Carbon accumulates in organo-mineral complexes after long-term liquid dairy manure application

• Liquid dairy manure application increased organic carbon stocks in the top 20 cm of soil.
• Mineral fertilization led to higher organic carbon stocks in the top 5 cm of soil.
• Liquid dairy manure favored carbon incorporation into organo-mineral complexes.
• The sand-size heavy fraction was very responsive to mineral fertilization and liquid dairy manure.

Quantifying and understanding the impact of animal manure on soil organic carbon (OC) is important for agronomic and environmental purposes. The influence of liquid manure on soil OC stocks has been less studied and was found to be more variable than that of solid manure. In addition, only a few studies have analyzed the effects of animal manure on stable fractions of soil OC. Our objective was to quantify OC in the whole soil and in specific physical fractions of organic matter (free and intra-aggregate light fractions, and sand and silt + clay-size heavy fractions) within a 0–50 cm soil profile after 17 years of applications of liquid dairy manure (LDM) and mineral fertilizer on a perennial grass sward. The mineral fertilizer and LDM were applied at nominally 200 and 400 kg mineral-N ha−1 year−1 (low and high rates, respectively) to tall fescue (Festuca arundinacea Schreb. var. Festorina), grown on a Monroe silt in a maritime climate near Agassiz, British Columbia, Canada. The LDM applications provided an average of 3.8 and 7.3 Mg C ha−1 year−1 for the low and high rates, respectively. The whole-soil OC stock was significantly higher in the top 20 cm of soils amended with LDM than either soils with mineral fertilizer or unamended soils. There were no differences in OC below 20 cm, which may be linked to the absence of C transfer at depth, or to an insufficient C input to offset a possible LDM-induced priming effect on soil OC mineralization. Compared to the unamended soil, mineral fertilization led to higher OC stocks in only the top 5 cm of soil. The application of LDM favored the incorporation of C into organo-mineral complexes rather than into the free light fraction of soil organic matter. The size separation of the heavy (density > 1.8 g mL−1) soil fraction revealed the presence of sand-size organo-mineral complexes that were responsive to treatments, and particularly to LDM application. This sand-size heavy fraction could be given greater scrutiny to detect more finely the effect of management changes on soil OC stocks.