Threshold nutrient levels for the birth and die of rootlets

Background/Question/Methods: Plant roots are modular that they are relatively independent in resource absorbing and responding to variable soil environment. According to the cost-benefit theory, survival and proliferation of individual roots depend upon their resource uptake related to the carbon costs of their construction and maintenance, with certain time-lag.  Following this line of thinking, we hypothesized: 1) a fine root will survive only as some available nutrient maintains at certain threshold level; 2) new roots emerge when the nutrient is above the level and last for a certain period; and 3) a root will decease when the nutrient is below the level and last for a certain period.  To examine the hypothesis, we designed a greenhouse experiment using Ailanthus altissima, Callistephus chinensis and Solidago Canadensis. The plants were grown individually in pots, and then three fine roots per plant were sorted and placed in three plastic vessels with one per vessel.  Three levels of nitrogen (NH4 & NO3): 0, 20 and 200 μg N/g soil, were applied in the three vessels. The roots were carefully exposed and photographed every 4 days, and ArcGIS was used to evaluate the root length changes. Repeated-measure ANOVA was used for statistics analysis. 

Results/Conclusions: New rootlet initiation (NRI) and root length growth (RLG) differed significantly among the three species and under the three N levels, and the interactions between species and N level were significant as well.  Both NRI and RLG were the least in the 0μg N treatment for A. ailanthus, and the highest in the 200μg N level for S. canadensis.  None were found deceased during the experiment period. These results demonstrated that the different species had different fine roots growth rates under the same N treatments as expected; and indicated that different species may have different threshold N levels. They further provided indirect partial supports for our first and second hypotheses, but failed in testing the third hypothesis.  We speculate that a sufficient examination of the hypotheses may require 1) the control of other vital resources such as water and other limiting nutrients; 2) considering the resource contrast between the treatment patches and the overall level; and 3) a longer experiment period.