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温室气体排放加剧全球气候变暖[1],导致系列环境问题频发,如水资源短缺[2]、区域干旱化加剧[3]等。氧化亚氮(N2O)是主要的温室气体之一,在百年尺度上,N2O造成全球变暖的潜力是二氧化碳(CO2)的265倍[4]。土壤是N2O的一个重要排放源[5],据估计,大气中80%~90%的N2O来源于土壤[6]。土壤中由微生物介导的硝化和反硝化过程是N2O主要产生途径,其中,施肥土壤是N2O排放量最高的土壤。有研究表明:氮(N)、磷(P)肥的大量施用会增加土壤气态氮、氧化氮(NO)、N2O等的排放[7-9]。因此,合理施肥对缓解全球气候变化显得极为重要。土壤N2O的排放不仅仅与施N、P肥相关,还受土壤含水量的影响,土壤含水量是影响N2O的排放的一种潜在因素[10]。有研究表明:土壤含水量的增加会降低土壤N2O的排放[11-12],也有研究表明:土壤含水量的增加会促进土壤N2O的排放[13-15]。聚丙烯酰胺是用强吸水性树脂制成的超高吸水保水能力的高分子聚合物。它可以吸收大量的水,然后形成水凝胶,且可以反复吸水。当土壤处于干旱时,它会缓慢的将水分释放,供土壤和植物利用[16-18],大幅度提高了用水效率。而且聚丙烯酰胺还具有改善土壤结构、低成本、使用方便等诸多优点,对农林产品的生产经营具有一定的实际意义。油茶Camellia oleifera是中国南方最重要的食用油料树种,主要分布于秦岭淮河以南至华南北部,以江西和湖南为主产区。江西省油茶产区属于典型的红壤地区,受亚热带季风气候的影响,夏季水热不同期。在这一时期内,油茶树进行产果,氮磷的缺乏会影响油茶的油脂转化,缺水会导致油茶落果,这将会限制油茶林的高产[19]。因此,保水剂聚丙烯酰胺的施用对油茶林的产果量有着很大意义。近年来,已有不少研究表明施氮、磷肥可以促进温室气体排放[7-9],聚丙烯酰胺可以增加土壤含水量,进而影响土壤N2O排放,那聚丙烯酰胺和氮、磷肥共同施用会如何影响油茶林土壤N2O排放?本研究通过添加不同肥料和不同用量聚丙烯酰胺,来探究聚丙烯酰胺和施肥对油茶土壤N2O排放的影响,为发展高效节水林业和缓解全球气候变化提供理论依据。
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样地位于江西农业大学科技园(28°45′53″N,115°50′10″E),属于亚热带湿润季风气候。年日照时间为1 723~1 820 h,日照率为40%,7−8月最高,2−3月最低;年平均气温约18 ℃,年降水量为1 600~1 700 mm,降水日主要集中在4−6月。土壤为典型红壤。
所用油茶苗是1年生实生苗,土壤采自未施肥油茶林。所施用的氮肥为硝酸铵(NH4NO3,分析纯),磷肥为磷酸氢二钠(Na2HPO4,分析纯)。保水剂聚丙烯酰胺(分析纯)为阴离子型,平均分子量不小于1 000万。土壤基本理化性质:有机碳13.01 g·kg−1、全氮1.46 g·kg−1、速效磷1.07 mg·kg−1、铵态氮1.87 mg·kg−1、硝态氮1.68 mg·kg−1、pH 5.76。
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盆栽试验于2018年4月17日至9月26日进行,盆栽容器为高18 cm,宽16 cm的圆形花盆,每盆土质量为1 500 g。试验共设置12个处理:C0+ck、C0+P、C0+N、C0+N+P、C1+ck、C1+P、C1+N、C1+N+P、C2+ck、C2+P、C2+N、C2+N+P。其中,C0、C1、C2分别为施用保水剂聚丙烯酰胺0、1.0、2.0 g·kg−1。每个处理3个重复,聚丙烯酰胺与土壤混施,氮磷肥加水混合均匀喷施。共施肥3次,分别为6月1日(0.050 g·kg−1 N,0.025 g·kg−1 P)、6月24日(0.040 g·kg−1 N,0.020 g·kg−1 P)和7月18日(0.040 g·kg−1 N,0.020 g·kg−1 P),共施氮肥0.130 g·kg−1,磷肥0.065 g·kg−1。
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采用静态暗箱法测定土壤N2O通量。静态箱体由箱体和底座2个部分组成,箱体是由有机玻璃制成的内径为18 cm,高为80 cm的圆柱体。箱体上侧的中心位置有1个小孔,用于放置温度计,用于记录箱体温度。箱体外部用锡箔纸进行包裹,保证在采气过程中内部是黑暗状态。底座由水进行密封,保证在抽气过程中箱内空间密闭。采样时把每盆油茶苗搬到底座上,再扣上箱体,用50 mL的医用注射器进行气体采集。采气过程中,注射器需反复抽拉10次,保证所采气体是混合均匀的[20]。气体采集时间自5月23日开始,7 d采集1次,每次施肥后的第1、3、6天进行气体采集,后逐渐延长采气时间。每次的气体采集时间为9:00−11:00,盖上箱体后的0、5、10、15 min分别抽取40 mL气体注入真空气袋。在气体采集过程中,用W.E.T Sensor Kit测定盆栽内土壤的温度和湿度。
N2O排放通量的测定:用气相色谱仪(安捷伦7890B)测定N2O质量浓度,并根据方程式计算N2O排放通量(L):
$L=P\times V\times \dfrac{\Delta c}{\Delta t}\times \dfrac{1}{R\times T}\times M\times \dfrac{1}{S}$ 。其中:P代表标准大气压力(Pa),V指箱内空间的体积(m3),Δc/Δt代表单位时间内N2O质量浓度的变化量(μg·L−1),R代表通用气体常数(m3·mol−1·K−1),T代表采气时箱内温度(K),M代表N2O的分子质量(g·mol−1),S代表箱体的底面积(m2)。N2O累积排放量根据以下公式计算:
