Tree rows change the soil biodiversity abundance and repartition within the first year of plantation at an experimental agroforestry site in Ramecourt (Northern France)
Résumé
It was largely documented that agroforestry systems promote soil biodiversity in agroecosystems but little is known about the timing and the origin of this improvement after the tree plantation. The aim of this study was to measure the change in soil biodiversity abundance and repartition during the first year of tree plantation at an experimental agroforestry (AF) site in northern France. The experimental site is located in Ramecourt (50° 22' N, 2° 17' E) on an 18-ha plot according to a randomized bloc design with 3 replicates. Modalities with or without nitrogen-fixing trees in AF treatment are compared with sole-crop (CC) and pure-forest control (FC) plots. In November 2018, one-year-old trees and shrubs were planted in 38 meters and 7 meters wide rows for AF and FC respectively. Within rows, 6 species of tall trees (Quercus robur, Carpinus betulus, Juglans regia x regia, Alnus glutinosa, Prunus avium, Robinia pseudoaccacia) were planted 8 meters apart and were intercalated every 1 meter by 9 species of shrubs (Castanea sativa, Cornus sanguinea, Acer campestris, Euonymus europaeus, Corylus avelana, Tilia cordata, Ligustrum vulgaris, Salix alba, Viburnum lantana). The density of tall trees is 50 and 430 trees ha-1 for AF and FC respectively. In AF, a spring barley was sown in February 2019 in the alley after a shallow stubble ploughing as well as in CC plots. In FC, cover plants composed of a mixture of melliferous and grass plant species were sown between tree rows in April 2019. The population of earthworms, micro and macroarthropods were measured from April to July 2019 using mustard extraction method, Berlese and Barber traps respectively. The collected organisms were counted, classified and identified thanks to a determination key based on morphological criteria. The abundance and the dominancy of each recognized families, genus or species were calculated as well as the Shannon-index for the diversity. A total number of 833 earthworms was collected in all plots. Anecic species were dominant (i.e. between 50% and 75% of the total number) whereas endogenic and epigeic species were common (i.e. between 25% and 50%) and very rare (< 5%) respectively. The abundance of earthworms was not significantly different between AF (25.3 ind. m-2), CC (28.6 ind. m-2) and TF (19 ind. m-2). The relative abundance of the same ecological category of worms was roughly the same for the three treatments. A total number of 85 microarthropods was counted. Mites and collembola represented in average 26% and 32% of the population respectively. The remaining individuals were composed of myriapods, enchytreids and insects. The calculated Shannon-index for the microarthropods was significantly higher in AF (1.03) and FC (1.19) than in CC (0.26). While 7 families of microarthropods were recorded in both AF and FC, only 3 families were noted in CC, suggesting that at this early stage of trees development, the population of microarthropods could benefit from more diverse habitat and food provided by tree lines. For macroarthropods, a total number of 11,005 individuals were collected during the 4 weeks of sampling. Some zoological groups such as Staphylinidae, Ephistemus, Bembidion and Araneae were only observed in AF. The abundance of macroarthropods decreased from the beginning to the end of sampling period in CC and FC whereas it remained stable in AF. This decrease was probably due to the hot and dry weather conditions in July 2019. It can be assumed that in AF, the microclimate and habitats, which were created together by tree rows and crop canopy cover, promoted the resilience of the installed ecosystem, allowing the maintenance of food chain predators. The drying up of cover plants in FC or the absence of tree rows in CC was not favourable to maintain macroarthropods populations over time. Our study showed that the micro and macro arthropods’ community abundance and repartition in soil react very quickly within one year after tree plantation in AF, thanks to the creation of favourable habitats for their development along the tree rows. Further work is necessary to confirm these tendencies in the following years and to identify their consequence in the ecosystem functionning.