Influence of soluble carbohydrates, free amino acids, and protein content on the decomposition of Lolium multiflorum shoots

The aim of the present investigation was to determine how the chemical composition of L. multiflorum (var. Imperial) shoots influences the initial dynamic decomposition phase. Decomposition in soil was monitored by taking semicontinuous respiration measurements with a resolution of 1 h. Shoots with six different total N contents (2.0-5.2%) were compared. The carbohydrate content of the shoots decreased with increasing total N content, whereas the free amino acid and protein contents increased. Two respiration peaks were observed during the 1-week-long incubation. Comparisons of C mineralisation curves for water-extracted and whole shoots showed that the first peak was derived from water-soluble plant components. Lengths of lag phases and values of specific microbial growth rates obtained from respiration curves of sugars and amino acids implied that these substances were mineralised during the time of the first respiration peak. Amounts of CO2-C evolved from the shoots during the first peak increased linearly as a function of the sum of the content of glucose, fructose, sucrose, fructans, and free amino acids in the shoots. The shoots with two highest N contents showed net N mineralisation during the first respiration peak, whereas at a lower N content there was net immobilisation. It was concluded that soluble carbohydrates and free amino acids were important C and energy sources for the decomposers during the time encompassed by the first respiration peak. The second peak was derived from both water-soluble and water-insoluble plant components, and the amount of C respired increased with an increasing protein content.