|Abstract or Summary
- Ryegrass residue consists of three main C fractions: readily available soluble C,
intermediately available cellulose and hemicellulose, and slowly available lignin.
Changes in chemical composition during decomposition influence rate of degradation as
well as composition of the microbial community involved. Use of ¹³C-labeled plant
material coupled with analysis of phospholipid fatty acids (PLEA) by isotope ratio mass
spectrometry results in a powerful tool for linking microbial community structure and C
cycling processes during decomposition.
The objective was to investigate the role of soluble C in the decomposition of
ryegrass straw. We wanted to determine (i) if the presence or absence of labile C in straw
affects C mineralization by the microbial community, (ii) if community structure would
differ based on the presence of labile C, and (iii) if community structure would shift as
Residue was added to soil microcosms at rates that reflect field loads. Treatments
were unleached straw (US), leached straw (LS), and leachate (L), plus an unamended
control (C). Added substrates had δ¹³C values between 120% and 180% the native soil
signature was 26%. Respiration was measured every 4 to 6 hours for the first 5 d, and
weekly thereafter. Destructive sampling took place after 0.6, 1 .6, 1 5, 1 8. 50, and 80 d of
incubation and microbial biomass '³C (MBC) and PLFAs were analyzed.
The soluble component of ryegrass straw strongly influenced C mineralization
and assimilation, as well as microbial community composition and dynamics. CO2
evolution rates and ¹³C signatures were similar in US and L during the first 3 d of
incubation. Most soluble C from leachate was consumed during that time, indicated by
the rapid decrease in δ¹³C value of CO2 evolved from L treatment. Substrate-derived C
moved quickly into and through the microbial biomass.
Distinct temporal shifts occurred in community composition. Early communities
in amended soils were dominated by short and branched-chain PLFAs such as 15:Oa.
Later samples contained more complex and longer PLFAs. 19:Ocy was an indicator for
late succession communities in US and L, and 18:2w6,9 characterized late samples in LS.
Soluble C affected when the temporal shift occurred in LS and L, communities shifted
earlier than in US. Lipids were differentially enriched with ¹³C. Fungi, as indicated by
18:2w6,9, were more effective at incorporating substrate C into cellular lipids, as this was
the most highly labeled of all PLFAs.