Effects of flooding on transformation of inorganic phosphorus fraction in calcareous soils
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摘要: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。
Abstract:ObjectivesWhen 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.
MethodsA 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.
ResultsSoil 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.
ConclusionsUnder 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.
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自发现施磷可以增产以来,投入农田的磷肥逐年递增[1-2]。诚然,连续的磷肥施用在提高产量的同时也增加了土壤的全磷和速效磷含量[3]。然而,磷肥施入土壤后大部分由于吸附等化学反应而固定于土壤中[4]无法被作物利用,不仅磷肥利用率可低至15%[5],造成大量资源浪费,而且施入土壤的磷可因淋溶或径流[6-7]进入水体引起水体富营养化等严重的环境问题。
据顾益初和蒋柏藩对石灰性土壤无机磷形态的分级方法,将土壤中无机磷分为Ca2-P、Ca8-P、Al-P、Fe-P、O-P和Ca10-P等形态[8]。我国北方石灰性土壤的无机磷主要以Ca-P为主[9],其中Ca2-P易被作物吸收,是作物的最有效磷源[10]。连续施磷肥后Ca2-P显著增加[3],但增加磷肥用量也显著增加了土壤中与Fe氧化物关系密切的Fe-P和O-P含量[11],并且在长期施磷条件下积累的无机磷转化为Fe-P和O-P的比例大于Ca2-P[12]。部分低磷土壤Fe-P仅占无机磷总量的3.2%~3.6%[13],但亦有研究显示Fe-P可占无机磷总量的19.4%~31.0%[14]。O-P也与Fe氧化物有关,可占土壤的14.3%~16.0%[13]。因此,提高与土壤中Fe氧化物相关的无机磷组分的转化过程就成为提高土壤潜在磷库周转效率的关键之一。
Fe不仅是土壤中常见且丰度最高的易发生氧化还原的过渡金属元素,而且Fe的还原可引起Fe氧化物的还原溶解[15-16]。因此,土壤中Fe的氧化还原可能影响Fe-P乃至O-P的形成与转化。一方面,研究认为淹水环境中Fe氧化物尤其是无定形和弱晶形Fe氧化物的异化还原可促进Fe-P的释放而增加磷有效性[17-19];另一方面,亦有研究认为淹水后土壤中的结晶态Fe氧化物含量明显下降,无定形Fe氧化物增加,磷吸附位点增多,导致吸附量大幅上升而降低磷的有效性[20]。有趣的是,土壤在淹水条件下并不只是单纯存在Fe的还原过程,可能存在由光合型Fe (Ⅱ) 氧化微生物介导的Fe氧化物先还原后再被氧化为Fe氧化物的过程[21-23]。因而,关于淹水条件下土壤磷生物有效性的变化是否与光照淹水条件下的Fe (Ⅱ) 氧化有关亦不明晰。鉴于此,本研究对长期施磷土壤分别在光照、避光条件下进行模拟淹水培养试验,通过分析淹水后土壤有效磷含量、无机磷形态转化,探究淹水对长期施磷土壤磷形态转化的影响;通过测试淹水土壤中Fe的氧化还原状况探索Fe氧化还原过程与无机磷形态转化的关系,以期明确土壤Fe氧化还原过程对磷形态转化的影响,为进一步利用Fe氧化还原过程的调控技术手段提高土壤惰性磷库的周转提供理论依据。
1. 材料与方法
1.1 供试材料
供试土壤于2018年9月采自河南科技大学农场 (33°35′~35°05′N,111°8′~112°59′E),该试验田设不施磷 (P0) 和施P2O5 180 kg/hm2 (合纯P 78.6 kg/hm2,简称P180) 2个处理,自2012年至取样连续6年采用相同施肥处理。土壤基本性质、大田耕作管理、田间小区设置等同参考文献[24],供试土壤类型为黄潮土,质地为中壤,pH7.56,有机质10.7 g/kg,P0处理全磷0.78 g/kg,P180处理全磷1.07 g/kg,碳酸盐含量21.2%。土壤样品采集时不考虑种植模式对土壤无机磷的影响,随机多点混合,采样深度0—20 cm。样品采集后,自然风干,人工剔除可见植物残体后,研磨过0.85 mm筛备用。
1.2 试验方法
