Inter-cropped perennial legumes in commercial eucalyptus spp. plantations enhance soil quality

Area planted with Eucalyptus spp. in Uruguay has largely increased in the last fi fteen years, mainly through afforestation of grasslands. Such changes in land use are known to affect soil properties, including increased soil acidity and exchangeable aluminum, decreased organic carbon content and water retention capacity, as well as changes in soil enzyme activities. The low mineralization rate of tree litter due to its high C:N ratio is suggested to be in the base of changes in soil properties. Our assumption was that the 2-year period between Eucalyptus grandis plantation and tree canopy closure allows the commercial forest to benefi t from the inclusion of an inter-cropped perennial legume. The aim of this research was to test if inter-cropping an adapted (native) legume can supply nitrogen from the atmosphere to the system, contributing to soil conservation through enhanced soil microbial activity and nutrient cycling, while improving tree growth. A fi eld experiment was carried out in a commercial forest area to assess the effects of perennial Desmanthus species (virgatus,   illinoensis and bicornutus) intercropped between tree rows on soil microbial biomass and soil enzyme activities (soil quality indicators); legume shoot dry matter and total nitrogen content; and tree diameter at breast height. Enhanced soil microbial biomass, soil respiration and hydrogenase activities were associated to legume inclusion. Both D. illinoensis and D. virgatus contributed comparable amounts of nitrogen to the system when inoculated with effective rhizobia.  Tree diameter 12 months after planting was 6.5 cm in average when Desmanthus spp. were intercropped and 5.6 cm when trees grew without the legume. The results indicate a direct and positive effect of intercropped legumes in Eucalyptus plantations on soil quality and early tree growth. An increase in soil microbial activities may lead to enhanced nutrient biogeochemical cycles, increased soil fertility and improved agroecosystem sustainability.