Capacity Nitrogen Saturation As a New Framework for Nitrogen Leaching in Urban Grasslands.

Nitrogen leaching from turfgrass has been extensively researched over the past four decades. With few exceptions, the majority of research suggests that nitrogen leaching poses minimal environmental risk when best management practices are followed. Mass balance studies suggest that nitrogen use efficiency hovers around 50% while fertilizer N recovered in leachate rarely exceeds 5% of applied N. A large portion of the remainder of nitrogen is often recovered in the soil. However, researchers have recently begun to characterize the accumulation of carbon and nitrogen in soil using field data and models, as turfgrass is often planted on soils that have low soil carbon content. These studies suggest that soil carbon and nitrogen accumulation is limited to the first 30 to 100 years following establishment (depending on soil, climate, and management).  Borrowing terms from Lovett and Goodale (2011), we propose that turfgrass leaching falls under two categories. Kinetic nitrogen leaching is the situation when the rate of applied nitrogen rate exceeds rate of soil and plant assimilation. This type of leaching has been thoroughly characterized by turfgrass research, and can be minimized by using best management practices (i.e. use slow release fertilizers, do not apply nitrogen before rain events, minimize nitrogen rates on sandy soils, etc.). The second type is capacity nitrogen leaching, defined as leaching that occurs when the annual amount of nitrogen exceeds the sink strength of the soil and vegetation. This type of leaching cannot be prevented by adjusting sources or timings. Only reductions in total nitrogen applied are expected to be effective.  It is likely that the majority of turfgrass leaching studies have been conducted on soils where sink strengths of soil and vegetation for N were large. Lawn fertilization in the United States has been widespread since the 1950s. Hence, after six decades it is possible or likely that many urban and suburban lawns are at or near soil nitrogen saturation levels.  Fertilizing these areas according to past paradigms of nitrogen leaching (kinetic saturation) may result in accelerated nitrogen leaching. Research to predict levels of nitrogen saturation in soils is a high priority for developing better fertilizer recommendations that are protective of environmental quality.