Shifts In The Balance Between Positive And Negative Internations Along An Environmental Water Gradient In A Controlled Experiment

There appears to be broad agreement in the literature that the balance between positive and negative interactions (1) may shift within a system and (2) may depend on the severity of abiotic stress in the system. It has been proposed that facilitation (or positive interactions) is more common in environments that are more stressful. Authors who report finding facilitation in arid systems often suggest that the positive effects of shading by a neighboring plant on the water status of a target plant have outweighed the negative effects of competition for light and water with the same neighboring plant. This study tests, in a controlled experiment, the hypothesis that the effect of shade on Glycine max plants can vary from negative to positive depending upon water availability. So, the changes of growth and its correlations with rhizosphere soil nutrient and root nodule were discussed in detail under drought condition here. Glycine max was grown in ten planting densities (0. 44, 0. 89, 1. 33, 1. 78, 4, 7. 11, 11.11, 16, 21. 78, 28. 44 individuals·m^-2) and two soil water levels (80±5% and 40±5% soil water capacity).Regardless of watering treatment, density effect significantly increased the mean individual biomass of Glycine max on low density; but Density effect significantly decreased the mean individual total biomass of Glycine max on high density .Water significantly increased the mean individual biomass of Glycine max, regardless of density treatment. There was a significant density vs. water interaction; the negative effect of density was not as large in the low watering treatment as it was in the high watering treatment. The results of this study suggest that whether the effect of density upon Glycine max is negative or positive depends upon density availability. Furthermore, instead of the effect gradually changing from negative to positive along a gradient of decreasing water availability, it appears that the effect changes abruptly from negative to positive.Result as follows: (1) The mean individual biomass,the mean height and thickness of the mean individual stem of lower planting densities increased with the increase of planting densities, but these of higher planting densities decreased with the increase of planting densities. The mean individual biomass of absence neighbour plant was less than that of the presence ones. Soil water deficit led to the decreases of these. From that, we can draw a conclusion shifts in the balance between positive and negative internations was induced by the planting densities.(2) Soil water deficit led to the increase of the content of total N of soil and soil organic matter vs. density, and it obviously heightened the content of total N of soil, but significantly lowered the content of soil organic matter. It showed that the elevation of planting density promoted the positive internations of the populations, and drought weakened the negative internations of the below ground part.(3) Soil water deficit led to the increase of the amount of root nodule, while it significantly depressed the the bulk of root nodule. And those were affected mainly by root positive and negative internations in low soil water levels had obvious effect on belowground activity vs. density.