Plant a tree, save a lake: Urban trees reduce groundwater nutrient pollution

Background/Question/Methods

Urban trees are known to enhance human well-being in many ways, from improving air quality to reducing crime rates, but less is understood about how urban trees can affect the water quality of local lakes and streams.  Many urban waterways suffer from excess nitrogen (N) and phosphorus (P) feeding algal blooms, which cause lower water clarity and oxygen levels, bad odor and taste, and the loss of desirable species.  The expansion and turnover of urban forests present a large-scale opportunity for homeowners, city foresters, and other land managers to select species that reduce nutrient pollution and improve the water quality and ecosystem service provisioning of local waterways.

In this study, we examine how common urban tree species affect N and P leaching to groundwater.   We sampled thirty-three trees of fourteen species, and seven open grassy areas, across three city parks in Saint Paul, Minnesota.  We installed lysimeters at 60cm depth to collect soil water and measure nutrient concentrations approximately biweekly, and installed tensiometers at 45cm and 75cm to measure matric potential gradients and calculate water flux.  We collected soil samples from 0-10cm, 10-20cm, 20-40cm, and 40-60cm as well as leaf, root, and leaf-litter samples, for carbon, nitrogen, and phosphorus analyses.

Results/Conclusions

A prolonged drought in 2011-2012 prevented lysimeter sampling during autumn litterfall and snowmelt to date. Nevertheless, data from July-August 2011 and April-June 2012 showed significant differences in total N and P concentrations in lysimeter water among grass, conifer, and hardwood sites, with trees reducing concentrations relative to turfgrass and hardwoods reducing them relative to conifers (TN mg/L±se: grass=8.7±1.7, conifer = 5.9±1.1, hardwood=3.5±0.8; p=0.004; TP µg/L±se: grass=116.4±22.4, conifer=73.8±18.4, hardwood=41.0±6.6; p=0.0004).  Total P concentrations in lysimeter water were significantly higher than expected for most soils, with a grand mean of 60µg/L, higher than the lake-eutrophication standard of 50µg/L.

Nitrogen in lysimeters showed a significant interaction with date and decreased during April-June 2012, whereas P showed no seasonal pattern so far.  Phosphorus concentrations were inversely related to leaf C:N ratio within conifers and hardwoods (p=0.007, R²=0.28), and not significantly correlated with leaf P, whereas N concen­trations were best predicted by a combination of surface soil C:N ratio (0-10cm) and 10d net mineralization rate (p=0.0002, R²=0.47).  These results suggest that both N and P leaching are under biotic control, though likely dominated by different mechanisms.