Biological Nitrogen Fixation in Tree Legumes of the Brazilian Semi-Arid
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Journal of Arid Environments 74 (2010) 344–349 Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv Biological nitrogen fixation in tree legumes of the Brazilian semi-arid caatinga A.D.S. Freitas a,*, E.V.S.B. Sampaio b, C.E.R.S. Santos a, A.R. Fernandes b a Universidade Federal Rural de Pernambuco, Departamento de Agronomia, Rua Dom Manoel de Medeiros, s/n, Recife (PE), 52171-900, Brazil b Universidade Federal de Pernambuco, Departamento de Energia Nuclear, Avenida Prof. Luı´s Freire, 1000, Recife (PE), 50740-540, Brazil article info abstract Article history: Information on biological nitrogen fixation (BNF) in dry forests is scarce, in spite of the importance of the Received 2 July 2008 process to nitrogen cycling in these systems. It can be used to improve management and to select species Received in revised form for revegetation and agroforestry systems. The BNF of tree legumes was estimated in four areas of the 12 March 2009 Brazilian dry forest (caatinga), analyzing potentially fixing species (target species) and non fixing species Accepted 15 September 2009 (reference species), using the 15N natural abundance methodology. The differences between the isotopic Available online 7 October 2009 signals of target and reference species permit the safe identification of N2-fixing plants and reasonably precise calculations of proportions of N derived from the air (%Ndfa). Species with great N fixation capacity Keywords: 2 Dry forest were identified, notably Mimosa tenuiflora, Mimosa arenosa and Piptadenia stipulacea. The mean biological Natural abundance fixation contributions to the plants were high, varying from 27 to 68%. However, the estimated quantities of À1 À1 Northeast Brazil N added annually to the leaf biomass in the systems were low (2.5 and 11.2 kg ha year ), due to low N-15 proportions of fixing plants in the vegetation communities. In situations of native vegetation regeneration, when the succession is dominated by fixing species, fixation could reach up to 130 kg haÀ1 yearÀ1. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Tropical dry forests are usually rich in legume species and grow in soils with low organic N; therefore, the contribution of biological Biological fixation is the principal process of nitrogen entry into fixation to these species could be high (Shearer et al., 1983; Ndiaye natural ecosystems and the fixation associated with vascular plants and Ganry, 1997; Raddad et al., 2005). However, not all legume usually contributes with the greatest quantities of added N species are capable of fixation and, among the fixing ones, the (Cleveland et al., 1999). However, estimates of these contributions proportion of fixed N varies greatly (Allen and Allen, 1981; Faria are scarce because the traditional methods of measuring fixation do et al., 1984, 1987, 1994; Faria and Lima, 1998; Boddey et al., 2000). not work in trees and for extended periods of time, except for the Even nodulated trees of the same species vary in fixation, with method based on variations of the 15N natural abundance. Even this a tendency of lower proportions in old growth vegetation than in method only allows reasonable estimates if there is a large differ- regrowth (Gehring et al., 2005). ence between the 15N abundance of fixing and non fixing plants, Brazil has one of the largest semi-arid areas in the world (about a condition not found in many ecosystems (Handley et al., 1994; 1 million km2), half of which covered with a native dry forest, called Roggy et al., 1999; Gehring and Vlek, 2004). Where the 15N condi- caatinga, with great richness and abundance of legume species tions are appropriate, most of the data refer only to the proportion (Queiroz, 2006). The nodulation capacity of most of these species is of fixed N in relation to the total N of single trees, without quan- unknown and no one determined the proportion of N derived from tifying the masses of fixed N in these trees (Boddey et al., 2000). As fixation in plants growing in this area. A previous work (Freitas, a consequence, the amounts added by N fixation of vascular plants 2008) showed that non fixing nitrogen caatinga plants are very to the ecosystems were estimated in very few cases and most of much enriched with 15N and that this enrichment is uniform, as them dealing with temperate forests (Cleveland et al., 1999). much spatially as between species. This isotopic standard is Among the rare ones dealing with tropical forests most referred to a promising condition for use of the 15N natural abundance meth- humid sites (Roggy et al.,1999; Gehring et al., 2005) and no one was odology for BNF estimates, because it facilitates the choice of found dealing with tropical dry forests. reference plants, the sampling scheme and the