淹水对石灰性土壤无机磷形态转化的影响

  【目的】  磷肥施入土壤后大部分转化为与铁氧化物关系密切的Fe-P和O-P，而淹水后土壤中铁的氧化还原过程可能影响与铁氧化物结合的磷的形态及有效性的变化。研究不同施磷处理下淹水土壤Fe (II) 、无机磷组分等的变化，以期明确淹水后土壤无机磷形态及磷有效性变化及其与铁氧化还原过程的关系。  【方法】  用不施磷土壤 (P0) 和连续6年施用P 180 kg/hm2的土壤 (P180) 进行室内模拟培养试验。将土壤装于西林瓶内，加水模拟淹水条件，西林瓶密封后，分别在避光或者光照条件下，于 (30 ± 1)℃恒温培养40天。测定供试土壤以及淹水培养土壤中的速效磷、无机磷以及不同形态无机磷组分含量，测定培养过程Fe (II) 的动态变化，以探讨磷形态转化与铁氧化还原过程的关系。  【结果】  施用磷肥显著增加土壤中的速效磷含量和无机磷总量，P0处理土壤速效磷含量为 (7.65 ± 1.65) mg/kg，P180处理土壤速效磷含量高达 (33.5 ± 2.01) mg/kg。施入土壤中的磷只有很小部分以Ca₂-P存在，主要以Ca₁₀-P、Ca₈-P、Al-P和Fe-P形态存在。避光淹水培养后，土壤速效磷含量增加，P0和P180处理土壤速效磷含量的增量分别为8.44、2.95 mg/kg。淹水培养降低了土壤Ca₈-P含量，提升了Fe-P、O-P、Al-P含量。光照和避光条件下P180处理土壤中Ca₈-P含量分别降低106.8、156.2 mg/kg，Fe-P含量分别增加23.4、47.0 mg/kg，O-P含量分别增加64.1、92.9 mg/kg，Al-P含量分别增加38.8、34.7 mg/kg，避光时Ca₈-P降幅以及Fe-P和O-P的增量均大于光照条件下。避光条件下，铁还原量和还原最大速率与Ca₈-P变化量之间存在显著负相关关系，与Fe-P、O-P增量之间存在显著正相关关系。  【结论】  淹水条件下，石灰性土壤中的Fe (Ⅲ) 还原形成Fe (Ⅱ) 和Fe (Ⅲ) 混合物，增加了铁氧化物的比表面积和磷吸附点，可促进Ca₈-P向O-P、Fe-P和Al-P转化。光照降低了Fe (Ⅲ) 的还原量，可能是Ca₈-P向O-P、Fe-P和Al-P转化率低的原因之一。

Effects of flooding on transformation of inorganic phosphorus fraction in calcareous soils

  【Objectives】  When applied to soil, phosphorus (P) is easily converted to Fe-P and O-P, which might be affected by the redox process of iron oxide. Here, we studied the valent state and form of iron under flooding condition, and its relationship with the transformation of inorganic P fractions.  【Methods】  A slurry incubation experiment was employed to simulate flooding condition in the laboratory, using calcareous soil receiving 0 and 78.6 kg/hm2 of P for 6 years (P0, P180). Soil samples were loaded into silling vials, sealed and incubated for 40 days at (30 ± 1)℃ under illumination and dark conditions. Available P and inorganic P fractions were measured before and after incubation. Soil Fe (Ⅱ) content was monitored regularly during the incubation process. The relationship between inorganic P fractions and the iron redox process was discussed.  【Results】  Soil available P increased dramatically as a result of P fertilization. Soil available P in P0 and P180 treatments were (7.65 ± 1.65) mg/kg and (33.5 ± 2.01) mg/kg. The applied P existed mainly as Ca₁₀-P, Ca₈-P, Al-P, and Fe-P fraction, and less than 1% existed as Ca₂-P. Flooding incubation in the dark increased soil available P by 8.44 mg/kg and 2.95 mg/kg in P0 and P180 treatments. After flooding incubation, the Ca₈-P content decreased while Fe-P, O-P and Al-P increased in P180 treatment. Under illumination and dark condition, Ca₈-P decreased by 106.8 and 156.2 mg/kg, Fe-P increased by 23.4 and 47.0 mg/kg, O-P increased by 64.1 and 92.9 mg/kg, and Al-P increased by 33.8 and 34.7 mg/kg, respectively. The observed decrease in Ca₈-P under dark conditions was higher than under illumination, while Fe-P and O-P recorded a higher increase in the dark condition than under illumination. We found a significant negative correlation ( P < 0.05) between the amount and maximum velocity of Fe (Ⅱ) reduction and the variation of Ca₈-P content, and a significant negative correlation with the increased amount of Fe-P and O-P contents.  【Conclusions】  Under flooding conditions, Fe (Ⅲ) is reduced to Fe (Ⅱ), leading to the formation of Fe (Ⅱ) and Fe (Ⅲ) mixtures. Consequently, the mixtures have a larger specific surface area and more P adsorption sites, leading to the formation of O-P, Fe-P, and Al-P fractions. Illumination decreases Fe (Ⅲ) reduction, which may be responsible for the lower transformation rate of Ca₈-P fraction to O-P, Fe-P, and Al-P fractions.