Proliferation of birch and spruce roots in patches of mineral and organic nitrogen

Background/Question/Methods Soils are often characterized by a significant spatial heterogeneity of resources, particularly of nitrogen (N). The ability to rapidly proliferate into these microsites is an important morphological characteristic that would facilitate acquisition of N. While responses of root proliferation to mineral N have been studied to some extent, less is known about similar responses to organic N. We examined intensity and rapidity of root proliferation of one-year old seedlings of birch (Betula pedula) and spruce (Picea pungens) into patches of high nitrate, ammonium, glycine and control. Individual seedlings in 4L pots with 6cm by 6cm Mylar windows on each of the four sides were simultaneously exposed to the patches. Each side of the pot received 20 mL of one of the following treatments: 10mM ammonium chloride (NH4Cl), 10mM potassium nitrate (KNO3), 10mM glycine (C2H5NO), and 10mM potassium chloride (KCl). Root length was traced on each window prior and on 1, 2, 3, 5, 7, 21 and 28 days after patch application. Relative growth rate (RGR) of each patch type and time interval was then used to evaluate the extent and rapidity of root proliferation compared to control windows. Results/Conclusions   Both species responded to N patches, but they differed significantly in timing and degree of responses. Birch reached maximum root proliferation rate relative to controls for all N forms within three days of the patch application, while spruce took nearly two weeks to reach the peak RGRs. In spruce, maximum root proliferation into glycine, ammonium and nitrate patches were 390, 350, and 420% higher than controls, respectively. These proliferation rates are not significantly different from one another and indicate an equal preference for all N forms. The respective RGR peaks in birch were 360, 340 and 250% where root proliferation in nitrate is significantly lower than the glycine and ammonium enriched microsites. Our results indicate that timing of root proliferation into high N patches differs significantly between species. Differential plasticity may be important for species that compete for the same patches or encounter short-lived patches. Both species also differ significantly in detecting and responding to different N forms. Both species respond to ammonium and glycine to the same extent, but spruce roots grow equally well in nitrate. Therefore, spruce may have an advantage over birch in high nitrate N patches.