Using near infrared spectroscopy (NIRS) to identify the contribution of earthworms to soil macroaggregation in field conditions

Abstract

Soil aggregates, the basic unit of structure in soils, regulate fundamental soil properties. However, a major obstacle to describing and modelling aggregate dynamics and associated processes is an almost complete ignorance of the origin of the different types of aggregates found in soils and their distribution in the soil matrix. We tested the utility of near infrared spectroscopy (NIRS) as a new tool to overcome this obstacle. For the first time, we studied the ability of NIRS to detect specific chemical and physical footprints left by different earthworms during the production of casts, and thereby to identify the origins of different casts in the same soil matrix in the field, in terms of the earthworm species that produced them. Casts collected in the field were identified by comparing their NIR spectral signatures to the signatures of macroaggregates produced by the same earthworm species living in the same soil in laboratory conditions. We showed clearly that casts of each earthworm species were characterized by a specific NIR spectral signature, resulting from quantitative and qualitative differences of OM in casts among species. The method allows evaluation of the contribution of each soil engineer to the macroaggregate composition of the soil matrix. PLS-DA models conducted on NIR spectral data showed that best discrimination among casts produced by different species was obtained when only young casts were taken into consideration. Heterogeneity of non-aggregated soils in field sites appears to be a principal limitation of the method.