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  土壤学报  2024, Vol. 61 Issue (1): 98-109  DOI: 10.11766/trxb202204030153
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引用本文  

李欢, 樊慧琳, 张佳敏, 等. 坡耕地红壤剖面磷的储存容量及其流失风险研究. 土壤学报, 2024, 61(1): 98-109.
LI Huan, FAN Huiling, ZHANG Jiamin, et al. The Phosphorus Storage Capacity and Phosphorus Loss Risk of Red Soil Profiles in Sloping Farmland. Acta Pedologica Sinica, 2024, 61(1): 98-109.

基金项目

国家自然科学基金项目(42077087)资助

通讯作者Corresponding author

王艳玲, E-mail:ylwang@nuist.edu.cn

作者简介

李欢(1997—),女,硕士研究生,主要从事土壤碳磷的环境效应研究。E-mail:516732955@qq.com
坡耕地红壤剖面磷的储存容量及其流失风险研究
李欢, 樊慧琳, 张佳敏, 王艳玲    
南京信息工程大学应用气象学院, 南京 210044
摘要:基于土壤磷储存容量的变化差异,准确评估与判断坡耕旱地与稻田红壤磷的剖面流失风险。以江西鹰潭孙家典型红壤小流域内位于坡上(T)、坡中(M)及坡下(B)的花生旱地(PU)及稻田(PF)各发生层土壤为对象,分析了各发生层土壤全磷(TP)、有效磷(Olsen-P)、磷饱和率(PSR)、磷吸持指数(phosphorus sorption index,PSI)及磷储存容量(Soil phosphorus storage capacity,SPSC)随剖面的变化特征及差异,估测了坡耕地红壤磷的剖面流失风险,探讨了土壤pH、Eh及容重(BD)等因子对坡耕地红壤发生层SPSC的影响差异。结果表明,不同坡位花生旱地与稻田红壤表层TP、Olsen-P含量均显著高于底层土壤,且稻田红壤TP与Olsen-P的剖面变异显著高于花生旱地剖面。与耕作表层(Ap1)相比,花生旱地底层土壤PSI显著增加了33.1%~146%,且随剖面深度增加而缓慢降低;而稻田红壤PSI则随着剖面深度增加而增大。花生旱地与稻田红壤(PF-M除外)PSR均随着剖面深度的增加而逐渐降低,SPSC变化范围分别为–89.2~298.3 mg·kg–1与–138.1~101.1 mg·kg–1。PU-T与PU-M剖面SPSC均为正值,且随剖面深度增加呈降低趋势;而PU-B(耕作层Ap2除外)剖面的SPSC均为负值。PF-T(氧化还原层Br2除外)、PF-M(Ap1除外)及PF-B剖面各发生层的SPSC均为负值,且随剖面深度增加变化显著。基于SPSC理论对坡耕红壤剖面土壤磷储量及其环境流失风险的评估,花生旱地土壤磷沿剖面及坡位的迁移与运移迹象明显,当土壤Olsen-P > 27.6 mg·kg–1时,花生旱地剖面土壤磷的流失风险将急剧增加,应及时采取管控措施。稻田红壤剖面各发生层均存在不同程度的磷流失风险,需立即停止施磷并及时采取有效措施。
关键词坡耕地    红壤    土壤磷储存容量    发生层    花生旱地    稻田    
The Phosphorus Storage Capacity and Phosphorus Loss Risk of Red Soil Profiles in Sloping Farmland
LI Huan, FAN Huiling, ZHANG Jiamin, WANG Yanling    
School of Applied Meteorology, Nanjing University of Information Science and Technology, Nanjing 210044, China
Abstract: 【Objective】The study aimed to assess and determine the P loss risk in red soil profiles of sloping upland and paddy fields based on the variation of soil phosphorus storage capacity (SPSC).【Method】The soil samples were collected from different pedogenic horizons in peanut uplands and paddy fields which were located at the top, middle and bottom of the slope in Sunjia small watershed, Ying tan, Jiangxi province of China. The profile variation characteristics of total P(TP), soil available P(Olsen-P), P saturation rate (PSR), P sorption index (PSI) and SPSC of tested soils were analyzed. Also, the effects of soil total carbon, total nitrogen, iron-aluminum oxides, pH, Eh and bulk density (BD) on the P loss risk of red soil profiles at different slope positions were evaluated.【Results】Soil TP and Olsen-P of the surface layer in peanut uplands and red paddy fields were significantly higher than those in the bottom layers and the profile variations of TP and Olsen-P in paddy fields were significantly higher than those in peanut uplands. Compared with the Ap1 layer, the PSI of peanut upland in the subsurface soils increased significantly by 33.1%~146%, and the PSI decreased with the increase of profile depth, but the PSI of paddy fields increased significantly with the increase of profile depth. The soil PSR of peanut upland and paddy field(except PF-M)decreased gradually with the increase of profile depth, while the variation ranges of SPSC were –89.2~298.3 mg·kg–1 and –138.1~101.1 mg·kg–1, respectively. The SPSC values of PU-T and PU-M were all positive, and they decreased with the increase of profile depth. Also, the SPSC value of all pedogenic horizons in PU-B(except Ap2)were all negative. The SPSC values of pedogenic horizons in PF-T (except Brs), PF-M (except Ap1), and PF-B were negative, and they changed significantly with the increase of profile depth.【Conclusion】Based on the assessment of soil phosphorus storage and its environmental loss risk in sloping red soil profiles by SPSC theory, the migration and transport signs of soil P in peanut upland along the profile and slope position are obvious. Once soil Olsen-P > 27.6 mg·kg–1, the risk of soil P loss in the peanut upland profiles will increase sharply, and control measures should be taken in time. The risk of P loss exists in each soil profile of the paddy fields, thus, proper management practices should be implemented during the application of P fertilizer.
Key words: Sloping farmland    Red soil    Soil phosphorus storage capacity    Pedogenic horizon    Peanut upland    Paddy field    

