Effect of snowpack on plant nitrogen dynamics in restored prairie ecosystems in southeastern Minnesota

Background/Question/Methods

Dramatic weather fluctuations that may lead to reduced snow cover and duration are predicted for northern temperate ecosystems in the next century.  Snow depth and melt timing affect a number of interrelated ecosystem processes, including microbial activity and plant biomass.  In prairies, nitrogen availability affects several critical ecosystem characteristics, such as plant species composition, plant productivity, and nutritional quality.  While assessing soil N availability is challenging, previous studies have found a strong correlation between enriched levels of foliar δ¹⁵N and increased rates of N-cycling.  This study aimed to quantify the effects of two winters (2008-2010) of snowpack depth on N availability in a restored southeastern Minnesota prairie by comparing snowpack (control) and snow removal (treatment) plots.  Aboveground biomass samples were collected in July and September 2010, and separated into grasses, legumes, and non-leguminous forbs (forbs).  Root biomass samples were also collected, and fine root hairs were separated out.  Grass, legume, forb, and fine root hair samples in all treatments were analyzed for C:N and δ¹⁵N. 

Results/Conclusions

Overall, forbs exhibited a stronger treatment effect than grasses.  Fall forb C:N was significantly higher in snowpack plots (P<0.05).  Three of the four plots in the summer showed the same trend as fall forb C:N data, with strong (P<0.05) or marginal (P<0.1) significance.  However, one prairie plot showed the opposite trend with significantly higher forb C:N in the snow removal treatment; this is likely due to a higher density of N-fixing legumes found in that plot.  Summer forb δ¹⁵N values in snowpack plots were more enriched than snow removal plots, suggesting increased N processing.  Overall, the treatment effect on forb δ¹⁵N across all prairie plots was marginally significant (P<0.1).  These results suggest that N availability is lower in snowpack plots, likely due to increased microbial processing and subsequent uptake of the soil N pool under snowpack during winter months. While there was no trend in fine root hair δ¹⁵N, the fine roots were enriched relative to foliar δ¹⁵N and had visible ectomycorrhizal fungi associations. Previous studies have shown that ectomycorrhizal fungi tend to be enriched in δ¹⁵N and subsequently deplete foliar δ¹⁵N, accounting for our unusually depleted foliar δ¹⁵N values.