Trophic interactions in cryptic belowground systems are difficult to assess, either experimentally or by direct observation. We used lipid analysis to determine feeding strategies in Collembola raised on various bacterial, fungal, plant, or nematode diets. Dietary fatty acids (FAs) were conserved and transferred through the trophic cascade into the neutral lipids of consumers. The presence of vaccenic type FAs (ω7 family) was indicative of a bacterial diet in general. More specifically, methyl‐branched (iso, anteiso) and cyclic forms of FAs were markers for consumption of gram‐positive and gram‐negative bacteria, respectively. Fungal‐feeding Collembola comprised a higher proportion of linoleic acid, whereas the profile of plant feeders showed an increased amount of oleic acid. The FA 20:1 ω9 was only present in Collembola with nematodes as prey. Based on the assigned marker FAs, field populations of Collembola in three deciduous forest stands were ascribed to feeding guilds (i.e., fungivores, bacterivores, herbivores, predators). In conclusion, FA biomarkers provide a high‐resolution method to define feeding strategies of decomposer invertebrates and to determine their diets in situ. Lipid analysis has considerable potential as a new tool in soil food web studies.

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Subsurface microorganisms are essential constituents of the soil purification processes associated with groundwater quality. In particular, soil enzyme activity determines the biodegradation of organic compounds passing through the soil profile. Transects from surface soil to a depth of 3.5 m were investigated for microbial and chemical soil characteristics at two groundwater recharge sites and one control site. The functional diversity of the microbial community was analyzed via the activity of eight enzymes. Acid phosphomonoesterase was dominant across sites and depths, followed by L-leucine aminopeptidase and beta-glucosidase. Structural [e.g. phospholipid fatty acid (PLFA) pattern] and functional microbial diversities were linked to each other at the nonwatered site, whereas amendment with nutrients (DOC, NO(3)(-)) by flooding uncoupled this relationship. Microbial biomass did not differ between sites, whereas microbial respiration was the highest at the watered sites. Hence, excess nutrients available due to artificial groundwater recharge could not compensate for the limitation by others (e.g. phosphorus as assigned by acid phosphomonoesterase activity). Instead, at a similar microbial biomass, waste respiration via overflow metabolism occurred. In summary, ample supply of carbon by flooding led to a separation of decomposition and microbial growth, which may play an important role in regulating purification processes during groundwater recharge.

Subsurface microorganisms are crucial for contaminant degradation and maintenance of groundwater quality. This study investigates the microbial biomass and community composition [by phospholipid fatty acids (PLFAs)], as well as physical and chemical soil characteristics at woodland flooding sites of an artificial groundwater recharge system used for drinking water production. Vertical soil profiles to c. 4 m at two watered and one nonwatered site were analyzed. The microbial biomass was equal in watered and nonwatered sites, and considerable fractions (25-42%) were located in 40-340 cm depth. The microbial community structure differed significantly between watered and nonwatered sites, predominantly below 100 cm depth. Proportions of the bacterial PLFAs 16:1omega5, 16:1omega7, cy17:0 and 18:1omega9t, and the long-chained PLFAs 22:1omega9 and 24:1omega9 were more prominent at the watered sites, whereas branched, saturated PLFAs (iso/anteiso) dominated at the nonwatered site. PLFA community indices indicated stress response (trans/cis ratio), higher nutrient availability (unsaturation index) and changes in membrane fluidity (iso/anteiso ratio) due to flooding. In conclusion, water recharge processes led to nutrient input and altered environmental conditions, which resulted in a highly active and adapted microbial community residing in the vadose zone that effectively degraded organic compounds.

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