Ja. Parrotta et al., APPLICATION OF N-15-ENRICHMENT METHODOLOGIES TO ESTIMATE NITROGEN-FIXATION IN CASUARINA-EQUISETIFOLIA, Canadian journal of forest research, 24(2), 1994, pp. 201-207
The N-15-enrichment technique for estimating biological nitrogen fixat
ion in Casuarina equisetifolia J.R. and G. Forst. was evaluated under
field conditions in single-species and mixed-species plantings (with a
nonfixing reference species, Eucalyptus Xrobusta J.E. Smith) between
ages 6 and 24 months in Puerto Rico. Trenched and untrenched quadrats
within the plantations were labelled at 6-month intervals with N-15-en
riched ammonium sulfate at a rate of 0.2 g N-15.m(-2).year(-1) (2.0 g
N.m(-2).year(-1)). Analyses of foliar and whole-tree (weighted average
) N-isotope ratios, based on periodic nondestructive and whole tree ha
rvests, were used to estimate the proportion of nitrogen derived from
biological dinitrogen fixation (PNDFA) and total nitrogen derived from
fixation (TNDFA) in Casuarina. These results were compared with those
obtained using the nitrogen difference method. The N-15-enrichment te
chnique yielded consistent estimates of biological nitrogen fixation i
n Casuarina when either foliar or whole-tree N-isotopic data were used
to calculate PNDFA and TNDFA. Estimates of PNDFA in Casuarina were si
milar for trenched monoculture and mixed-species quadrats, and in tren
ched and untrenched quadrats where the reference species (Eucalyptus)
was interplanted with the N-fixing species. However, the results indic
ate that eucalyptus grown in small, untrenched monoculture quadrats is
an inappropriate reference for estimating PNDFA in Casuarina. During
the first 2 years after plantation establishment, Casuarina obtained f
rom 48 to 67% of its nitrogen from the atmosphere based on foliar and
whole-tree sampling. This amounted to between 82 and 94 kg.ha(-1).year
(-1) in the monoculture treatment and between 39 and 62 kg.ha(-1).year
(-1) in the mixed stands with Eucalyptus. These results also were in c
lose agreement with estimates made using an N-difference method.