Nitrogen retention in urban lawns and forests

Lawns are a dominant cover type in urban ecosystems, and there is concern about their impacts on water quality. However, recent watershed-level studies suggest that these pervious areas might be net sinks, rather than sources, for nitrogen (N) in the urban environment. A N-15 pulse-labeling experiment was performed on lawn and forest plots in the Baltimore (Maryland, USA) metropolitan area to test the hypothesis that lawns are a net sink for atmospheric-N deposition and to compare and contrast mechanisms of N retention in these vegetation types. A pulse of N-15-NO3-, simulating a precipitation event, was followed through mineral soils, roots, Oi-layer/thatch, aboveground biomass, microbial biomass, inorganic N, and evolved N-2 gas over a one-year period. The N-15 label was undetectable in gaseous samples, but enrichment of other pools was high. Gross rates of production and consumption of NO3- and NH4+ were measured to assess differences in internal N cycling under lawns and forests. Rates of N retention were similar during the first five days of the experiment, with lawns showing higher N retention than forests after 10, 70, and 365 days. Lawns had larger pools of available NO3- and NH4+; however, gross rates of mineralization and nitrification were also higher, leading to no net differences in NO3- and NH4+ turnover times between the two systems. Levels of N-15 remained steady in forest mineral soils from day 70 to 365 (at 23% of applied N-15), but continued to accumulate in lawn mineral soils over this same time period, increasing from 20% to 33% of applied N-15. The dominant sink for N in lawn plots changed over time. Immobilization in mineral soils dominated immediately (one day) after tracer application (42% of recovered N-15); plant biomass dominated the short term (10 days; 51%); thatch and mineral-soil pools together dominated the medium term (70 days; 28% and 36%, respectively); and the mineral-soil pool alone dominated long-term retention (one year; 70% of recovered N-15). These findings illustrate the mechanisms whereby urban and suburban lawns under low to moderate management intensities are an important sink for atmospheric-N deposition.