2.2 Experimental design and soil microcosm incubation

The air dried soil was adjusted to 60% of the water hold capacity (i.e. 18%) and sieved (mesh size 4 mm) prior to soil microcosm incubations, according to the AFNOR standard (AFNOR XP U 44–163). Each soil microcosm contained the equivalent of 25 g of dry soil placed in hermetically closed 1 L glass jars to allow the CO₂ produced to accumulate in the headspace; the air of the headspace was entirely renewed each time a measurement was taken (once a week). The soil microcosms were subjected to three different treatments each in three replicates: i) soil alone as a control (CS), ii) soil mixed with 120 mg of straw (SS) simulating 4.8 kg of wheat residues per m² and corresponding to an input of 50 mg of orgC and 0.94 mg of total N, and iii) soil mixed with 120 mg of straw and the BS (SBS). This last treatment was prepared by applying 100 mg of the BS to 25 g of straw, and 120 mg of this mixture was then incorporated in the soil. In the SBS treatment, the additional orgC and total N amounts due to the BS input were negligible (100 μg and 10 μg, respectively) compared to both the orgC and total N amounts due to the straw input and the initial contents of the soil (245 mg orgC and 22.5 mg total N per microcosm, respectively). The initial raw materials (soil, straw and BS at t = 0) and the three soil microcosm treatments (CS, SS and SBS), each performed in three replicates, were characterized for their soil OM, orgC and total N contents, as well as for their microbial biomass and pH_water (Table 2). Soil microcosms were equipped with a NaOH trap to collect the evolved CO₂ and incubated in the dark for 49 days at 28°C. Soil incubations were performed for seven weeks (49 days). After 3, 7, 14, 21, 28, 42 and 49 days of incubation, the CO₂ trapped in 10 ml of 0.5 N NaOH was quantified by titration with 0.1 N HCl according to the AFNOR standard (AFNOR XP U 44–163) and the soil moisture was maintained by replacing the weight loss with sterile water. At the end of incubation, the pH of the soil microcosms were measured in water with a pH meter using a soil-to-water ratio of 1:5 (Table 2) and the soil samples were stored at -20°C for further chemical and microbial analyses. The soil microbial communities were analyzed after seven weeks of incubation, when the communities of microbial decomposers were presumed to be well established and stable in each soil treatment.

The whole measurements are expressed on a soil dry weight basis (d.w).

*Negatives values for the microbial biomass carbon were obtained for BS and straw and are explained in the materials and methods section (2.2). ND = Not Determined.