Accelerated soil nitrogen cycling in response to a whole ecosystem acid rain mitigation experiment

Acid deposition has declined substantially over the last thirty years in the developed world. In forested watersheds previously impacted by acid deposition, evidence suggests that soils are slowly beginning to recover their alkalinity and base cation fertility. Research in these recovering ecosystems suggests that these changes in soil chemistry may result in decreased ecosystem retention and greater hydrologic export of nitrogen (N), although the drivers behind this enhanced export remain poorly understood. A whole-watershed acid rain mitigation experiment at Hubbard Brook Experimental Forest (New Hampshire, USA) offers a unique opportunity to examine how forest N cycling might change as soil recovery progresses over the coming decades. In this experiment, researchers added 1168 kg ha⁻¹ of wollastonite (CaSiO₃) to an 11.8 ha watershed in 1999, leading to sustained increases in soil pH and increasing Ca fertility. By 2008, the experimental watershed began to export significant quantities of nitrogen, becoming a net nitrogen source for the first time since measurements began in 1963. We sought to understand whether shifts in soil N cycling could explain this watershed phenomenon. Across repeated soil sampling campaigns in 2015 and 2016, we found that soil inorganic nitrogen pools in the treated watershed were between 1.8 and 2.6 times higher (p < 0.001) than in a nearby reference watershed. Gross N mineralization rates and immobilization rates were ∼40% higher in the litter layer (Oie horizon) of the treated watershed(p < 0.001). We conclude that an accelerated N cycle in this litter layer, with faster turnover between organic and inorganic N pools, likely resulted in faster replenishment of inorganic N pools that were susceptible to hydrologic loss, resulting in higher N export. Nitrogen cycling rates were uncorrelated with soil acid-base properties, suggesting that direct geochemical controls are not the primary driver of the altered N cycle.