Effect of compost application rate on carbon degradation and retention in soils

We investigated the effect of a single compost application at two rates (50 and 85 Mg ha-1) on carbon (C) degradation and retention in an agricultural soil cropped with maize after 150 d. We used both C mass balance and soil respiration data to trace the fate of compost C. Our results indicated that compost C accumulated in the soil after 150 d was 4.24 Mg ha-1 and 6.82 Mg C ha-1 for 50 and 85 Mg ha-1 compost rate, respectively. Compost C was sequestered at the rate of 623 and 617 g C kg-1 compost TOC for 50 and 85 Mg ha-1 compost dose, respectively. These results point to a linear response between dose of application and both C degradation and retention. The amount of C sequestered was similar to the total recalcitrant C content of compost, which was 586 g C kg-1 compost TOC, indicating that, probably, during the short experiment, the labile C pool of compost (414 g C kg-1 of compost TOC) was completely degraded. Soil respiration measured at different times during the crop growth cycle was stable for soils amended with compost (CO2 flux of 0.96 ± 0.11 g CO2 m-2 h-1 and 1.07 ± 0.10 g CO2 m-2 h-1, respectively, for 50 and 85 Mg ha-1), whereas it increased in the control. The CO2 flux due to compost degradation only, though not statistically significant, was always greatest for the highest compost doses applied (0.22 ± 0.40 g CO2 m-2 h-1 and 0.33 ± 0.25 g CO2 m-2 h-1 for the 50 and 85 Mg ha-1 compost dose, respectively). This seems to confirm the highest C degradation for the 85 Mg ha-1 compost dose as a consequence of the presence of more labile C. Unlike other studies, the results show a slight increase in the fraction of carbon retained with the increase in compost application rate. This could be due to the highly stable state of the compost prior to application, although it could also be due to sampling uncertainty. Further investigations are needed to better explain how the compost application rate affects carbon sequestration, and how characterization into labile and recalcitrant C can predict the amount of C sequestered in the soil.