Is P more limiting than N in young northern hardwood forests?

Poster Number: 
242
Presenter/Primary Author: 
Melany Fisk
Co-Authors: 
Tera Ratliff, Ruth Yanai, Ed Rastetter, Brendan Naples, Tim Fahey, Farrah Fatami, Steve Hamburg

Although temperate forests have long been thought to be primarily nitrogen limited, resource optimization theory suggests that ecosystem productivity should be co-limited by multiple nutrients. In northeastern North America, air pollution and forest harvesting disturbance elevate N availability and contribute to the likelihood of P limitation. We explored the relative limitation by N and P in northern hardwood forests by simulating productivity with the Multiple Element Limitation Model (MEL) and by testing several indices of nutrient availability and acquisition, in replicate young (26-30 years) and mature (>100 years) forests in the Bartlett Experimental Forest (BEF), NH. The model predicted a greater response of aboveground productivity to N+P than N or P alone. In older stands, MEL predicted a greater response to N than to P addition, but in younger stands, the supply of N from detritus was predicted to be sufficient to create P limitation. Field measurements were consistent with this pattern of limitation. Fine roots of trees foraged preferentially for P in young forests and for N in mature forests. Phosphatase activity and net N mineralization rates were higher in soils of young than mature forests. Resin-available P did not differ between young and mature forests, but lower bicarbonate-extractable P in young forests suggests transfer of organic P to available pools, consistent with the idea of higher P mobilization in young forests. Foliar retranslocation was higher for P than for N in two of three tree species, and microbial N:P ratios averaged 29:1 in the mineral soil, suggesting P limitation to microorganisms that may cause competition for P with plants. Our results suggest that P could now be more limiting than N especially in young northern hardwood forests.