Afforestation of pastures with Pinus radiata influences soil carbon and nitrogen pools and mineralisation and microbial properties

In New Zealand, Pinus radiata D. Don is frequently planted on land under pasture primarily for production forestry, but with the added advantage of potentially offsetting carbon dioxide (CO2) emissions from energy and industrial sources. Conversion of pasture to P. radiata plantations can, however, result in lowered contents of soil carbon (C) at some sites. We here examine the effects of this land-use change on soil C and nitrogen (N) pools, and on microbial properties involved in the cycling of these nutrients, at 5 paired sites, each with an established pasture and P. radiata plantation. Four sites had first-rotation trees aged 12–30 years and the other site second-rotation trees aged 20 years. In mineral soil at 0–10 cm depth, total and microbial C and N, extractable C, CO2-C production, and, generally, net mineral-N production were lower under P. radiata than under pasture; differences were significant (P < 0.05), except for total and extractable C at 2 sites. Differences between these land uses were less distinct in soil at 10–30 cm depth. On an area basis, total C in 0–30 cm depth soil was lower under P. radiata than under pasture at most sites, but significantly lower at only one site. Total N, microbial C and N, and CO2-C and net mineral-N production were, however, again generally significantly lower under P. radiata. These ecosystem differences were less marked, although still present, except for CO2-C production, when forest litter (LFH material) was included in the area calculations. Overall, our study suggests that afforestation with P. radiata leads to a reduction in total N, microbial biomass, and microbial activity, but a less consistent effect on soil C storage after one rotation.