A system of large soil columns was used to examine water and ion transport dynamics and salt accumulation in the soil profile, with special reference to the effects of the root absorptive functions of crops. The soil column system was equipped with on-line systems for measuring the upward flux of water (i.e. the evapotranspiration rate) and the soil water content. The distribution and accumulation of ions in the soil profile and the crop xylem sap were analyzed using ion chromatography. Comparing water loss between a corn cropped column with the uncovered soil surface and a corn cropped column with the soil surface covered to inhibit soil surface evaporation indicated that root water absorption (i.e. transpiration) accounted for about 75% of the evapotranspiration. This suggests that root water absorption is the most significant driving force for transporting water and ions from groundwater to the root zone. The major essential ions for crop growth were actively absorbed by roots and were largely absent from the root zone. In contrast, soluble salts such as NaCl were scarcely absorbed and largely remained in the root zone. Furthermore, soil surface evaporation drove the upward movement of water and salt from the root zone to the soil surface, which increased salt accumulation at the soil surface. Thus, salt accumulation is caused mainly by active water uptake and selective ion absorption by crop roots, and these effects can be enhanced by the salinity of groundwater and the higher evaporative demand found in arid and semi-arid regions.

View full abstract

Effects of environmental changes on organic carbon and nitrogen partitions between zooxanthellate corals and host were studied with Montipora digitata. Nubbins from corals were kept at 21°C (March) or 28°C (July) with and without nitrate enrichment (5μmol L^-1) for 6 days, then experimental groups were moved to 28°C (March) or 33°C (July) and kept for 3 days under 12-hour light condition. Nubbins with neither enrichment nor heat increment were used as control. Particulate organic carbon and nitrogen (POC and PON) in hosts and symbionts were measured along with nitrate in incubation medium. Nitrate in incubation bottles decreased to less than 0.5μmol L^-1 within 24 hours of incubation. PON in host and symbiont for the experimental groups was statistically similar to those for the control both in March and July. POC in symbiont for combined stress (enrichment and heat stress) was significantly lower than that for the control in March. Host POC/PON for the combined stress group was higher than that for the control in March. Symbiont POC/PON for the enrichment group and host POC/PON for all experimental groups were significantly lower than those for the control in July. These results indicated that nitrate uptake by symbiont and photosynthate translocations from symbiont to host were influenced by nitrate enrichment and/or heat stress. Different POC/PON responses between March and July suggest that temperature is an important factor for evaluating the effect of nitrate enrichment and heat stress on POC/PON in M. digitata.

View full abstract