Satinleaf (Chrysophyllum oliviforme), a medium-sized tree in the Sapotaceae family, can reach heights of up to 45 feet and has a spread of approximately 25 feet. It is renowned for its unique and attractive foliage. Native to Florida, satinleaf is a prized choice in South Florida landscaping, often featured as a standout lawn specimen or integrated into shrub borders and naturalized settings (Gilman et al., 2019). Its popularity in urban landscaping is due to its aesthetic appeal and versatility in various environments (Meerow et al., 2020). In urban landscapes, satinleaf plays a crucial role by providing shade, enhancing air quality, and adding significant visual appeal to cityscapes.

Achieving optimal nitrogen (N) fertilization involves balancing nutrient supply with plant demand, a task complicated by the difficulty of accurately predicting both factors (Colaço and Bramley, 2018; Khoddamzadeh and Dunn, 2016; Fallahi et al., 2000). Satinleaf trees have moderate to high nitrogen needs and perform best in soils enriched with organic matter (Koeser et al., 2017). Therefore, tools like optical sensors are essential for managing nitrogen levels effectively.

Typically, only a small fraction of applied nitrogen is absorbed by crops, with the excess prone to environmental loss, leading to various ecological issues. Unused nitrogen can leach below the root zone or be lost through runoff (Randall and Goss, 2008; Cameron et al., 2013; Freidenreich et al., 2019), resulting in nitrate (NO₃ ⁻) accumulation in natural water bodies (Pulido-Bosch et al., 2000; Ju et al., 2006). Elevated nitrate levels in water bodies are linked to human health risks (Addiscott, 1996) and eutrophication (Cameron et al., 2013). Furthermore, excess nitrogen contributes to environmental degradation through nitrous oxide (N₂O) emissions, which exacerbate global warming, and ammonia (NH₃) volatilization, which enriches natural ecosystems with nitrogen (Hartz, 2006; Meisinger et al., 2015; Galloway et al., 2008; Tilman et al., 2001).

Environmental protection and water pollution, especially in South Florida with its high precipitation rates and the prevalence of harmful algal blooms, have become critical issues. The restoration of the Everglades, a unique and vital ecosystem, has taken on added importance. This region faces significant challenges due to nutrient runoff, which contributes to ecological degradation and water quality concerns. Sustainable nitrogen management practices are therefore essential to safeguard the Everglades and surrounding water bodies.

Optical sensor technology has emerged as a key tool for optimizing nitrogen fertilization and mitigating its environmental impact. These sensors enable non-destructive, efficient assessments of crop nitrogen status, supporting informed fertilizer management decisions. Among the most widely used tools are the GreenSeeker™ handheld sensor (Trimble Navigation Ltd., CA), the SPAD meter (SPAD-502, Konica Minolta, Japan), and the atLEAF chlorophyll sensor (FT Green LLC, DE) (Basyouni et al., 2015; Khoddamzadeh et al., 2023).

The GreenSeeker™ sensor calculates the normalized difference vegetation index (NDVI) using active red and near-infrared light, providing insights into plant health and nitrogen levels. The SPAD meter evaluates chlorophyll content by analyzing light transmittance through leaves, offering a reliable indicator of nitrogen concentration. Similarly, the atLEAF chlorophyll sensor provides chlorophyll readings with the added benefit of digital integration for enhanced data analysis. These tools are not only adaptable across various growth stages but also require minimal labor, making them practical for large-scale implementation (Fox and Walthall, 2015; Tremblay et al., 2012).

This study aimed to determine the optimal nitrogen fertilizer dose for satinleaf by monitoring chlorophyll content using optical sensor technology. Additionally, it sought to evaluate the correlation between sensor readings and nitrogen content in satinleaf plants, identifying the fertilizer rate that minimizes nutrient runoff and supports sustainable agriculture in South Florida.

Nitrogen is a vital nutrient that significantly influences plant growth and development, primarily as a core component of chlorophyll in leaves. Chlorophyll levels directly affect leaf area, biomass, plant height, and water usage. Inadequate nitrogen can cause deficiency symptoms that negatively impact plant health, productivity, and commercial value. On the other hand, excessive nitrogen application can lead to toxicity, resulting in stunted growth and poor plant quality. Over-application also raises production costs and poses environmental risks due to nutrient leaching and runoff, leading to contamination of water bodies and ecosystems (Khoddamzadeh and Dunn, 2016; Basyouni and Dunn, 2013). This study utilized optical sensors as a non-destructive method to estimate chlorophyll content and assess nitrogen status, aiming to identify the optimal fertilizer dosage for Satinleaf that balances growth and environmental sustainability.

