Short-term effects of biochar and compost on soil microbial community, C and N cycling, and lettuce (Lactuca sativa L.) yield in a Mediterranean environment

Despite the use of biochar as a soil amendment has been widely studied as a promising strategy for climate change mitigation, to date, due to the extremely heterogeneous nature of this material, there is not a general agreement of the scientific community about its soil amendment potential. In the present paper, a comprehensive approach has been adopted to test the short-term effects of biochar alone or in combination with compost on soil biochemical properties related to C and N cycling functions, microbial community, and lettuce (Lactuca sativa L.) yield in a Mediterranean environment. With this purpose, a randomized block field trial was set up within an experimental area of about 500 m2 . The four treatments tested were: 1) Control (C), no treatment; 2) Compost (CMP); 3) Biochar (BIO); 4) Compost-Biochar mixture (CMP + BIO in the ratio 9:1 w/w). After 45 days of cultivation, the biomass of well-developed lettuce plants was increased from 2- to 2.5-fold in all amended soils with respect to the control one. The CMP amended soils showed a higher effect with respect to the BIO amended soils on enzymatic activities (increase in β-glucosidase, SEI, synthetic enzymatic index, and butyrate esterase) and microbial community structure evaluated by the Ester-linked Fatty Acid Methyl Ester (El-FAME) profiling (increase in G- bacteria, total fungi, and AMF). The increase in the activity and abundance of the microbial community in CMP amended soils could indicate that the organic matter provided by the compost is more readily mineralized than that added with the biochar. Moreover, the BIO amended soils showed the lowest qCO2 and ecoenzymatic C:N ratio and the highest qMIC (1.4 %), indicating a C immobilization process into the microbial biomass and a greater limitation for N with respect to C, offset by an increase in N-cycling enzyme activity. In the short term, even if the number of well-developed plants increased in all treated soils, a boost of soil C and N cycling was evident with CMP and BIO amendments respectively. Conversely, the combination of CMP with BIO (CMP + BIO) did not provide any synergic effect, probably because requiring longer periods. For these reasons repeated combined treatments can be suggested to reach a cumulative effect on soil quality over time using a balanced C:N ratio in the mixture amendment.