HR: 09:45h
AN: B51B-08    [PDF]
TI: Factors Influencing Changes in Soil Carbon Following Afforestation of Pastures With {\it Pinus radiata} in New Zealand
AU: * Tate, K R
EM: tatek@landcareresearch.co.nz
AF: Landcare Research, Private Bag 11052, Palmerston North, xxxx New Zealand
AU: Scott, N A
EM: nscott@whrc.org
AF: Woods Hole Research Center, PO Box 296, Woods Hole, MA 02543 United States
AU: Parshotam, A
EM: parshotama@landcareresearch.co.nz
AF: Landcare Research, Private Bag 11052, Palmerston North, xxxx New Zealand
AU: Halliday, J C
EM: joannehalliday@hotmail.com
AF: University of New South Wales, School of Biological, Earth and Environmental Sciences, Sydney, NSW 2052 Australia
AU: McMurtrie, R E
EM: r.mcmurtrie@unsw.edu.au
AF: University of New South Wales, School of Biological, Earth and Environmental Sciences, Sydney, NSW 2052 Australia
AU: Ross, D J
EM: rossdj@landcareresearch.co.nz
AF: Landcare Research, Private Bag 11052, Palmerston North, xxxx New Zealand
AB: Since 1992, afforestation with {\it Pinus radiata} D.Don in New Zealand has led to the establishment of over 500,000 ha of new plantation forests, about 85% of which are on grazed pastures. These forests accumulate carbon (C) in vegetation rapidly, storing on average about 230 Mg C ha$^{-1}$ during a 30-year rotation. Although mineral soil C levels can decline following afforestation, the consequences of afforestation on mineral soil C pools are poorly understood. Our observations at several sites throughout New Zealand suggest that afforestation decreases soil C and N cycling rates, increases soil C:N ratios, and lowers soil temperatures. Using the G'DAY ecosystem model, we investigated the interactions between N cycling processes and soil C storage at a site where soil C content had not changed following afforestation. Our simulations, in contrast, showed an increase in soil C following afforestation, primarily as a result of increased above-ground inputs and soil C:N ratio. We concluded that G'DAY overestimated C inputs from the forest floor to the mineral soil. We then used the ROTHC soil C model to examine changes in plant C inputs at another site where mineral soil C declined with afforestation. Below-ground forest C inputs were estimated assuming a steady-state relationship between litterfall and soil respiration; these inputs increased linearly between years 6 and 12, after which they remained constant at 1.53 Mg C ha$^{-1}$y$^{-1}$ until harvest at year 26. Measured changes in soil C (0-20 cm) and estimated below-ground inputs were then used to estimate above-ground inputs (as a proportion of total litterfall (3.81 Mg C ha$^{-1}$y$^{-1}$)) to the mineral soil. Our results suggest that 10% of forest floor inputs entered the mineral soil over one rotation. Using these results, and estimates of slash and weed production during/after harvest, we found that mineral-soil C stocks would likely continue to decline during second and third rotations of {\it P. radiata}; the magnitude of this decline depended in part on slash inputs to the soil. How afforestation changes mineral-soil C could depend on previous pasture management; soil C losses could be higher when afforestation occurs on highly productive pastures. While slash inputs after harvest help minimize soil C losses, they do not compensate for the ecosystem changes in C inputs viz. primarily below-ground under pasture and above-ground under forest.
DE: 0400 Biogeosciences
DE: 1600 GLOBAL CHANGE (New category)
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting