Nutrients and carbon fate in two lowland contrasting soils amended with compost

The present study investigated the transient effects of compost application combined with minimum tillage followed by traditional fertilization in two contrasting agricultural soils of the Po River plain, where both nutrients leaching and carbon cycle were monitored for three years (2016–2019) via several campaigns of soil and groundwater sampling and gas emissions. The results of this work highlight that the effects of compost on nutrients leaching is transient and is soon lessened over time if new incorporations do not take place. Nutrients leaching was minimal on shallow groundwater quality with the following decreasing order: nitrate > ammonium > nitrite > phosphate. Moreover, the nutrients distribution profiles (nitrate, nitrite, ammonium and phosphate) also depended on the groundwater fluxes and on boundary conditions (geogenic sources and previous fertilizations). The soil organic carbon results highlight that its increase (approximately 1 ± 0.4% in both soils) was temporary and was not anymore discernible after three years since compost incorporation. The carbon dioxide emissions behave like the soil organic carbon, with an initial increase (approximately 66 ± 17 mM/m2/d in both soils) due to mineralization and a subsequent decrease, while no methane and hydrogen sulphide emission were detected. Moreover, carbon dioxide fluxes were also driven by the soil with nearly doubled fluxes in the site with a higher soil organic carbon baseline.