Management induced nutrient losses have minimal impact on a sensitive Zululand soil

Dovey SB. Titshall L
Publication Type
Technical Note
Publication Year

Sustainable timber supply from a limited land resource necessitates management practices that aim to maintain an optimal soil nutrient supply to growing trees. Research to understand management impacts on soil productivity is  required to inform site-specific risk and management decisions.  The effects of post-harvesting soil management operations were assessed on growth and soil properties on a low carbon, dystrophic sandy soil in the subtropical environment of the Zululand coastal ecosystem. Management options were experimentally tested through treatments that included residue retention, residue burning, whole tree removal and stemwood nutrient replacement through fertiliser addition. Soil, litter and tree chemical fluxes were measured for a year  before harvesting, throughout re-establishment and after eight-years growth of the subsequent crop. Large nutrient and biomass losses were directly attributed to stemwood harvesting, but were increased mostly through whole tree removal and to a lesser extent by residue burning. Whole tree removal increased nutrient losses two-fold for nitrogen (N), phosphorous (P) and potassium (K), six-fold for calcium (Ca), and five-fold for magnesium (Mg) compared to stemwood removal. Micronutrient losses were quadrupled for zinc (Zn), copper (Cu), manganese (Mn), and iron (Fe). Residue burning increased N losses two-fold, Ca by 1.4 times and Mg two-fold compared to stemwood removal. Micronutrient losses ranged between 1.5 and 3.0 times that of stemwood removal. No growth differences occurred in the subsequent crop between the various residue treatments despite the large nutrient losses reported. However, fertiliser addition increased growth to canopy closure by 30% compared to all other treatments with these gains decreasing, becoming non-significant by rotation end. Although some soil and litter changes occurred prior to canopy closure, no treatment differences were detected in soil, litter chemistry or litter mass at eight years after planting. The lack of growth response to nutrient loss and unchanged soil nutrient levels under contrasting treatments gives evidence towards the resilience of the site and soil under current management practices. This is due to retention of large quantities of nutrient in the litter layer and soil (root system), large atmospheric deposition inputs and the likely deep root uptake that retains and replenishes top soil nutrient stocks. These above factors and rapid nutrient turnover masked the effects of large losses as is evident from single rotation tree growth responses. The large net nutrient loss cannot be sustained over multiple rotations.