Where has the fertilizer gone? Closing the nutrient budget for a eucalyptus fertilization experiment in southern China

Sammanfattning: An increasing demand for wood products in China has resulted in large areas invested in fast-growing tree plantations of eucalyptus. Eucalyptus plantations are often associated with an intensive management including fertilization. By understanding the effects of fertilization and where in the ecosystem nutrients are accumulated a more sustainable forest management could be achieved. In this study, a nutrient budget including all biomass and soil components was created for Eucalyptus urophylla. The examined nutrients were nitrogen, phosphorus and potassium. The nutrient budget was created for a control and a fertilized treatment which had been fertilized with; 830 kg ha-1 nitrogen, 408 kg ha-1 phosphorus and 736 kg ha-1 potassium as NPK fertilizer during six consecutive years. Results showed that fertilization had contributed to a 20% significantly larger tree biomass. The results also indicated a higher nutrient content in the fertilized treatment than in the control. Depending on nutrient and treatment, the nutrients in understory vegetation accounted for 11-17 % of the total amount of nutrients in the biomass. The main part of the nutrients in the eucalypt ecosystem was found in the mineral soil (85-97%). Over time nutrients decreased in the soil and instead accumulated in the biomass. Furthermore, the results indicated that significant amounts of nitrogen and potassium in the fertilized treatment had leached out. Phosphorus had instead accumulated in the soil. The results also indicated more organic matter and available nutrients in the soil probably due to higher amounts of nutrients and biomass in the fertilized treatment. More organic matter and available nutrients in the soil could lead to improved soil conditions with higher water holding capacity and increased nutrient retention. Fertilization could therefore have a long-term positive effect on the soil leading to a higher productivity.