 yr−1 rate of urea caused turfgrass quality to decline under shade (80%) compared to the lower N rates (49 and 98 kg ha−1 yr−1). In this study, we found that the high rate of urea (294 kg ha−1 yr−1) had the overall worst turfgrass quality in 2017 (Figure 10b). This was due to the high rate of urea (294 kg ha−1 yr−1) causing fertilizer burn to the treated plots even though they were irrigated. Chapman and Walker [39] found watering in the urea (221 kg ha−1 yr−1) following application was instrumental in reducing fertilizer burn on the putting green. Fertilizer burn was less severe in that study due to the lower N rate of urea. The high rate of Milorganite (294 kg ha−1 yr−1) consistently had the highest turfgrass quality rating throughout the season. The low rate of Milorganite (147 kg ha−1 yr−1) had the second best turfgrass rating (7 out of 12 sampling dates). Fertilizer burn was not an issue with the Milorganite fertilizer even at the high rate due to its slow release of N into the soil.

4. Conclusions

The impact of tree shade on the emissions of both CO2 and N2O was strong and consistent across both years of the study, with the turfgrass exposed to full sun fluxing more soil CO2 and N2O than the plots with tree shade. Both CO2- and N2O-fertilized plots fluxed more than the unfertilized plots. Additionally, urea fluxed more soil N2O than the Milorganite fertilizer treatment across both years of the study. We also observed a higher flux of soil CH4 in the tree-shaded plots in 2016, which was associated with high soil moisture content; thus, the sun plots served as a net sink for soil CH4. Carbon dioxide drives the overall global warming potential of these management techniques, suggesting that tree shade on turfgrass could reduce the global warming potential of turfgrass systems.
Turfgrass color was higher on the green located in the sun except in late summer where the shade from trees provided relief from the higher air and canopy temperatures. Fertilizer applications of both urea and Milorganite greatly improved turfgrass color. Turfgrass quality was higher for the green located in the shade. Milorganite fertilizer applications improved turfgrass quality, whereas urea applications (294 kg ha−1 yr−1) decreased turfgrass quality over the 2017 growing season where on several occasions the overall quality of turf fell below acceptable ratings (<6.0). Urea applications at the high rate (294 kg ha−1 yr−1) caused fertilizer burn on the greens. Judicious use of N fertilizer on greens, especially fast-release fertilizers like urea, is highly recommended when managing putting greens in the shade. However, when more N is needed to improve turfgrass color and quality, slow-release N sources like Milorganite should be part of any BMP for shaded putting greens. The environmental impacts of having shaded greens are reduced soil CO2 and N2O flux. Our results also show an improvement in turfgrass quality with the Milorganite N fertilizer on shaded greens. Thus, our study shows decreasing soil CO2 and N2O flux can be achieved while also improving the quality of the turfgrass.

Author Contributions

Conceptualization, K.E.C. and K.S.W.; methodology, K.E.C. and K.S.W.; resources, K.E.C. and K.S.W.; writing—original draft preparation, K.E.C. and K.S.W.; writing—review and editing, K.E.C. and K.S.W.; visualization, K.E.C. and K.S.W.; supervision, K.E.C. and K.S.W.; project administration, K.E.C. and K.S.W.; funding acquisition, K.E.C. and K.S.W. All authors have read and agreed to the published version of the manuscript.

Funding

Funding for this project was provided by the Minnesota Turf and Grounds Foundation and by the University of Minnesota Crookston.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