detection of significant differences between the d15N of fixing and non-fixing plants (Ho¨gberg, 1997; Boddey et al., 2000). The estimation of biological nitrogen fixation (BNF) in plants of * Correspondence author. R. Ja´der de Andrade, 404/1801, Recife (PE), 52061-060, Brazil. Tel.: þ55 81 21267979. this area would increase the knowledge of fixation in native vege- E-mail address: [email protected] (A.D.S. Freitas). tation of semi-arid areas. Identifying high fixing species adapted to 0140-1963/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2009.09.018 A.D.S. Freitas et al. / Journal of Arid Environments 74 (2010) 344–349 345 these areas may contribute to regevetation efforts, better manage- Table 2 ment of native vegetation as pastures or fallow in shifting cultivation Characteristics of the soils under caatinga vegetation in four chosen municipalities in the States of Paraı´ba (PB) and Pernambuco (PE), Brazil. and selection of trees to be used in agroforestry systems. Moreover, a few places have data on legume proportion and on above ground Characteristics Municipality (state) biomass production which could be used to estimate the quantities Remı´gio Santa Teresinha Caruaru Serra Talhada of N fixed in the vegetation. Considering that, apparently, this (PB) (PB) (PE) (PE) information is not available for any native vegetation in tropical Soil type Regolithic Litholic Yellow Luvisol semi-arid areas, it would provide the first data on the importance of Neosol Neosol Argisol fixation in similar areas, allowing comparisons with those of humid pH (water) 4.4 5.8 5.2 6.8 P (mg dmÀ3) 8.4 7.3 2.8 4.9 tropical (Roggy et al.,1999; Gehring et al., 2005) and temperate areas Sand (g kgÀ1) 725 623 654 651 (Cleveland et al., 1999). Silt (g kgÀ1) 122 224 223 227 Thus, the objectives of this work were to determine BNF in tree Clay (g kgÀ1) 153 153 123 122 legumes of caatinga, using the 15N natural abundance method- ology, to identify species with great fixing potential and to make the first estimates of the quantities of N added annually to the leaf nodulation capacity or without information on nodulation (Allen biomass of this vegetation. We hypothesized that: (1) the 15N and Allen, 1981; Faria et al., 1984, 1987, 1994, Faria and Lima, 1998). natural abundances of fixing legume trees are significantly lower Samples were collected from all the target species found in each plot than those of non fixing species, allowing good estimates of their and also from four reference species (Table 3). These reference BNF; and (2) BNF represents large proportions of total leaf nitrogen species were two non legume species (Croton sonderianus Muell. in some plants and species. Arg. and Aspidosperma pyrifolium Mart.) and two non nodulating legume species (Bauhinia cheilanta (Bong.) Steud and Caesalpinia 2. Material and methods pyramidalis Tul.). They were selected because of their wide distri- bution and high density in the caatinga vegetation areas. Due to the 2.1. Studied areas absence of plants of some species in some plots, the number of plants of the species were, in most cases, lower than the number of Four fragments of native caatinga were selected in four plots in each site. municipalities of the semi-arid regions of the states of Paraiba (PB) Leaf material from different branches was collected from adult and Pernambuco (PE). Remı´gio (PB) and Caruaru (PE) are localized plants with diameters at breast height larger than 3 cm. After in the Agreste physiographic zone and Santa Teresinha (PB) and collection, completely expanded healthy green leaves were Serra Talhada (PE) are localized in the Serta˜o zone, and these zones selected, forming a composite sample of around 100 to 200 g of reflect the different water availability conditions for plants (inten- leaves per plant. In all the areas the samples were collected during sity and duration of water deficit period) in the fragments studied the rainy season. From all the sample species, botanical material (Table 1). was collected for comparison with the collections deposited in Descriptions of flora composition and vegetation structure were herbarium, of floristic surveys carried out by Alcoforado Filho et al. published for the forest fragments of Caruaru (Alcoforado Filho (2003), in Caruaru, Ferraz et al. (2003), in Serra Talhada, and Pereira et al., 2003), Serra Talhada (Ferraz et al., 2003) and Remı´gio (Pereira et al. (2002), in Remı´gio. et al., 2002). Number of species, height and diameter of plants are greater in the Agreste sites than in Serra Talhada. There is no published description of the caatinga fragment of Santa Teresinha, 2.3. Determination and statistical analysis but number of species, height and diameter of plants are slightly lower than those of Serra Talhada, probably reflecting a lower water The leaf material was oven dried at 60 C for 72 hours and then availability.