Trinsoutrot et al. 2000

Trinsoutrot I, Recous S, Mary B, Nicolardot B. 2000. C and N fluxes of decomposing 13C and 15N Brassica napus L.: effects of residue composition and N content. Soil Biology and Biochemistry 32: 1717-1730.

These authors studied the decomposition process by soil microorganisms when isotope-labelled crop residues were added to soil. The crop used, oilseed rape Brassica napus (also known as canola) varies its nitrogen content of tissues, and the C:N ratio, depending on levels of N inputs by fertilization. This allows variation in input organic matter quality by manipulation of growing conditions; in this experiment, both carbon and nitrogen inputs to the plant included stable-isotope labels, in the form of 13C-CO2 and 15N-KNO3. Plant residues were added to soils and incubated for 168 days.

Initial C:N ratio and especially the labile-C fraction of organic-matter inputs are major controls of both the rate of decomposition and fate of matter through the system. Additionally, temperature, particle size of residues, and water content in the soil also strongly influence decomposition processes.

Here, N mineralization (the formation of NO3- and NH4+ pools in the soil from organic-N precursors) occurred in two phases. In the early phase, up to about 3 weeks, the N cycle resulted in net mineralization. Later, mineral N pools were depleted and N was immobilized, that is, incorporated into the tissues of microbial cells.

Carbon dioxide release during the experiment occurred through two pathways. The more direct route was rapid mineralization of organic matter, which I interpret as non-incorporation of organic matter by microbes, consuming such material but metabolizing it rapidly through respiration. The second, presumably slower route was through metabolization of material after incorporation into cells through respiration. Either way, the ultimate fate of much of the organic-C in the residues was release as CO2.

Differences in the N-content of residues affected decomposition rates early in the experiment, but by about 4 months the differences between high-N and low-N residues had evened out. Only a small fraction of labelled N from residues ended up in soil mineral-N pools; the majority was either immobilized into microbial cells or remained in recalcitrant organic matter fractions. Immobilization of unlabeled, SOM-derived N was enhanced by the addition of C through a substitution effect.

These authors conclude that 15N labelling was fraught with difficulties, and both under- and overestimated some pools and processes. However, the use of their model, named NCSOIL, improved their ability to trace the fate of added material through the system. This paper represents a study similar in some ways to our planned course activity in the special topics in soil science course, fall 2010.