Carbon and nitrogen stocks in a native pasture and an adjacent 16-year-old 'Pinus radiata' D. Don. plantation in Australia

Conversion of pastures to plantation forests has been proposed as a means to increase rates of carbon (C) sequestration from the atmosphere thereby reducing net greenhouse gas emissions from human activities. However, several studies have indicated that soil C stocks decrease after planting conifer (mainly pine) trees into pasture. This loss of soil C detracts from the role that plantation forests can play in net C sequestration. Here, we used a paired site (a grazed native pasture with the C₄ grass 'Themeda triandra' dominant, and an adjacent 16-year-old 'Pinus radiata' plantation) to compare all C and nitrogen (N) pools (including soil, litter on the floor, below-ground and above-ground biomass) in the two ecosystems and to estimate the rate of C sequestration after the land use change from the native pasture to the pine plantation. Soil C and N stocks from soil surface down to 1 m under the pine plantation were significantly less than under the native pasture by 20% (57.3 Mg C ha⁻¹ vs. 71.6 Mg C ha⁻¹) and 15% (5.6 Mg N ha⁻¹ vs. 6.7 Mg N ha⁻¹), respectively. Much more C and N was stored in litter on the floor in the pine plantation than in the native pasture (8.0 Mg C ha⁻¹ vs. 0.03 Mg C ha⁻¹, and 119.0 kg N ha⁻¹ vs. 0.9 kg N ha⁻¹), and in biomass (95.0 Mg C ha⁻¹ vs. 2.5 Mg C ha⁻¹ and 411.5 kg N ha⁻¹ vs. 62.8 kg N ha⁻¹). Carbon stored in coarse tree roots was alone sufficient to compensate the C loss from soil after the land use change. Much more C and N was deposited annually to above-ground litter in the pine plantation than in the native pasture (2.18 Mg C ha⁻¹ year⁻¹ vs. 0.22 Mg C ha⁻¹ year⁻¹, and 32.8 kg N ha⁻¹ year⁻¹ vs. 5.9 kg N ha⁻¹ year⁻¹), but less to below-ground litter (through fine root death) (2.71 Mg C ha⁻¹ year⁻¹ vs. 3.57 Mg C ha⁻¹ year⁻¹ and 38.9 kg N ha⁻¹ year⁻¹ vs. 81.4 kg N ha⁻¹ year⁻¹). The shift in net primary production from below-ground dominance to above-ground dominance after planting trees onto the pasture, and the slower turnover of litter in the plantation, played a key role in the reduction in soil C in the plantation ecosystem. In conclusion, planting pine trees onto a native temperate Australian pasture sequestered a significant amount of C (net 86 Mg C ha⁻¹, averaging 5.4 Mg C ha⁻¹ year⁻¹) from the atmosphere in 16 years despite the loss of 14 Mg C ha⁻¹ from the soil organic matter.