Torrefied biomass effects on soil organic matter dynamics in a soil-plant system

The return of thermally converted crop residues by torrefaction (biotorr) or pyrolysis (biochar) may alter soil organic matter (SOM) composition and therefore influence its turnover with effects on (i) nutrient availability and (ii) carbon retention. We evaluated these effects in a four-month greenhouse pot experiment using an alkaline silty soil amended with raw rapeseed straw (RS), torrefied straw at 230 °C (BT230) and 280 °C (BT280) and biochar at 400°C (B400) at a rate of 10t ha^-1. Untreated soil was included as a control (CON). We monitored nitrogen, phosphorus and potassium uptake by plant shoots monthly and soil solution pH and dissolved organic carbon (DOC) biweekly. At the end of the experiment, we assessed microbial functional diversity with Biolog EcoPlates^TM. Our results indicated that nutrient uptake converged across treatments within four months, although BT230 supported the highest nitrogen uptake, compared to CON. BT280 sustained elevated DOC beyond the initial peak, whereas B400 released minimal DOC but increased soil organic carbon and soil C/N, indicating enhanced carbon retention. Microbial functional diversity at the end of the experiment did not differ significantly among treatments. These findings suggest that biotorr can supply soluble carbon and support SOM turnover without compromising nutrient dynamics, whereas biochar primarily promotes SOM stabilization and short-term carbon retention. Understanding these mechanisms is essential for designing SOM-based crop residue management strategies that sustain fertility while enhancing climate resilience.