Maximising carbon sequestration through mixing compost in moist soil

Abstract

Organic amendments to soil often enhance soil organic matter (SOM) decomposition, also referred to as a priming effect (PE). However, decomposition of the organic amendment itself could also be altered when mixed in soil, with important consequences for the net soil carbon (C) balance and nutrient cycling, but this has rarely been explored. We examined decomposition and gross nitrogen (N) mineralisation of compost (C:N = 29) and SOM (C:N = 12) separately and when mixed, at two different moisture levels (45 and 85% water holding capacity) in a 42-day laboratory incubation study. As observed by many others, compost addition enhanced decomposition of SOM (i.e., causing a PE) at both moisture levels. In contrast, decomposition of compost was strongly reduced when mixed with soil, particularly at the highest moisture level, which more than compensated for the PE. These results indicate that the microbial decomposer community had a greater preference for decomposing SOM than decomposing compost when mixed, possibly because decomposition of SOM released more N than decomposition of compost in meeting the microbial demand for N. This was supported by higher gross N mineralisation rates at the end of the incubation period in compost-soil mixtures compared to what would be expected from gross N mineralisation rates in their separate components. It is also possible that mineral adsorption of organic compounds derived from compost caused greater protection against microbial decomposition of compost, and thus reduced compost decomposition when mixed into soil. We conclude that mixing compost in soil could result in greater net soil C gains compared to having compost and soil separate, particularly under moist conditions.