The effect of gases generating from buried rubbish on planted trees and the methods of improvement

Rubbish treatment is a serious problem that many cities are confronted with today. As a way of treatment, large-scale reclamation has been carried out in recent years. Most of newly reclaimed lands are used for parks because of the soft ground and being located close to downtown areas. However, the buried rubbish soon actively generates CH₄ gas in reclaimed land. Furthermore, as CH₄ gas is resolved into CO₂ and H₂O by the aerobic bacteria in the surface soil layer, a deficiency of oxygen in the soil causes the trees to die or grow feeble. We practiced the following experimental research for the purpose of improving soil conditions for trees in rubbish buried site, in Tsurumi-Ryokuchi Park in Osaka City<BR>1. The effect of “gassed soil” on the roots of the trees<BR>Sapium sebiferum were sowed on five plots in which “gassed soil” was replaced by amended sandy soil with peat moss content of 10% to a depth of 15cm (Fig. 1). The “gassed soil” on all the plots was as hard as sandstone. Eight months later, even on plots with the greatest generation of gas, there were very few dead seedlings though the growth was poor. This was because oxygen permeated easily into the surface soil layer (Fig. 2). However, the root extension to deeper “gassed soil” were checked irrespective of the shoot growth (Photo 1). Apparently, it is impossible to expect better growth so we must replace “gassed soil” with normal soil before planting, whether gas generation is continuing or not.<BR>2. Effect of ventilation and the growth of tree<BR>The planted plots of different ventilation types shown in Fig. 1 were prepared to observe the growth of trees and the root system. Several plots were allotted to each type, and located irregularly because the generation of gas fluctated according to the location. However, for G-type we selected locations in which gas generating was active. Two or three years old saplings were planted in May 1977. They were Sapium sebiferum, Cry ptomeria japonica, Quercus glauca and Zelkova serrata. The first two species and the remainder weredug up in December 1977 and 1978 respectively.<BR>Figs 5 and 6 show the close relation between the growth of trees and the soil air conditions at a depth of 30cm, with Quercus and Zelkova. It is recognized that each speciesbegins withering at 13% and 10% O₂ concentration respectively, and both die at less than 5%. However, both show relatively good growth at more than 17.5%. In regard to the effect of ventilation, G-type attained the most excellent results (Photo. 3). However, there was a large variation in results between the other types (Photo. 4). Although there was no change in amended soil of G-type, the other types resulted in very bad growth, and the brown sandy soil changed into bluish gray, very compact and reduced soil at the depths of 8 to 15cm. Judging from the soil survey, this is considered to be because of the structure of undergroud like that of Fig. 4.<BR>Accordingly, when planting under the “gassed soil” conditions, we must replace the soil with better soil to the depth which will make sufficient root extension possible. Moreover, in soil in which gas generating is continuing, normal growth can not be attained without preventing gas permeation from the surrounding soil.