Influence of biochar and compost and their interaction on carbon and nitrogen mineralization in a Saharan sandy soil

Biochar and compost co-application in agricultural soils is of growing interest due to their complementary nature, combining high carbon (C) sequestration potential and nutrient inputs. An organically depleted sandy soil sampled in an oasis in southern Tunisia was selected to evaluate the impact of date palm-based biochar and compost as soil amendments and to assess the biodegradability of these organic products sole or in mixtures. Carbon and Nitrogen (N) mineralization dynamics in soils amended or not with biochar and/or compost were monitored during 112 days. Soil samples were taken at the start and after 56 and 112 days of incubation for soil organic matter quantification and characterization using Rock-Eval® thermal analysis.

The apparent mineralized organic carbon fraction of the compost was ≈16.7 % after 112 days. The mineralization rate constants of the labile fractions of sole compost and biochar were 0.012 and 0.037 day⁻¹ respectively following model fittings on C mineralization data. The proportion of their recalcitrant fractions were 77 and 98 % of initial C for compost and biochar, respectively. The comparison of model fits obtained for products alone vs mixtures showed no differences, suggesting no significant interaction between biochar and compost in mixtures on C mineralization. However, in the soil amended with pre-incubated biochar-compost mixtures, lower cumulated CO₂ emissions were found compared with the same products inputs applied individually. The nitrogen supply of the compost was estimated at 53 ± 28 kg N.ha⁻¹ in the short-term in the soil amended with ≈77 t.ha⁻¹ of date palm compost. Biochar pre-incubated with compost increased the soil N supply to 106 ± 45 kg N.ha⁻¹ compared with the unamended soil.

The low thermal stability of the initial pure compost highlighted the low stabilization of the organic matter during the composting process. The high salt content of the compost does not seem to inhibit organic matter decomposition in a saline non-sodic soil. However, improving the composting process with date palm residues is needed to optimise its carbon stability, and to avoid salinization of the soil and water in arid areas.