Study of the effect of temperature on the cycling of carbon in a forest ecosystem at Mount Taranaki : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Soil Science, School of Agriculture and Environment, Palmerston North, New Zealand

Abstract

Soil organic matter (OM) represents one of the largest reservoirs of carbon (C) on the global scale. It is therefore crucial to understand the potential response of these C stocks to global warming. Global mean surface temperature is likely to increase by between 1.4 °C and 3.1 °C by the end of the 21st century (2081–2100), relative to 1986–2005 range, and it is anticipated that any warming-induced C emissions from soils will further drive planetary warming. However, there is disagreement on the potential feedbacks of soil organic C to climate warming, due to the complexity of the relationship between climate warming and soil C. The objective of this study was therefore to assess how changes in temperature affects the cycling of soil OM in a thermo-sequence at the Egmont National Park in Taranaki. Soil samples were collected at four sites (in a transect of increasing altitudes, ranging from 512 m to 1024 m asl) down to 40 cm depth, at depth increments of 5 cm, using PVC pipes of 5 cm Ø. Additional soil samples were taken for a general chemical characterisation of the soils at time 0. The soil columns were incubated for 190 days at four different temperatures (5°C, 15°, 25°C and 35°C) using a 0.25 M NaOH solution to trap CO₂ with soil moisture maintained at field capacity. A three-pool C model was used to determine the rate of C decay in the C fractions/pools. The results showed that, in general, altitude did not have a significant effect on either C stocks or cumulative C efflux at the end of the laboratory incubation. Cumulative C efflux was ~3 times larger (significant at P<0.05) at the highest temperature (e.g., 0.015 t C/ha/day for topsoil layer) compared with the lowest temperature (0.005 t C/ha/day for topsoil layer). At all temperatures and sites, the topsoil layer had the largest C efflux (ranging from 0.015 to 0.005 t C/ha/day) compared with the deeper layers (averaged between 0.006 to 0.002 t C/ha/day). The Q₁₀ values (averagely 1.47-1.35) revealed that all soil layers were temperature sensitive. All three C pools considered (fast, intermediate, slow) were temperature sensitive, though C efflux in the slow pool was very small (< 0.00006 t C/ha/day). We attributed the higher C efflux in the topsoil to the presence of more labile C enriched in necromass, weaker interaction of organic ligands with mineral components and high microbial abundance. Our findings showed that a rise in temperature enhanced the decomposition of soil OM even at the deepest layer, where mineral protection is largest. Also, the organic C at all C pools, soil layers, and altitudes were shown to be temperature sensitive.