『Abstract
Plant productivity in many tropical savannas is phosphorus limited.
The biogeochemical cycling of P in these ecosystems, however,
has not been well quantified. In the present study, we characterized
P stocks and fluxes in a well-preserved small watershed in the
Brazilian Cerrado. As the Cerrado is also a fire-dominated ecosystem,
we measured the P stocks and fluxes in a cerrado stricto sensu
plot with complete exclusion of fire for 26 years (unburned plot)
and then tested some predictions about the impacts of fire impacts
on P cycling in an experimental plot that was burned three times
since 1992 (burned plot). The unburned area is an ecosystem with
large soil stocks of total P (1,151 kg ha-1 up to 50
cm depth), but the largest fraction is in an occluded form. Readily
extractable P was found up to 3 m soil depth suggesting that deep
soil is more important to the P cycle than has been recognized.
The P stock in belowground biomass (0-800 cm) was 9.9 kg ha-1.
Decomposition of fine litter released 0.97 kg P ha-1
year-1. Fluxes of P through bulk atmospheric deposition,
throughfall and litter leachate were very low (0.008, 0.006 and
0.028 kg ha-1 year-1, respectively) as was
stream export (0.001 kg ha-1 year-1). Immobilization
of P by microbes during the rainy season seems to be an important
mechanism of P conservation in this ecosystem. Fire significantly
increased P flux in litter leachate to 0.11 kg ha-1
year-1, and added 1.2 kg ha-1 of P in ash
deposition after fire. We found an increase of P concentration
in soil solution at 100 cm depth (from 0.03 μg l-1
in unburned plot to 0.3 μg l-1 in the burned plot).
In surface soils (0-10 cm) of the burned plot, fire decreased
the concentrations of extractable organic-P factions, but did
not significantly increase inorganic-P fractions. The reduction
of extractable soil organic P in the burned plot in topsoil and
the increase of P in the soil solution at greater depths indicated
a reduction of P availability and may increase P fixation in deep
soils. Repeated fire events over the long term may result in significant
net loss of available forms of phosphorus from this ecosystem.
Keywords: Nutrient cycling; Savanna; Prescribed burning; P sequential
extraction; Stream chemistry』
Introduction
Materials and methods
Study area
Belowground biomass, litter, ash and soil sampling
Nutrient determinations in biomass and ash samples
Analyses of soil P fractions
Solution sampling, analyses and flux estimates
Streamwater sampling and flux estimates
Statistical analyses
Results
Phosphorus fluxes and stocks in the unburned plot
Effect of fire on P cycling
Discussion
Soil P
Effects of fire on soil P
P fluxes
Conclusions
Acknowledgments
References