Plants Inhibit Nitrification Response to Calcium Additions in a Northern Hardwood Forest.

Concern about the role of acidic deposition in the long-term depletion of Ca in soil and vegetation led to the initiation of watershed, plot and mesocosm-scale Ca addition experiments at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. An 11.8 ha watershed (W1) was treated with 850 kg Ca/ha of the  Ca-silicate mineral (wollastonite) to restore the Ca that was estimated to have leached from the ecosystem by 50 years of acidic deposition.  Small (5 m x 5 m) field plots and plant-free mesocosms were established in 2006 with four treatments involving fertilization with Ca (850 or 4,250 kg/ha) as wollastonite, P (50 kg/ha) as NaH₂PO₄ and P + 850 kg/ha Ca.  Results from W1 and the small field plots have shown declines in soil inorganic nitrogen (N) and potential net N mineralization, and no change in either net or gross nitrification rates.  However, in plant-free mesocosms, there have been marked increases in nitrification in response to Ca additions.  These differences between plots with and without plants suggest that plants are inhibiting a nitrification response to Ca additions either by outcompeting nitrifiers for ammonium or by helping heterotrophs to outcompete nitrifiers for ammonium by providing carbon through root exudation or turnover.  More broadly, these results suggest that plants have responded more strongly to the Ca additions than microbes, outcompeting microbes for N and leading to decreased N flow though the microbial community.