Sensitivity of soil hydrogen uptake to natural and managed moisture dynamics in a semiarid urban ecosystem.

The North American Monsoon season (June-September) in the Sonoran Desert brings thunderstorms and heavy rainfall. These rains bring cooler temperature and account for roughly half of the annual precipitation making them important for biogeochemical processes. The intensity of the monsoon rains also increase flooding in urban areas and rely on green infrastructure (GI) stormwater management techniques such as water harvesting and urban rain gardens to capture runoff. The combination of increased water availability during the monsoon and water management provide a broad moisture regime for testing responses in microbial metabolism to natural and managed soil moisture pulses in drylands. Soil microbes rely on atmospheric hydrogen (H) as an important energy source in arid and semiarid landscapes with low soil moisture and carbon availability. Unlike mesic ecosystems, transient water availability in arid and semiarid ecosystems has been identified as a key limiting driver of microbe-mediated H uptake. We measured soil H uptake in rain gardens exposed to three commonly used water harvesting practices during the monsoon season in Tucson AZ, USA. static chamber measurements were used to calculate H uptake in each of the three water harvesting treatments (stormwater runoff), (stored rooftop runoff), and (used laundry water) compared to an unaltered treatment to assess the effects of water management practices on soil microbial activity. In addition, soils were collected from each treatment and brought to the lab for an incubation experiment manipulating the soil moisture to three levels capturing the range observed from field samples. H fluxes from all treatments ranged between -0.72 nmol m s and -3.98 nmol m s over the monsoon season. Soil H uptake in the greywater treatment was on average 53% greater than the other treatments during pre-monsoon, suggesting that the increased frequency and availability of water in the greywater treatment resulted in higher H uptake during the dry season. H uptake was significantly correlated with soil moisture (r = -0.393, = 0.001, df = 62) and temperature (r = 0.345, = 0.005, df = 62). Our findings suggest that GI managed residential soils can maintain low levels of H uptake during dry periods, unlike unmanaged systems. The more continuous H uptake associated with GI may help reduce the impacts of drought on H cycling in semiarid urban ecosystems.