$$ E=\sum _{i=1}^{n}\frac{\left({L}_{i}+{L}_{i+1}\right)}{2}\times \left({t}_{i+1}-{t}_{i}\right)\times 24{\text{。}}$$ 其中:L代表N2O排放通量(μg·m−2·h−1),i表示第i次进行气体采集,ti+1−ti代表2次采样的间隔天数,n代表气体采集的次数。
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用Excel 2010对数据进行统计和处理,利用JMP 9.0软件进行单因素方差分析,利用Origin 2017软件绘图。
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由图1和图2可见:随着时间的变化,不同处理土壤温度和N2O排放通量无明显差异。聚丙烯酰胺处理土壤含水量高于未添加聚丙烯酰胺,说明聚丙烯酰胺对土壤有很好的保水作用。
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由表1和图3可见:未施肥条件下,C1和C2处理土壤含水量较之C0,分别增加37.5%、53.2%(P<0.05);单施N条件下,土壤含水量从大到小依次为C2、C1、C0,与C0对比,C1和C2分别增加38.3%、61.4%(P<0.05);单施P条件下,C1和C2处理土壤含水量较之C0,分别增加51.4%、71.6%(P<0.05);施N+P条件下,C1和C2处理土壤含水量较之C0,分别增加65.4%、57.6%(P<0.05)。总体上,C1、C2处理土壤含水量均显著高于C0,说明聚丙烯酰胺可以有效提高土壤保水性能。
表 1 不同处理对土壤N2O排放通量、土壤含水量和N2O累积排放量的影响
Table 1. Effects of different treatments on soil N2O emission flux, soil moisture content and cumulative N2O emission
处理 df F N2O排放通量 土壤含水量 N2O累积排放量 N 1 5.41* 13.72** 1.35 P 1 6.27* 5.05* 5.41* N+P 1 2.23 4.19 0.49 C 2 0.66 200.89** 2.75 N+C 2 0.13 1.34 0.18 P+C 2 2.45 3.78* 2.24 N+P+C 2 3.13* 4.17* 1.71 说明:N表示施氮处理,P表示施磷处理,C表示施加聚丙烯 酰胺处理;*P<0.05,**P<0.01 -
由图4和图5可见:添加1.0 g·kg−1聚丙烯酰胺条件下,施N、P、N+P的土壤N2O排放通量较之未施肥土壤,分别增加56.0%、61.7%、40.7%(P<0.05);添加2.0 g·kg−1聚丙烯酰胺条件下,施P、N+P的土壤N2O排放通量较之未施肥土壤,分别增加38.7%、58.1%(P<0.05)。C0+ck、C1+ck、C2+ck处理表明:添加聚丙烯酰胺的土壤N2O排放通量要低于未添加的,但不显著,可能是聚丙烯酰胺施用的量较小。相对于只添加聚丙烯酰胺的处理,施肥和添加聚丙烯酰胺的,土壤N2O排放通量增加,说明聚丙烯酰胺与N、P之间存在某种作用促进土壤N2O排放。施P显著提高土壤N2O累积排放量(P<0.05),相较于ck增加13.3%。
Effects of water-retaining agent on soil nitrous oxide emission in Camellia oleifera forest under nitrogen and phosphorus addition
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摘要:
目的 化肥施用导致土壤氧化亚氮(N2O)排放增加,加剧了全球气候变化。在干旱和降水分配不均地区,土壤含水量是影响土壤N2O排放的关键因子,施用保水剂(如聚丙烯酰胺)可能影响土壤N2O排放。本研究目的是探究氮(N)与磷(P)肥添加下施用聚丙烯酰胺对土壤N2O排放的影响。 方法 以油茶Camellia oleifera林土壤为研究对象,设置不同处理,包括不同肥料添加[N、P、N+P、不施肥(ck)],不同聚丙烯酰胺用量(C0:0 g·kg−1,C1:1.0 g·kg−1,C2:2.0 g·kg−1)以及两者交互处理,利用静态箱-气相色谱法测定油茶苗生长期内土壤N2O排放。 结果 ①施用聚丙烯酰胺显著提高了油茶林土壤含水量(P<0.05),且土壤含水量随保水剂施用量的增加而增加。与C0相比,C1和C2土壤的含水量分别增加47.1%和57.4%,但施用聚丙烯酰胺不会促进土壤N2O排放(F=2.75,P>0.05)。②施磷肥显著提高土壤N2O累积排放量(P<0.05),相较于ck增加13.3%。③与只添加聚丙烯酰胺的土壤相比,1.0 g·kg−1聚丙烯酰胺分别与N、P、N+P肥混施处理的土壤N2O排放通量分别显著增加56.0%、61.7%、40.7% (P<0.05);2.0 g·kg−1聚丙烯酰胺与P、N+P肥混施处理的土壤N2O排放通量分别显著增加38.7%、58.1% (P<0.05)。 结论 施用聚丙烯酰胺不仅能有效提高油茶土壤保水能力,而且还不会促进油茶土壤N2O排放,有利于发展高效节水林业和缓解全球气候变化。