Natural abundance of 15N and 13C in nodulated legumes and other plants in the cerrado and neighbouring regions of Brazil

Leaves from over 1000 Brazilian native plants growing in the cerrado and neighbouring regions were sampled for C and N content. Half of these were analy- sed for 15N and further samples for a3C and ash content. Nodulated legumes from all three sub-families were in- cluded, together with two types of reference plant, non- nodulated legumes and non-legumes. Particular emphasis was placed on the large caesalpinioid genus Chamaecris- ta which is here for the first time reported to fix nitrogen in its native habitats. Woody and herbaceous species of this and other nodulated genera, with the exception of the mimosoid tree Stryphnodendron, showed evidence of nitrogen fixation. Amounts fixed were site-specific as was the 15N signature of reference plants. There was no evidence that nodulated legumes had higher leaf N than non-nodulated legumes: both were higher than non-le- gumes. Several species of Chamaecrista from section absus and species of Stryphnodendron had carbon con- tents of 50-55%, higher than previously reported for leaves. This was coupled with low (1-3%) ash contents. The 13C values of plants with >49% C were significantly more negative than those with <49% C: most species in the former group were woody and most in the latter group herbaceous. Mimosa pudica was unusual in having a wide range of percent C, percent ash and 13C values; these parameters were significantly correlated. It is con- cluded that Brazilian native legumes can fix significant amounts of nitrogen in the nutrient-poor cerrado soils. Consideration of mineral and lipid nutrition will be nec- essary in order fully to understand relations between ~3C, carbon content and other physiological parameters.