Droughts often affect the soil ecosystem directly. To address this question, a series of prototype observation experiments were designed in this study. The objective of this study was to identify changing tendencies of microbial biomass carbon content and the proportion of microbial biomass carbon in soil organic carbon under different drought conditions. All the soil samples were collected from experiments fields about 10 cm far away from the rhizosphere of the maize, and the content of microbial biomass carbon was selected as indicator for identifying effects of extreme drought on agriculture soil ecosystem. The results showed that the optimum mass water content of soil for microbial biomass carbon was 19.5 % and the demarcation point of microbial biomass carbon to drought was 14.3 %, which could be used to demonstrate alters and degradation of soil ecosystem and the irrigation requirement of crops. Meanwhile, sustainability of different drought soil ecosystems was also evaluated after rainstorm with rehabilitation. The results suggested that soil ecosystem could recover after interfered by moderate drought, and its tolerance to drought, as well as its function and activity was improved. Soil system could not adapt to severe drought stress, could barely recover and restore from extreme drought within a few days, and its function and structure were also damaged. From this view, we could conclude that mass water content of soil should be kept above 10 % to avoid soil system function and structure being destroyed. Nevertheless, these findings suggested that

Abstract. Content of microbial biomass carbon was selected as indicator for identifying effects of extreme drought on agriculture soil ecosystem. Through a series of prototype observation experiments, changing tendencies of microbial biomass carbon content and the proportion of microbial biomass carbon in soil organic carbon were identified. The optimum mass water content of soil for microbial biomass carbon was 19.5% and the demarcation point of microbial biomass carbon to drought was 14.3%, which could be used to demonstrate alters and degradation of soil ecosystem as well as the irrigation requirement of crops. We evaluated sustainability of different drought soil ecosystems after experiencing rainstorm with rehabilitation. The results suggested that soil ecosystem which was interfered by moderate drought could recover and its tolerance to drought was improved, as well as its function and activity. Soil ecosystem could barely recover from severe drought and could not adapt to severe drought stress. Soil ecosystem could not restore from extreme drought within a few days, the function and structure were damaged. We came to the conclusion that mass water content of soil should kept above 10% to avoid destroying function and structure while soil ecosystem would better be watered when mass water content was lower than 14.3% in order to maintain high productivity.

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