Multiple element limitation in northern hardwood ecosystems (MELNHE): updates from the world's longest-running NxP nutrient addition experiment in a temperate forest

Background/Question/Methods

The productivity of temperate forests on glaciated soils has been assumed to be N limited, although theory suggests that ecosystem productivity should be co-limited by multiple nutrients, and anthropogenic N deposition should be tipping these forests towards P limitation. Experimental tests of nitrogen vs. phosphorus limitation in temperate forests are needed to better understand the capacity of ecosystems to balance the acquisition of limiting resources. In 2011, in the White Mountains of New Hampshire, USA, we began a full-factorial N x P fertilization experiment in 13 stands of young, mid-aged, and mature northern hardwoods in three sites that span a range of native soil fertility: Hubbard Brook, Jeffers Brook, and Bartlett Experimental Forest. Our objectives include elucidating mechanisms that allow trees and forest communities to maintain balanced nutrition in addition to identifying whether N or P is more limiting to above- and belowground productivity and whether N-P co-limitation is common.

Results/Conclusions

By 2015, diameter growth of trees had responded more to P addition than to N addition, especially in mid-aged and mature stands, but large trees responded most to N addition, and by 2019, N appeared to be more limiting than P across all age classes. Root growth responded more to N than to P addition in mature stands and will soon be assessed in mid-aged stands. Soil respiration was reduced by N addition, and this reduction increased over time, while enhancement of soil respiration by P addition increased over time. We are monitoring many aspects of ecosystem function and welcome collaborators interested in making additional measurements in what is already the world’s longest-running NxP nutrient addition experiment in a temperate forest.