This work could not have been completed without the hours of work by our laboratory technicians Tamara Luna and Wade Wallace. In addition, many undergraduate research students worked on this project including Karen Soi Choi, Eui Young Kim, Laura Hicks, Camila Costa, Kaori Suda, Alexis Strong, LaRyssa Nelson, Maddie Giese, Bruna Just, Rachel MacDowell, Hee In Moon, Nathan Hvidsten, Jason Scherer, and Heidi Reitmeir.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Shade (a) and sun (b) putting greens used in this study. The percent shade covered was determined by quantifying the area shaded in this image vs. the area that was not shaded (Google Earth). The shaded green was 98.8% shaded by trees (bottom right: Colorado Blue Spruce; upper left: Japanese Lilac, and 2 American Elms) surrounding the green and the sun green was in 100% sun.
Figure 1. Shade (a) and sun (b) putting greens used in this study. The percent shade covered was determined by quantifying the area shaded in this image vs. the area that was not shaded (Google Earth). The shaded green was 98.8% shaded by trees (bottom right: Colorado Blue Spruce; upper left: Japanese Lilac, and 2 American Elms) surrounding the green and the sun green was in 100% sun.
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Figure 2. Total rainfall (mm) and mean air temperatures (°C) during the growing season (May–October) of 2016–2017. Total rainfall is indicated by a bar chart (left axis) and mean air temperature is indicated by a line graph (right axis). Weather data were collected by the Grand Forks International Airport (Grand Forks, ND, USA).
Figure 2. Total rainfall (mm) and mean air temperatures (°C) during the growing season (May–October) of 2016–2017. Total rainfall is indicated by a bar chart (left axis) and mean air temperature is indicated by a line graph (right axis). Weather data were collected by the Grand Forks International Airport (Grand Forks, ND, USA).
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Figure 3. Canopy coverage (shade vs. sun) on carbon dioxide (CO2) emissions in 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD. All cases of significant differences are Sun > shade except 30 September 2016, 14 October 2016, and 5 October 2017.
Figure 3. Canopy coverage (shade vs. sun) on carbon dioxide (CO2) emissions in 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD. All cases of significant differences are Sun > shade except 30 September 2016, 14 October 2016, and 5 October 2017.
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Figure 4. Fertilizer source (control, Milorganite, urea) on carbon dioxide (CO2) emissions for 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD.
Figure 4. Fertilizer source (control, Milorganite, urea) on carbon dioxide (CO2) emissions for 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD.
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Figure 5. Canopy coverage (shade vs. sun) on nitrous oxide (N2O) emissions for 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD. All cases of significant differences are sun > shade except 8 June 2016 and 31 August 2017.
Figure 5. Canopy coverage (shade vs. sun) on nitrous oxide (N2O) emissions for 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD. All cases of significant differences are sun > shade except 8 June 2016 and 31 August 2017.
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Figure 6. Fertilizer source (control, Milorganite, urea) on nitrous oxide (N2O) emissions for 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD.
Figure 6. Fertilizer source (control, Milorganite, urea) on nitrous oxide (N2O) emissions for 2016 (a) and 2017 (b). * Means are significantly different at the 0.05 level according to LSD.
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Figure 7. Canopy coverage (shade versus sun) on turfgrass color (NDVI) in 2016 (a) and 2017 (b). Turfgrass color, the normalized difference vegetation index (NDVI) measurements can range from −1 to 1, with higher values indicating greater plant health. * Means are significantly different at the 0.05 level according to LSD.
Figure 7. Canopy coverage (shade versus sun) on turfgrass color (NDVI) in 2016 (a) and 2017 (b). Turfgrass color, the normalized difference vegetation index (NDVI) measurements can range from −1 to 1, with higher values indicating greater plant health. * Means are significantly different at the 0.05 level according to LSD.
