Changes in soil humic pools after soil application of two-phase olive mill waste compost

The use of appropriate amendments derived from two phase olive mill wastes (TPOMW) can represent a suitable option to maintain and restore C levels in agricultural soils under Mediterranean climates. We evaluated soil organic matter stabilisation pathways among different humic pools in a Calcaric Cambisol amended with 2% (40 Mg ha− 1) of TPOMW composting mixtures of different composition and at different degrees of stabilisation: starting mixture, after 14 weeks of composting (thermophilic stage) and after 30 weeks (mature compost). Non-humified soil organic C and two different fractions of humic acids (HA), namely free HA (biochemically stabilised) and bound HA (biochemically and chemically stabilised) were obtained by sequential extraction with NaOH and alkaline Na4P2O7 after 90 and 150-days incubation. HA were characterised by thermal analysis, size exclusion chromatography (HPLC-SEC), FTIR and 13C CPMAS-TOSS NMR. Amendments promoted incorporation of altered lignin structures, carbohydrate moieties and N-containing compounds into free HA and to a lesser extent into the bound HA, and increased the proportion of high MW fractions. There was an average increase of 40% for non-humic C in the free C fraction even after 90 days of incubation under optimum conditions for mineralisation. Augmentation of bound C resulted in an average increase of about 0.7 Mg ha− 1 of humic C in amended soils. This increase is important as it contributes to one of the more inert soil C pools and could represent a useful indicator for soil C stabilisation.

► Soil stabilisation pathways of olive mill wastes were monitored by sequential extraction of soil humic fractions.
► The lignocellulosic nature of TPOMW promoted the incorporation of lignin and carbohydrate moieties into the pre-existing soil HA.
► The composition and stabilisation degree of the amendments governed its biochemical stabilisation (free HA C).
► The humic C bound to soil minerals (chemically stabilised) resulted in an average increase of about 0.7 Mg ha− 1.