Effect of a six-year Biochar Amendment on Water Productivity and Nitrogen Utilization of Maize and Comprehensive Soil Fertility

In the short term, biochar effectively retains water and nutrients, thereby enhancing water productivity and nitrogen (N) use efficiency, consequently increasing crop yield. Over time, however, the ability of biochar to regulate water and N may diminish, leading to changes in its mechanisms for enhancing yield. Therefore, the time-dependent effects of aged biochar on yield enhancements need to be assessed. We conducted a two-year field experiment using a split-plot design with varying periods of biochar addition as the main plots, denoted as one year (Y₁), two years (Y₂), five years (Y₅), and six years (Y₆), and three addition rates as the subplots, denoted as no biochar addition (C₀), 6 t·hm⁻² biochar (C₆), and 12 hm⁻² biochar (C₁₂). The results showed that under identical conditions, short-term biochar addition significantly outperformed medium- to long-term addition in enhancing maize yield, water productivity, N-use efficiency, and soil fertility index (SFI). There was no significant difference between the Y₆C₆ treatment and the control with no biochar addition, however high biochar addition may help mitigate this decline. Structural Equation Model (SEM) analysis demonstrated a positive correlation between increases in soil NH₄⁺-N and NO₃⁻-N content and SFI. Additionally, nitrate nitrogen (NO₃⁻-N) content positively affected water productivity. However, with extended periods of biochar addition, the effect of NO₃⁻-N on both SFI and water productivity weakens, whereas that of ammonium nitrogen (NH₄⁺- N) on SFI intensifies, influencing yield. Therefore, C₁₂ treatment not only improves yield, water productivity, and N-use efficiency but also mitigates the reduction of positive effects on crops and soil after medium- and long-term addition of biochar.