The results indicated that the T3 effectively promoted Satinleaf growth, as evidenced by an increase in plant height and higher chlorophyll content measured by SPAD. This treatment also enhanced total nitrogen levels in both the leaves and soil substrate. These findings are consistent with those reported by Costa et al. (2023) for Cocoplum plants, where treatment 3 (30-15-15) treatment was also found to be the optimal nitrogen dose for promoting growth without excessive nutrient loss. These outcomes suggest that moderate nitrogen doses can provide sufficient nutrients for optimal plant growth while minimizing nutrient loss through runoff.

The results also show that the Control treatment provided higher values for SPAD, compared with the with higher doses. This may have happened due to high doses of nitrogen can generate severe vegetative growth, which can lead to the dilution effect where the concentration of chlorophyll per leaf unit decreases due to the increase in leaf area. This can also explain the why T4 has higher N concentrations in the leaf dry mass compared to T5.

The T4 achieved the highest total nitrogen concentration at 60 DPFA, while the highest total carbon content was recorded in the T1 at 150 DPFA. Furthermore, T1 also provided lower chlorophyll values (SPAD and atLEAF) compared to the other treatments, which shows that this plant was efficient in absorbing carbon, but was not efficient in converting it into chlorophyll through photosynthesis.

As observed in this study, chlorophyll content measured by SPAD and atLEAF sensors increased over time following fertilizer application, while NDVI values showed a decline towards the end of the experiment. The highest fertilizer dose T5, resulted in elevated salt concentration, electrical conductivity (EC), and total nitrogen levels in the runoff samples. This indicates substantial nitrogen loss due to runoff, which can harm the environment. A more sustainable approach to nitrogen management involves using moderate doses, such as the T3, which supports effective plant growth while reducing the risk of environmental contamination. Similar patterns were observed in studies on other native and non-native plants in South Florida, including Cocoplum (Costa et al., 2023) and Cacao (Khoddamzadeh and Souza Costa, 2023).

In general, all plants were watered manually, which may have resulted in different amount for each plant. However, for the nutrient runoff analysis, a methodology was followed in which the size of the pots was considered and the same amount was added to all the plants, in addition the saturation point was the same for all treatments and the water was observed the flowing through the pots. Therefore, the large amount of nutrient runoff may have been primarily due to the amount of fertilizers applied, thus showing the difference between the high fertilizer treatments and the control.

Correlation analysis revealed significant negative correlations between SPAD readings and total carbon at 60 days post-fertilizer application (DPFA) and between total nitrogen and total carbon at 120 and 180 DPFA. A strong negative correlation was also found between SPAD and NDVI. These negative correlations suggest that as one parameter increases, the other decreases, reflecting the complex correlation between these variables. Khoddamzadeh and Souza Costa (2023) also noted similar negative correlations between total nitrogen and total carbon in Cacao plants grown under different fertilization rates, further emphasizing the need to balance these nutrients for optimal plant health and growth.

This study demonstrates that Satinleaf (Chrysophyllum oliviforme) plants can thrive with lower nitrogen fertilizer doses, which also help mitigate nutrient runoff pollution. The treatment with 30 g (15 g applied twice in November and March; T3) proved to be the most effective, enhancing plant growth and nitrogen levels in both soil and plant tissues. In contrast, the 45 g (15 g applied twice; T5) treatment resulted in substantial nutrient loss through runoff, making it less ideal for sustainable urban landscape management.

The findings underscore the importance of integrating sensor technologies, such as GreenSeeker™, SPAD meter, and atLEAF chlorophyll sensors, into fertilization management strategies. These tools enable precise monitoring of nitrogen levels, optimizing fertilization to reduce environmental impacts, minimize costs, and prevent over-fertilization.

Future research should focus on improving the sensitivity and accessibility of sensor technology to better serve landscape professionals. Additionally, further investigations into how sensor-based fertilization management can be adapted to diverse urban environments and plant species will enhance its practical application. These advancements can contribute significantly to more sustainable urban landscape practices and environmental resilience.