Temporal patterns of nitrogen retention in calcium treated and reference watersheds at the Hubbard Brook Experimental Forest

Background/Question/Methods  

Since the preindustrial era, the flux of nitrogen from the atmosphere to land and aquatic ecosystems has nearly doubled. Many ecosystems are sensitive to nitrogen saturation, and elevated inputs of nitrogen can be detrimental to biodiversity and impact ecosystem services. Our study focuses on a northern temperate forest ecosystem which has historically received elevated atmospheric acid and nitrogen deposition. We report on long-term (1985-present) changes in the concentrations and fluxes of nitrogen species in atmospheric deposition and from experimental watersheds. We examined soil solution and stream chemistry from watersheds 1 (W1) and 6 (W6) at Hubbard Brook Experimental Forest, New Hampshire, USA. Watershed 6 serves as a reference, while W1 was treated with wollastonite (CaSiO₃) in 1999 to replenish available calcium lost due to leaching acid deposition. Soil solution samples, from the Oa, Bh, and Bs horizons, were collected monthly from zero-tension lysimeters at varying elevations within the watersheds. Stream water samples were also collected at locations corresponding to lysimeter elevations. Monthly solute fluxes were determined by multiplying the average solute concentration (µmol/L) by the monthly discharge (m/month) calculated by the “Brook90” model. Annual fluxes (mmol/m²-yr) were calculated from the sum of the monthly fluxes.

Results/Conclusions

Atmospheric nitrogen deposition at Hubbard Brook was elevated early in the study but has decreased markedly due to decreases in nitrate (NO₃^-) since the early 2000s. Drainage fluxes of all nitrogen species were highest in Oa horizon and decreased with soil depth to streamwater. Annual fluxes for total nitrogen (TN), dissolved organic nitrogen (DON), and NO₃^- in W1 have increased over the past two decades, while ammonium has decreased. In 2012, W1 had particularly high fluxes of TN, NO₃^-, and DON in the Oa horizon, with values of 203 mmol/m²-yr, 93 mmol/m²-yr, and 112 mmol/m²-yr, respectively. High fluxes of these solutes were also evident in the Bh and Bs horizons. At W6, TN and DON fluxes decreased in the Oa and Bh horizons over the study period, but increased in the Bs horizon. In stream water after 2012, TN and NO₃ fluxes decreased in W6, while fluxes have remained elevated in W1. Patterns of DON loss are generally consistent with DOC. Divergent patterns of nitrogen leaching between W1 and W6 suggests that improvements in the calcium status of soil alters the watershed processing of nitrogen.