Earthworms and management affect organic matter incorporation and microaggregate formation in agricultural soils

Through their feeding activities and cast production, earthworms influence both aggregate turnover and soil organic matter (SOM) dynamics. We studied the impact of earthworm activity on soil macro- and microaggregation and SOM incorporation in different farming systems. Dry-sieved aggregates (4¿12.5 mm) of a permanent pasture (PP), a conventional arable field (CA) and an organic arable field (OA) (0¿10 and 10¿20 cm depth) were separated into different aggregate fractions, which were analyzed for organic C and N. The separation was based on macromorphological characteristics that reflect the dominant process of aggregate formation and the degree of degradation. We distinguished: two classes of biogenic macroaggregates (fresh casts and welded casts); one class of physicogenic macroaggregates (angular to subangular blocky macroaggregates); and an intermediate fraction (rounded to subrounded macroaggregates). The structural arrangement of mineral particles and organic matter and the quantitative contribution of particulate organic matter (POM) and microaggregates were studied in thin sections. Total organic C contents tended to be higher in biogenic than in physicogenic macroaggregates of the PP and OA soils, whereas the reverse was found for the CA soil. Comparison of the different macroaggregate types in thin sections revealed that the worm-made macroaggregates of the PP soil were considerably enriched in fine POM and microaggregates, in which large amounts of organic matter were intimately mixed with fine mineral material. By contrast, worm casts of the CA and OA soils were hardly enriched in POM and microaggregates. Our study demonstrated that earthworms can directly initiate the formation of microaggregates, which in turn affects the physical protection of SOM against microbial decay. Farming practices that stimulate earthworm activity may thus constitute an important aspect of sustainable agricultural management. However, the much smaller amounts of POM and microaggregates present in worm casts of CA and OA than PP indicate a different impact of earthworms on C stabilization depending on land use. Further research is needed to elucidate the exact management conditions that favour the beneficial effects of earthworms on soil microstructure and associated SOM dynamics. In this respect, the use of micromorphological techniques in addition to chemical and physical analyses was shown to be very valuable