**1. Introduction**

A well-managed, aesthetically pleasing landscape is associated with wellbeing in many residential landscapes, and the area of turfgrass in the United States is larger than that for any irrigated crop [1]. For many urban homeowners in the United States, turfgrass is the predominant lawn cover. To maintain a healthy, attractive turfgrass cover, regular fertilization, irrigation, and pest control management plan is often established [2]. Besides the associated economic implications, the fate and potential loss of nutrients (nitrogen and phosphorus) from fertilized turfgrass have important implications for aquatic ecosystems since nutrients mobilized by leaching and/or runoff may impair receiving waterbodies through eutrophication and algal proliferation [3–5].

While numerous studies have indicated that nutrient losses via leaching and runoff are minimal from healthy, properly maintained turfgrass and that turfgrass lawns are sinks of nitrogen (N) in urban watersheds [6–8], turfgrass fertilizers are increasingly targeted by management practices and policies aimed at reducing anthropogenic nutrient inputs to aquatic ecosystems [9]. For example, in Florida, more than 50 counties and municipalities have enacted fertilizer ordinances that often prohibit any application of N- and phosphorus (P)-bearing fertilizers to urban lawns during Florida's summer rainy season (June to September) each year [10,11]. The premise behind these bans is that summer rains may lead to increased leaching and runoff losses of N and P applied as fertilizer to lawns, in turn leading to increased anthropogenic nutrient loading to nearby waterbodies. To date, the efficacy of the fertilizer ban ordinances has not been demonstrated, and they remain controversial strategies for urban nutrient management [12,13].

**Citation:** Bukomba, J.; Lusk, M.G. Spatial Variability in Inorganic Soil Nitrogen Production in a Mixed-Vegetation Urban Landscape. *Nitrogen* **2022**, *3*, 118–127. https:// doi.org/10.3390/nitrogen3010009

Academic Editor: Jacynthe Dessureault-Rompré

Received: 18 January 2022 Accepted: 10 March 2022 Published: 11 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

To address urban nutrient management needs, we need not only more studies on the mechanisms and extent of nutrient mobilization from urban landscapes but also a better understanding of soil nutrient dynamics in these landscapes [14,15]. Nutrients associated with soil organic matter pools have been identified as one source of N and P that may be mobilized from urban lawns to stormwater runoff [5,16,17]. We also need research focused on a wider variety of potential urban landscape covers, such as mulches or ornamental plants other than turfgrass. Only a few studies have investigated differences in soil nutrient cycling between turfgrass monoculture lawns and lawns with a mix of species. In one such study, Erickson et al. [8] compared N leaching and runoff from a St. Augustine turfgrass monoculture versus a mixed-species lawn in Florida. For both lawn types, N losses via surface runoff were minimal, but leaching losses were substantial, especially for the mixed-species lawns, which lost up to 48.3 kg N ha−<sup>1</sup> via leaching. By comparison, the turfgrass lawns in the study lost 4.1 kg N ha−<sup>1</sup> via leaching, indicating that the turfgrass was more effective than mixed-species vegetation for preventing N leaching from lawns. In another study, Amador et al. [18], studied pore water nitrate (NO3 −) concentrations at 60 cm soil depth under turfgrass versus various landscape covers, including flowers, shrubs, and unplanted mulched beds. In their study, flowers, managed turfgrass, and ornamental deciduous and evergreen trees represented a lower risk of NO3 − loss from the soil than unplanted mulched areas, which the authors recommended should be used sparingly in urban landscapes because of the potential for NO3 − leaching to groundwater. Amador et al. [18] suggested that these unplanted mulched areas were more susceptible to NO3 − leaching because there was no plant sink for N produced by mineralization of soil organic N. That study also noted that unplanted mulched landscape beds lost via leaching nearly twice the NO3 − input to the landscape through atmospheric deposition, making the unplanted mulched areas net sources of NO3 − to the underlying groundwater.

The sparse studies on soil N dynamics in mixed-species urban landscapes are in line with the framework for urban soil ecology presented by Byrne [19], in which the heterogeneity commonly found in urban landscapes gives rise to a specific "habitat structure," or a unique composition of physical matter with consequent unique effects on local ecological variables. Variation in habitat structure gives rise to differences in soil pH, moisture content, microbial populations, temperature, and vegetation cover, which in turn may all cause variations in the N cycling processes within an urban landscape [15,20–22]. This means that distinct patches may emerge, creating a landscape mosaic where ecological variables can vary at scales of just a few meters or less. In this study, we selected a mixed-species urban landscape with this type of small-scale patchiness in vegetative cover and investigated the spatial variability of inorganic N production in soils. We hypothesized that differences in landscape cover at a spatial scale of meters would result in varying levels of soil inorganic N production, as measured by nitrification and net N mineralization rates. We tested this hypothesis by evaluating N cycling processes in soils under traditional turfgrass and several common ornamental alternatives to turfgrass.

This work is important because it helps constrain and fill knowledge gaps related to N cycling processes in urban soils by focusing on small-scale differences that are common in urban landscapes. While previous studies mentioned above [8,18] have focused solely on N leaching from mixed-vegetation landscapes, this work adds the body of knowledge by focusing on N cycling processes, namely mineralization, and nitrification. Typically, urban lawns and other green spaces are broadly categorized as "lawn" or "turfgrass," when in reality, they are seldom turfgrass monocultures but instead a mosaic of various vegetative types and ground covers. As we grapple with water quality degradation associated with excess nutrients from urban landscapes, research models may be used to predict the transformations and movement of N in urban soils—and accounting for the expected mosaic of variable N processing in urban soils can lead to improvements in those modeling efforts. For example, in Florida, the Nitrogen Source Inventory and Loading Tool (NSILT) is a model used to predict the fate of fertilizer N applied to urban landscapes. The model is based on the best available data on N inputs and N transformations in urban soils but

is informed only by data on N cycling in turfgrass soils, without consideration for other land cover types (e.g., flower beds or mulched areas) [23]. In this work, we show how N cycling processes in a single urban landscape can be highly variable, and we argue that efforts to constrain the fate of urban nutrient sources can be improved by greater attention to mixed-vegetation landscape scenarios as well as data at finer spatial scales.
