Decomposition of Summer Active Perennial Grass Residues: What Controls It?.

Carbon sequestration under perennial grasses is thought to be greater than under annual systems due to lower residue quality leading to lower decomposition rate, higher residue input and lower disturbance in the perennial system. However, many of these components and the importance of biotic-abiotic interactions have not been adequately assessed, esp. in the Mediterranean region of southern Australia.

A one year decomposition experiment was conducted using litter samples from summer active perennial grass species (Panicum grasses - Megathyrsus maximus cv Petrie, M. maximus cv Gatton and Rhodes grass - Chloris gayana cv Katambora) in field experiments in South Australia and Western Australia. Litter samples from Wheat and Barley were also included. Chemical composition of initial residues was measured using Solid State ¹³C NMR. Samples retrieved at regular intervals (8 samplings) were analysed for rate of decomposition, C and N turnover, bacterial and fungal community composition (16S rRNA and ITS amplicon sequencing), abundances of functional genes such as Chitinase, nifH and amoA, microbial biomass and microbial activity.

Rate of decomposition was greater for the perennial grass residues compared to the cereal crop stubble and the decomposition of roots was slower than shoot residues. Litter chemistry differences had significant influence on decomposition and rate of decomposition was negatively related to C:N ratio. Proteobacteria, actinobacteria and bacteroidetes and ascomycetes were the most abundant bacterial and fungal phyla. Composition of bacterial phyla Firmicutes, Verrucomicrobiae and Actinobacteria significantly contributed to the variation in the decomposition of different residues. In general, chitinase gene (family 18 Group A) abundance was higher with grass residues compared to the cereal residues. Differences in the rate of decomposition between the plant types are discussed in relation to variation in microbial diversity and microbial activity and litter chemistry to determine the key drivers of decomposition.