Multiple ecosystem services in urban lawns: Nitrogen retention and aesthetics

Background/Question/Methods

Lawns are an important and abundant ecological feature of cities. Lawns can provide multiple ecosystem services and should be managed particularly for cultural and regulating services. The capacity to provide these services is affected by heterogeneity in light, soil conditions, and fertilizer use within and among lawns. We tested the impact of light availability (i.e., amount of solar radiation), soil organic matter (SOM) level, and fertilizer use on lawn aesthetics – a cultural service – and nitrogen (N) retention – a regulating service. We used 15 existing lawns under the same management regime that spanned a 1.1 to 5.8% soil carbon (C; a measure of SOM) gradient. Plots were arrayed across light gradients within each lawn with all plots (N=51) ranging from 18-98% of the annual solar radiation under open sky conditions. Plots were split into paired 1 m² subplots: a control and a fertilized treatment receiving 150 kg N·ha^-1·y^-1. The capacity for lawns to retain N via plant pools was measured as above-and belowground biomass and mowing yield. Lawn aesthetics – determined by turf color and density - were assessed using a hand-held NDVI meter for color (i.e., greenness) and by quantifying the density of aboveground biomass.

Results/Conclusions

Retention of N in biomass pools and lawn aesthetics were affected by all three drivers tested – light availability, soil C, and fertilizer use. Light availability was the dominant factor; it strongly correlated with increases in aboveground biomass, total biomass and mowing yield, as well as declines in NDVI values – decreasing greenness. Soil C was positively correlated with mowing yield. Fertilizer use slightly decreased total biomass, but increased mowing yield and NDVI values. Biomass responses were significant, ranging more than an order of magnitude for all measures. Total dry biomass ranged from 1.9 to 20.4 Mg∙ha^-1. Assuming 3% N content, total biomass N ranged from 56 to 610 kg N∙ha^-1, comparable to the range found between desert and temperate forest biomes. Mowing yield ranged from <0.1 to 22 kg dry biomass·ha^-1·wk^-1 during the peak growing season.  Density of aboveground biomass, relevant to lawn aesthetics, ranged from 25 to 699 g·m^-2. In summary, lawns exhibit a wide range of ecosystem functioning under environmental and management conditions known to vary within and among urban lawns. This variation has consequences for how to manage to achieve multiple ecosystem services in lawns.