Abstract

Growth rate, resource partitioning, and several biological traits related to biological N2 fixation for six native or non-native tree species were compared using 15N isotope dilution techniques. The trees were field grown for six years in a semiarid mediterranean-climate region with five to six months a year of absolute drought. Trees were tested as candidates for new agroforestry systems being developed in central Chile to improve soil fertility and land 'health', while also increasing productivity and profitability for landowners and animal breeders. Four nitrogen-fixing legume trees (NFTs) were tested: Acacia caven (Mol.) Mol. Prosopis alba Griseb., P. chilensis (Mol.) Steuntz. emend. Burk., and Tagasaste (Chamaecytisus proliferus L.f. subsp. palmensis (Christ.) Kunkel). Additional, non-nitrogen-fixing trees were the slow-growing native Huingán (Schinus polygamus (Cav.) Caberera and the fast-growing European Ash (Fraxinus excelsior L.). Among the NFTs, highly contrasting patterns in biological nitrogen fixation (BNF) were detected, for Ndfa (proportion of N derived from atmosphere), nodule efficiency (NE = gN fixed g-1 nodules), and N content in leaves, stems and roots. Tagasaste produced 2.5-25 times more biomass and fixed 4.5 to 30 times more atmospheric nitrogen than the South American Acacia and Prosopis species. Ndfa reached 250 g plant-1 in Tagastaste, in the sixth year, with NE = maximum 2.68 in the 4th year, and 1.12 in the 6th year. In contrast, Acacia caven had by far the highest NE of the four NFTs - 12.13 in the 4th year and 6.6 in the 6th year. Whereas BNF in Tagasaste peaked in the fourth year, and declined thereafter, BNF in Acacia caven increased steadily over six years. Fraxinus excelsior and Schinus polygamus had growth rates and biomass accumulation intermediate between that of Tagasaste and the South American NFTs. Results are discussed in relation to agroforestry, restoration of soil fertility, and ecological and economic rehabilitation of damaged ecosystems and landscapes.