Nutrient Input, Turnover, and Extraction in an Agroforestry System in Rio Grande do Sul (Brazil)

Peter Trüby1, Francine Calil2, Mauro Schumacher3
1 Albert-Ludwigs-Universität Freiburg
2 Universidade Federal de Goias
3 Universidade Federal Santa Maria RS

P 3.20 in Fluxes between the atmosphere and ecosystems

Poster Session 2 on Tuesday, 16:30-18:00

In Rio Grande do Sul large areas of pasture have been changed to plantations or agroforestry systems with Eucalyptus. The resident population initiated a very emotional discussion about the sustainability and the ecological risks of this kind of land use.  A soil degradation is supposed. This might be caused by the extraction of nutrients if harvesting Eucalyptus and crops within short rotation periods.

Our project focused on the nutrient budget investigating the nutrient cycling by measuring input from atmosphere, internal cycling, loss by seepage water, and harvesting biomass. Open field bulk precipitation, throughfall and seepage water were collected for more than 3 years and analyzed for pH, N, P, K, Mg, Ca, S, and Cl. For calculating nutrient extraction by harvesting biomass a nutrient inventory was done.

Nutrient extraction by harvesting Eucalyptus plantations and crops of sunflower and sorghum was the most important component in the balance of the whole ecosystem. This holds even true if only grains and wood were taken out. Burning of the residues or extracting further biomass as leafs, stalks or twigs would increase the loss strongly. Beside of other nutrient losses only the extraction of biomass caused strongly negative ecosystem balances for all nutrients.

The nutrient input by open field bulk precipitation is in the range of 2 – 20 kg a-1 ha-1. Although the site was far away from industry and motorways nitrogen input was 18 kg a-1ha-1. Compared with the industrialized Middle Europe the input was high. The main compound was ammonium which was up to 60 % of total nitrogen. Fertilizer use in agriculture is supposed to be the main source of nitrogen input. Generally the atmospheric input contributed only marginally to the nutrient balances of the whole ecosystem.  For estimating the amount of seepage water a climatic water balance model was used. The annual seepage water flow was estimated on 400 mm a-1. The calculated loss of nutrients by seepage water was in the same order of magnitude as the atmospheric input. For N, Mg, S output by seepage water was higher than the atmospheric input, indicating a negative balance even if neglecting the loss by harvesting crops or timber. In contrast atmospheric potassium input was higher than the output by seepage water. This was probably caused by an intensive fertilizer usage in the surrounding areas or by wind transported spray of througfall from the plantations in the vicinity.    

The results show that nutrient regime in agro-silvi-pastoril systems is changed substantially by the extraction of nutrients. Thus, sustainability can be guaranteed only by an adequate fertilization. Due to the short rotation periods atmospheric nutrient input and chemical weathering of primary silicates are insufficient to compensate the nutrient loss by harvesting biomass.  A fertilization management based on a nutrient inventory is recommended.

Letzte Änderung 01.04.2014