Root distribution as function of litter layer management in eucalypt stands

The distribution of root system in the soil profile is an important factor for water and nutrient acquisition in planted forests. Harvest residue and litter layer management can alter nutrient availability to the plants, reflecting in shifts of root system distribution in the soil profile. In the present study, we aimed to evaluate the effect of litter layer management in root density and its correlation with depth, soil organic matter (SOM), and soil macro and micronutrients in fast-growing eucalypt plantations. We hypothesized that the presence of litter layer increases root density in the first soil layers while its absence results in higher root density in deeper soil layers. To this end we carried out an experiment in a eucalypt stand (Eucalyptus urophylla x Eucalyptus grandis; 3.3 x 1.82 m spacing; one-year-old trees) in the region of Telêmaco Borba, Paraná – Brazil. The Köppen climate classification is Cfb and the soil is classified as dystrophic Red Latosol. The treatments consisted of presence (+R) or absence (-R) of litter layer. The residue was composed by the previous rotation litter layer that remained in the forest after harvest. The experiment followed a randomized block design, with four replications. Root density (g dm^-3) was evaluated in one representative tree per plot at each treatment and replication. We used a 5.3 cm diameter auger to collect root samples from 0-10, 10-20, 20-40 and 40-60 cm depths in seven predefined points in the planting row and interrow. For each sampling point and depth, roots were manually separated, washed to remove soil and other impurities, and subsequently oven dried at 65 °C to determine total dry mass. Additionally, SOM, soil macro and micronutrient contents were analyzed in each soil layer. Pearson’s correlation (α=5%) was performed between root density, depth, SOM, soil macro and micronutrients. Contrary to our hypothesis, the results showed that the presence of litter layer did not impacted root density in the superficial layer (0-20 cm), but resulted in a substantial increase of this trait in deeper soil layers (20-40 cm and 40-60 cm). The increase in root density observed for deeper soil layers was possibly a consequence of the higher nutrient availability favored by the presence of litter layer. The increase of root density in subsurface is important not only to increase tree water and nutrient use efficiency, but also to promote C sequestration in deeper soil layers. Our results highlight the importance of maintaining the litter layer in the field as reasonable strategy for a more sustainable management of fast-growing eucalypt forests.