Contrasting Effects of Straw and Straw-Derived Biochar Amendments on Soil Aggregates and Phosphorus Uptake in a Rice–Rice Cropping System

The long-term effects of straw and biochar amendments on soil aggregation, C:N:P stoichiometry of soil aggregates, and rice phosphorus (P) uptake in double-rice systems remain unclear. We conducted a seven-year double-rice field experiment comparing mineral fertilization only (NPK), low and high rice straw incorporation (LS and HS; 3 and 6 t ha−1 per season, respectively), and two wheat straw-derived biochar applications (LB and HB; 24 and 48 t ha−1, respectively). Biochar was applied once at the beginning of the experiment, whereas straw was incorporated after each rice season. HS increased mean weight diameter (MWD), whereas both HS and HB increased the proportion of >2 mm aggregates, with the increase under HB not accompanied by higher MWD. Biochar, particularly HB, increased C:P and C:N ratios, whereas N:P ratios differed among aggregate size fractions depending on the amendments. LS and HS increased late-rice straw P uptake relative to NPK, whereas HB decreased it. The N:P ratio in the 1–2 mm aggregate fraction was positively associated with straw, grain P concentrations, and straw P uptake, whereas the N:P ratio in the 0.053–<0.25 mm fraction was negatively associated with straw P uptake and MWD. Forward selection in Redundancy analysis (RDA) identified degree of P saturation (DPS) and average aggregate N:P ratio as significant explanatory variables for rice P concentrations and uptake. Overall, these findings highlight the importance of aggregate-size-specific N:P ratios for understanding rice P uptake responses to straw and biochar amendments.