试验采用泥浆恒温厌氧培养方法,各称取过0.85 mm筛的P0和P180处理风干土3.000 g若干份于10 mL西林瓶中,每份加入3 mL去离子水,盖上橡胶塞,充N2 5 min排出空气后加铝盖密封。每个处理一半的西林瓶置于光照培养箱 (宁波莱福,FPG3) (30 ± 1)℃作光照处理,一半的西林瓶置于隔水式恒温培养箱 (上海博讯,GSP-9270MBE) (30 ± 1)℃作避光处理,连续培养40天。培养过程中间隔取样测定各处理Fe (Ⅱ) 的动态变化,测定培养前后全磷、速效磷等指标含量,并对无机磷形态进行分级测定。
1.3 测试指标与方法
Fe (Ⅱ) 的测定:每次各处理取样1瓶摇匀,吸取0.4 mL泥浆加入装有4.6 mL 0.5 mol/L HCl的带盖聚乙烯管中 (加入泥浆前后称重),(30 ± 1)℃下避光浸提24 h,每个处理3次重复,浸提液过0.22 μm滤膜后,采用邻菲啰啉比色法测定Fe (Ⅱ) 含量[16]。全磷采用HClO4–H2SO4法测定,速效磷采用0.5 mol/L NaHCO3溶液浸提—钼蓝比色法测定[25],碳酸钙含量采用中和滴定法测试[25]。无机磷形态分级测定采用顾益初等[8]提出的分级方法提取,提取后采用钼蓝比色法测定。
1.4 数据处理
避光培养Fe (Ⅱ) 的增长曲线符合Logistic函数模型,函数公式如下:
Ct=a1+be−kx 铁氧化还原过程的关键参数[a为土壤中Fe (Ⅱ) 的最大累积量 (铁还原量),b为无量纲模型参数,x为培养时间(天),k为反应速率常数]参考相关文献[22]中的方法进行计算。数据采用Microsoft Excel 2016、Origin 9.0和IBM SPSS Statistics 23进行分析处理。
2. 结果与分析
2.1 长期施磷土壤中速效磷及无机磷形态的变化
长期施磷显著提升了土壤中速效磷的含量。淹水培养前河南科技大学农场6年不施磷土壤速效磷含量为 (7.65 ± 1.65) mg/kg (所有不施肥土壤磷含量的均值和标准差),经过6年连续施磷后土壤速效磷含量高达 (33.5 ± 2.01) mg/kg,增加了337.8% (图1)。
不施磷土壤无机磷主要以Ca10-P、Ca8-P的形态存在,Ca10-P和Ca8-P分别占无机磷总量的46.5%和26.1%,Al-P、Fe-P和O-P分别占无机磷总量的8.6%、7.3%和10.5%,Ca2-P仅占无机磷总量的1.00%。
连续6年施磷后土壤无机磷总量增加了290.5 mg/kg,其中,Ca10-P、Ca8-P、Al-P、Fe-P、O-P和Ca2-P分别显著增加了164.2、42.5、36.6、16.3、16.3和14.6 mg/kg,分别占无机磷增量的56.5%、14.7%、12.6%、5.63%、5.60%和5.00% (图2)。说明施入土壤的磷转化为不同的无机磷组分,Ca2-P的比例只增加了4个百分点,Ca10-P增加的量最多,其次为Ca8-P、Al-P、Fe-P。
2.2 淹水培养土壤速效磷及无机磷形态的变化
淹水后土壤的速效磷含量显著增加 (图3)。经过40 天淹水以后,P0、P180处理避光条件下速效磷含量分别增加了8.44和2.95 mg/kg,光照条件下P0处理速效磷含量增加了4.88 mg/kg,P180处理无显著变化 (图3)。
避光淹水培养40 天后,P0和P180处理无机磷总量分别增加了9.90%和8.46%,光照淹水培养40 天后分别增加了13.1%和9.60%。避光及光照条件下,P180处理淹水培养40 天后,Ca2-P分别增加了11.7、5.37 mg/kg,Al-P分别增加了34.7、38.8 mg/kg,Fe-P分别增加了47.0、23.4 mg/kg,O-P分别增加了92.9、64.1 mg/kg,Ca10-P分别增加了47.6、63.6 mg/kg,而Ca8-P分别减少了156.2、106.8 mg/kg (图4)。光照条件下Ca2-P、Fe-P、O-P的增量小于避光条件下,P0和P180处理Ca2-P增量较避光条件下分别降低了31.6%和54.3%,Fe-P增量分别降低了37.4%和50.3%,O-P增量分别降低了22.4%和30.9%,且Ca8-P的降幅也较避光条件下分别降低了37.0%和31.6%。表明避光淹水条件更有利于无机磷组分的变化。淹水培养后Al-P增加了13.7~38.8 mg/kg,避光条件下P0和P180处理Al-P增量较光照条件下分别降低了26.8%和10.5%。
避光淹水后Ca8-P变化量与Al-P、Fe-P和O-P变化量间均存在极显著负相关关系 (表1),光照淹水后Ca8-P变化量与Al-P和O-P变化量间均存在极显著负相关关系 (表2)。