基于磷饱和率(Phosphorus saturation ratio,PSR)理论建立的土壤磷储存容量(Soil phosphorus storage capacity,SPSC)的概念可以有效表征土壤在达到磷流失环境阈值前可继续容纳外源磷的容量及现存土壤磷的流失潜能与风险[1]。SPSC = 0是判断土壤是磷的源与汇的转折点,当SPSC < 0时,土壤是磷源、不能继续容纳外源磷;当SPSC > 0时,土壤是磷汇、可以继续容纳外源磷[2-3]。与以往评估土壤磷流失风险的指标土壤有效磷(Olsen-P)、磷饱和率(PSR)等相比,SPSC不仅能捕获土壤低磷吸附能力引起的流失风险,还能有效预测土壤磷流失前可继续容纳磷的含量,能有效避免因土壤磷含量高而固持能力低对土壤磷流失风险造成的错误估计[14]。南方红壤区是我国重要的粮食生产基地,强大的固磷容量和较低的磷素利用率是农业生产上的主要障碍因子,不合理耕作、过量施肥等因素是导致土壤遗存磷流失、水环境污染的重要诱因,更有春蚀、秋旱的自然因素加剧土壤剖面的磷流失[5-6]。因此,准确评估坡耕地红壤剖面磷的储存容量及其流失潜力是提高红壤区土壤磷的农学效率及降低土壤磷流失风险的重要方面。

目前,对土壤磷流失损失的研究大多集中在表层土壤,而忽视了底层土壤的磷迁移与流失[7],当土壤表层磷含量超过饱和吸附点时,易产生剖面淋溶损失,而底层土壤磷流失对地下水体富营养化的贡献也是不可忽视的[8-9]。Nair等[10-11]的研究表明SPSC理论既适用于表层土壤又适用于底层土壤,可用于表征土壤剖面磷素迁移与流失风险。Nair和Harris[12]还发现径流水中磷的浓度与SPSC之间存在显著负相关关系,而通过相邻土体SPSC值的差异可以追踪任何受磷影响的土壤中磷的迁移与流失情况。当土壤表层SPSC > 0时,土壤磷淋失进入剖面底层的风险较低,而当某一剖面层的SPSC < 0时,则该剖面层深度的地下水磷含量会显著增加[13]。Nair和Harris[12]对沙质土壤剖面SPSC的评估表明地下水位升高至氧化还原层(Bh)以上会引起Bh层磷的流失风险。但仅从某一剖面层SPSC值的大小判断土壤地下水中磷的环境风险是片面的,还应考虑到土壤整体剖面中SPSC的变化[14],使得土壤磷的管理更具有科学性[15]

