Klaus Spohrer

Influence of Water Supply on CO2 Concentration in the Rootzone of Split-Root Potted Longan Trees

Longan trees are irrigated in Thailand as fruit growth takes place during the dry season. Due to the scarcity of water resources, ways for water saving irrigation are investigated. As deficit irrigation was found to have a high water saving potential, the focus was on the investigation of plant stress responses to drought, which can be used for optimizing the deficit irrigation regime. Five split-root potted longan trees in sand culture were subjected to partial rootzone drying PRD, and during six months the CO2 concentration in the rootzone was measured by rootzone probes and compared to a well-watered control. The CO2 efflux from the rootzone was found to be well correlated to the moisture regime in the substrate. However, it was necessary to correct the measured values by the values from the control to obtain a significant correlation between CO2 concentration and soil moisture. The main observed external factor influencing CO2 was the ambient temperature. It was shown that the CO2 efflux from the soil can be used as a non-destructive method for drought stress monitoring, but continuous measurement will be necessary to externalize disturbing environmental effects.

Variability of Soil Resources in Northern Thailand

There is still much of great value to discover about soil variability in the highlands of northern Thailand. Part of this chapter tries to review the state of knowledge of the highland soils and part presents results which have been produced within subproject B1 of SFB 564. This project follows a long-term strategy in which the variability of soil and water balance is researched beginning at a field scale and ending at a regional level. The same approach is also followed here.