Forest trees can respond to nutrient limitation by allocating carbon belowground. Roots can proliferate at localized scales in small patches of soil nutrients to “forage” for limiting nutrients. At broader scales more representative of an entire forest stand, trees can more uniformly allocate carbon belowground to higher root growth. Forest productivity in our northern hardwood study sites increased in response to phosphorus (P) addition, indicating P limitation. Therefore we hypothesized that fine roots forage for apatite (a primary P mineral), whereas plot-level root growth is less in plots with more available P. We also asked whether plot-level root growth declines further in response to elevated nitrogen (N) and P together, given that N could become secondarily limiting where P availability is high. We examined fine root responses to nutrient availability by quantifying root growth into cores of homogenized root-free soil (ingrowth cores) in three mature stands that have received four fertilization treatments (none, N, P, and N+P). We tested the plot-level fine root response to nutrient availability and root foraging for apatite. We also tested root uptake of apatite-derived P by measuring root Lanthanum concentration, which is commonly used as an indicator of root uptake of apatite dissolution products.
Results/Conclusions
We found higher root growth in cores with added apatite compared to cores without apatite, in plots receiving no N or P fertilization, indicating foraging by roots for apatite-derived P. Higher Lanthanum in roots from cores that contained added apatite, compared to apatite-free control cores, suggests that root foraging activities increased the uptake of apatite-derived P. Root growth into unamended cores did not respond to plot-scale fertilization with P or N+P, but was higher in response to N fertilization. While P limitation of aboveground productivity is supported by our fine root foraging results, four years of fertilization does not appear to have reduced root growth at the plot scale. In fact, fine root growth was higher where N alone was added, but not where N was added in combination with P. The contrasting effects of our N vs N+P treatments lead us to hypothesize that elevated N increases root growth in response to plant demand for P.