大量研究证明SPSC理论优于Olsen-P、PSR等指标,能有效评价土壤磷储量及其环境流失风险[16-19],但国内对SPSC的应用与发展仍显不足,尤其是在铁铝氧化物含量丰富、水热变化剧烈、磷流失风险极大的红壤区,还未有应用SPSC理论评估土壤磷流失风险的研究[20-22]。且当前的研究仅关注了土壤表层,而忽视了底层土壤或剖面土壤磷的管理[23]。因此本研究以江西鹰潭孙家典型红壤小流域内不同坡位的花生旱地与稻田红壤各发生层土壤为研究对象,分析与探讨坡耕地红壤剖面SPSC的变化差异及影响因素,评估花生旱地与稻田红壤剖面磷的储存容量及其流失风险,为红壤坡耕地磷肥的有效利用及磷流失风险的有效管控提供理论依据。

1 材料与方法 1.1 研究区概况

孙家典型红壤小流域观测站距鹰潭农田生态系统国家野外科学观测研究站(28°15'20''N,116°55'30'' E)约4 km,该小流域区域面积约为50.5 hm2,坡度小于8°,主要成土母质为第四纪红黏土。花生旱地与稻田是流域土地的主要利用方式,花生的种植期为每年的4月上旬至8月下旬;水稻种植以双季稻为主,分别在每年4月上旬与7月中旬种植。花生旱地及稻田的施肥量如表 1所示。

表 1 孙家小流域花生旱地与稻田的肥料施用量 Table 1 Application amounts of fertilizers in peanut uplands and paddy fields of Sunjia small watershed
1.2 剖面土壤样品的采集与制备

2018年11月,在孙家小流域内按坡度变化分别选取地势较为平坦的花生旱地及稻田样地各三处,分别为坡上花生旱地(PU-T,20~30a)、坡中花生旱地(PU-M,20~30a)、坡下花生旱地(PU-B,20~30a)及坡上中期稻田(PF-T,50~60a)、坡中新稻田(PF-M,20~30a)、坡下老稻田(PF-B,400~500a)。在每个样地中心位置平整地表后,按长×宽为2 m×1 m标准挖土壤剖面至母质层(或出水层),并以向阳面作为观察面,垂直削平后观察土壤的颜色、质地、紧实度、结构等变化,以此划分出各发生层次并做好标记(表 2)。然后,自下而上地采集各发生层土壤样品,每个发生层平行采集8~10个土壤样品,混匀后带回室内,去除小石砾及动植物残体,经自然风干并磨细过2 mm和0.149 mm筛,保存备用。

表 2 花生旱地和稻田土壤剖面发生层简述 Table 2 A brief description of the pedogenic horizons in peanut uplands and paddy fields
1.3 土壤测试磷及基本理化性质分析

土壤全磷(TP)采用硫酸-高氯酸(H2SO4-HClO4)消解,土壤有效磷(Olsen-P)采用盐酸氟化铵(0.025 M HCl + 0.03 M NH4F,pH = 2.7)法提取,土壤水溶性磷(CaCl2-P)采用0.01 mol·L–1 CaCl2溶液浸提(液土比为10︰1),消解液及提取液中的磷用钼锑抗比色法测定。土壤容重(BD)采用环刀法测定;土壤pH采用电位法测定(液土比为2.5︰1);全碳(TC)、土壤全氮(TN)及碳氮比(C/N)使用碳氮元素分析仪(Vario EL cube,Elementar,德国)测定。非晶质态铁铝氧化物(a-Fe2O3a-Al2O3)采用酸性草酸铵溶液(pH = 3.17)提取,游离态铁铝氧化物(f-Fe2O3f-Al2O3)采用DCB(连二亚硫酸钠-柠檬酸铵-重碳酸氢钠)法提取,提取液中的铁铝浓度采用电感耦合等离子光谱发生仪(ICP-OES)测定。以上测试指标的具体实验操作步骤参照《土壤农化分析》[24]。供试土壤剖面基本理化性质如表 3所示。

表 3 花生旱地和稻田土壤剖面基本理化性质 Table 3 Soil profile basic physical and chemical properties in peanut uplands and paddy fields
1.4 土壤磷饱和率(PSR)与磷储存容量(SPSC)的测定与计算

