干湿交替频率影响下非饱和带有机氮转化的生物学机制

Biological mechanism of dry-wet alternation frequency affecting organic nitrogen transformation in vadose zone

为明晰非饱和带中溶解性有机氮(DON)在干湿交替频率影响下的转化机制,以大沽河流域集约化农业区为研究区,开展了不同干湿交替频率的室内批次试验,分析了氮素含量及其影响因素(土壤理化性质、微生物指标)的动态变化特征,通过冗余分析识别了氮素生物转化的驱动因子,基于环境因子、功能基因和氮素转化之间的关系建立了路径模型。结果表明:施加DON后快速发生氨化,干期NO_(3)^(-)-N逐渐累积,而湿期NO_(3)^(-)-N逐渐转化为NO_(2)^(-)-N和NH_(4)^(+)-N,不同频率下干期均以硝化为主、湿期均以反硝化为主;随着非饱和带干湿循环次数从1次提高至3次,每个干期NO_(3)^(-)-N累积量有所减少,但湿期NO_(3)^(-)-N消耗量减少更为显著,导致最终土壤中NO_(3)^(-)-N的赋存含量(每千克干土壤中NO_(3)^(-)-N的质量)从14.28 mg/kg提高至34.16 mg/kg,频繁干湿交替增加了DON对地下水硝酸盐污染的潜在风险;随着干湿交替频率的提高,干期硝化菌属Nitrospira和湿期反硝化菌属Pseudarthrobacter相对丰度均逐渐下降,相关功能基因丰度也呈现下降趋势;含水率、氧化还原电位和溶解性有机碳会影响氮素转化相关基因表达,从而间接调节干湿交替下DON的转化行为。

To clarify the transformation mechanism of dissolved organic nitrogen(DON)in vadose zone under alternating dry and wet conditions,an intensive agricultural region within the Dagu River Basin was used as the study area,and indoor experiments were performed to analyze dynamic changes in nitrogen content and its influencing factors(soil physical and chemical properties and microbial indicators)under varying frequencies of dry-wet cycles.The driving factors of nitrogen transformation were identified through redundancy analysis,and a path model was established based on the relationships among environmental factors,functional genes,and nitrogen transformation.The results show that DON undergoes rapid ammonification,with an accumulation of NO_(3)^(-)-N during the dry period that subsequently transforms into NO_(2)^(-)-N and NH_(4)^(+)-N during the wet period,suggesting a dominance of nitrification processes during the dry period and denitrification processes during the wet period across all dry-wet alternation frequencies.With an increase in the number of dry-wet cycles in vadose zone from one to three times,the cumulative amount of NO_(3)^(-)-N decreases during the dry period,while NO_(3)^(-)-N depletion has a more pronounced decrease during the wet period,resulting in the residual NO_(3)^(-)-N content(the mass of NO_(3)^(-)-N in per kilogram of dry soil)increasing from 14.28 mg/kg to 34.16 mg/kg.Therefore,frequent dry-wet alternations increase the potential risk of DON exacerbating groundwater nitrate pollution.The values of relative abundance of the nitrifying genus Nitrospira in the dry period and the denitrifying genus Pseudarthrobacter in the wet period gradually decrease with the increase of the dry-wet alternation frequency,and the values of abundance of related functional genes also show a downward trend.Water content,redox potential,and dissolved organic carbon affect the expression of nitrogen transformation-related genes,thereby indirectly regulating the transformation behavior of DON under dry-wet alternations.