The Fate of Soil Organic Carbon from Compost: A Pot Test Study Using Labile Carbon and 13c Natural Abundance

Recycled organic waste (OW) can be a valuable nutrient source for plant cultivation; however, knowledge is poor regarding its effect on soil carbon conservation, especially in the frame of organic-mineral fertilisation succession. In this study, four composts, green waste (GWC), anaerobically digested bio-waste (DC), sludge (SSC), and bio-waste (BWC), were compared (10 and 20 Mg volatile solids ha−1) in a ryegrass pot test over two growing cycles (112 + 112 days), along with an unamended control (Ctrl) and a chemical reference (Chem), with and without mineral nitrogen (N) fertilisation. At the end of the two growth cycles, the pot soil was analysed for total- (TOC) and labile-carbon (CL) as well as for 13C isotope natural abundance (δ13C and Δ13C vs. Chem). At day 112, the pot test showed that Ctrl and Chem gained poor TOC (8.48 g kg−1), lower than the compost at both 10 and 20 Mg volatile solids ha−1 (10.01 vs. 11.59 g kg−1). At day 224, a deep soil TOC depletion occurred in the pot soil treated with GWC, DC and BWC at both levels (-10 and -20). However, all the compost treatments showed more depleted soil d13C vs. the references, especially Chem, thus revealing relevant compost-derived carbon conservation. Regarding the compost treatments, the carbon management index (CMI) increased over time, indicating high soil functionality, also showing a good relationship with δ13C, suggesting a probable increase in relative lignin which could have been linked to carbon conservation and increased functionality.