Suppression of hydrolytic enzyme activities by short-term aeration of periodically anoxic soils: Evidence from upland ecosystems

Land-use change and duration of agricultural practice cause shifts in the soil physical structure and porosity. These changes, in turn, may hamper soil aeration, restrict gas exchange, and therefore, cause establishment of anoxic conditions in upland soils. Such periodic fluctuations of aeration control the activity of microbial extracellular hydrolytic enzymes. Earlier studies revealed a clear suppression of key hydrolytic enzymes due to a short-term aeration (i.e., oxygen exposure) of established anoxic ecosystems such as rice paddies. However, it remains unclear whether the suppression also occurs in upland soils with periodic anoxicity. This key gap was addressed by measuring the kinetic parameters and in situ activities of hydrolytic β-glucosidases and acid phosphatases under oxic and anoxic conditions in forest, grassland, and cropland mineral soil after 3–5 days of anoxic pre-incubation. Short-term (2 h) aeration suppressed β-glucosidase and acid phosphatase activities by up to two times as compared with activities under anoxic conditions. The suppressive effects of aeration on the maximum reaction rate of enzymatic activity (V_max) were more pronounced in the cropland (by 1.6–2.4 times) than in the forest (by 1.3–1.7 times) soil. Our findings indicate the importance of fluctuations in redox conditions in the soil organic matter transformation catalyzed by hydrolytic enzymes, particularly in scenarios where the frequency and/or severity of flooding events increase due to changes in land use and/or climatic conditions. The mechanism of the suppression of hydrolytic enzyme activities has been demonstrated to be sustainable in lowland and upland ecosystems, yet further studies are required to elucidate the biogeochemical basis for this phenomenon.