Land-use change and soil fertility: a New Zealand perspective.
Leo M. Condron, Lincoln University, Agriculture and Life Sciences, PO Box 84, Lincoln University, Canterbury, New Zealand
This presentation provides an overview and commentary on specific issues relating to the effects and implications of recent and ongoing large-scale changes in land-use in New Zealand on soil fertility and soil quality. This will contrast changes in soil fertility and quality that occurred as a consequence of afforestation of large areas of hill and high country grassland during the 1980s and 1990s with more recent conversion of land from plantation forestry to grassland. Results from a number of paired-site comparison studies have revealed that establishment of short-rotation plantation forests on land developed under improved grassland consistently resulted in significant decreases in topsoil organic matter and associated nutrients (nitrogen, sulphur, phosphorus) with concomitant increases in levels of plant-available nutrients. This occurred despite increased soil acidity under trees compared with grassland, and indicated an overall improvement in soil fertility which was mainly attributed to mineralization of organic nutrients accumulated in soil under fertilised grazed pasture. This in turn reflected dramatic changes in the amounts, forms and spatial and temporal distribution of organic matter and nutrient returns under forestry compared with grazed pasture, and associated shifts in the composition and activities of soil fauna, including mycorrhizas. In recent years, the improved viability of pastoral farming and continued decreases in returns from forestry have resulted in a marked decline in new forest plantings and the conversion of land-use from forestry to grazed pasture. Conversion from established forest to sustainable productive grassland presents a number of challenges with respect to soil fertility and quality. These include dealing with the consequences of ongoing decomposition of large quantities of woody debris in the soil on nutrient availability (principally nitrogen), together with assessment and management of lime and fertiliser inputs.