Nitrogen fertilisation reduces grass-induced N2 fixation of tree seedlings from semi-arid savannas
Coexistence of trees and grasses in nutrient-poor arid savannas may result in competition for soil N. While grasses may be more effective than woody plants in acquiring N from the soil, some leguminous woody species rely on N2 fixation. We assessed the role of N2 fixation in the N-budget of Acacia mellifera seedlings by varying N supply and grass competition.
The contribution of N2 fixation to the N-budget of Acacia mellifera seedlings with varying N supply and grass competition was determined by measuring growth, nutrient concentrations, and 15N values.
Tree seedlings were 4-fold taller and had 20-fold more biomass in the absence of grass. Tree foliar δ15N was lower with (−0.25 ± 0.2‰, n = 9) than without grasses (5.2 ± 0.1‰, n = 64). The contribution of N2-fixation to the N budget decreased with increasing N supply. Greater reliance on N2-fixation by trees in the presence of grasses did not result in greater biomass accumulation or tissue [N] relative to tree seedlings grown without grass competition. Tree seedlings competing with grass had significantly more negative δ13C (−29.5 ± 0.6‰) than seedlings without grass competition (−28.8‰ ± 0.5‰).
Induction of N2-fixation by grass may have resulted from competition for nutrients. N2-fixation enables tree seedlings to compensate for limited soil N and survive grass competition at a critical and vulnerable developmental stage of germination and establishment.
KeywordsAcacia mellifera Bush encroachment Carbon isotope discrimination δ15N values Nitrogen fixation Semi-arid savanna Tree-grass coexistence WUE
Vanessa Stuart and Seth Hakizimana are thanked for technical assistance. Financial assistance from the National Research Foundation of South Africa to DW is gratefully acknowledged. This research was partially supported by the International Foundation for Science, Stockholm, Sweden through a grant to JRK. Slow-release fertiliser was donated by AGLUKON Spezialduenger GmbH & Co. KG, Düsseldorf, Germany through Grovida (Durban, SA). We thank the Stable Isotope Unit at the University of Cape Town for isotope analyses. John Lanham and Ian Newton are thanked for carrying out the mass spectrometer analyses. Mary K. Seely commented on versions of the manuscript. We thank the anonymous reviewers for their comments that considerably improved this ms.
- Allen ON, Allen EK (1981) The Leguminosae: a source book of characteristics, uses and nodulation. Macmillan, LondonGoogle Scholar
- Carlsson G (2005) Input of nitrogen from N2 fixation to northern grasslands. Dissertation, Swedish University of Agricultural Sciences, UmeåGoogle Scholar
- Crews TE (1999) The presence of nitrogen fixing legumes in terrestrial communities: evolutionary vs. ecological considerations. Biogeochemistry 46:233–246Google Scholar
- Deans JD, Ali OM, Lindley DK, Abdelnour HO (1993) Rhizobial nodulation of Acacia tree species in Sudan: soil inoculum potential and effects of peat. J Trop For Sci 6:56–64Google Scholar
- De Klerk JN (2004) Bush encroachment in Namibia. Report on phase 1 of the bush encroachment research, monitoring and management project. Ministry of Environment and Tourism, Windhoek, NamibiaGoogle Scholar
- Frost PG, Medina E, Menaut JC, Solbrig O, Swift M, Walker BH (1986) Response of savannas to stress and disturbance. Biology International Special Issue, 10, IUBS, ParisGoogle Scholar
- Hellsten A, Huss-Danell K (2000) Interaction effects of nitrogen and phosphorus on nodulation in red clover (Trifolium pratense L.). Acta Agric Scand B 50:35–142Google Scholar
- Hooper DU, Johnson L (1999) Nitrogen limitation in dryland ecosystems: responses to geographical and temporal variation in precipitation. Biogeochemistry 46:247–293Google Scholar
- Hudak AT, Wessman CA (2001) Textural analysis of high resolution imagery to quantify bush encroachment in Madikwe Game Reserve, South Africa, 1955-1996. Int J Remote Sens 22:2731–2740Google Scholar
- Kambatuku JR, Cramer MD, Ward D (2012) Overlap in soil water sources of savanna woody seedlings and grasses. Ecohydrol, published online. doi: 10.1002/eco.1273
- Rutherford MC (1997) Categorization of biomes. In: Cowling RM, Richardson DM, Pierce SM (eds) Vegetation of southern Africa. Cambridge University Press, Cambridge, pp 91–98Google Scholar
- Sankaran M, Hanan NP, Scholes RJ, Ratnam J, Augustine DJ, Cade BS, Gignoux J, Higgins SI, Le Roux X, Ludwig F, Ardo J, Banyikwa F, Bronn A, Bucini G, Caylor KK, Coughenour MB, Diouf A, Ekaya W, Feral CJ, February EC, Frost PGH, Hiernaux P, Hrabar H, Metzger KL, Prins HHT, Ringrose S, Sea W, Tews J, Worden J, Zambatis N (2005) Determinants of woody cover in African savannas. Nature 438:846–849PubMedCrossRefGoogle Scholar
- Scholes RJ, Hall DO (1996) The carbon budget of tropical savannas, woodlands and grasslands. In: Breymeyer AI, Hall DO, Melillo JM, Agren GI (eds) Global change: effects on coniferous forests and grasslands. SCOPE V56. Wiley, Chichester, pp 69–100Google Scholar
- Shearer G, Kohl DH (1986) N2- fixation in field settings: estimations based on natural 15N abundance. Aus J Plant Physiol 13:699–756Google Scholar
- Skarpe C, Bergström R (1986) Nutrient content and digestibility of forage plants in relation to plant phenology and rainfall in the Kalahari, Botswana. J Arid Environ 11:147–164Google Scholar
- Trinogga J (2010) Arid savanna shrubs affect soil chemistry and vegetation cover. Dissertation, Friedrich Schiller University, JenaGoogle Scholar
- Unkovich MJ, Pate JS, Lefroy EC, Arthur DJ (2000) Nitrogen isotope fractionation in the fodder tree legume tagasaste (Chamaecytisus proliferus) and assessment of N2 fixation inputs in deep sandy soils of western Australia. Aus J Plant Physiol 27:921–929Google Scholar
- Walter H (1939) Grassland, savanne und busch der ariden teile Afrika in ihrer okologischen bedingtheit. Jahrb Wissen Bot 87:750–860Google Scholar
- Walter H (1964) The vegetation of the earth in ecophysiological examination [Die Vegetation der Erde in ökophysiologischer Betrachtung]. Gustav Fischer Verlag, StuttgartGoogle Scholar
- Walter H (1971) Ecology of tropical and subtropical vegetation. Oliver and Boyd, EdinburghGoogle Scholar
- Zapata F, Axman H (1995) 32P isotopic techniques for evaluating the agronomic effectiveness of rock phosphate materials. Nutr Cycl Agroecosyst 41:189–195Google Scholar