Fig. 3. The ¹³C values of CO₂ evolved from the no-plant control (thin solid lines), wheat (dotted lines), and soybean (thicker solid lines) treatments with three levels of fertilization (Zero NPK, square symbol; low NKP, circle symbol; high NPK, triangle symbol) and four sequential samplings. Error bars are two standard errors, and some error bars are too small to be seen.