Legacy of soil health improvement with carbon increase following one time amendment of biochar in a paddy soil – A rice farm trial.

• Rice fields of a clayey paddy without and with biochar 6 years compared in a farm. • Soil carbon increased and soil aggregation enhanced in biochar field. • Microbial growth and enzyme activities promoted greatly in biochar field. • Beneficiary by microbial community composition changes in biochar field. • Legacy of improved soil health for better production over years in biochar field. • Mechanistic link between C increase, aggregation and microbial promotion unraveled. "4 per 1000 Initiative" launched at COP21 has strongly recommended that increasing soil organic carbon (SOC) is a global imperative. However, the strategies to improve agricultural soil functioning and health by an increase in soil C have been poorly addressed. In a rice farm trail, topsoil samples were collected in 2015 respectively in a field amended with biochar (BA) (equivalent to 10 t ha−1) in 2009 and an adjacent field with no biochar (NB). Soil aggregation, biochemical activities and microbial communities of the samples were analyzed with microscopic, micro-biochemical and molecular assays respectively. Compared to NB field, SOC storage increased by 45%, total and available nitrogen pool enhanced by approximately 30% while the bulk density decreased and soil pH unchanged in BA field. A 25% increase in mean weight diameter of water stable aggregates was observed in the BA compared to NB field. Correspondingly, microbial biomass nitrogen and enzyme activities in BA field was enhanced both by approximately 30% compared to NB field. Furthermore, with community structure altered, a moderate (32%) increase in total bacterial abundance and a significant decrease in microbial abundance of amino acid metabolism and fungal pathotrophs were observed in BA field compared to NB field. This could link to the higher (10%) grain yield with lower yield inter-annual variability in BA field compared to NB field, reported previously. All these changes clearly demonstrated a legacy of paddy soil health improvement over years following one time biochar amendment. Overall, addition of biochar to the clayey nutrient rich paddy soil could sustain an increase in SOC, soil aggregation and soil health functioning, with positive changes in microbial community over years. Thus, carbon increase as per required by the "4 per 1000 Initiative" could be a mechanistic driver to enhance soil fertility and improve soil-(plant) health for ensuring food production in world rice agriculture facing land degradation and climate change impacts, which could be assisted by biochar from crop residue in an approach of circular economy. [ABSTRACT FROM AUTHOR]