Agro-forestry management of Paulownia plantations and their impact on soil biological quality: The effects of fertilization and irrigation treatments

Abstract Short-rotation plantations for biomass production are wood systems in which fast growing tree species are grown under intensive agricultural practices to achieve high biomass yields. Paulownia sp. is an extremely fast-growing tree species which is commonly used to produce biomass for energetic purposes. This tree species has a great nutrient and water demand, and the establishment of its plantations may affect soil fertility and quality. For this fact, this study is focused on the evaluation of the responses of the soil microbial community – in terms of biomass, community structure, and activity – in a Paulownia plantation submitted to a semiarid climate. Particular attention was paid to the impact of the soil fertilization (mineral fertilizer or an organic-mineral amendment, BAN, obtained from the forestry industry) and level of irrigation on the soil microbial community. For both irrigation regimes, a significant increase in TOC was observed with the addition of the BAN to the soil. The content of water soluble C in the soil was affected by both factors (fertilization and irrigation) and their interaction. Both enzyme activities related with the C and P cycles in soil and dehydrogenase activity were significantly affected by the fertilization, irrigation, and their interaction. Regardless of the irrigation level, the microbial respiration rate was highest with the BAN treatment. The content of total PLFAs and PLFA biomarkers of microbial groups showed no significant differences between the fertilization treatments. However, the total PLFAs and the Gram-positive, fungal, and saturated PLFA concentrations differed significantly between the irrigation levels. In general, the use of residues generated in the forest biomass industry (pine bark and biomass ash) as a soil amendment had a positive effect on the soil microbial activity without altering the structure of the soil microbial community. Considering the water deficit of semiarid areas, such as SE Spain, the low level of irrigation tested in this study would be enough to maintain both the soil microbial activity and suitable production of biomass by Paulownia trees.