称取过2 mm筛的风干土1.00 g于三角瓶中(100 mL),然后加入0.2 mol·L–1的草酸铵提取液50 mL(pH = 3.17),用橡皮塞将瓶口塞紧后装入里红外黑的布袋里防止光化学反应,在室温(25 ± 2℃)条件下震荡2 h后,离心并过0.45 μm滤膜。使用电感耦合等离子光谱发生仪(ICP-OES)测定提取液中的铁(Fe)、铝(Al)及磷(P)含量。分别按式(1)和式(2)计算得到PSR与SPSC:

$ {\text{PSR = }}\frac{{\frac{{{{\text{P}}_{{\text{0x}}}}}}{{{\text{31}}}}}}{{\frac{{{\text{F}}{{\text{e}}_{{\text{0x}}}}}}{{{\text{56}}}} + \frac{{{\text{A}}{{\text{l}}_{{\text{0x}}}}}}{{{\text{27}}}}}} $ (1)
$ {\rm{SPSC = (mg}} \cdot {\rm{k}}{{\rm{g}}^{ - 1}}{\rm{) = (Threshold PSR - Soil PSR)}} \times {\rm{(}}\frac{{{\rm{F}}{{\rm{e}}_{{\rm{Ox}}}}}}{{56}}{\rm{ + }}\frac{{{\rm{A}}{{\rm{I}}_{{\rm{Ox}}}}}}{{27}}{\rm{)}} \times {\rm{31}} $ (2)

式(1)与式(2)中,POx、FeOx、AlOx为酸性草酸铵提取的磷、铁及铝的含量(mg·kg–1),31、56及27分别为磷、铁及铝的原子质量。

1.5 土壤磷吸持指数(PSI)的测定与计算

称取1.00 g过2 mm筛的风干土,置于50 mL离心管中,加入20 mL磷浓度为75 mg·L–1的0.01 mol·L–1 CaCl2溶液(磷的加入量为1.5 g·kg–1),同时加入2~3滴甲苯防止微生物活动。将离心管加盖并置于25 ± 2℃的室温下,用往复振荡仪振荡18 h后,离心过0.45 μm滤膜,滤液中的磷浓度(C)用钼锑抗比色法测定,并根据式(3)计算PSI:

$ {\text{PSI(mg}} \cdot {\text{k}}{{\text{g}}^{ - {\text{1}}}}{\text{) = }}\frac{{\text{X}}}{{{\text{lgC}}}} $ (3)

式中,X为土壤磷吸附量(mg·kg–1),即加入磷的量减去滤液中磷的量;C为平衡溶液中磷的浓度(mg·L–1)。

1.6 数据处理与计算

使用Excel 2019软件进行数据计算,使用IBM SPSS Statistics 26软件进行数据统计与分析,采用Origin 2021软件进行绘图。

2 结果 2.1 坡耕地红壤全磷及有效磷的剖面变化

花生旱地与稻田红壤全磷(TP)含量剖面变化范围分别为0.14~0.42 g·kg–1及0.06~0.72 g·kg–1,且均随着剖面深度的增加而逐渐降低(图 1)。与Ap1层相比,花生旱地与稻田红壤底层土壤的TP含量分别显著降低了42%~56%与26%~91%(图 1)。花生旱地三个剖面间的TP含量无显著差异,而稻田红壤Ap1层的TP含量变异较显著,大小变化顺序依次为PF-T > PF-B > PF-M,PF-B底层土壤的TP含量则显著高于PF-T和PF-M(图 1)。

图 1 花生旱地(PU)与稻田(PF)全磷和有效磷的剖面变化 Fig. 1 Profile changes of soil total P and available P in peanut upland(PU)and paddy field(PF)

花生旱地与稻田土壤有效磷(Olsen-P)含量也均随着剖面深度的增加而呈逐渐降低趋势,变化范围分别为11.7~56.4 mg·kg–1及31.9~59.9 mg·kg–1。与Ap1层相比,花生旱地与稻田红壤底层土壤的Olsen-P含量分别显著降低了24.3%~68.8%与9.4%~38.9%(图 1)。花生旱地PU-B剖面的Olsen-P含量显著高于其他两个剖面,而稻田红壤三个剖面间无显著差异(图 1)。

