Decomposition of crop residues provides the primary source of new C in agricultural soil, and understanding residue C decomposition in different soils is important for developing and promoting practices that best sustain a soil's productive capacity. Stable isotope probing of the microbial community using ¹³C is a powerful tool for tracking residue C as it flows through the microbial biomass. As part of a larger long-term study that investigates the rate and pathways of litter decay, we studied the incorporation of ¹³C from barley residue into the microbial biomass using phospholipid fatty acid analysis (PLFA).

Study sites under contrasting climatic conditions were located at Ottawa, ON (humid) and Lethbridge, AB (semi-arid), Canada. In situ microcosms were established in which labelled residues were either surface-applied or incorporated into the soil (10 cm deep). At approximately 0.5, 1 and 2 yr, microcosms were destructively sampled, divided into two 5-cm increments, and analyzed for ¹³C enrichment of microbial PLFAs. At Ottawa about twice as much ¹³C was detected in the microbial PLFAs where residue was incorporated vs. surface-applied after 0.5 yr but this difference disappeared after 1 yr. At Lethbridge approximately half as much label was recovered in the PLFAs from the surface-applied vs. incorporated residue treatment after 2 yr and a greater overall proportion of the residue ¹³C was detected in the microbial biomass at Lethbridge compared to Ottawa.

After about 1 yr, the proportion of residue-derived C that was assimilated decreased for fungi but increased for bacteria and was not affected by residue placement. Overall, the fungal biomass was more heavily labeled with ¹³C than the bacterial biomass. Residue placement affected microbial community structure even after 2 yr, and this effect was stronger under more arid conditions.