表 1 避光培养结束各形态无机磷变化量相关关系矩阵Table 1. Correlation matrix of changes in inorganic P fraction contents after dark incubation项目 Item Olsen-P Ca2-P Ca8-P Al-P Fe-P O-P Ca2-P −0.144 Ca8-P 0.761** −0.360 Al-P −0.828** 0.435 −0.875** Fe-P −0.631** 0.483* −0.788** 0.822** O-P −0.725** 0.517* −0.943** 0.821** 0.784** Ca10-P −0.557* −0.211 −0.420 0.454 0.271 0.327 注(Note):表中相关系数为 Person 相关系数 The correlation coefficient in the table is Person correlation coefficient. *—P < 0.05, **—P < 0.01. 表 2 光照培养结束各形态土壤无机磷变化量相关关系矩阵Table 2. Correlation matrix of changes in inorganic phosphorus fraction contents of soil after illuminated incubation项目 Item Olsen-P Ca2-P Ca8-P Al-P Fe-P O-P Ca2-P 0.501* Ca8-P 0.698* 0.213 Al-P −0.615** −0.041 −0.873** Fe-P −0.136 0.088 −0.422 0.392 O-P −0.593** −0.004 −0.837** 0.841** 0.567* Ca10-P −0.265 −0.181 −0.405 0.467 −0.208 0.312 注(Note):表中相关系数为 Person 相关系数 The correlation coefficient in the table is Person correlation coefficient. *—P < 0.05; **—P < 0.01. 各无机磷形态变化量的相关分析表明,避光淹水后速效磷变化量与Ca8-P变化量间存在极显著正相关关系,与Al-P、Fe-P、O-P和Ca10-P变化量间存在极显著负相关关系 (表1)。光照淹水后速效磷变化量与Ca2-P、Ca8-P变化量间存在显著正相关关系,与Al-P、O-P变化量间存在极显著负相关关系 (表2)。
避光淹水后Al-P、Fe-P和O-P变化量之间均存在显著正相关关系,光照淹水后Fe-P变化量仅与O-P变化量间存在显著正相关关系。此外避光淹水后Ca2-P变化量与Fe-P和O-P变化量间存在显著正相关关系。
2.3 淹水培养过程中土壤铁氧化物的氧化还原
避光淹水后,土壤中发生了Fe (Ⅲ) 的还原,Fe (Ⅱ) 含量持续增加,P0和P180处理铁还原量分别达5.74和6.18 mg/g (图5)。而在光照淹水条件下,培养前5天土壤Fe (II) 含量持续增加,分别达到最高含量2.32和2.56 mg/g后逐渐减少,显示体系内铁氧化物先被还原,然后部分Fe (Ⅱ) 被再次氧化 (图5)。避光时P180处理Fe (Ⅲ) 还原量、最大速率、速率常数均显著高于P0处理 (表3),相较于P0处理,P180处理还原量高0.44 mg/g,还原最大速率高0.05 mg/(g·d)。光照时P0处理的铁还原量与P180处理无显著差异,但P180处理的Fe (Ⅱ) 氧化量较P0处理高0.38 mg/g (表4)。
表 3 避光Fe (III) 还原过程关键参数Table 3. Key parameters of Fe (III) reduction process under dark condition处理
TreatmentFe (Ⅲ) 还原量 (mg/g)
Reduction amount of Fe (Ⅲ)速率常数
Rate constant最大速率 [mg/(g·d)]
Max velocity决定系数
R2P值
P-valueP0 5.74 ± 0.29 b 0.17 ± 0.03 b 0.25 ± 0.03 b 0.96 < 0.01 P180 6.18 ± 0.25 a 0.19 ± 0.03 a 0.30 ± 0.03 a 0.97 < 0.01 注(Note):同列数据后不同字母表示处理间差异达到显著水平 (P < 0.05) Values followed by different small letters in a column indicate significant difference between treatments (P < 0.05). 表 4 光照条件下Fe (III) 还原及Fe (II) 氧化过程关键参数Table 4. Key parameters of Fe (III) reduction and Fe (II) oxidation process under illumination处理
TreatmentFe (Ⅲ) 还原量
Reduction amount of Fe (Ⅲ)
(mg/g)Fe (Ⅱ) 氧化开始时间
Initial days for Fe (Ⅱ) oxidationFe (Ⅱ) 氧化量
Fe (Ⅱ) oxidation amount
