Kirschbaum, M.U.F., Harms, B., Mathers, N.J., Dalal, R.C. (2008).
Soil carbon and nitrogen changes after clearing mulga (Acacia aneura)
vegetation in
Abstract.
In the work reported here we examine the changes in soil (organic)
carbon and nitrogen that are observed after converting a stand of
nitrogen-fixing mulga trees (Acacia
aneura) to buffel-grass (Cenchrus
ciliaris) pasture that contained no nitrogen-fixing legumes. A range of
previously reported field measurements was compared against the output of CenW
3.1, a reformulated version of the CENTURY model.
The model successfully reproduced the observed
patterns of soil carbon, C:N ratios and nitrogen mineralisation rates under
mulga vegetation. This included relatively small changes in carbon
concentration down to 1 m, C:N ratios of around 11–13 across all soil depths,
substantial nitrogen mineralisation rates to a depth of 90 cm and, after
clearing, an on-going decrease in soil organic carbon and nitrogen stocks.
Interpretation of experimental observations was made
difficult by the addition of a large amount of ‘dead’ organic matter from
killed mulga roots after clearance. This material may be excluded through
sieving (to 2 mm) in measurements taken shortly after tree removal, but may be
included in later-year sampling as the partly decomposed material might be able
to pass through sieves. Past work has usually ignored consideration of dead
coarse roots. For the site carbon budget, changes in live biomass and surface
litter significantly outweighed the small changes in soil organic carbon, and
changes in decaying coarse roots were quantitatively more important than
changes in other organic carbon pools.
Modelled nitrogen mineralisation rates were lower
under buffel-grass than those under mulga and showed significant year-to-year
variations that were in line with varying rainfall. It showed no consistent
trend over the first 20 years after clearing because the effect of decreasing
nitrogen stocks was balanced by an increase in organic matter quality with the
change from lignin-rich mulga litter to buffel-grass litter with lower lignin
concentration. Nitrogen mineralisation rates gradually decreased thereafter as
nitrogen stocks continued to decrease but litter quality stabilised.
A scenario analysis showed that soil carbon and
nitrogen trends could be affected by changing the nitrogen budget through
inclusion of legumes or cessation of nutrient removal by grazing animals.
Inclusion of legumes was needed to halt the decline in soil nitrogen and to
ensure the long-term maintenance, or increase, in nitrogen stocks.
Keywords: CenW; deforestation; mulga; land-use change; model; soil carbon; soil nitrogen.