黑土累积磷的释放动力学特征及主要影响因素

  【目的】  长期大量施用磷肥致使农田土壤磷快速累积，磷肥增产效率和利用率下降。研究不同施肥措施下土壤累积磷的释放动力学特征及其机制，为累积磷的活化利用和磷肥高效施用提供理论依据。  【方法】  黑土长期定位试验位于黑龙江省哈尔滨市，在5个不同处理采集土样，分别标记为P1、P2、P3、P4和P5，其Olsen-P含量依次为15.5、20.2、93.2、199和216 mg/kg。采用0.5 mol/L NaHCO₃连续浸提法测定5个土壤样品中有效磷的释放量，以5个动力学方程拟合或描述有效磷的释放动力学。采用Hedley法测定了浸提前后土壤活性磷(Resin-P、NaHCO₃-P_i、NaHCO₃-P_o)、中活性磷(NaOH-P_i、NaOH-P_o、DilHCl-P)和稳性磷(ConcHCl-P_i、ConcHCl-P_o、Residual-P)含量。  【结果】  连续浸提过程中，供试5个土壤的Olsen-P释放均呈先快后慢的趋势，在浸提1400 min (10次)时基本达到平衡。以米氏方程(Michaelis)、一级方程、Elovich方程、抛物线方程和幂函数方程描述有效磷的释放动力学特征，拟合方程的决定系数(R2)均在0.964及以上(P<0.01)，其中又以米氏方程和一级方程为佳，其R2分别为0.9955和0.9922。由米氏方程可知，供试5个土壤的有效磷最大释放量分别为59.8、60.9、332.6、589.7和717.0 mg/kg，随Olsen-P含量的增加而增加，Olsen-P每增加1 mg/kg，磷释放量平均增加3.12 mg/kg；5个土壤有效磷的最大释放量占土壤全磷的比例分别为13.68%、14.86%、45.44%、60.67%和66.45%，也随Olsen-P含量的增加而增加，但土壤浸提残留磷(全磷与可提取有效磷之差)均为373.99 mg/kg左右，说明施磷对土壤残留磷的影响较小，长期施肥累积的磷具有较高的活性。一级方程和Elovich方程表明，黑土有效磷的释放是以扩散为主的动力学过程。连续浸提后，土壤活性磷平均减少了82.9% (67.8%～93.5%)，中活性磷平均减少了34.6% (17.2%～52.0%)，稳定性磷无显著变化。活性磷中的Resin-P和NaHCO₃-P_i显著降低了89.8% (78.5%～96.0%)和84.5% (72.9% ～91.3%)，这表明Resin-P和NaHCO₃-P_i的有效性最高。土壤活性磷库、中稳态磷库、磷释放的动力学方程参数与土壤有机质、Olsen-P、全磷呈显著正相关，与pH呈显著负相关，说明土壤累积磷的释放过程主要受到土壤活性磷、中活性磷、土壤有机质、pH和全磷含量的影响。  【结论】  黑土固定的残留磷量受施肥的影响较小，施肥累积的磷大部分可以释放，尤其Resin-P和NaHCO₃-P_i的有效性最高。黑土中磷的释放符合米氏方程和一级方程，磷释放量随Olsen-P水平的增加而增加。土壤有机质、pH和全磷含量是磷释放过程的主要影响因素。

Release kinetics characteristics and the main drivers of phosphorous accumulated in black soil

  【Objectives】  Long-term phosphate (P) fertilization leads to P accumulation in soil, decreasing yield and P fertilizer use efficiency. We studied the release kinetic characteristics of the accumulated P to provide a scientific basis for P fertilizer management.  【Methods】  The long-term fertilization experiment was located in Harbin, Heilongjiang Province. Soil samples from different treatments were marked P1, P2, P3, P4, and P5, corresponding the soil Olsen-P contents were 15.5, 20.2, 93.2, 199, and 216 mg/kg, respectively. The five soil samples were extracted continuously with 0.5 mol/L NaHCO₃ for the determination of the release of available P, and the release dynamics were fitted or described using different kinetic equations. The soil labile P fraction (Resin-P, NaHCO₃-P_i, NaHCO₃-P_o), moderate labile P fraction (NaOH-P_i, NaOH-P_o, DilHCl-P), and stable P fraction (ConcHCl-P_i, ConcHCl-P_o, Residual-P) before and after the extraction were analyzed using the Hedley method.  【Results】  In all the five treatments, the available P was released fast at beginning and then slowed down, and reached equilibrium after 10 repeated extraction in 1400 minutes. The dynamics of available P release were illustrated by Michaelis equation, first-order equation, Elovich equation, parabola equation and power function equation, with the coefficient of determination (R2) above 0.964. Michaelis and first-order equation were preferable as the R2 values were 0.9955 and 0.9922, respectively. From the Michaelis equation, the maximum available P releases of the five treatments were 59.8, 60.9, 332.6, 589.7, and 717.0 mg/kg, every 1 mg/kg increase of Olsen-P produced 3.12 mg/kg more P release. The maximum release proportion of soil total P was increased with the Olsen-P content which were 13.68%, 14.86%, 45.44%, 60.67%, and 66.45% for the five treatments, respectively, however, the difference between total P and extractable P in treatments was about 373.99 mg/kg, indicating that the stability of extractive residual P in soil. The first-order equation and the Elovich equation showed that the release of available P was a kinetic process dominated by diffusion. After continuous extraction, soil labile P fraction decreased by 82.9%, moderate labile P fraction decreased by 34.6%, and stable P fraction did not change significantly. In the labile P fraction, Resin-P and NaHCO₃-P_i decreased by 89.8% and 84.5%, respectively. The kinetic equation parameters of soil active P pool, stable P pool and available P release were significantly and positively correlated with organic matter, Olsen-P and total P while negatively correlated with pH, indicating that the release process of soil accumulated P was mainly affected by soil labile P, moderate labile P, soil organic matter, pH, and total P content.  【Conclusions】  The stable P in black soil was less affected by fertilization, most of the accumulated P was released, with the highest release proportion in fraction of Resin-P and NaHCO₃-P_i. The release dynamics of P fitted well by Michaelis equation and first-order equation, and the release amount of P increased with the increase of Olsen-P content in black soil. The main factors driving the release process are soil organic matter, pH, and total P content.