(mg/g)Fe (Ⅱ) 氧化速率
Velocity of Fe (Ⅱ) oxidation
[mg/(g·d)]P0 2.32 ± 0.50 a 12 0.88 ± 0.27 b 0.03 ± 0.01 b P180 2.56 ± 0.76 a 12 1.26 ± 0.38 a 0.04 ± 0.01 a 注(Note):同列数据后不同字母表示处理间差异达到显著水平 (P < 0.05) Values followed by different small letters in a column indicate significant difference between treatments (P < 0.05). 淹水前后无机磷组分变化量与铁氧化还原参数的相关分析结果 (表5)显示,避光淹水过程铁还原量和还原最大速率均与速效磷和Ca8-P的变化量呈显著负相关关系,与Al-P、Fe-P和O-P的变化量呈显著正相关关系。光照条件下铁氧化还原参数与无机磷组分变化之间并无显著相关关系。
表 5 铁氧化还原参数与土壤各无机磷形态变化量相关关系矩阵Table 5. Correlation matrix between iron redox parameters and changes in inorganic P fractions of soil项目 Item Olsen-P Ca2-P Ca8-P Al-P Fe-P O-P Ca10-P 避光铁还原量 Fe reduction in darkness −0.826* 0.689 −0.954** 0.957** 0.912* 0.963** 0.537 避光铁还原最大速率 Max velocity of Fe reduction in darkness −0.936** 0.085 −0.897* 0.835* 0.833* 0.826* 0.896* 光照铁还原量 Fe reduction under illumination −0.023 0.537 0.046 0.060 −0.515 −0.057 0.167 光照铁氧化量 Fe oxidation under illumination −0.243 0.433 −0.349 0.438 −0.276 0.304 0.550 光照铁氧化速率 Velocity of Fe oxidation under illumination −0.032 0.506 −0.089 0.184 −0.501 0.061 0.467 注(Note):*—P < 0.05; **—P < 0.01. 3. 讨论
本试验长期施磷土壤速效磷平均含量相较于不施磷土壤提升了337.8%,李新乐等[26]进行的连续施磷6年的试验中,土壤速效磷含量也提升了164.7%~335.9%。不施磷肥土壤中以Ca8-P和Ca10-P为主,占无机磷总量的72.6%,而长期施磷后这两个磷组分占无机磷总量的71.2%,说明不论是否施磷肥,这两个无机磷组分均占绝对优势。
在模拟田间条件的淹水培养试验中,土壤Ca8-P含量显著降低,而其他形态无机磷含量增加 (图4),表现为淹水促使Ca8-P向其他形态磷转化,与田间土壤Ca8-P的变化一致。已有研究表明,Ca8-P可不成比例的转化为Ca2-P和Ca10-P[27]。本试验淹水培养结束后Ca2-P和Ca10-P的变化量与Ca8-P变化量之间无显著的相关关系 (表1,表2),说明Ca8-P并不是全部向Ca2-P和Ca10-P转化,而是更多地向Al-P、Fe-P转化,这可能是因为淹水后Ca8-P向Ca2-P转化,Ca2-P溶解产生PO43–,而PO43–可被晶型氧化铁、铝转化的非晶型氧化铁、铝吸附,从而增加了Al-P、Fe-P[28-29]。以往研究表明,Fe (Ⅱ) 可与土壤中的PO43–结合形成蓝铁矿[Fe3 (PO4) 2·nH2O][30-32],而且高浓度的磷有利于蓝铁矿的形成[33]。淹水避光培养后,土壤pH降低至7.39,部分Fe (Ⅲ) 还原为Fe (Ⅱ)。Fe (Ⅱ) 与Fe (III) 形成的Fe (Ⅱ)-Fe (Ⅲ) 混合氢氧化物具有很大的比表面积和更多的P吸附位点,增加了吸附PO34–的能力[34],表现为Fe-P含量增加。另外,淹水后PO43–可取代铝氧化物表面Al―OH (H) 基团的―OH从而吸附在铝氧化物表面[35]使Al-P增加,也有可能Al3+与土壤中的有机质结合吸附P形成“Al-有机质-P”络合物从而增加Al-P[17]。本试验避光培养结束Ca8-P变化量与Al-P、Fe-P变化量间存在极显著的负相关关系 (表1、表2),也从侧面说明了淹水后土壤可能存在Ca8-P向Al-P、Fe-P的转化。
