The interactive effects of soil freezing and nitrogen deposition on winter and growing season nitrogen retention

Background/Question/Methods

In northern temperate regions, climate warming is predicted to decrease the proportion of precipitation that falls as snow. Reduced snow cover can increase soil freezing, causing microbial lysis, disruption of soil aggregates, and damage to roots, ultimately leading to decreased ecosystem N retention. Coupled with increased atmospheric N deposition and ecosystem N saturation over the next century, an increased quantity of N may be transferred from terrestrial to aquatic systems. The objective of this study was to investigate the interactive effects of soil freezing and N deposition on N retention over one year. We added ¹⁵N to plots treated with combinations of snow removal and N addition, then sampled the plots in fall, the end of winter, and the next fall to assess cumulative N losses from the soil and plant pools. In addition, we added ¹⁵N to a duplicate set of plots after snow melt and at peak biomass to assess short-term uptake of simulated N deposition at these times. We predicted that soil freezing would increase N losses over the winter, and that these accelerated losses would continue over the growing season as a result of damage to the root system. We also predicted that N addition would exacerbate N losses.

Results/Conclusions

Soil freezing significantly decreased retention of ¹⁵N enrichment from the soil pool over winter; however, there was no added treatment effect on this ¹⁵N over the growing season, and no effect of N addition on winter and growing season losses of this ¹⁵N. Soil freezing had no effect on short-term retention of ¹⁵N added just after snowmelt, whereas N addition decreased retention of this added ¹⁵N. Short term retention of ¹⁵N added at peak plant biomass was reduced by ~45% in snow removal plots, due to reduced plant biomass. In contrast, retention of ¹⁵N added at peak biomass increased by ~30% in N addition plots, due to increased plant biomass. Our findings highlight the potential importance of extreme climate events over winter in modifying ecosystem N retention over the next century.