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Figure 8. Nitrogen fertilizer treatment on turfgrass color (NDVI) for 2016 (a) and 2017 (b). Turfgrass color, the normalized difference vegetation index (NDVI) measurements can range from −1 to 1, with higher values indicating greater plant health. There were 5 fertilizers treatments: MILH = Milorganite, 294 kg N ha−1 yr−1, MILL = Milorganite, 147 kg N ha−1 yr−1, UREH = urea, 294 kg N ha−1 yr−1, UREL = urea, 147 kg N ha−1 yr−1, UNT = unfertilized control, 0 kg N ha−1 yr−1. * Means are significantly different at the 0.05 level according to LSD.
Figure 8. Nitrogen fertilizer treatment on turfgrass color (NDVI) for 2016 (a) and 2017 (b). Turfgrass color, the normalized difference vegetation index (NDVI) measurements can range from −1 to 1, with higher values indicating greater plant health. There were 5 fertilizers treatments: MILH = Milorganite, 294 kg N ha−1 yr−1, MILL = Milorganite, 147 kg N ha−1 yr−1, UREH = urea, 294 kg N ha−1 yr−1, UREL = urea, 147 kg N ha−1 yr−1, UNT = unfertilized control, 0 kg N ha−1 yr−1. * Means are significantly different at the 0.05 level according to LSD.
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Figure 9. Canopy coverage (shade versus sun) on turfgrass quality on turfgrass quality (1–9 visual scale) in 2016 (a) and 2017 (b). Turfgrass quality is a visual scale (1–9); where 1 = completely brown dead turf, 6 = minimally acceptable turf, and 9 = optimum uniformity, density, and greenness. * Means are significantly different at the 0.05 level according to LSD.
Figure 9. Canopy coverage (shade versus sun) on turfgrass quality on turfgrass quality (1–9 visual scale) in 2016 (a) and 2017 (b). Turfgrass quality is a visual scale (1–9); where 1 = completely brown dead turf, 6 = minimally acceptable turf, and 9 = optimum uniformity, density, and greenness. * Means are significantly different at the 0.05 level according to LSD.
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Figure 10. Nitrogen fertilizer treatment on turfgrass quality (1–9 visual scale) for 2016 (a) and 2017 (b). Turfgrass quality is a visual scale (1–9); where 1 = completely brown dead turf, 6 = minimally acceptable turf, and 9 = optimum uniformity, density, and greenness. There were 5 fertilizers treatments: MILH = Milorganite, 294 kg N ha−1 yr−1, MILL= Milorganite, 147 kg N ha−1 yr−1, UREH = urea, 294 kg N ha−1 yr−1, UREL = urea, 147 kg N ha−1 yr−1, UNT = unfertilized control, 0 kg N ha−1 yr−1. * Means are significantly different at the 0.05 level according to LSD.
Figure 10. Nitrogen fertilizer treatment on turfgrass quality (1–9 visual scale) for 2016 (a) and 2017 (b). Turfgrass quality is a visual scale (1–9); where 1 = completely brown dead turf, 6 = minimally acceptable turf, and 9 = optimum uniformity, density, and greenness. There were 5 fertilizers treatments: MILH = Milorganite, 294 kg N ha−1 yr−1, MILL= Milorganite, 147 kg N ha−1 yr−1, UREH = urea, 294 kg N ha−1 yr−1, UREL = urea, 147 kg N ha−1 yr−1, UNT = unfertilized control, 0 kg N ha−1 yr−1. * Means are significantly different at the 0.05 level according to LSD.
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Table 1. Mean and median canopy temperature, soil temperature, and soil moisture by year, canopy coverage.
Table 1. Mean and median canopy temperature, soil temperature, and soil moisture by year, canopy coverage.
Canopy
Temperature		Soil
Temperature		Soil
Moisture
YearTreatmentMeanMedianMeanMedianMeanMedian
°C°CVWC %
2016
Canopy Coverage
Shade17.0b16.7517.3b18.530.1a30.7
Sun23.2a23.2022.0a24.421.7b20.8
Source of Variationdf df df
   Shade vs. sun1***1***1***
   Fertilizer2NS2NS2NS
   Shade * fertilizer2NS2NS2NS
2017
Canopy Coverage
Shade16.8b16.7520.0b18.626.1a27.9
Sun21.2a21.2021.5a21.719.6b19.0
Source of Variationdf df df
  Shade vs. sun1***1***1***
  Fertilizer2NS2NS2NS
  Shade * fertilizer2NS2NS2NS
Shade = turf is shaded majority of day; sun = turf is never shaded; VWC % = volumetric water content. Within columns, means followed by a different letter are significantly different according to LSD (0.05). No letter means no significance. * Significant at the 0.05 probability level; *** significant at the 0.001 probability level. NS, nonsignificant.
Table 2. Mean and median soil CO2, N2O, CH4 flux by year, canopy coverage, and fertilizer.
Table 2. Mean and median soil CO2, N2O, CH4 flux by year, canopy coverage, and fertilizer.
CO2 N2O CH4
YearTreatmentMeanMedianMeanMedianMeanMedian
g CO2-C m−2 h−1µg N2O-N m−2 h−1µg CH4-C m−2 h−1
2016
Canopy Coverage
