Mycorrhizal associations of dominant tree species help explain variation in forest ecosystem response to N deposition

Background/Question/Methods

Chronic nitrogen (N) deposition has been linked to changes in carbon (C) and N retention and loss from temperate forest ecosystems.  However, there is little consensus on the why some forests become N saturated while others do not. We hypothesized that the mycorrhizal association of the dominant tree species is an important variable mediating how forests respond to N deposition.  Arbuscular mycorrhizal (AM) trees are associated with labile litter, exist in high N soils, and depend on soil microbes to access available N.  In contrast, ectomycorrhizal (ECM) trees may rely on organic forms of N as the primary source of N and be less able to utilize and exploit N inputs from deposition.  Hence, we hypothesized that ECM forests would exhibit symptoms of N saturation more rapidly than AM forests. 

We present the results from N addition experiments in seven ECM-dominated and seven AM-dominated forest sites in southern Indiana.  AM sites are dominated by sugar maple, tulip poplar, and cherry tree species while ECM sites are dominated by oak, hickory, and beech trees.   Paired plots for each forest type were left as controls or fertilized monthly with ammonium sulfate and sodium nitrate from May-October at a rate of 50 kg N ha^-1 yr^-1.

Results/Conclusions

Nitrogen saturation theory suggests that deposition-induced N enrichment of leaf litter should result in increased N mineralization rates in N-limited forests.  However, the addition of inorganic N may enhance microbial mining of recalcitrant C compounds, subsequently increasing the flux of organic N to ammonium.  We found that N addition increased N mineralization rates in the upper (0-5 cm) soil layers in ECM plots (p=0.016), but not AM plots (p=0.495).  In contrast, there were no changes in nitrification rates in AM or ECM plots, though nitrification rates were much higher in the upper soils of AM plots (.95 ug N g soil^-1 day^-1) than in ECM plots (-0.02 ug N g soil^-1 day^-1).  All results were similar for lower (5-15 cm) soils, though they were dampened.  Overall, N addition did not change N cycling rates in AM plots, likely due to high intrinsic rates of N cycling in AM-dominated forests.  Our results indicate that N deposition can lead to accelerated N cycling in ECM plots but not in AM plots, suggesting that ECM-dominated forests may experience N saturation at a more rapid rate than AM-dominated forests.