Nitrogen (N) limitation is pervasive across global forest types and is a major constraint on net primary production, carbon sequestration and nutrient cycling rates. Because they have access to the atmospheric N pool, symbiotic N-fixing trees should have a competitive advantage in nitrogen limited environments. Some work has hypothesized that symbiotic N fixers require more P than non-fixers, and other work has hypothesized that symbiotic N fixers are better able to acquire P than non-fixers because their excess fixed N can be used to synthesize more phosphatase enzymes. Both of these hypotheses would predict that N fixers would have higher phosphatase activity than non-fixers. Previous studies exploring the relationship between P and N fixation have shown mixed results, and have focused mostly on tropical forests. To investigate this relationship in temperate forests, we measured phosphatase enzyme activity in the soil under N-fixing trees (Robinia pseudoacacia) and non-fixing trees (Betula nigra) across four fertilizer treatments: control (trace quantities of isotopically-labeled nitrogen), medium N (100 kg N/ha/yr), high N (150 kg N/ha/yr), and high N + high P (150 kg P/ha/yr), in a field site in Black Rock Forest, NY. We hypothesized that N fixers would produce more extracellular phosphatase enzymes than non-fixers to avoid P limitation when they have sufficient resources to sustain N fixation.
Results/Conclusions
At our temperate forest site in NY, we found no significant difference in phosphatase enzyme activity between N-fixing and non-fixing tree species in any of our fertilization treatments. Phosphatase activity was 13.07 (umol/g soil/hr) vs. 16.11 (umol/g soil/hr) for N fixers and non-fixers in our control treatment, 20.47 vs. 18.76 in medium N, 12. vs. 15.53 in our high-N treatment, and 15.34 vs. 18.0 in our high N+ high P treatment. Phosphatase activity was not significantly different for trees in our P addition treatment (N+P) than plants growing in ambient P conditions (all other treatments). The lack of differences seen here could indicate a lack of P limitation, as might be expected at this site. Alternatively, our results could indicate that Robinia pseudoacacia does not rely on phosphatase production to overcome P limitation.