Plants inhibit nitrification response to calcium additions in a northern hardwood forest

Background/Question/Methods

Concern about the role of acidic deposition in the long-term depletion of calcium (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.  We expected that the Ca addition and associated pH increase would stimulate microbial nitrogen cycling.  Surprisingly, we observed declines in soil inorganic nitrogen (N) and potential net N mineralization, and no change in either net or gross nitrification rates in the treated watershed.  To explore these responses in more detail, 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, phosphorus (P) (50 kg/ha) as NaH₂PO₄ and P + 850 kg/ha Ca.  Our objectives were to investigate the role of plants and P as factors controlling microbial response to Ca additions. 

Results/Conclusions

In contrast to field results from W1 and the small field plots, we observed marked increases in nitrification in response to Ca additions in plant-free mesocosms.  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.  Net mineralization and microbial respiration were not affected by Ca additions, and no process responded to P addition. Laboratory assays confirmed that mineralization, nitrification and respiration all increase in response to pH increases, furthering the idea that plants inhibit microbial response to pH increases under field conditions.  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.