title: Soil Nitrogen Dynamics Under Adjacent Native Forest and Hoop Pine Plantations
Creators: name: Burton, Joanne
Publisher: name: Griffith University
Year: 2007
Subjects: subject: soil nitrogen dynamics
subject: native forest
subject: hoop pine plantations
subject: South-east Queensland
subject: forest industry
subject: land-use change
subject: Yarraman State Forest
subject: residue management
subject: soil

description: Single-species plantation forests have become the dominant source of inputs for the Queensland forest industry. Almost a quarter (50,000 ha) of the Queensland plantation estate is accounted for by plantations of the nitrogen (N) demanding species, hoop pine (Araucaria cunninghamii). The majority of the hoop pine estate was originally native forest, and is currently moving into the second rotation phase. The future of plantations of this N demanding species is dependent on the long-term maintenance of soil N cycling and availability. Land-use change can impact soil N dynamics; however there is currently limited knowledge of how the land-use change from native forest (NF) to first rotation (1R) hoop pine plantation and subsequent second rotation (2R) hoop pine plantation, and the associated disturbance due to site preparation have influenced soil N transformations and availability. The objectives of this study were to examine the impact of land-use change from 1) NF to 1R hoop pine plantation, and 2) 1R hoop pine plantation to 2R hoop pine plantation on soil N dynamics. The impact of the current 2R residue management strategy was also examined. The study was conducted in adjacent NF, 1R hoop pine plantation, and 2R hoop pine plantation (5-year old) in Yarraman State Forest, south-east Queensland. A laboratory incubation using the 15N isotope dilution method was undertaken in order to examine the impact of land-use change and residue management on gross N transformations. Results showed that land-use change had a significant impact on soil N transformations. The conversion of the NF to the 1R hoop pine plantation significantly reduced the availability of NH4+-N and NO3- -N. It also decreased the rate of gross N mineralisation (measured under anaerobic conditions) and gross nitrification (measured under aerobic conditions). This result was related to lower soil, litter and root C:N ratios in the NF compared to the 1R hoop pine plantation, indicating a reduction in organic matter quality associated with the land-use change. The conversion of 1R to 2R hoop pine plantation resulted in an increase in the gross rate of ammonification. This was attributed to an increase in mineralisation of native organic N associated with changes in soil physical conditions and microclimate as a result of harvesting. Residue management was found to have no significant influence on the soil N transformations in the 2R plantation approximately five years after establishment. A second study focused on quantifying the impact of land use and residue management on soil soluble organic N (SON) pools using a variety of extraction methods, including water, hot water, 0.5 M K2SO4, 2 M KCl and hot KCl. Both land use and residue management were found to have a significant influence on the size of soil SON pools. The conversion of NF to 1R hoop pine plantation tended to result in a decrease in the amount of soil SON and the potential to produce SON. This reduction coincided with increased soil, litter and root C:N ratios, and may therefore be the result of a decline in organic matter quality and quantity. The conversion of 1R to 2R hoop pine plantation generally resulted in a reduction in the amount of SON. Residue management also had a significant influence on soil SON pools, which tended to be higher in windrows of harvest residues than in tree rows. The impact of land-use change on the size, activity, and composition of the soil microbial community was examined using fumigation-extraction, CO2 respiration, and community level physiological profiling (CLPP) techniques. Landuse change from NF to 1R hoop pine plantation resulted in a reduction in microbial biomass and activity, and a shift in soil microbial community composition. While the conversion from 1R to 2R hoop pine plantation appeared to have no significant influence on the size and activity of the soil microbial community, there were some indications of a difference in community composition. Soil microbial biomass and activity tended to increase as the quality and quantity of organic matter input increased. An 18-month field-based study was conducted using the in-situ incubation method to examine the impact of land-use change on seasonal N dynamics. The results of this study were consistent with results from the laboratory studies. In general, the rate of N transformations and size of soil mineral N pools and microbial biomass were lower in the 1R soil compared to the NF soil. The 1R soils tended to have lower total C and total N, and higher C:N ratios compared to the NF soil, indicating that lower rates of N transformation and N availability in the 1R soil may be a result of significant reductions in organic matter quality and quantity. While the difference in the rates of net N mineralisation and net nitrification among the plantation soils were statistically insignificant, over the 18-month sampling period more N was mineralised and nitrified in the 2R soil compared to the 1R soil. Residue management also influenced the total amount of N transformed over the sampling period, with more N tending to be mineralised and nitrified in soil under windrowed residues compared to soil under tree rows. Seasonal fluctuations in soil N dynamics tended to be controlled by temperature and soil moisture. From these results, it was concluded that land-use change and residue management had a significant impact on soil N dynamics. This was possibly associated with shifts in the quality and quantity of organic inputs, soil microbial properties and microclimate conditions. Results from this study indicate that land-use change and residue management may have implications for the long-term productivity of the soil resource. Future studies are required to improve the understanding of the chemical and biological mechanisms driving changes in soil N dynamics.