Biochar but not earthworms enhances rice growth through increased protein turnover

The aim of this work was to compare the effects of biochar and earthworms on rice growth and to investigate the possible interactions between both. In addition to classic macroscopic variables we also monitored some leaf-level cellular processes involved in protein turnover. Both biochar and earthworms significantly increased shoot biomass production. However, biochar had a higher effect on the number of leaves (+87%) and earthworms on leaf area (+89%). Biochar also significantly increased the leaf turnover. At the cellular level, biochar but not earthworms enhanced protein catabolism by an increase in leaf proteolytic activities. This could be related to the increased expression of three of the six genes tested related to protein catabolism, one serine protease gene OsSP2 (+24%), one aspartic acid protease gene, Oryzasin (+162%) and one cysteine protease gene OsCatB (+257%). Furthermore, biochar also enhanced the expression level of two genes linked to protein anabolism, coding for the small and large subunits of rubisco (+33% and +30%, for rbcS and rbcL, respectively), the most abundant protein in leaves. In conclusion, our data gives evidence that biochar increased rice biomass production through increased leaf protein turnover (both catabolism and anabolism) whereas earthworms also increased rice biomass production but not through changes in the rate of protein turnover. We hypothesize that earthworms increase nitrogen uptake at a low cost for the plant through a simultaneous increase in mineralization rate and root biomass, probably through the release in the soil of plant growth factors. This could allow plants to accumulate more biomass without an increase in nitrogen metabolism at the leaf level, and without having to support the consecutive energy cost that must bear plants in the biochar treatment.