Nitrogen fertilisation reduces grass-induced N2 fixation of tree seedlings from semi-arid savannas
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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.
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