Nitrogen allocation in Mojave Desert winter annuals

Nitrogen contents and allocation were examined in winter annuals at two Mojave Desert sites near Boulder City, Nevada. Application of 10 g N m^-2 as NH₄NO₃ increased production 0- to 7-fold in species growing on a sandy soil (an Entisol) but fertilizer had no effect on plants on an alluvium (an Aridisol). Tissue nitrogen comprised 0.09-3.5% of dry weight with the lowest concentrations found in vegetative organs of nitrogen-responsive plants. During development, nitrogenpoor species showed only minor changes in nitrogen concentration and allocation compared with more nitrogen-rich species. Maximum reproductive nitrogen allocation varied among species from 43 to 67%, while reproductive biomass allocation was 31 to 51%. Fertilizer increased reproductive biomass allocation by 7 to 16%, reproductive nitrogen concentrations by 120 to 260%, and eaf and root nitrogen concentrations by 200 to 615% in nitrogen-deficient plants. Nitrogen-poor plants appear to allocate nitrogen to reproduction at the expense of vegetative organs throughout the life cycle.