NATURAL-ABUNDANCE OF N-15 AND C-13 IN NODULATED LEGUMES AND OTHER PLANTS IN THE CERRADO AND NEIGHBORING REGIONS OF BRAZIL

Leaves from over 1000 Brazilian native plants growing in the cerrado a nd neighbouring regions were sampled for C and N content. Half of thes e were analysed for N-15 and further samples for C-13 and ash content. Nodulated legumes from all three sub-families were included, together with two types of reference plant, nonnodulated legumes and non-legum es. Particular emphasis was placed on the large caesalpinioid genus Ch amaecrista which is here for the first time reported to fix nitrogen i n its native habitats. Woody and herbaceous species of this and other nodulated genera, with the exception of the mimosoid tree Stryphnodend ron, showed evidence of nitrogen fixation. Amounts fixed were site-spe cific as was the N-15 signature of reference plants. There was no evid ence that nodulated legumes had higher leaf N than non-nodulated legum es: both were higher than non-legumes. Several species of Chamaecrista from section absus and species of Stryphnodendron had carbon contents of 50-55%, higher than previously reported for leaves. This was coupl ed with low (1-3%) ash contents. The C-13 values of plants with greate r than or equal to 49% C were significantly more negative than those w ith <49% C: most species in the former group were woody and most in th e latter group herbaceous. Mimosa pudica was unusual in having a wide range of percent C, percent ash and C-13 values; these parameters were significantly correlated. It is concluded that Brazilian native legum es can fix significant amounts of nitrogen in the nutrient-poor cerrad o soils. Consideration of mineral and lipid nutrition will be necessar y in order fully to understand relations between C-13, carbon content and other physiological parameters.