Synergistic and antagonistic interactions among organic amendments of contrasted stability, nutrient availability and soil organic matter in the regulation of C mineralisation

• The application of unstabilised manures can synergistically increase soil microbial activity.
• Nutrient addition in N poor soils can increase or decrease microbial activity depending on the abundance of labile C.
• Nutrient addition can increase soil organic C retention in N rich soils while it depletes soil C in N poor soils.
• The addition of stabilised organic matter reduces soil microbial activity irrespective of nutrient availability.

Organic amendments to agricultural soils may increase soil organic matter reserves. However, manure amendment may stimulate soil microbial activity and soil organic matter decomposition. The stability of manure and soil organic matter and the availability of nutrients are crucial for the regulation of soil organic matter decomposition. Therefore, we aimed to study the effects of adding stabilised or unstabilised manures combined with mineral nutrients (N–NO3, P–PO4, K and S–SO4) on carbon mineralisation in two agricultural (sandy loam) soils with contrasted organic matter stocks and N availability. These two soils were incubated alone and after adding unstabilised farmyard manure (FYM) or stabilised manure (compost), combined with the addition of mineral nutrients. Mineralisation of C (CO2 evolution) increased after FYM addition while it decreased after compost addition as compared to the control soils. When mineral nutrients were added to soils amended with FYM, C mineralisation synergistically increased only in soils with low nitrogen availability. In these soils, the addition of nutrients may promote the mobilization of soil C, while in soils with high N availability the synergistic mobilization of C on adding FYM may be reduced. In contrast, the depressed C mineralisation after the addition of compost was little affected by nutrient availability. We suggest that recalcitrant compounds in compost may be stabilised in soil particles and humic substances with little effects of nutrient availability. In contrast, the incorporation of FYM was mainly driven by microbial transformations and affected by nutrient availability. Finally, we conclude that the application of stabilised organic matter may be a good practice for the built up of humus in a wide range of arable lands with particular significance for land reclamation in organic matter depleted soils.