光照淹水前期Fe (Ⅲ) 还原为Fe (Ⅱ),后期部分Fe (Ⅱ) 被再次氧化为Fe (Ⅲ),前期铁还原量小于避光淹水处理且土壤pH升高至7.94。这可能是光照培养结束Fe-P含量虽然增加但其增量较避光淹水时P0和P180处理分别低37.4%和50.3%的主要原因。虽然Fe (Ⅱ) 与磷酸盐的结合强度不如Fe (Ⅲ)[36-37],但Fe (Ⅱ) 水合氧化物往往具有更高的比表面积和更多潜在的磷酸盐吸附位点[38],可能结合更多的磷;另外,光照条件Fe-P增量小于避光也可能与其Fe (Ⅱ) 含量始终低于避光条件,具备较少可以沉淀磷的Fe (Ⅱ) 有关。
淹水后增量最大的无机磷形态是O-P,增量可达92.9 mg/kg。O-P被认为是部分无定形的磷酸铁铝被Fe2O3膜包裹而形成的[4, 39]。虽然避光淹水是一个铁还原环境,但是与Fe (Ⅱ) 共沉淀或者因专性吸附于Fe (Ⅱ) 水合氧化物表面而增加的Fe-P,可能覆盖包裹于专性吸附了磷的铁氧化物表面而引起O-P的增加,淹水后O-P增量与Al-P和Fe-P增量间存在极显著的正相关关系虽然从侧面说明了这一点,但仍需要进一步研究O-P的微观结构及铁的价态构成来进一步证实。
虽然淹水过程主要通过影响铁的氧化还原而影响Fe-P与Ca-P之间的相互转化,且Al-P的增量小于Fe-P,但Al-P不仅显著增加了13.7~38.8 mg/kg (图4),而且在亚铁产生量少的光照条件下,P0处理和P180处理的Al-P增量较避光分别高了26.8%和10.5%。由此可见土壤中的铝氧化物可能也参与了淹水后铁与钙对磷的竞争固定。然而铝、铁、钙对磷的竞争关系仍然需要通过竞争吸附试验等进一步明确。
本试验培养结束后无论是避光还是光照条件,Ca8-P的减少量均小于其他磷形态的增量之和,表现为无机磷含量在淹水40天后增加了8.46%~13.1%。Bai等[40]研究有机磷矿化特征时发现黄河入海三角洲沉积物培养6天后无机磷含量由603.4 mg/kg显著增加至641.7 mg/kg,其原因是微生物氧化有机物而造成无机磷的释放。Bünemann[41]评价土壤有机磷矿化时发现,土壤有机磷矿化速率一般介于0.1~2.5 mg/(kg·d),草地和森林土壤有机磷的矿化速率可以高达12.6 mg/(kg·d),且矿化对土壤无机磷供应的贡献与土壤有机碳含量呈正相关。也有研究表明在厌氧环境下存在有机磷的矿化过程[42],然而本试验条件下有机磷矿化对淹水后无机磷增量的贡献如何仍需要进一步明确。P180处理有效磷增量低于P0处理,可能与P180处理显著促进了铁的还原 (图5、表3和表4),而且铁还原促进了Fe-P和O-P等不易被0.5 mol/L NaHCO3浸提的磷的形成有关,速效磷变化量与Fe还原量、Fe-P变化量间存在显著的负相关关系也说明了这一点。但也有研究依据沉积物中有效磷与铁浓度的显著相关性,说明了磷的有效性是因为Fe (Ⅲ) 的还原而增加[43]。故而淹水后土壤磷有效性变化的原因,仍需要进一步从淹水后有机磷矿化、土壤中铁、铝、钙相对含量及其对磷的竞争固定方面进一步明确。
4. 结论
施用磷肥可显著增加土壤中有效磷含量和无机磷总量。施入土壤中的磷只有很小部分以Ca2-P形态存在,主要以Ca10-P、Ca8-P、Al-P和Fe-P形态存在。淹水条件下,石灰性土壤中的Fe (Ⅲ) 还原,形成了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,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。
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表 1 避光培养结束各形态无机磷变化量相关关系矩阵
Table 1 Correlation matrix of changes in inorganic P fraction contents after dark incubation
项目 Item Olsen-P Ca2-P Ca8-P Al-P Fe-P O-P Ca2-P −0.144 Ca8-P 0.761** −0.360 Al-P −0.828** 0.435 −0.875** Fe-P −0.631** 0.483* −0.788** 0.822** O-P −0.725** 0.517* −0.943** 0.821** 0.784** Ca10-P −0.557* −0.211 −0.420 0.454 0.271 0.327 注(Note):表中相关系数为 Person 相关系数 The correlation coefficient in the table is Person correlation coefficient. *—P < 0.05, **—P < 0.01. 表 2 光照培养结束各形态土壤无机磷变化量相关关系矩阵
Table 2 Correlation matrix of changes in inorganic phosphorus fraction contents of soil after illuminated incubation
