环境因子对水稻土硝酸根异化还原过程速率和分配的影响

Effects of Environmental Factors on Rate and Partitioning of Dissimilatory Nitrate Reduction Processes in Paddy Soils

以五常、常熟和雅安水稻土为研究对象,通过室内泥浆培养,利用基于膜进样质谱仪(Membrane Inlet Mass Spectrometer,MIMS)的15N示踪技术,探究了温度、pH、NO_(3)^(–)浓度、C/N、Fe^(2+)和S2–浓度对三种水稻土反硝化和硝酸根异化还原成铵(Dissimilatory nitrate reduction to ammonium,DNRA)速率及二者占硝酸根还原过程相对贡献的影响。结果表明,在所研究的稻田土壤中,反硝化是NO_(3)^(–)异化还原过程的主导途径,占比87.97%~91.73%,而DNRA仅占8.27%~12.03%。反硝化和DNRA速率随温度升高均呈指数增长,且DNRA占NO_(3)^(–)异化还原的比例(RDNRA)也随温度升高呈增长趋势。反硝化和DNRA速率分别在pH为7或者8.5时最高,相对于碱性环境(4.92%~14.67%),酸性环境中RDNRA(6.24%~15.56%)更高。反硝化和DNRA速率与NO_(3)^(–)浓度之间关系符合米氏方程,且反硝化的最大速率(Vmax)和米氏常数(Km)均大于DNRA。与未加碳源对照组相比,C/N为2.5时,反硝化速率显著提高了22%~35%;C/N大于2.5时,DNRA速率显著提高了74%~199%。三种土壤中,Fe^(2+)添加和S2–添加处理中呈现出类似的趋势,均在低浓度电子供体(即Fe^(2+)和S2–浓度分别为300~500μmol·L^(-1)和50~62.5μmol·L^(-1))时呈现出最高的反硝化速率,而DNRA速率达到峰值则需要更高浓度的电子供体(即Fe^(2+)和S2–浓度分别为800~1000μmol·L^(-1)和100~125μmol·L^(-1))。综上可知,环境因子可显著影响NO_(3)^(–)异化还原过程的速率及分配,其中高温、高C/N、高浓度Fe^(2+)和S2–有利于更多的NO_(3)^(–)分配给DNRA过程,而高浓度NO_(3)^(–)会提高NO_(3)^(–)向反硝化过程的分配。上述研究结果深化了对水稻土NO3–异化还原过程分配的认识,对于探寻潜在农学措施提高DNRA过程的分配比例,进而提高土壤中氮素的固持和提高稻田氮肥利用率具有重要的科学意义。

【Objective】Denitrification and dissimilatory nitrate reduction to ammonium(DNRA)are the two competing nitrate reduction pathways that remove the available nitrate,both of which are affected by inter-related environmental factors.Understanding how environmental factors regulate nitrate partitioning in these two competing processes is of great significance for the optimization of nitrogen management in paddy fields.【Method】Using 15N-tracing technique in combination with membrane inlet mass spectrometer(MIMS),a series of laboratory incubation experiments were performed to investigate the effects of different environmental factors including temperature(5,15,20,25,and 35℃),pH(5,6,7,8.5,and 9.5),NO_(3)^(–)concentration(50,100,150,200,and 300μmol·L^(–1)),C/N(0,2.5,5,12,and 30),Fe^(2+)(0,300,500,800,and 1000μmol·L^(–1)),and S2–(0,50,62.5,100,and 125μmol·L^(–1))on denitrification and DNRA rates and their partitioning in nitrate reduction in three paddy soils(Wuchang,WC;Changshu,CS;Ya’an,YA).【Result】Denitrification was the predominant pathway(87.97%–91.73%),whereas DNRA only contributed to 8.27%–12.03%of the total dissimilatory nitrate reduction in all treatments.Denitrification and DNRA rates increased exponentially with increasing temperature,as well as the DNRA/(Denitrification+DNRA)ratio(RDNRA).The highest denitrification and DNRA rates occurred at the pH of 7 and 8.5,respectively,and RDNRA was higher in an acidic environment(6.24%–15.56%)than under an alkaline environment(4.92%–14.67%).The response of denitrification and DNRA rates to nitrate concentrations fitted well with the Michaelis-Menten relationship,in which the Vmax and Km of denitrification were larger than those of DNRA.In the three paddy soils,compared with the treatment without glucose addition,denitrification rates were significantly increased by 22%–35%at the C/N ratio of 2.5.Following the C/N ratio increased to>2.5,DNRA rates were enhanced by 74%–199%.In terms of Fe^(2+)and S2–addition treatments,denitrification rates were the highest in the low levels of electron donors(300–500μmol·L^(–1)Fe^(2+)and 50–62.5μmol·L^(–1)S2–),whereas more electron donors(800–1000μmol·L^(–1)Fe^(2+)and 100–125μmol·L^(–1)S2–)were required when DNRA reached the peak rates.【Conclusion】By exerting different effects on rates of denitrification and DNRA,temperature,pH,NO_(3)^(–),C/N ratio,Fe^(2+),and S2–concentration significantly changed the partitioning between denitrification and DNRA.Specifically,relatively high temperature,C/N,Fe^(2+),and S2–concentration favored nitrate partitioning to DNRA,while denitrification dominated the nitrate reduction process in environments with a relatively high NO_(3)^(–)concentration.Collectively,our results provide comprehensive information in terms of regulation of environmental factors on nitrate partitioning between DNRA and denitrification in paddy soils.This deepens our understanding of nitrate reduction processes and provides a scientific basis for increasing nitrogen use efficiency by favoring the nitrate partitioning to DNRA in rice fields.