Drought and rewetting events enhance nitrate leaching and seepage-mediated translocation of microbes from beech forest soils

Abstract

Nitrification in forest soils is often associated with increased leaching of nitrate to deeper soil layers with potential impacts on groundwater resources, further enhanced under scenarios of anthropogenic atmospheric nitrogen deposition and predicted weather extremes. We aimed to disentangle the relationships between soil nitrification potential, seepage-mediated nitrate leaching and the vertical translocation of nitrifiers in soils of a temperate mixed beech forest in central Germany before, during and after the severe summer drought 2018. Leaching of nitrate assessed below the litter layer and in 4, 16 and 30 cm soil depth showed high temporal and vertical variation with maxima at 16 and 30 cm during and after the drought period. Maximum of soil potential nitrification activity of 4.4 mg N kg-1 d-1 only partially coincided with maximum nitrate leaching of 10.5 kg N ha-2. Both ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) were subject to translocation by seepage, and AOB decreased at least by half and AOA increased by one to three orders of magnitude in their abundance in seepage with increasing soil depth. On the level of the total bacterial population, an increasing trend with depth was also observed for Cand. Patescibacteria while Bacteroidetes were strongly mobilized from the litter layer but poorly transported further down. Despite stable population densities in soil over time, abundances of AOA, AOB and total bacteria in seepage increased by one order of magnitude after the onset of autumn rewetting. Predicted future higher frequency of drought periods in temperate regions may result in more frequent seepage-mediated seasonal flushes of nitrate and bacteria from forest soils. Moreover, the observed translocation patterns point to taxon-specific differences in the susceptibility to mobilization, suggesting that only selected topsoil derived microbial groups are likely to affect subsoil or groundwater microbial communities and their functional potential.

The economical lifestyle of CPR bacteria in groundwater allows little preference for environmental drivers

“…Exposure to oxygen is not exceptional for Cand. Patescibacteria, as oxic soils are the main source for their vertical translocation into shallow groundwater [17,24]. Cand.…”

“…These traits may be responsible for their high abundance in nutrient-limited groundwater habitats, which are mainly anoxic. Interestingly, oxic surface soils are a major source of CPR bacteria inhabiting modern groundwater (stored within last 50 years) [23], as these organisms are easily mobilized into soil seepage water [17,24], but their metabolic traits to cope with oxidative stress are largely unknown. Divergent trends in the preference for several hydrochemical parameters or specific host preferences seem to result in the differentiation of CPR bacteria in groundwater [17].…”

Groundwater bacterial communities evolve over time, exhibiting oscillating similarity patterns in response to recharge

“…The Hainich CZE groundwater microbiome, dominated by Cand. Patescibacteria, Proteobacteria and Nitrospira, is distinct from the soil microbiome in the local recharge areas (Herrmann et al, 2019; Krüger et al, 2021). Acidobacteria, Actinobacteria, and Planctomycetes, making up approximately 40% of the soil bacterial community, are either not mobilized into the seepage at all, or in low abundance (Herrmann et al, 2019).…”