2.2 坡耕地红壤磷吸持指数(PSI)的剖面变化

花生旱地与稻田红壤剖面的土壤磷吸持指数(PSI)变化范围分别为621~2156 mg·kg–1与561~1849 mg·kg–1,花生旱地剖面土壤PSI的大小变化顺序为:PU-B < PU-M < PU-T,而稻田红壤三个剖面间(除Brs层外)差异不显著,且稻田红壤剖面的PSI显著低于花生旱地剖面(图 2)。与Ap1层相比,花生旱地底层土壤PSI显著增加了33.1%~146%,其中Ap2层增加最显著,其余各层土壤随剖面深度的增加而降低;而PF-T的Brs层、PF-M的Br2层及PF-B的Br层土壤PSI则分别增加了154%、32.4%及81.2%(图 2)。

图 2 花生旱地(PU)与稻田(PF)土壤磷吸持指数的剖面变化 Fig. 2 Profile changes of P sorption index in peanut upland(PU)and paddy field(PF)
2.3 坡耕地红壤磷饱和率(PSR)的剖面变化

花生旱地及稻田红壤剖面(除PF-M外)的PSR均随着剖面深度的增加而降低,Ap1层土壤PSR大小依次为:PU-T < PF-M < PU-M < PF-T < PU-B < PF-B(图 3)。与Ap1层相比,花生旱地剖面底层土壤PSR含量显著降低了10.2%~92.3%;PF-B及PF-M剖面底层土壤PSR分别显著降低与增加了25.9%~31.0%及33.1%~202.5%,而PF-T的Br1层PSI显著增加了12.1%,而其他各发生层的则显著降低了5. 4%~33.8%(图 3)。

图 3 花生旱地(PU)与稻田(PF)土壤磷饱和率的剖面变化 Fig. 3 Profile changes of P saturation ratio in peanut upland(PU)and paddy field(PF)
2.4 坡耕地红壤磷储存容量(SPSC)的剖面变化

土壤磷饱和率阈值(Threshold PSR)的确定是准确计算土壤磷储存容量(SPSC)的关键,即将土壤磷饱和率(PSR)与水溶性磷(CaCl2-P)进行分段线性拟合后获得的拐点值,即为Threshold PSR。由图 4可知花生旱地与稻田红壤的Threshold PSR = 0.09,即当PSR < 0.09时,CaCl2-P随PSR增大缓慢增加;当PSR > 0.09时,CaCl2-P含量随PSR增大急剧增加(图 4)。

图 4 花生旱地与稻田土壤磷饱和率阈值 Fig. 4 Threshold P saturation ratio(Threshold PSR)of peanut upland and paddy field

基于Threshold PSR = 0.09计算了花生旱地与稻田红壤各发生层的SPSC值,由图 5可以看出,花生旱地与稻田土壤剖面SPSC变化范围分别为–89.2~298.3 mg·kg–1与–138.1~101.1 mg·kg–1之间。PU-T及PU-M剖面的SPSC均为正值,而PU-B剖面(除Ap2层外)的则为负值;与PU-M剖面相比,PU-T剖面各层的SPSC值显著增加了49.6%~291.3%(图 5)。花生旱地的Ap1层SPSC均显著低于底层土壤,且PU-T与PU-M底层土壤SPSC则随剖面深度增加呈显著降低趋势,而PU-B(Ap2层除外)的SPSC随剖面深度增加无显著变化(图 5)。PF-T(Brs层除外)、PF-M(Ap层除外)及PF-B剖面各发生层的SPSC均为负值,且随剖面深度的增加变化显著(图 5)。

注:SPSC为土壤磷储存容量;相同大写字母表示不同剖面的相同发生层间不显著,不同小写字母表示同一剖面不同发生层间差异显著(P < 0.05)。  Note: SPSC, Soil phosphorus storage capacity; Different uppercase letter means no significant difference in the same pedogenic horizon between different profiles, and the different lowercase letter means significant difference between different pedogenic horizons in the same profile(P < 0.05). 图 5 花生旱地与稻田剖面的磷储存容量(SPSC)变化 Fig. 5 Profile changes of soil phosphorus storage capacity(SPSC)in peanut upland and paddy field
2.5 坡耕地红壤剖面SPSC的影响因素

