Influence of solid vs. liquid digestate application on soil organic matter accrual and stabilization, and microbial communities evolution
Authors: Sinatra Martina , Giannetta Beatrice , Galluzzi Giorgio , Tondello Alessandra , Stevanato Piergiorgio , Squartini Andrea , Zaccone Claudio 
Published: February 2023

The decline of soil organic matter (SOM) as well as the depletion of micro and macronutrients are among the most serious threats facing many agricultural soils of the world. Consequently, in many countries, soil amendments are increasingly originating from a wide range of organic wastes, as a win-win strategy to mitigate global warming while increasing soil fertility. Digestate, a by-product of the anaerobic digestion generally characterized by both high biological stability and content of nutrients (including nitrogen &#8211; N &#8211;), is often used as a fertilizer and/or organic amendment. However, a safe and functional application of digestate in agriculture requires an in-depth understanding of its possible impacts on the native SOM, on carbon sequestration mechanisms, as well as on soil microbial communities.The aim of this work was to test the influence of solid vs. liquid digestates on both SOM accrual and the distribution of SOM in particulate organic matter (POM) and mineral associated organic matter (MAOM) pools.The experiment was set up as a randomized complete block design with two factors, namely solid and liquid digestate, with 3 replicates. A control treatment (no digestate), two depths (0-15 and 15-30 cm) and a digestate application at a rate of 340 kg of potentially available N ha-1 yr-1 were considered. Soil samples were collected few days following the amendment (t0), and after 3 (t3) and 6 (t6) months. All samples (54) were characterized for pH, EC, bulk density, total organic C (TOC), total N (TN), texture and major and trace elements. Moreover, the abundance of key genes related to the N cycle was assessed by quantitative polymerase chain reaction (PCR). Finally, POM and MAOM fractions were isolated and characterized by elemental analysis (CHNS).Preliminary data suggest that, after 6 months from the amendment, soil organic C (SOC) stock in the first 15 cm of depth was not affected by digestate application, whereas a significant influence was observed for TN, especially when liquid digestate was added. On the opposite, the application of digestate had a significant influence on both SOC and TN stock at 15-30 cm of depth. Moreover, while the distribution of SOC between MAOM and POM was exclusively affected by the application of digestate, independently of depth, the distribution of TN between SOM pools was influenced by both the treatment and the time. In particular, the time factor seems to play an important role on TN concentration in the MAOM. As regards the abundance of bacterial genes, the solid digestate conferred an increase to those for nitrification (amoA), denitrification (nirK, nosZ) and total bacteria (16S), particularly at 15-30 cm of depth. The effect reached its maximum after three months from the application and decreased near control levels after six months.&#160;AcknowledgementsSM and CZ thank Cattolica Agricola soc. agr. a r.l. and Iniziative Biometano s.p.a. for allowing the experimental activity.