Understanding the dynamics of N deposition in fragmented forests

Background/Question/Methods

Anthropogenic nitrogen (N) emissions associated with motor vehicles, power plants, agriculture, and other human activities, have resulted in elevated rates of N deposition to forest ecosystems on a global scale (Galloway 1998; Galloway and Cowling 2002; Galloway et al. 2004). Thus, forests near livestock farms can experience very high levels of N deposition leading to negative effects on the ecosystem (Lopez et al., 2018). Once deposited to these fragmented forests, N in its various forms can be subsequently distributed below the canopy by various mechanisms, including throughfall (i.e., that precipitation that interacts with foliar and woody surfaces in the canopy or passes through canopy gaps en route to the forest floor) (Dunkerley, 2000; Xiao et al., 2000, Zhang et al., 2016). For this study our main hypothesis is how N deposition to forest fragments, as evidenced by throughfall N composition, will vary in accordance with adjacent land use. We conducted this study in three woodlots on the East Lansing campus of Michigan State University (MSU) , three transects were established perpendicular to the western forest edge at each woodlot with a total of 24 throughfall collectors and 1 bulk collector at each site.

Results/Conclusions

Our results show a spatial pattern as well in throughfall N within the sites were the study woodlots were influenced both by relative to the edge and by local tree species compositions around each collection point. Stand species composition also had a significant effect on NO3-concentrations in throughfall having opposites effects: NO3- concentrations were found to increase with increasing Tilia americana abundance and decrease with increasing Quercus rubra abundance. There was no significant effect of distance from the woodlot edge on NH4+ concentrations (P= 0.22) , flux (P=0.67) or enrichment ratio (P=0.55). For the study we accounted the temporal variability of throughfall by looking at site, rainfall intensity and antecedent dry periods but only site was significant for the concentrations, fluxes and enrichment ratios of NO3- and NH4. In conclusion we found that species composition and the forest structure affects the throughfall. The results presented will help to fill the data gaps in the literature on how there is species effect that influence nutrients in out forest. Ultimately strengthen our current understanding of the transport and fate of deposited in the subcanopy help and the importance of trees in our urban forest and there role in forest hydrology.