Irrigation and fertilization effects on foliar and soil carbon and nitrogen isotope ratios in a loblolly pine stand.

摘要

We examined delta 13C and delta 15N in needle (current and 1-year-old) and soil samples collected on two occasions (July and September 1999) from a 15-year-old loblolly pine (Pinus taeda) stand in an irrigation and fertilizer application experiment conducted in North Carolina, USA, to investigate whether these treatments leave specific isotope signals in the samples and thus to infer the effects of treatments on C and N cycling. Irrespective of foliar age or sampling date, carbon isotope discrimination ( Delta ) was increased by irrigation, and decreased by fertilizer application. The carbon isotope discrimination model suggested that increased Delta by irrigation was due to decreased water use efficiency (WUE) through increased stomatal and/or mesophyll conductance. The decreased Delta by fertilizer application and published gas exchange measurements at the same site suggest that water stress resulting from increased water demand in the fertilized plots improved WUE through increased stomatal control of water loss. Foliar delta 15N values were not affected by irrigation, but were increased by fertilizer application. The delta 15N value of soil NH4+ were higher than those of NO3- and were positively correlated with foliar delta 15N, suggesting that tree uptake of NH4+ was one of the factors affecting foliar delta 15N. We conclude that irrigation and fertilizer application affected delta 13C and delta 15N values in plant and soil samples, reflecting changed C and N cycling patterns and water use efficiency in the studied loblolly pine stand..