We investigated microbial colonization of field-grown, ¹³C-labeled, crimson clover and ryegrass straw residues incorporated into soil and incubated under field conditions from late summer (September) to the following early summer (mid-June). At time intervals, residues were recovered from the soil and their phospholipid fatty acids (PLFA) extracted, quantified, and ¹³C content determined.  By October, branched PLFA (i15:0, a15:0, i16:0) appeared on clover residue that were undetected on the ryegrass residue. By November, the same branch-chain PLFA appeared on ryegrass residue and NMS (nonmetric multidimensional scaling) analysis showed similar microbial community structures on both residues. By April, additional PLFA (16:1ω5, 16:1ω7, cy17:0, 18:0, 18:1ω7) had appeared on both residues indicating a dominant effect of environmental conditions over residue quality on community composition shifts during winter and/or early spring. Between April and June, community structure shifted on both types of residue, with both increases and decreases detected in the quantities of various PLFA. From November onward, ryegrass straw- C almost invariably accounted for a lower percentage of PLFA-C than did clover straw-C providing circumstantial evidence for a greater contribution of soil C to microbial communities associated with the N-poor ryegrass.  Coupling¹³C with PLFA analysis provides greater insights into the temporal response of the residue-colonizing microbial community to C inputs of different quality and environmental factors.