相关分析表明,花生旱地剖面的SPSC与pH、Eh值分别呈显著正、负相关关系(P < 0.05),与BD呈极显著负相关关系(P < 0.01);稻田红壤的SPSC与pH、Eh、BD等理化因子无显著相关性(表 4)。可见,花生旱地与稻田红壤两个不同种植体系下,影响土壤剖面SPSC的主控因子数量差异较大,BD、pH、及Eh均为旱地红壤的影响因素(表 4)。

表 4 花生旱地与稻田基本理化性质与SPSC的相关性 Table 4 Correlation between basic physicochemical properties and SPSC in uplands and paddy fields of red soil
3 讨论

土壤全磷(TP)和有效磷(Olsen-P)是评价土壤磷素供应潜力与活力的重要指标[25-27]。花生旱地与稻田红壤表层的TP及Olsen-P的累积量显著高于底层土壤,且均随剖面深度增加而降低(图 1),这主要是受施肥等人为因素的影响导致了磷素在表层土壤中大量累积,且随剖面深度的增加,磷的累积量显著降低[28-2916]。本研究中,稻田红壤剖面TP含量均显著高于红壤旱地(图 1),主要是因为水稻根系的大量残留与肥料的长期施入,导致稻田土壤中有机质的累积量显著增加[30],同时,有机质矿化分解释放出大量的磷酸盐也能显著增加表层土壤磷的累积[21]。虽然,土壤TP能有效反映土壤磷素的赋存及肥力状况,但对人为活动和环境因子的响应较为迟钝,尤其是无法在短时间内反映土壤磷储存容量及有效性的变化[31]。与TP相比,Olsen-P对人为耕作及施肥等行为的响应更加迅速,但Olsen-P亦难以准确评估土壤磷的储存容量[32]。基于土壤磷饱和率(PSR)计算的土壤磷素储存容量(Soil phosphorus sorption capacity,SPSC)值,不仅可以准确判断土壤磷的流失风险,还可以有效估算土壤磷达到环境流失风险阈值前土壤可继续容纳外源磷的数量[1-2]。当SPSC > 0时,土壤是磷的“汇”,反之则是意味着土壤是磷的“源”,土壤磷具有环境流失风险[1-2]

基于SPSC值,可以发现坡耕的花生旱地与稻田红壤磷不仅可以沿剖面进行垂直运移淋失,还可以随坡位以地表径流的方式进行迁移损失(图 5)。本研究发现,坡耕花生旱地土壤剖面相对应发生层的SPSC值均随着坡位的降低而显著降低,即各发生层土壤磷流失风险会随坡位降低而呈增大趋势[33];而坡上及坡中花生旱地剖面SPSC均为正值(图 5),表明这两处剖面土壤均为磷汇、土壤磷不存在向外流失或淋失风险;而坡下旱地(除Ap2层外)各发生层土壤的SPSC均为负值(图 5),则表明坡下花生旱地表层及底层土壤均为磷源、土壤磷均存在流失风险;而且表层土壤磷流失风险显著高于底层土壤,但底层土壤淋失的磷可能并不完全来源于表层土壤,而是底层土壤磷素累积量高或底层水流的淋溶作用导致[1534],也可能是花生旱地底层土壤的横向地下水流使磷素随水运移而绕过Ap2层土壤对磷的截留,进而增加了旱地底层土壤磷向外流失的风险[1]。可见,降雨等水分过程将会加剧旱地坡下土壤磷的流失风险,需及时采取有效措施对土壤磷素流失进行阻控,以降低土壤磷对周围环境的污染。

土壤磷素有效磷(Olsen-P)的多少不仅影响着土壤磷素的有效供应量,还影响土壤磷的流失的潜能与风险,通常将20 mg·kg–1视为作物生长的最佳Olsen-P值,当Olsen-P > 40 mg·kg–1时,土壤磷即存在流失风险且会影响水体环境质量[35]。本研究中将花生旱地剖面土壤Olsen-P与SPSC进行拟合得到(图 6):当SPSC = 0时,Olsen-P = 27.6 mg·kg–1(> 20 mg·kg–1),已超出作物最佳需磷量,也就意味着土壤中多余的磷流失风险增大。因此,根据SPSC与Olsen-P的关系,可将花生旱地土壤磷的管理分为四个阶段:(1)磷经济高效阶段(SPSC > 281 mg·kg–1或Olsen-P ≤ 20 mg·kg–1),此时土壤是磷汇,可增加磷肥施用量,能有效提高作物对磷的利用效率;(2)磷经济低效阶段(0 mg kg–1 < SPSC ≤281 mg·kg–1或20 mg·kg–1 < Olsen-P ≤ 27.6 mg·kg–1),此时土壤是磷汇,土壤有效磷供应水平较高,但作物利用效率较低,具有潜在的磷流失风险;(3)磷经济无效阶段(–41.3 mg·kg–1 < SPSC ≤ 0 mg·kg–1或27.6 mg·kg–1 < Olsen-P ≤ 40 mg·kg–1),此时土壤为磷源,磷肥利用效率低,且存在磷的流失;(4)磷高流失风险阶段(SPSC ≤ –41.3 mg·kg–1或Olsen-P > 40 mg·kg–1),此时土壤为磷源,磷流失风险极高,应立即停止施磷并采取环境保护措施。

