石小娟(1994-), 女, 主要从事水污染处理研究, E-mail:
Shi Xiaojuan (1994-), main research interest: water pollution treatment, E-mail:
Guo Jinsong (corresponding author), professor, doctorial supervisor, E-mail:
农业面源氮素已成为影响三峡库区环境安全的主要因素,但有关农业面源氮污染研究并未深入区分氮污染主要来自何种农业用地,同时以什么方式进入三峡库区。以三峡库区紫色土农用坡地为研究对象,对典型农耕模式下碳铵、尿素和复合肥的氨挥发特征以及小流域内氮素收支平衡进行分析,以期探究氨挥发对三峡库区氮污染的影响。采用原位受控对照实验的范式进行研究,结果表明,在典型农耕模式下,三峡库区紫色土氨挥发速率表现为:复合肥最低,变化最平缓;尿素的峰值出现滞后,下降缓;碳铵的峰值出现较早,下降快。小流域内尿素的氨挥发率为8.82%~18.37%,碳铵为17.86%~30.70%,复合肥为2.56%~3.86%。施肥种类的氨挥发率大小为:碳铵>尿素>复合肥,典型用地的氨挥发率大小为:水田>果林>旱地。对流域内氮收支平衡分析,发现小流域内化肥是氮素最主要的输入,氨挥发是主要的输出,土壤氮素残留严重,增加了氮素流失风险。从环保角度考虑,降低三峡库区碳铵使用频率、减少旱地和果林施肥量、优化氮肥施用结构是减少氨挥发的有效途径,氨挥发率的减少对三峡库区氮污染防治具有重要意义。
Agricultural non-point source nitrogen has become the main source of environmental safety in the Three Gorges Reservoir. However, investigation on agricultural non-point source nitrogen pollution has not been studied in depth which agricultural land was the main source, and how to enter Three Gorges Reservoir. Purple soil sloping ploughland in Three Gorges Reservoir was taken as the studied object to explore the effect of ammonia volatilization on nitrogen pollution. The ammonia volatilization characteristics of ammonium bicarbonate, urea and compound fertilizer in typical farming modes, and the nitrogen budget in a small watershed were investigated. The study was conducted by using a paradigm of controlled experiments in situ. The results show that the ammonia volatilization flux of compound fertilizer is the lowest and the change is the most gradual. Meanwhile, the peak value of ammonia volatilization flux of urea lags behind and decreases slowly, while that of ammonium bicarbonate appears earlier and decreases faster. The ammonia volatilization loss ratio of urea, ammonium bicarbonate and compound fertilizer in the watershed are 8.82%~18.37%, 17.86%~30.70% and 2.56%~3.86%, respectively. Besides, the ammonia volatilization loss ratio is in order:ammonium bicarbonate > urea > compound fertilizer, and the ammonia volatilization loss ratio between typical land use is in order:paddy field > fruit forest > dry land. Moreover, it is found that chemical fertilizer and ammonia volatilization are the most important nitrogen inputs and outputs in the watershed, and soil nitrogen residues are serious, which increases the risk of nitrogen loss. From the perspective of environmental friendliness, reducing the frequency of ammonium bicarbonate use, reducing the amount of fertilizer applied to dry land and fruit forest, and optimizing the application structure of nitrogen fertilizer in the Three Gorges Reservoir are effective ways to reduce ammonia volatilization. The reduction of ammonia volatilization is of great significance for the prevention and control of nitrogen pollution in the Three Gorges Reservoir.
