Long term trends in nitrate leaching and nitrous oxide flux from urban grasslands

Abstract:

There is great interest in and concern about gaseous and hydrologic losses of nitrogen from urban grasslands (lawns), which are defined as "ecosystems dominated by turf-forming species created and maintained by humans for aesthetic and recreational (not grazing) purposes." In the National Science Foundation funded urban Long Term Ecological Research project in Baltimore, there was an initial assumption that lawns would be “hotspots” of nutrient pollution in the urban landscape due to high rates of fertilizer application and low potential for retention of these nutrients in lawn vegetation and soils. To evaluate these assumptions, we established long-term study plots with lysimeters to measure nitrate leaching and in situ chambers to measure nitrous oxide flux on lawns with different management regimes and made comparisons with forested reference areas. Initial analyses (1998 – 2005) showed that exports of nitrogen to water and air were lower than expected, although still environmentally important, due to a surprisingly high capacity for nitrogen retention in lawn soils and vegetation. Here we present longer-term analyses (1998 – 2018) that have clarified some differences and produced some new surprising results. First, the longer term analyses shows that grasslands clearly have more nitrate leaching than forests. However, fertilized inputs do not always led to more nitrate leaching, raising questions about the complexities of nitrogen retention in these ecosystems. Second, in contrast to nitrate leaching, grasslands do not have higher nitrous oxide emissions than forests, and again, fertilizer effects were less marked than expected. We hypothesize that high nitrogen retention in urban grasslands is driven by an active carbon cycle with high plant productivity and nitrogen uptake and high production of organic matter that facilitates microbial immobilization. High rates of total soil respiration in lawns support this hypotheses. Continued long term monitoring and ancillary mechanistic studies of urban grasslands are needed to understand the biogeochemical dynamics and improve the environmental performance of this important ecosystem type.