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 (C-L) as well as for C-13 isotope natural abundance (delta C-13 and Delta C-13 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 d(13)C 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 delta C-13, suggesting a probable increase in relative lignin which could have been linked to carbon conservation and increased functionality.