Secondary soil salinization in urban lawns: Microbial functioning, vegetation state, and implications for carbon balance

Deicing agents cause soil salinization and degradation in urban areas. We assessed the capacity of urban lawns to maintain carbon sequestration and nutrient cycling with increasing soil salinity. The sensitivity to soil salinity of the main ecosystem players: plants and microorganisms were assessed considering their complex interactions between each other and environment. The effects of low and moderate soil salinization by common deicing agent (NaCl) were evaluated in mesocosms planted with two urban lawns: Lolium perenne and mixture of grasses. Mesocosm-, plant-, and soil-level gas exchange were assessed on a short-term (days) and long-term (months) scales. Microbial response was characterized by analyzing the microbial properties and activities of nine enzymes. Carbon balance remained independent on the salinity due to cancelling effect of lowered gross primary production (GPP, -20%), decreased C input by plants into the soil (-40% for mixture) balanced by slower microbial decomposition of organic matter (-20%) and so, lower soil respiration (-35%). GPP declined as a long-term response by a combination of stomatal constraint on photosynthesis with leaf respiration increase. Toxic effects of salinization on soil respiration were observed only for temperatures above 15°C. Microbial community with high C:N ratio (common for fungi) was the most sensitive to salinization. The death of microbial biomass (-31% for Lolium) and cell lysis increased soil enzyme activities (+38% for Lolium). We conclude that C balance of urban lawns remain homeostatic at secondary salinization. Temperature effects and plant-microbial interactions will determine C and nutrients cycling under salinity stress in urban lawns.