三峡水库是长江中下游水环境安全保障的关键区域,农业面源氮污染是影响水环境安全的重要因素之一[
中国农耕土地氮肥施用量大,但利用率低,其中,1%~47%随着氮挥发进入大气[
系统地研究区域性氮素的输入和输出等收支过程是合理、有效地理解一个区域氮循环的重要手段,也是对其环境效应评价的关键[
新政小流域位于重庆市忠县石宝镇,是三峡库区心腹区域,其中,紫色土耕地占80%,坡上果林、坡下水田和旱地、坡底水田的土地利用模式在三峡库区具有代表性。本文以新政小流域紫色农用坡地为研究对象,通过野外原位实验研究三峡库区紫色土典型农耕模式下化肥氮的氨挥发特征,以及氨挥发对氮素平衡的影响,以期为三峡库区紫色土氮收支的研究及氮污染的防治提供理论指导。
选择新政小流域(108°10′E30°25′N)作为研究实验区。小流域属亚热带东南季风气候,四季分明,日照充足,雨量充沛,年均气温19.2 ℃,降雨量1 150 mm,无霜期约320 d,适宜水稻、小麦、玉米、蔬菜等农作物生长。小流域种植类型主要为坡上果林、坡下旱地和水田、坡底水田,总面积为45.47 hm2,其中,果林占55.64%,旱田占24.85%,水田占19.51%。
对新政小流域化肥使用情况调查发现,复合肥、尿素及碳铵是农用肥中主要的氮肥,单季单位面积施用氮肥折纯氮量为225 kg/hm2。为了减小不同季节耕作条件和气候条件对实验结果的影响,便于实验观测,实验控制氮肥类型、氮肥用量及施肥时间相同。
选择果林、旱地和水田各20 m2作为小流域典型农耕模式下的实验样地。各实验样地均设置1个对照组(不施肥)和3个实验组(单施复合肥、尿素、碳铵),每个实验组设置3个重复,共12个实验样本。其中,尿素、碳铵和复合肥与小流域正常农用肥来源一致,均购于当地市场,含氮量分别为46.4%、17.1%和14.0%。样地编号及具体施用氮肥量见
实验设计
The experiment design
序号 | 编号 | 耕地类型 | 氮肥类型 | 折纯氮量/(kg·hm-2) |
01 | PF00 | 水田 | 无 | 000 |
02 | PF01 | 水田 | 尿素 | 225 |
03 | PF02 | 水田 | 碳铵 | 225 |
04 | PF03 | 水田 | 复合肥 | 225 |
05 | FF00 | 果林 | 无 | 000 |
06 | FF01 | 果林 | 尿素 | 225 |
07 | FF02 | 果林 | 碳铵 | 225 |
08 | FF03 | 果林 | 复合肥 | 225 |
09 | DL00 | 旱地 | 无 | 000 |
10 | DL01 | 旱地 | 尿素 | 225 |
11 | DL02 | 旱地 | 碳铵 | 225 |
12 | DL03 | 旱地 | 复合肥 | 225 |
采用李宗新等[
田间氨挥发捕获器示意图
Schematic diagram of in-situ ammonia volatilization collector
施肥后,随即在各实验样地随机放置3个氨挥发收集装置。于每天17:00对样品进行采集。连续采集一周后,在第2、3周,每隔2 d或3 d采样一次,最终将采样时间间隔延长至7 d,直至监测不到氨挥发为止。用于隔绝外界气体的海绵,肉眼观察其干湿程度,大约3~7 d更换1次。将采集的样品密封保存,带回实验室,浸于300 mL 1 mol/L的KCl溶液中,振荡1 h,获得浸提液。氨氮浸提液采用纳氏试剂光度法测定(HJ535-2009)。
氨挥发量
式中:
氨挥发速率
式中:
氨挥发累积量
式中:
氨挥发率
式中:
实验数据采用Excel2016、SPSS21.0和Origin8.0进行数据分析和绘图。
氨挥发速率
Ammonia volatilization flux
在施肥后1~6 d内,FF01氨挥发速率出现两个峰值,第1个是第3 d的小高峰5.76 kg/hm2/d,第2个为第5 d的峰值7.12 kg/hm2/d,6 d后氨挥发速率低于0.29 kg/hm2/d,且随着时间的增加而降低。DL01在施肥后氨挥发速率只出现1个峰值,即第5 d的峰值6.07 kg/hm2/d,随即下降。PF01和DL01的变化趋势相似,在施肥后,氨挥发速率迅速增加,并在施肥后的第4 d达到峰值13.15 kg/hm2/d,然后缓慢下降。由此可见,小流域紫色土在施用尿素后的第4~5 d氨挥发速率最大,3种样地氨挥发速率的大小依次为:水田>果林>旱地。
FF02、DL02和PF02的氨挥发速率变化规律比较一致,均表现为施肥后氨挥发速率迅速增加,第3 d达到峰值,分别为15.55、21.11、38.69 kg/hm2/d,随后均迅速下降。由此可见,耕地在施用碳铵后第3 d氨挥发速率达到峰值,随后下降,样地间氨挥发速率的大小依次为:水田>旱地>果林。
