Abstract
The African grass Andropogon gayanus Kunth. is invading Australian savannas, altering their ecological and biogeochemical function. To assess impacts on nitrogen (N) cycling, we quantified litter decomposition and N dynamics of grass litter in native grass and A. gayanus invaded savanna using destructive in situ grass litter harvests and litterbag incubations (soil surface and aerial position). Only 30% of the A. gayanus in situ litter decomposed, compared to 61% of the native grass litter, due to the former being largely comprised of highly resistant A. gayanus stem. In contrast to the stem, A. gayanus leaf decomposition was approximately 3- and 2-times higher than the dominant native grass, Alloteropsis semilata at the surface and aerial position, respectively. Lower initial lignin concentrations, and higher consumption by termites, accounted for the greater surface decomposition rate of A. gayanus. N flux estimates suggest the N release of A. gayanus litter is insufficient to compensate for increased N uptake and N loss via fire in invaded plots. Annually burnt invaded savanna may lose up to 8.2% of the upper soil N pool over a decade. Without additional inputs via biological N fixation, A. gayanus invasion is likely to diminish the N capital of Australia’s frequently burnt savannas.
Highlights
Tropical savannas are globally significant, pyrogenic ecosystems[1]
We tested the hypothesis that A. gayanus invasion increases grass litter decomposition and the total quantity of N released from the litter N pool, when compared to native savanna, and that this impacts on the soil N pool
We confirm that Andropogon gayanus invasion results in marked changes to litter decomposition and concomitant N fluxes
Summary
Tropical savannas are globally significant, pyrogenic ecosystems[1]. Despite generally oligotrophic soils, savannas contribute 30% of the global terrestrial net productivity[2]. Past studies have found that in other ecosystems, high biomass invasive grasses tend to have higher rates of litter decomposition than native species, resulting in accelerated N cycling in invaded ecosystems[19,20] This is generally attributed to differences in litter quantity and quality[19], but may be due to an altered decomposition microenvironment[19,21]. At soil surface and aerial positions, to examine the potential mechanisms driving differences between in situ litter decomposition of the standing litter, differences in litter quality and habitat We used these rates to examine the impact of invasion on the soil total N pool over a 10-year period for three fire frequency scenarios
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