  Shade0.32B0.2843.510.914.27A0.35
  Sun0.40A0.3636.918.7−17.92B−2.08
Fertilizer
  Milorganite0.36ab0.3234.5b22.03.100.11
  Urea0.37a0.3267.2a17.34.28−1.40
  Control0.32b0.280c5.4−24.67−0.37
Source of Variationdf df df
   Shade vs. sun1***1NS1*
   Fertilizer2*2***2NS
   Shade * Fertilizer2NS2NS2NS
2017
Canopy Coverage
  Shade0.30B0.2713.0B12.383.35.8
  Sun0.45A0.4543.6A21.2−57.4−7.6
Fertilizer
  Milorganite0.41a0.3634.719.3−5.32.7
  Urea0.37b0.3232.920.386.71.3
  Control0.34b0.305.65.4−87.7−1.3
Source of Variationdf df df
  Shade vs. sun1***1*1NS
  Fertilizer2**2NS2NS
  Shade * fertilizer2NS2NS2NS
Control = no fertilization; urea = urea fertilizer; Milorganite = Milorganite fertilizer; shade = turf is shaded majority of day; sun = turf is never shaded. Means in the same column for each canopy coverage (shade and sun) followed by a different uppercase letter are significantly different according to Fisher’s protected LSD t-test (p = 0.05). Means in the same column for each fertilizer treatment followed by a different lowercase letter are significantly different according to Fisher’s protected LSD t-test (p = 0.05). No letter means no significance. * Significant at the 0.05 probability level; ** significant at the 0.01 probability level; *** significant at the 0.001 probability level. NS, nonsignificant.
Table 3. Percent global warming potential contributions of CO2, N2O, CH4 flux by year, canopy coverage, and fertilizer.
Table 3. Percent global warming potential contributions of CO2, N2O, CH4 flux by year, canopy coverage, and fertilizer.
CO2N2OCH4
YearTreatmentGWPMeanMean
2016
Canopy Coverage
Shade97%3%0
Sun98%2%0
Fertilizer
Milorganite98%2%0
Urea96%4%0
Control100%00
2017
Canopy Coverage
Shade98%1%1%
Sun97%2%1%
Fertilizer
Milorganite98%2%0%
Urea97%2%1%
Control100%00
Control = no fertilization; urea = urea fertilizer; Milorganite = Milorganite fertilizer; shade = turf is shaded majority of day; sun = turf is never shaded.
Table 4. Turfgrass color and turfgrass quality yearly means for 2016 and 2017.
Table 4. Turfgrass color and turfgrass quality yearly means for 2016 and 2017.
Turfgrass ColorTurfgrass Quality
NDVI1–9 Visual Scale
2016201720162017
Canopy Coverage
Shade0.81B0.867.6A7.6A
Sun0.83A0.867.3B7.2B
Fertilizer
MILH0.82bc0.87a7.6a7.9a
MILL0.81cd0.85b7.4b7.6b
UREH0.83a0.86ab7.5ab7.3b
UREL0.82ab0.86ab7.5ab7.4b
UNT0.80d0.84c7.3b6.9c
Source of Variation
Shade vs. sun***NS*****
Fertilizer ******NS***
Shade * fertilizerNSNSNSNS
Turfgrass color, the normalized difference vegetation index (NDVI) measurements can range from −1 to 1, with higher values indicating greater plant health. Turfgrass quality is a visual rating of 1–9; where 1 = completely brown dead turf, 6 = minimally acceptable turf, and 9 = optimum uniformity, density, and greenness. Means in the same column for each canopy coverage (shade and sun) followed by a different uppercase letter are significantly different according to Fisher’s protected LSD t-test (p = 0.05). Means in the same column for each fertilizer treatment (MILH = Milorganite, 294 kg N ha−1 yr−1, MILL= Milorganite, 147 kg N ha−1 yr−1, UREH = urea, 294 kg N ha−1 yr−1, UREL = urea, 147 kg N ha−1 yr−1, UNT = unfertilized control, 0 kg N ha−1 yr−1) followed by a different lowercase letter are significantly different according to Fisher’s protected LSD t-test (p = 0.05). No letter means no significance. *, **, ***, and NS refer to significance at 0.05, 0.01, 0.001, and nonsignificant, respectively.
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Chapman, K.E.; Walker, K.S. Shade and Nitrogen Fertilizer Effects on Greenhouse Gas Emissions from Creeping Bentgrass Putting Greens. Horticulturae 2024, 10, 832. https://doi.org/10.3390/horticulturae10080832

AMA Style

Chapman KE, Walker KS. Shade and Nitrogen Fertilizer Effects on Greenhouse Gas Emissions from Creeping Bentgrass Putting Greens. Horticulturae. 2024; 10(8):832. https://doi.org/10.3390/horticulturae10080832

Chicago/Turabian Style

Chapman, Katy E., and Kristina S. Walker. 2024. "Shade and Nitrogen Fertilizer Effects on Greenhouse Gas Emissions from Creeping Bentgrass Putting Greens" Horticulturae 10, no. 8: 832. https://doi.org/10.3390/horticulturae10080832

APA Style

Chapman, K. E., & Walker, K. S. (2024). Shade and Nitrogen Fertilizer Effects on Greenhouse Gas Emissions from Creeping Bentgrass Putting Greens. Horticulturae, 10(8), 832. https://doi.org/10.3390/horticulturae10080832

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