Near infrared spectroscopy (NIRS) to estimate earthworm cast age

As ecosystem engineers, earthworms are major actors of soil aggregation, a process that drives the delivery of ecosystem services by soils. However, persistence of soil aggregates produced by earthworms, their degradation rates, and their role in the dynamics of soil organic matter (SOM) and nutrients remain poorly known. In this experiment, near infrared (NIR) spectral signatures were measured in subterranean casts of the endogeic earthworm Aporrectodea caliginosa, incubated in controlled laboratory conditions for different periods of time. In parallel, dynamics of total amounts of C and N were assessed in ageing casts. As casts aged, NIR spectral signatures went through three main stages in the maturation process: (1) rapid changes in the NIR signal during the first 48 h, (2) a maturation period from days 3-30 with much slower change in NIR spectral signatures and (3) a further stage of maturation (days 45-90), where cast signals converged towards those of control soil. The first two axes of the PCA corresponded closely to the C and N content, respectively, of the casts. C and N contents in casts remained higher than those in control soil during the whole incubation time. Drying of casts halted the mineralization of organic matter, resulting in lack of change in the NIR spectral signal as long as casts were kept dry.