The forest grows but the ecosystem leaks: Calcium enrichment increases both forest biomass and nitrogen export

Background/Question/Methods

Decades of watershed ecosystem research have given rise to a conceptual model of ecosystems that assumes vegetation growth exerts primary control on the retention and export of limiting nutrients from ecosystems. This conceptual model has been developed primarily through vegetation removal experiments or through succession gradient studies. In 1999, researchers of the Hubbard Brook Experimental Forest initiated a whole-watershed experiment to reverse the long-term soil acidification and base cation loss associated with acid rain. Calcium was added (1189 kg ha^-1 Ca²⁺) as the pulverized mineral wollastonite (CaSiO₃) to an entire forested watershed (W1).  As expected soil and stream pH and acid neutralizing capacity (ANC) increased significantly. Forest biomass, leaf area index and total evapotranspiration all increased following the whole watershed Ca enrichment, providing convincing proof that long-term soil Ca²⁺ depletion had been in large part responsible for the decline in sugar maple growth in northeastern forests. We anticipated that this enhanced forest growth would be associated with reduced exports of limiting nutrients. We used watershed mass balance comparisons between the experimental watershed and adjacent, unmanipulated reference watersheds to assess whether Ca fertilization has further enhanced the already strong capacity for N retention in these watersheds 

Results/Conclusions

Contrary to our expectations, nitrate export from the treated watershed began to exceed those of the untreated neighboring watersheds and as of 2012, the streams draining the Ca²⁺ enriched watershed have NO₃^- concentrations that are as high as those measured at the peak of acid deposition back in the 1970’s but accompanied by much lower sulfates, positive ANC, and an absence of Al³⁺. By 2013, annual inorganic N losses from the experimental watershed were 30 times higher than from the adjacent reference watershed, a proportional increase only seen previously in the case of whole watershed clear-cutting experiments. This increasing NO₃^- flux is not observed in any other experimental watershed and is occurring despite declines in NO₃^- deposition to the watershed and increases in forest growth. The discovery that Ca²⁺ enrichment can convert a watershed from a net sink to a net source of N suggests that the primacy of plant control over watershed nutrient cycles in our conceptual models needs to be reconsidered.