Examining nutrient co-limitation in northern hardwood forests

Recent meta- analyses reveal that ecosystem productivity is often co-limited by nitrogen (N) and phosphorus (P). Our study tests co-limitation in temperate hardwood forests with the goal of elucidating mechanisms by which biota adjust to the availability of different nutrients to maintain balanced nutrition. We are studying 13 forest stands in central New Hampshire, located in the Hubbard Brook Experimental Forest, the Bartlett Experimental Forest, and in the White Mountain National Forest near Jeffers Brook. Each of these stands contains a full factorial of 50X50m plots that in 2011 began receiving 30 kg N/ha/yr, and/or 10 kg P/ha/yr. Results prior to fertilizer treatments revealed wide variation in nutrient availability across forest stands and suggested that biota optimized resource use to increase the acquisition of one nutrient (N or P) where availability of the other was high. Enzyme production appeared to be an effective long-term mechanism promoting balanced N and P availability, with phosphatase enzyme activity and resin-available P increasing in relation to N availability across stands. Allocation to foraging by fine roots is another means by which plants appeared to balance acquisition of different nutrients, with preferential foraging for N where N availability was low and for P or Ca where N availability was high. These and other plant-soil processes will be evaluated to improve our interpretation of forest response to fertilization treatments. In the first year of treatment we found synergistic effects of N+P on the availability of soil NH₄ and NO₃, suggesting that the processes mediating soil nutrient availability are co-limited by N and P. Our initial results indicate multiple interactive feedbacks in soil that must be considered together to understand biotic responses to changes in the relative availability of multiple nutrients.