Kirschbaum, M.U.F., Guo, L.B and Gifford, R.M. (2008). Observed and modelled soil carbon and
nitrogen changes after planting a Pinus
radiata stand onto former pasture. Soil Biology and Biochemistry 40:
247-257.
Abstract.
After reforesting pasture land, it is often observed that soil carbon
stocks decrease. The present work reports findings from a site near
The model indicated that over the first 18 years after
forest establishment, the site lost about 5.5 tC ha-1 and 588 kgN ha-1
from the soil. The C:N ratio of soil organic matter did not change in a
systematic manner over the observational period. Carbon and nitrogen stocks
contained in the biomass of the 18-year old pine stand exceeded that of the
pasture by 88 tC ha-1 and 393 kgN ha-1. An additional 6.1
tC ha-1 and 110 kgN ha-1 accumulated in above-ground
litter. These changes, together with the vertical distribution of carbon and
nitrogen in the soil, agreed well with the observation at the site. It was
assumed that over 18 years, there was also a loss of 86 kgN ha-1
from the ecosystem because of normal gaseous losses during nitrogen turn-over
and a small amount of nitrogen leaching. Those losses could not be replenished
in the pine system without symbiotic biological nitrogen fixation, and there
were no fertiliser additions.
A simple mass balance approach indicated that the
amount of nitrogen accumulating in plant biomass and the litter layer plus the
assumed nitrogen loss from the site matched the amount of nitrogen lost from
the soil organic nitrogen pool. This reduction in soil nitrogen, together with
an unchanged C:N ratio, provided a simple and internally consistent explanation
for the observed reduction of soil carbon after reforestation. It supports the
general notion that trends in soil carbon upon land-use change can often be
controlled by the possible fates of available soil nitrogen.
Keywords: CenW, C:N ratio; Model; Nitrogen; Pasture; Pine; Reforestation; Soil carbon..