PARENT SESSION
Oral Session # 19: Biogeochemistry II: Grasslands.
Presiding: JT Lennon
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 105.

Ecological stoichiometry of horticulture: Consequences of pruning and water for plant nutrient use efficiency.

Lewis, David^{*,1, 2}, Stabler, Linda³, Martin, Chris³, ¹ Center for Environmental Studies, Tempe, AZ, USA² Biology Department, Tempe, AZ, USA³ Plant Biology Department, Tempe, AZ, USA

ABSTRACT- We used an experiment to test the hypothesis that factors encouraging plant growth reduce nutrient use efficiency (NUE), measured as carbon : nitrogen (C:N) and carbon : phosphorus (C:P) ratios. In an arid, urban ecosystem (Phoenix, AZ), replicate individuals of both Texas sage (Leucophyllum frutescens) and oleander (Nerium oleander) were treated with high vs. low irrigation crossed with no pruning vs. pruning once per 6 weeks. Foliar tissue chemistry was measured seasonally, including damp, cool and dry, hot periods. From our hypothesis, we predicted that (i) NUE is reduced by pruning, (ii) NUE is reduced by irrigation, (iii) low-water conditions offset the reduction of NUE caused by pruning, and (iv) any irrigation X pruning interaction is only evident during drought. Prediction (i) was partially supported. Pruning reduced C:N of Texas sage and C:P of oleander. Prediction (ii) was fully supported, as irrigation reduced both nutrient ratios of both species. Prediction (iii) was also supported. The reduction in Texas sage C:N caused by pruning was offset by seasonal drought, and the reduction in oleander C:P caused by pruning was offset by withholding irrigation. Prediction (iv) was supported by oleander. For this species, the reduction in NUE caused by pruning could be offset by withholding irrigation, but this phenomenon was more pronounced during the natural drought condition of July than during the moist monsoon and winter rain period. Because varied horticultural practices are widespread throughout urban ecosystems, these plant-level modifications of NUE may profoundly effect ecosystem level material cycling.

Key words: stoichiometry, horticulture, nutrient use efficiency, urban ecosystem