Abstract
A field experiment was established at Notto in Senegal, in a sandy dune soil, to evaluate the percentage of nitrogen derived from the atmosphere (Ndfa %) by a 3yr old Casuarina equisetifolia plantation using both the total nitrogen difference (TND) and the natural 15N abundance methods (NA). Ndfa % based on the total nitrogen difference method was 33 ± 10 when 20 inoculated and 20 uninoculated trees were sampled, 32 ± 8 when the sampling comprised 8 inoculated trees with 8 uninoculated C. equisetifolia as reference trees and 24 ± 8 with Eucalyptus camaldulensis as reference trees. Ndfa % based on the natural abundance method was 39 ± 12, the sampling comprising 8 uninoculated and 8 inoculated C. equisetifolia trees and 37 ± 12 when E. camaldulensis was used as reference tree instead of C. equisetifolia. A complementary experiment simulating field conditions was carried out in another sandy soil (Bel-Air) with the same tree species. It showed that Ndfa % was 37 ± 14 (TND method) or 39 ± 6 (NA method). Thus the TND and NA methods appear to provide similar estimations, a conclusion which can probably be extrapolated to soil types similar to the Senegalese sandy soil used in this work. To calculate the Ndfa % using the natural abundance method, it was necessary first to estimate the isotopic enrichment factor associated with dinitrogen fixation (ϵ fix) of C. equisetifolia. This was found to be −0.98 + 0.5%. (calculation based on two clones α and β). Based on the mean Ndfa % (38%), the amount of nitrogen fixed in the semi arid condition of Senegal was 15 kgN ha −1 year −1. This modest amount of nitrogen fixed was attributed to the low soil fertility and to the fact that the Notto station is far from the sea and consequently does not benefit from the high air humidity in the coastal zone.
Published Version
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