Since 1992, afforestation with Pinus radiata D. Don in New Zealand has led to the establishment of over 600 000 ha of new plantation forests, about 85% of which are on fertile pastures used previously for grazing sheep and cattle. While this leads to rapid accumulation of carbon (C) in vegetation, the effects of afforestation on soil C are poorly understood. We examined key soil C cycling processes at the (former) Tikitere agroforestry experimental site near Rotorua, New Zealand. In 1973, replicated stands of P. radiata (100 and 400 stems/ha) were established on pastures, while replicated pasture plots were maintained throughout the first 26-year rotation. In 1996, soil C and microbial biomass C in 0–0.10 m depth soil, in situ soil respiration and net N mineralisation, and soil temperature were lower in the forest than in the pasture, and tended to decline with increasing tree-stocking density. In the 400 stems/ha stands, mineral soil C (0–0.50 m depth) was lower than in the pasture (104 and 126 Mg C/ha, respectively; P < 0.01). Carbon accumulation in the forest floor during the first rotation of these forest stands was 12 Mg C/ha. Using the Rothamsted soil C model (Roth-C), we examined how changes in plant C inputs following afforestation might lead to changes in soil C content to 0.30 m depth. Steady-state pasture inputs of 9.0 Mg C/ha.year were estimated using Roth-C; these C inputs were assumed to decrease linearly during the first 12 years following tree establishment (until canopy closure). Below-ground C inputs in the forest were estimated using steady-state relationships between litterfall and soil respiration; these inputs were assumed to increase linearly between years 1 and 12, after which they remained constant at 1.53 Mg C/ha.year until harvest. Measured changes in soil C (0−0.30 m) during the first rotation, in conjunction with the below-ground inputs, were used to estimate above-ground inputs (as a proportion of total litterfall [3.81 Mg C/ha.year]) to the soil. Our results suggest 10% of litterfall C over one rotation actually entered the mineral soil. Using these results and estimates of additional C inputs to the soil from harvest slash and weeds following harvest, we found mineral-soil C stocks would continue to decline during second and third rotations of P. radiata; the magnitude of this decline depended in part on how much slash enters the mineral soil matrix. We confirmed our modelling approach by simulating soil C changes to within 8% over 19 years following afforestation of pasture at another previously studied site, Purukohukohu. Whether afforestation leads to an increase or decrease in mineral-soil C may depend on previous pasture management; in highly productive pastures, high C inputs to the soil may maintain soil C at levels that cannot be sustained when trees are planted onto these grasslands.
Aspen and white spruce productivity is reduced by organic matter removal and soil compaction