图 6 花生旱地Olsen-P与SPSC的拟合关系 Fig. 6 Correlation fitting relationships between SPSC and Olsen-P in peanut upland

稻田红壤剖面磷也随坡位发生变化,但受干湿交替的水分变化及沿坡向水的流动影响,稻田剖面土壤磷的运移过程更加复杂多变,并不像旱地剖面表现出明显的阶梯状变化(图 5)。坡上(Brs层除外)、坡中(Ap层除外)及坡下稻田剖面各发生层的SPSC均为负值,土壤均为磷的源,土壤磷均存在着不同程度的流失风险;而且坡上与坡下稻田表层土壤磷的流失风险均显著高于底层土壤(图 5),这可能与土壤表层常年施肥、磷素累积量相对高于底层土壤,土壤磷吸持能力逐渐降低、土壤磷饱和率逐渐增大等原因有关[7]。而当稻田灌水或复水时,将加快水耕表层、犁底层及氧化还原层的磷素垂直向下运移或淋失。但由于稻田土壤容重大且易被压实,导致在底层土壤渗透性较差,当地下水垂直流动受到阻碍并在渗透性较差的土层上形成饱和带,就会迅速产生沿斜坡向下的地下侧向流,进而使坡上土壤中的磷素被运送到坡中及坡下的土壤中[34]。正如本研究发现随剖面深度增加,坡上中期稻田剖面土壤磷流失风险显著降低,底层土壤(C层)磷几乎没有流失风险;而坡中新稻田底层土壤磷的流失风险则显著增大(图 5)。坡中新稻田水耕表层的SPSC值虽为正值,但土壤磷的流失风险随剖面深度的增加而逐渐增大,且C层土壤磷的流失风险最大(图 5)。可见,稻田红壤底层土壤磷的流失风险显著高于表层土壤,即意味着稻田土壤剖面存在磷素的纵向迁移[13],且磷的纵向迁移淋失甚至会损坏并突破了具有较高磷储存能力的氧化还原层土壤,需立即调整现有的施肥量及耕作模式,或采取其他补救措施[123]。水流的纵向移动是剖面磷流失和淋失的重要影响因素[81225],稻田土壤通常建立在深层不透水的紧实土壤之上,水稻种植过程中反复的干湿交替过程增强了土壤剖面的垂直淋洗,也极易造成剖面磷的流失,进而增加下层土壤磷流失风险[26],孙家小流域的坡中新稻田植稻年限较短(20~30a),还未成为稳定的稻田体系,土壤孔隙大,土壤磷易受水流影响而有明显向底层迁移的迹象[27]。因此,稻田红壤、尤其是植稻年限相对较短的稻田土壤磷的流失风险相对较大,应根据土壤的供磷容量与供磷效率及时地调整施肥策略与管理措施,以便有效地阻控稻田磷素的流失、增加稻田土壤磷的利用效率。

4 结论

花生旱地土壤磷沿剖面及坡位的迁移与运移迹象明显,当土壤Olsen-P > 27.6 mg·kg–1时,花生旱地剖面土壤磷的流失风险将急剧增加,应及时采取管控措施。稻田红壤剖面均为磷的源,各发生层土壤磷均存在不同程度的流失风险,需立即停止施磷并及时采取有效措施以提高稻田土壤遗存磷的利用率。无论是花生旱地还是稻田红壤,均可以依据SPSC值的变化对土壤磷达到流失风险阈值前土壤可继续容纳外源磷的安全容量及土壤磷的流失风险进行预测与判断。

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