Factors affecting Co₂evolution from the lower plow layer of grassland soil

Abstract

Microbial activities in the lower plow layer from 11 various fields were compared with those in the uppermost layer. The soils examined were divided into two types: Tenpoku heavy clay soil and Tokachi volcanic ash soil. Soils were taken with stainless steel core samplers and the amounts of in situ CO2 evolution were simulated by incubation test. Amounts of microbial substrates in soils were represented by CO2 evolved from the disturbed soils. Simulated amounts of in situ CO2 evolved from the lower plow layer (10-15 cm in depth) were significantly smaller than those from the uppermost layer (0-5 cm in depth). This was partly due to the decrease of the microbial counts with increasing age of the grassland and to the smaller amounts of substrates in the lower layer. However, the lower ratios of the simulated amount of in situ CO2 evolution (D) to the index of microbial substrates (C) in the lower layer than in the uppermost layer indicated that the amount of CO2 evolution from the lower plow layer was small in proportion to the amount of microbial substrates present. In fact, the simulated amounts of in situ CO2 evolution in the lower layer did not correlate with the index of microbial substrates, though a significant correlation was obtained in the uppermost layer. These results suggest that some other factors may depress microbial activities in the lower layer. Compared with volcanic ash soil, D/C ratios in the heavy clay soils were lower due to the lower ratios of gaseous phase. Furthermore, the D/C ratios were positively correlated with the ratios of the gaseous phase. Therefore, the gaseous phase, which affects the oxygen flow to the lower plow layer must be a critical factor for soil microbial activity. Limitation in the oxygen supply to soil microorganisms is likely to restrict the microbial decomposition processes in the lower plow layer of grasslands cn heavy clay soils.