Does Chronic N Addition Increase P Limitation in Northeastern U.S. Forest Soils?.

The Bear Brook Watershed in Maine is a paired watershed study composed of an experimentally acidified watershed (West Bear, WB) treated with (NH₄)₂SO₄ (25.2 kg N ha^-1 yr^-1) for 23+ years and an untreated reference watershed (East Bear, EB). Twenty permanent plots are distributed across the two watersheds and were used to determine if chronic treatments in WB increase P limitations on N dynamics. Within each permanent plot, one subplot was treated in early June 2012 with 100 kg P ha^-1, and a second subplot served as reference. Organic (O) and mineral (min) soil samples were collected throughout the 2012 growing season (5 dates) to examine the potential differences in extractable NH₄⁺-N and NO₃^--N between P-fertilized and non-P fertilized subplots in both the treated and reference watersheds.

Overall, extractable NO₃^--N concentrations from the O and mineral horizons did not differ between P-fertilized and reference subplots (P = 0.961, 0.200). Extractable O NH₄⁺-N concentrations, however, were higher (P < 0.001) in the reference than in the P-treated subplots, but no difference was detected (P = 0.139) between treatments in the mineral horizon. Sampling time influenced the P treatment effect on inorganic N similarly in both watersheds. Mean O NH₄⁺-N concentrations in the reference subplots ranged from 0.7 to 1.5 (EB), and from 1.5 to 3.2 (WB), times higher than in the P treated subplots. The lowest difference between subplots was found two weeks (mid-June 2012), and highest difference two months (early August 2012), after treatment.

In conclusion, P-enrichment did not increase extractable inorganic N in both horizons at both sites.  These results suggest that acute P additions either suppressed N mineralization, or stimulated alterative soil sinks for mineralized N.  Research on microbial processes and N dynamics is underway to further define responses to P in these long-term experimental watersheds.