分析复合肥的氨挥发速率发现,整体变化幅度不大,在0.00~3.40 kg/hm2/d之间波动。FF03和DL03呈现相同的氨挥发规律,在施肥后第3 d达到小高峰,分别为2.39、1.94 kg/hm2/d,随即在第4 d下降,然后,在第5 d达到峰值3.05、2.72 kg/hm2/d,5 d后氨挥发速率缓慢降低。PF03的氨挥发速率在第3 d已达到峰值3.40 kg/hm2/d,随后呈波浪下降。
综上所述,3种肥料在不同农耕模式下的氨挥发特征为:复合肥的氨挥发变化平缓,氨挥发速率最低;碳铵的氨挥发在第3 d出现峰值,随后快速下降;而尿素的氨挥发峰值滞后于碳铵,在第4~5 d出现,然后缓慢下降。
氨挥发累积量
Accumulation of a mmonia volatilization
进一步分析发现,各个不同样地的氨挥发累积量随着实验时间的增加明显增加,除了碳铵在施肥3 d后氨挥发累积量趋向平缓,另两种氮肥在施肥7 d后趋向平缓,说明氨挥发在施肥后7 d内基本完成。其中,碳铵的氨挥发积累量最高,分别为41.64、46.31、71.52 kg/hm2/d(按果林、旱地、水田顺序,下同);在施肥后的1~3 d,氨挥发累积量迅速增加,第3 d氨挥发累积量分别占总挥发量的73%、90.3%和92.4%。在4~7 d,氨挥发积累量变化较小,7 d后,氨挥发累积量趋于平稳。尿素的氨挥发累积量相对碳铵而言增加相对较缓,1~7 d,氨挥发累积量缓慢增加,7 d之后,氨挥发量与对照基本持平。复合肥的氨挥发累积量最低,监测期间,变化范围为7~10 kg/hm2,整体呈增加的趋势,但变化幅度不大。
分析还发现,复合肥氨挥发累积量呈一个缓慢增加的趋势;而尿素和碳铵的氨挥发累积量表现为两个阶段,一个是施肥后立即进入的快速增加阶段,另一个是3~7 d后进入的缓慢增加阶段,这与Chen等[
研究采用式(4)计算氨挥发净损失率,即氨挥发率。小流域典型农耕模式下氨挥发率如
氨挥发率
Ammonia volatilization loss ratio
对比3种典型农耕模式发现,果林的氨挥发率为3.86~17.86%,旱地为2.56%~19.81%,水田为3.76%~30.70%,说明同一农耕模式下,施加不同的氮肥,氨挥发率差异大,氨挥发率主要受氮肥类型影响。对3种农耕模式下施加不同氮肥后氨挥发率分别进行比较发现,水田施加3类氮肥后的氨挥发率均最大,而旱地和果林3类氮肥氨挥发率相当。由此可知,水田的氨挥发损失最大,旱地和果林的氨挥发损失总体相当。
氮素收支平衡分析是合理、有效地理解小流域氮循环的重要手段,也是对其环境效应评价的关键。氨挥发作为小流域氮素输出的途径之一,其相对程度只有通过氮素平衡分析才能获得。因此,以小流域3种典型农耕模式作单独子系统,分别估算流域内果林、旱地和水田的氮素收支情况,具体估算方法及过程见刘京等[
新政小流域坡耕地氮收支结构图
Nitrogen budget structure diagram in Xinzheng Watershed
新政小流域典型农耕模式下氮的表观平衡
Apparent N balance in typical land of Xinzheng Watershed
用地 |
输入/(kg·a-1) | 合计 | 输出/(kg·a-1) | 合计 | ||||||||||||||||
化肥 | 大气 |
生物 |
非生物 |
母岩 |
种子 | 人畜 |
有机 |
秸秆 |
淋溶 | 径流 |
水土 |
氨挥 |
反硝 |
秸杆 |
秸杆 |
农产品 |
农产品 |
|||
注:“—”表示未产生或产生量忽略不计的氮素输入或输出。 | ||||||||||||||||||||
果林 | 11 759.19 | 762.75 | — | 379.40 | — | — | 880.89 | — | 7.09 | 13 789.32 | 880.89 | — | — | 1 238.72 | 1 749.50 | 346.10 | 130.58 | 1 309.97 | 1 763.88 | 7 419.64 |
旱地 | 9 946.83 | 341.58 | 624.31 | 169.50 | 4 521.13 | 1.28 | 675.34 | 7.24 | 9.06 | 16 296.27 | 675.34 | 214.14 | 1 101.69 | 3 457.37 | 1 016.32 | 155.00 | 471.26 | 953.90 | 1 492.02 | 9 537.03 |
水田 | 1 793.16 | 267.50 | — | 266.14 | — | — | 472.66 | 5.69 | 20.23 | 2 8525.38 | 472.66 | 157.55 | 0 182.19 | 0 508.66 | 151.80 | 121.38 | 20.64 | 1 348.90 | 573.81 | 3 537.59 |
