The response of juvenile and small adult western juniper (Juniperus occidentalis) to nitrate and ammonium fertilization

Abstract

Effects of nitrate-N and ammonium-N fertilization on foliar nitrogen concentrations, carbon dioxide assimilation, leaf conductance, transpiration, intercellular carbon dioxide, xylem pressure potentials, specific leaf mass, and growth were measured on naturally established juvenile and small adult Juniperus occidentalis growing in central Oregon where ammonium-N was the dominant form of soil nitrogen throughout the summer. Fertilization with both forms of nitrogen increased foliar nitrogen concentrations in juveniles and small adults in May; concentrations in small adult foliage from the nitrate-N treatment were still higher than controls in September. Both forms of nitrogen reduced carbon dioxide assimilation and potential photosynthetic nitrogen-use efficiency. The negative relationship between nitrogen addition and carbon dioxide assimilation was more apparent in juveniles than in small adults; negative effects were more pronounced in the ammonium-N treatment than in the nitrate-N treatment. Leaf conductance and transpiration were also reduced, but reductions of water loss were greater than were negative effects of fertilization on carbon gain; thus water-use efficiency of juvenile and small adults increased in May and July. The depression in gas exchange processes was detectable for a longer period during the summer in juveniles than in small adult J. occidentalis. Branchlet elongation of juvenile and small adult J. occidentalis was increased with nitrate-N and ammonium-N addition, but variability reduced significance levels. Juniperus occidentalis appears to be adapted to utilize low, ambient levels of soil nitrate at the research site and did not preferentially utilize ammonium. Key words: gas exchange, water relations, foliar nitrogen, growth.