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.