B5 - Effects of ENSO droughts and land-use practices on soil C, N, P dynamics in a tropical rainforest and agroforestry systems in Central Sulawesi, Indonesia

Abstract
The project aims to characterize the response of soil nutrient (N, P) and soil carbon storage and dynamics to experimental drought and land-use practices in Central Sulawesi, Indonesia. In a throughfall exclusion experiment the impacts of a moderate and a severe drought and subsequent re-wetting cycles in a natural tropical forest and in a cacao agroforest will be analysed with regard to soil organic matter (SOM) decomposition and N, P and C turnover and availability. Responses of nutrient availability to different land-use management practices such as N-fertilisation and weeding activities are studied in cacao agroforestry systems. By this project we hope to improve our knowledge about relevant processes for nutrient dynamics and the preservation of forest and agricultural ecosystem functions and services.

Summary
Especially in South-eastern Asia, seasonal droughts are supposed to increase in severity through more frequent El Nino-Southern Oscillation (ENSO) episodes hypothesising to cause significant changes of soil nutrient cycling and availability, particularly of the nitrogen and carbon budget of natural forests and agroforestry systems. Changes in rainfall regime and subsequent depletion of soil moisture are expected to affect above and below ground litter input to the soil altering soil organic matter (SOM) pools and fractions as well as microbial-mediated decomposition and mineralisation processes, which govern the biochemical cycling of nutrient elements. To date, the knowledge about these processes and their response to a changing environment especially in the tropics is still imperfect.

The core activities of this project are centred around the throughfall exclusion experiment, where in a natural forest and in cacao agroforest the impacts of moderate and severe droughts and re-wetting cycles will be analysed with regard to SOM decomposition and C, N and P dynamics and availability.

Measurements cover the determination of C, N and P input and output fluxes at the ecosystem boundaries as well as their internal cycling within the soil system. The ecosystem input is determined by bulk precipitation, fluxes to the ground are tracked by throughfall and aboveground litter input. Lysimeters underneath the litter layer quantify the element input into the mineral soil, and solution samples from the mineral soil characterize the matter output fluxes with seepage water. Flux measurements are supplemented by base analyses on soil samples encompassing total C, N and P stocks, different pools of SOM, extractable bioavailable nutrients, cation exchange capacity (CEC), pH and soil physical parameters in corresponding soil depths. In addition to drought-induced effects on nutrient cycling, the nutrient status responding to land-use practices such as weeding and N fertilization is investigated. Here, soil analyses are carried out before and after manipulation treatment covering a variety of agroforestry plots. As an integrating final step of this project, field data on organic carbon dynamics and driving climatic variables obtained from the field study are used in a modelling approach applying DyDOC (Dynamic
DOC model). By means of DyDOC, different scenarios of environmental changes and their effects on soil organic carbon dynamics are simulated.

This project aims to broaden the understanding of the internal cycling of nutrients and their response to environmental and agricultural constraints in tropical land-use systems, which is of central importance for nutrient preservation and thus of maintaining forest and agriculture functions and services.