Soil incorporation of digestate represents a common practice to dispose the solid residues from biogas producingplants. Although the digestate constitutes a residual biomass rich in partially decomposed organic matter andnutrients, whose content is often highly variable and unbalanced, its potential fertilizer value can vary considerablydepending on the recipient soil properties. The aim of the work was to assess short-term changes in the fertilitystatus of two contrasting agricultural soils in Southern Italy (Calabria), olive grove on a clay acid soil (TypicHapludalfs) and citrus grove on a sandy loam slightly calcareous soil (Typic Xerofluvents), respectively locatedalong the Tyrrhenian or the Ionian coast. An amount of 30 t ha-1 digestate was incorporated into the soil byploughing. Unamended tilled soil was used as control. The following soil physical, chemical and biochemicalvariables were monitored during the experimental period: aggregate stability, pH, electrical conductivity, organicC, total N, Olsen-P, N-NH4+, N-NO3-, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN)and the mineralization quotient (qM). Moreover, in the olive grove soil CO2 emissions have been continuouslymeasured at field scale for 5 months after digestate incorporation. Digestate application in both site exerted asignificant positive effect on soil aggregate stability with a greater increase in clay than in sandy loam soil. Overthe experimental period, digestate considerably affected the nutrient availability, namely Olsen-P, N-NH4+, NNO3-,along with the electrical conductivity. The soil type increased significantly the soil N-NH4+ content, whichwas always higher in the olive than in citrus grove soil. N-NO3- content was markedly increased soon after theorganic amendment, followed by a seasonal decline more evident in the sandy loam soil. Moreover, soil propertiesas CaCO3 content and the pH selectively affected the Olsen-P dynamics. No appreciable variation was recordedin total C and N pools. Interestingly, amendment with digestate altered the soil microbial community size in bothsoils as MBC and MBN were increased, although the response was more evident in the clay soil (olive) than in thesandy loam (citrus) one. The considerably higher qM observed in the clay soil suggests that the C mineralizationwas selectively stimulated in this soil. This finding was confirmed by the increase of CO2 emissions. As a wholeour results show that digestate application selectively stimulated soil C dynamics and determined an unbalancednutrient release, strongly depending on the soil physical-chemical properties. The use of digestate can thereforerepresent an interesting strategy for managing the soil fertility in Mediterranean agroecosystem soils, provided thatdigestate and recipient soil properties are carefully taken into account.
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