项目 Item Olsen-P Ca2-P Ca8-P Al-P Fe-P O-P Ca2-P 0.501* Ca8-P 0.698* 0.213 Al-P −0.615** −0.041 −0.873** Fe-P −0.136 0.088 −0.422 0.392 O-P −0.593** −0.004 −0.837** 0.841** 0.567* Ca10-P −0.265 −0.181 −0.405 0.467 −0.208 0.312 注(Note):表中相关系数为 Person 相关系数 The correlation coefficient in the table is Person correlation coefficient. *—P < 0.05; **—P < 0.01. 表 3 避光Fe (III) 还原过程关键参数
Table 3 Key parameters of Fe (III) reduction process under dark condition
处理
TreatmentFe (Ⅲ) 还原量 (mg/g)
Reduction amount of Fe (Ⅲ)速率常数
Rate constant最大速率 [mg/(g·d)]
Max velocity决定系数
R2P值
P-valueP0 5.74 ± 0.29 b 0.17 ± 0.03 b 0.25 ± 0.03 b 0.96 < 0.01 P180 6.18 ± 0.25 a 0.19 ± 0.03 a 0.30 ± 0.03 a 0.97 < 0.01 注(Note):同列数据后不同字母表示处理间差异达到显著水平 (P < 0.05) Values followed by different small letters in a column indicate significant difference between treatments (P < 0.05). 表 4 光照条件下Fe (III) 还原及Fe (II) 氧化过程关键参数
Table 4 Key parameters of Fe (III) reduction and Fe (II) oxidation process under illumination
处理
TreatmentFe (Ⅲ) 还原量
Reduction amount of Fe (Ⅲ)
(mg/g)Fe (Ⅱ) 氧化开始时间
Initial days for Fe (Ⅱ) oxidationFe (Ⅱ) 氧化量
Fe (Ⅱ) oxidation amount
(mg/g)Fe (Ⅱ) 氧化速率
Velocity of Fe (Ⅱ) oxidation
[mg/(g·d)]P0 2.32 ± 0.50 a 12 0.88 ± 0.27 b 0.03 ± 0.01 b P180 2.56 ± 0.76 a 12 1.26 ± 0.38 a 0.04 ± 0.01 a 注(Note):同列数据后不同字母表示处理间差异达到显著水平 (P < 0.05) Values followed by different small letters in a column indicate significant difference between treatments (P < 0.05). 表 5 铁氧化还原参数与土壤各无机磷形态变化量相关关系矩阵
Table 5 Correlation matrix between iron redox parameters and changes in inorganic P fractions of soil
项目 Item Olsen-P Ca2-P Ca8-P Al-P Fe-P O-P Ca10-P 避光铁还原量 Fe reduction in darkness −0.826* 0.689 −0.954** 0.957** 0.912* 0.963** 0.537 避光铁还原最大速率 Max velocity of Fe reduction in darkness −0.936** 0.085 −0.897* 0.835* 0.833* 0.826* 0.896* 光照铁还原量 Fe reduction under illumination −0.023 0.537 0.046 0.060 −0.515 −0.057 0.167 光照铁氧化量 Fe oxidation under illumination −0.243 0.433 −0.349 0.438 −0.276 0.304 0.550 光照铁氧化速率 Velocity of Fe oxidation under illumination −0.032 0.506 −0.089 0.184 −0.501 0.061 0.467 注(Note):*—P < 0.05; **—P < 0.01. -
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