Abstract:
Sustainable management of planted forests requires maintenance of soil function
to promote 1) root growth; 2) storage and cycling of water and nutrients; 3) optimal gas
exchange; and 4) biological activity. Soil quality measurements can be used to monitor
short- and long-term changes in these soil functions, allowing managers to track
sustainability elements over time.
I measured selected key soil processes (nitrogen (N) mineralization,
decomposition, and soil water dynamics) which regulate soil function during the final
year of growth (year 4) on highly stocked plots (40,000 stems ha⁻¹) at ten sites distributed
across New Zealand in order to investigate impacts of soil disturbance (low vs. high),
fertilization (fertilization vs. none), and tree species selection (Pinus Radiata D.Don vs.
Cupressus lusitanica Mill.) on those soil processes. Secondly, I developed a quantitative
forest soil quality index (FSQI) that is applicable across the complex environmental
gradient in New Zealand.
Treatment effects on N mineralization and decomposition were site-specific.
Effects of soil disturbance depended on initial soil physical conditions, rainfall patterns,
or soil fertility levels. Fertilization effects were most often observed at sites with low
fertility. Tree species effects were more pronounced at sites with low rainfall, low soil N,
and low sum of bases (Ca, Mg, K, Na). Response of soil water dynamics to treatments
was consistent across the gradient. P. radiata had more optimal conditions compared to
C.lusitanica, whereas fertilized plots had more days with wet soil conditions and fewer
days with dry soil conditions compared to unfertilized plots. These results suggest that
site-specific recommendations are necessary for appropriate management of N
mineralization and decomposition processes, whereas a more general approach is
sufficient when considering soil water dynamics.
The FSQI developed through this research was based on sufficiency levels of key
soil (A horizon depth, bulk density, C:N ratio, Olsen P) and climate (air temperature)
indicators for forest productivity, and provided data for calculation of forest soil quality
of both P. radiata and C. lusitanica plantations, with and without fertilization. The final
FSQI was correlated with tree growth and soil processes, and provides a useful tool for
measuring soil resource sustainability elements in New Zealand.
