NUTRIENT MANAGEMENT OF EUCALYPT PLANTATIONS IN SOUTH CHINA

Eucalypts are very popular for revegetation of many parts of South China because of their capacity to tolerate degraded sites and infertile soils, their fast growth potential and their ability to coppice. More than 1,500,000 ha of plantations have been established, increasing annually by about 150,000 ha. The productivity is very low, only about 25-50% of the average productivity of eucalypt plantations in the world. Most of the land available for eucalypts has been degraded over recent centuries with extensive loss of the A horizon. The soils are generally oxisols and utilsols, which are acidic, highly leached and deeply weathered. Phosphorus deficiency is very severe and its availability is reduced because of limited compatible ectomycorrhizal fungi. Nitrogen supply is limited because of low organic matter content in the topsoil. This paper reviews a decade of field trials in South China; undertaken as part of several bilateral research programs in plantation forestry, concerning the use of fertilisers, harvest residue management and inoculation with eucalypt-compatible ectomycorrhizal fungi. One of the key questions addressed is whether the productivity of plantation eucalypts in South China can be increased to the world average by nutrient management across a rotation. Some of the major findings include: application of superphosphate at establishment substantially increased growth, tree survival and nutrient uptake. The P recovery by tree uptake was between 6-35% and decreased as the quantity of P applied increased. Phosphorus applied in the first rotation resulted in elevated levels of available, total and organic P in the topsoil in the second rotation. Coppice was less responsive to superphosphate than seedlings/cuttings to P in the first rotation, indicating the potential importance of P storage in below-ground biomass. Only a small number of the introduced ectomycorrhizal fungi increased stand volume and the effect was sometimes temporary, possibly due to nursery practices impairing mycorrhizal development and poor fungal persistence after outplanting. A diverse range of species and isolates of ectomycorrhizal fungi should be screened for specified site conditions in order to identify functionally superior commercial isolates. Reduction in the amount of residue removed during harvest increased tree growth by increasing available nutrient supply and reducing the loss of soil organic C after tree planting. Intercropping with N-fixing trees enhanced tree growth and increased litterfall. More than 80% of nutrient accumulation by trees is lost via current harvesting practices. We conclude that productivity of plantation eucalypts in south China can be improved to 10-20 m3/ha/yr, the average productivity in the world, by nutrient management. Recommended site management (land preparation, weed control, fertilizer rates) options are discussed for the first rotation. Harvest residue retention and coppice regeneration are recommended as operational practices in the second rotation. This research indicates that the productivity of well-managed plantations should be sustainable whereas poor management practices result in serious yield declines across rotations and continued soil degradation. Operational manuals are being developed that incorporate these findings.