Original ArticleSoil CO2 emission partitioning, bacterial community profile and gene expression of Nitrosomonas spp. and Nitrobacter spp. of a sandy soil amended with biochar and compost

- Compost affected soil respiration partitioning and increased soil N.
- Compost promoted bacterial diversity and nitrification-involved key gene expression.
- Biochar did not alter soil respiration partitioning and fertility.
- Biochar and compost synergistically increased soil organic-C derived respiration.
- Biocompost stimulated key gene expression of Nitrosomonas and Nitrobacter spp.

We performed a 18-month experiment on 0.496 m3 pots filled with a sandy soil in which 1-year old nectarine trees were grown. With four replicates, the following amendment strategies were compared: a) unamended control; b) biochar (20 g fw kg−1); c) compost (76.8 g fw kg−1) and d) biocompost (mixing biochar and compost at the same rates of the previous two strategies). Amendments were applied at planting and only unamended and merely biochar-amended soils were fertilized with mineral inputs. Soil mineral N availability was regularly measured and at the end of the experiment, we determined total C and N concentration of soils and aged biochar fragments. Total soil respiration (RTOT) was separated into soil organic-C derived (RSOC) and rhizosphere (RR) respiration by the trenching method. Total soil DNA was extracted from samples collected after 6 and 18 months and bacterial community analysis was carried out by denaturing gradient gel electrophoresis (DGGE). Expression of nitrification key genes of ammonia monooxygenase (AMO) and nitrite oxidoreductase (NOR) and the relative abundance of specific bacterial community (Nitrosomonas spp. and Nitrobacter spp.) were determined by Real Time PCR on soil samples collected at 6, 12, 15, 16 and 18 months since amendments incorporation. The addition of compost significantly promoted soil mineral N, bacterial diversity and the relative expression of nitrification process related key genes. Furthermore, compost enhanced RSOC likely due to the stimulation of the microbial community by providing labile C sources. Conversely, changes due to the mere addition of biochar were negligible. However, biochar had no detrimental effects, rather it promoted gene expression involved in the nitrification process. A synergic effect between the two amendments emerged in the total soil C and N concentration and in the RSOC, leading to a significantly higher cumulative CO2. Although the source of the additional CO2 rate remains uncertain, a priming effect induced by biochar on the labile compost-derived C-fractions is hypothesized. Compost reduced the relative richness of Arthrobacter spp. in soil, while Actinomadura flavalba, Saccharomonospora viridis, Thermosporomyces composti and Enterobacter spp. were peculiar of the biocompost profile which increased band richness. Biocompost significantly increased the relative abundance of Nitrosomonas spp. and Nitrobacter spp. and both AMO and NOR key genes expression levels. The mixture of biochar and compost seems effective to induce agronomical benefits, although environmental concerns (e.g. additional CO2 emissions) require further investigations.