We evaluate the relative importance of trophic control in the soil micro-food web of two geographically distinct but structurally similar scrub oak forests, one in Florida (FL) and one in New Jersey (NJ).  We experimentally applied allochthonous nitrogen as 0 Kg Ha^-1 Yr^-1 (deionized water control), 35 Kg Ha^-1 Yr^-1 and 70 Kg Ha^-1 Yr^-1 in monthly increments over the course of one year to replicated 1m² plots situated at the base of a reference scrub oak tree (Quercus myrtifolia in FL and Q. ilicifolia in NJ).  We measured bacterial and fungal biomass as well as density of soil animals including collembola, oribatid and predatory mites.   

Results/Conclusions

Allochthonous nitrogen did not affect the biomass or density of any of the sampled functional groups at either experimental site.  However, the FL site supports a greater biomass of bacteria and fungi than NJ, and the NJ site supports a greater density of all soil animal groups than FL.  We correlated microbial biomass and soil animal density with abiotic soil characteristics including, total C, total N, soil moisture, depth of organic horizon, and concentration of soluble nutrients (NO₃, NH₄ and PO₄).  We found evidence for top down control by soil animals on microbial biomass.   At the same time, we also found evidence for bottom up control because NO₃ and PO₄ positively correlate with microbial biomass.  Our results show that top-down and bottom-up control may be working simultaneously in naturally occurring soil food webs.   However, geographic context plays an important role.  We found differences in soil food web structure between two forests that have similar above ground biotic structure, but which appear to have very different abiotic drivers.