果林、旱地和水田氮素年输入量分别为13 789.32、16 296.27、2 825.38 kg/a,其中,单位面积氮肥输入量为464.79、880.25、202.16 kg/hm2/a,分别占总输入氮量的85.28%、61.04%和63.47%(
此外,流域内果林、旱地和水田单位面积氮输出量分别为293.27、843.98、398.83 kg/hm2/a,均高于全国单位面积损失氮量87.1 kg/hm2/a[
小流域内氮输入总量为723.80 kg/hm2/a,输出总量为450.72 kg/hm2/a。土壤残留氮量为273.07 kg/hm2/a,分别是氮素输出总量和输入总量的0.61倍和0.38倍。其中,果林和旱地分别有251.77、598.16 kg/hm2/a的氮素残留于土壤中,加大了氮素面源污染风险。
小流域典型农耕模式下,氨挥发特征为:复合肥的氨挥发变化平缓,氨挥发速率最低;碳铵的氨挥发在第3 d出现峰值,随后快速下降;而尿素的氨挥发峰值滞后于碳铵,在第4~5 d出现,然后缓慢下降。针对氨挥发过程进一步研究发现,施肥后,3种肥料氨挥发速率均随着监测时间的延长呈现先增加后降低的趋势,氨挥发速率峰值出现在施肥后的第3~5 d。相比而言,碳铵的氨挥发速率峰值出现最早,尿素峰值出现滞后,而复合肥的氨挥发速率整体较平缓,无明显峰值出现。
施肥后,氨挥发特征不同主要与肥料性质有关。从3种氮肥的变化趋势来看,碳铵属于速效性肥料,主要以NH4+♂形态存在,易分解为NH3、CO2和H2O 3种气体挥发到大气中。由于施肥期间温度较高,氨挥发非常迅速,在施肥后第3 d即达到峰值,第4 d因温度降低的原因,加上施入土壤的铵与土壤胶体形成结合态铵离子,使得氨挥发速率降低。尿素施入土壤后,需要在脲酶的作用下水解为碳酸铵或碳酸氢铵,进而再产生氨挥发。因此,尿素的氨挥发峰值滞后于碳铵。此外,果林在施加尿素后1~6 d出现两个峰值,这是因为,在实验正式开始前,果林土壤本来氮素残留量少,加上其翻耕少、孔隙度低、容重高、砂粒含量高[
实验观测的后期,尿素和碳铵的氨挥发速率均呈现降低的趋势,原因是由于土壤中有机质分解产生大量有机酸和腐殖酸,使土壤pH值下降,并促使土壤对NH4+的吸附增强,进而抑制氨挥发。这间接说明土壤有机质的增加可以有效降低土壤中的氨挥发损失。李燕青[
各样地土壤氨挥发速率存在显著差异,在一定程度上反映出土壤、气候、耕作方式等环境条件对农田氨挥发的影响[
分析结果可知,流域内果林、旱地和水田施用尿素后氨挥发率分别为10.49%、8.82%和18.37%。可见,尿素在水田中氨挥发率最高,同样的情况也出现在碳铵中,这可能与水田的湿度有关。田昌等[
流域内果林、旱地和水田的氨挥发总量分别为1 238.7、3 457.4、508.7 kg/a,分别占总氮输出量的16.70%、36.25%和14.38%,表明氨挥发是小流域氮损失的最主要途径之一。与其他研究结果相比,小流域氨挥发损失明显高于崔健等[
造成小流域氨挥发率偏高的原因为:1)传统的施肥方式。实地调研表明,小流域施肥主要为表土施肥,缺少翻耕等农事活动,肥料裸露在表土上。研究表明,传统的施肥方式不利于土壤固定氮素,也不利于植物吸收,再加上表层光照强、温度高、空气流通性好等原因,大量肥料氮通过氨挥发而损失[
由此可见,在保持产量的情况下,适当减少施肥量,优化流域肥料结构,采取氮肥配合磷钾肥等复配施用方式,提高种植科技水平等是降低氮肥氨损失量的有效途径。此外,配施缓释剂或缓释肥等新型肥料也是降低氨挥发损失的途径之一。减少氨挥发氮素流失能够减缓三峡库区氮污染,使三峡库区水体富营养化从源头上得到治理。
1) 三峡库区紫色土典型农耕模式下氨挥发速率表现为:复合肥的氨挥发变化平缓,氨挥发速率最低;碳铵的氨挥发在第3 d出现峰值,随后快速下降;而尿素的氨挥发峰值滞后于碳铵,在第4~5 d出现,然后缓慢下降。复合肥氨挥发累积量呈一个缓慢增加的趋势;而尿素和碳铵的氨挥发累积量,表现为两个阶段,一是施肥后立即进入的快速增加阶段,一是3~7 d后的缓慢增加阶段。
2) 对于肥料种类而言,碳铵的氨挥发率为17.86%~30.70%,尿素为8.82%~18.37%,复合肥为2.56%~3.86%。施肥种类间的氨挥发率大小依次为:碳铵>尿素>复合肥。于农耕模式而言,果林的氨挥发率为3.86%~17.86%,旱地为2.56%~19.81%,水田为3.76%~30.70%。典型农耕模式间的氨挥发率大小依次为:水田>果林>旱地。
3) 小流域氮素收支估算发现,氨挥发是小流域氮流失的主要途径之一,典型农耕模式下,水田、果林和旱地的氨挥发损失分别占氮输出总量的14.38%、16.70%和36.25%,占氮损失的44.84%、77.23%和69.20%。
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