Effect of Raw and Composted Anaerobic Digestates from Sewage Sludge and Biowaste on Ryegrass Phosphorous Availability and Soil Carbon Management

Purpose: Organic waste (OW), such as anaerobic digestates and compost, can provide nitrogen (N) and valuable crop available phosphorous (P) and is a potential alternative to chemical P fertilizers, which is a non-renewable resource. However, little is known about the short- to medium-term P supply capacity of OW within the framework of optimal soil organic carbon management. Methods: In this study, four products were investigated: a sewage sludge digestate (D1) and biowaste digestate (D2), in addition to their respective composts (C1 and C2). These products were compared at 170 kg N ha-1 along with a chemical fertilizer (Chem) and digestate from animal manure (DM) in two successive tests on potted ryegrass (84 + 84 days), without (N-) and with (N+) mineral N supply (170 kg N ha-1). Their agronomic performance and apparent bioavailable P (ABP mg pot-1) were assessed. In addition, the variation in the total organic carbon (TOC), labile carbon (CL) and carbon management index (CMI) was monitored in potting soil at the end of the two growth cycles. Results: The pot test showed that after 84 days, Chem mostly exploited its ABP (95%), similar to DM, resulting in a low P inheritance over 168 days, while the compared products (D1; C1; D2; C2) ensured long-lasting P release. CL was more sensitive than Corg to changes occurring during the test, thus proving that biowaste products, especially compost (C2), gained the best CMI. Conclusions: This promising approach may provide comprehensive insight into rational P fertilization via OW soil distribution and can provide important information on its effect on overall soil carbon management, a currently notable issue, especially in the Mediterranean region.