图5表1参35 Abstract:Objective The application of chemical fertilizer accelerates soil nitrous oxide (N2O) emission and intensifies global climate change. Soil moisture content is a key factor affecting soil N2O emission in arid and uneven rainfall distribution areas, so the application of water retaining agent (such as polyacrylamide) affects soil N2O emission. The purpose of this study is to explore the effect of polyacrylamid on soil N2O emission under the application of N and P fertilizer. Method Taking Camellia oleifera forest soil as the research object, different treatments were set, including different fertilizer additions (N, P, N+P, ck), different polyacrylamide dosage (C0: 0 g·kg−1, C1: 1.0 g·kg−1, C2: 2.0 g·kg−1) and their interaction. The soil N2O emission during the growth of C. oleifera seedlings was determined by static chamber gas chromatography. Result (1) The application of polyacrylamide significantly increased the soil moisture content of C. oleifera (P<0.05), which increased with the increase of the application amount. Compared with C0, the soil moisture content of C1 and C2 increased by 47.1% and 57.4% respectively, but the application of polyacrylamide did not promote soil N2O emission (F=2.75, P>0.05). (2) The application of P fertilizer significantly increased the cumulative soil N2O emission (P<0.05), which increased by 13.3% compared with ck. (3) Compared with the soil supplemented only with polyacrylamide, the N2O emission fluxes of soil treated with 1.0 g·kg−1 polyacrylamide and N, P, N+P fertilizer increased significantly by 56.0%, 61.7%, and 40.7%, respectively (P<0.05). The N2O emission fluxes of soil treated with 2.0 g·kg−1 polyacrylamide and P, N+P fertilizer increased significantly by 38.7% and 58.1%, respectively. Conclusion The application of polyacrylamide in C.oleifera soil can effectively improve soil water holding capacity, but will not promote soil N2O emission, which is conducive to the development of efficient water-saving forestry and the mitigation of global climate change. [Ch, 5 fig. 1 tab. 35 ref.] -
表 1 不同处理对土壤N2O排放通量、土壤含水量和N2O累积排放量的影响
Table 1. Effects of different treatments on soil N2O emission flux, soil moisture content and cumulative N2O emission
处理 df F N2O排放通量 土壤含水量 N2O累积排放量 N 1 5.41* 13.72** 1.35 P 1 6.27* 5.05* 5.41* N+P 1 2.23 4.19 0.49 C 2 0.66 200.89** 2.75 N+C 2 0.13 1.34 0.18 P+C 2 2.45 3.78* 2.24 N+P+C 2 3.13* 4.17* 1.71 说明:N表示施氮处理,P表示施磷处理,C表示施加聚丙烯 酰胺处理;*P<0.05,**P<0.01 -
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