Assessing the effect of organic mulches with different carbon-nitrogen ratios on soil resource availability, ponderosa pine growth and chemical defenses /by Javier Lugo-P{acute}erez.

Some plant defense hypotheses postulate a physiological trade-off of photosynthate allocation between growth and secondary compounds determined by the amount of resources available for plants. Applying organic mulches is a common landscaping practice that can significantly influence plant resource availability. I proposed a mechanistic pathway explaining how the impact of organic mulches on soil water and nitrogen content can be reflected in patterns of plant resource uptake, carbon assimilation and carbon allocation between growth and chemical defenses. I also installed a microcosm experiment to compare the effect of organic mulch blends with different physicochemical composition on soil water and nitrogen content, plant stomatal conductance and foliar nitrogen content. I also measured plant net assimilation rate, relative growth rate and foliar monoterpene content. Results showed that mulch composition affected soil water and organic matter content, but the amount of soil inorganic nitrogen was unaffected. Increase in soil water content was proportional to the mulch layer thickness increased by the proportion of bark chips in the mulch blends. Soil organic matter increased proportionally to the amount of compost in the mulch. At the individual tree level, mulch composition failed to affect the amount of foliar nitrogen. However, a proportional contribution of the compost to the foliar nitrogen was documented using the foliar isotopic composition of {esc}p15{esc}sN. Organic mulches can help to conserve soil water content, but the efficiency varies with their thickness and substrate composition. Mulches used in this experiment lacked a significant effect on ponderosa pine stomatal conductance, foliar assimilation rate, plant relative growth rate and foliar monoterpene content. These results contradict general assumptions about beneficial contributions of mulching on plants. However, across years of study, an increment in relative growth rate and a reduction in foliar assimilation rate and monoterpene content supported the predicted trade-off between growth and differentiation. These studies show that the different organic mulches used can alter soil resource availability and potentially affect plant defensive traits. However, under the existing soil conditions, mulch influences on soil and plant were not enough to have a physiological effect on ponderosa pine trees.Thesis (Ph. D., Plant Science)--University of Idaho, December 2007