Linkages between detritus-based ("brown") food webs and producer-based ("green") food webs are critical components of ecosystem functionality, but these linkages are difficult to study because soil is opaque and brown food webs are poorly characterized. We used the well-defined detritus-based food web that forms in water-filled leaves of the pitcher plant Sarracenia purpurea to directly study how food web structure affects nitrogen (N) transformation and N-uptake by the plant itself. We used isotopically-enriched prey (detritus) and soluble inorganic N to test three predictions of the hypotheses that N uptake efficiency (UE) by S. purpurea is enhanced by the presence of a complete food web in its pitchers: (1) presence of top trophic levels increases UE of prey-derived, but not inorganic, N; (2) UE is contingent on availability of different forms of N; and (3) congeneric Sarracenia species that do not host food webs differ from S. purpurea in UE of prey-derived N. Surprisingly, none of these predictions were borne out in a 3-month greenhouse experiment. We conclude that the higher trophic-level members of this brown food web actively process detritus, but it is the activity of the microbial component of this web that ultimately influences N-availability in S. purpurea.
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