Seven-year straw and biochar amendments modulate soil pore structure, nutrient availability, and nitrogen partial factor productivity

Straw and biochar have shown potential to enhance soil structure, increase nutrient availability, improve crop productivity, and reduce reliance on chemical fertilizers. However, their cumulative effectiveness as partial fertilizer substitutes over extended periods remains unclear. This study evaluated rice (Oryza sativa L.) straw and its biochar as partial fertilizer substitutes on soil pore structure, nutrient supply, pH, root growth, yield, and nitrogen partial factor productivity (PFP_N-chem) in a 7-year field trial in Northeast China. The experiment included five treatments: (1) 100% chemical NPK fertilizer (NPK), (2) low-dose biochar (LB: 1.5 Mg ha⁻¹ year⁻¹), (3) high-dose biochar (HB: 3.0 Mg ha⁻¹ year⁻¹), (4) low-dose straw (LS: 4.5 Mg ha⁻¹ year⁻¹), and (5) high-dose straw (HS: 9.0 Mg ha⁻¹ year⁻¹). Chemical NPK application rates in the straw and biochar treatments were adjusted to maintain equivalent total nutrient level. After 7 years, both LB and LS attained average rice yields (LB: 6.7; LS: 7.6 Mg ha⁻¹) similar to NPK (7.3 Mg ha⁻¹), though initial yields were lower than NPK. This parity resulted from enhanced macroporosity and pore connectivity, which promoted root growth to compensate for reduced nitrogen availability. Specifically, LS exhibited 42.4% greater macroporosity (100–500 µm), 19.3% longer roots, and 54.8% higher root biomass than LB, yielding superior PFP_N-chem (+27.3%) with a 16% chemical N fertilizer reduction. However, high doses (HB/HS) led to average yield declines (22.8% and 13.1% lower than NPK). These findings highlight the potential of low-dose straw and biochar as sustainable strategies for improving soil quality and reducing fertilizer dependency.