黑土根际土壤有机碳及结构对长期施肥的响应

Changes of soil organic carbon and its structure in rhizosphere of black soil under long-term fertilization

由于人类农业措施的干扰,氮肥和畜禽粪污大量输入到黑土中,对土壤有机碳库产生了较大负面影响。根际有机碳在调控土壤碳循环和养分转化中发挥着重要的作用。探明根际有机碳对不同养分的生态响应,可为不同施肥处理下黑土农田生态系统碳固持和农田可持续利用提供理论依据。该研究以黑土长期定位试验为基础,采集长期不施肥(CK)、常量氮(N)、二倍量氮(N2)、常量有机肥(M)、二倍量有机肥(M2)、常量有机肥+常量氮(MN)、二倍量有机肥+二倍量氮(M2N2)7个处理下大豆根际土壤,分析了根际有机碳和活性有机碳特征,同时利用固态核磁共振技术分析其光谱特征。结果表明,N、M2、MN和M2N2处理的根际土壤有机碳含量显著高于非根际水平,且以MN和M2N2处理的根际效应最显著,分别比非根际增加了18.3%和26.7%。分析核磁共振光谱显示,与非根际土壤相比,根际土壤具有较高的烷氧基碳比例和较低芳香碳比例,表明根际效应能够改变土壤有机碳结构比例。与不施肥处理相比,大部分施肥处理提升了黑土根际有机碳含量,其中以氮肥马粪配施和二倍量马粪(M2N2)处理提升幅度最高。由核磁共振图谱可知,M2和M2N2处理均增加根际土壤难降解成分烷基碳比例、芳香基碳比例、烷基碳与烷氧基碳比值、芳香碳与总碳比值,而MN处理仅增加烷基碳比例、烷氧基碳比例以及烷基碳与烷氧基碳比值。二倍量氮肥(N2)处理降低烷基碳比例、芳香碳比例和烷氧基碳比例,根际土壤难降解成分降低,不利于土壤固碳,同时证明固态^(13)C-核磁共振技术结合半定量分析能够准确地分析不同有机碳结构组分变化,深刻认识根际土壤有机碳的稳定机制。

A large amount of nitrogen fertilizer and manure can be input into the arable land of black soil in agricultural production,thus resulting in a serious negative impact on the soil’s organic carbon pool.Nearly 40%of plant primary products are transferred to the soil as substantial rhizodeposits.There is also an important carbon source for the soil microorganism surrounding the root of the plant.The rhizosphere soil can be often the zone of the highest soil microbial activity,further regulating the soil carbon cycle and nutrient transformation in terrestrial ecosystems.Therefore,it is necessary to clarify the responses of soil organic carbon content and structure to the intensification of nitrogen fertilizer and manure.In this study,a 38-year(1979-2017)long-term fertilization experiment of wheat-soybean-maize rotation was conducted at Harbin County,Heilongjiang Province,China.The rhizosphere soil of soybean was collected from nine treatments,including no fertilization(CK),nitrogen fertilizer(N),two times of nitrogen fertilizer(N2),horse manure(M),two times of horse manure(M2),horse manure plus nitrogen fertilizer(MN),and two times of horse manure plus nitrogen fertilizer(M2N2).Each treatment was performed on the 36 m^(2)(4 m×9 m)plot area.The soil organic carbon and labile organic carbon were also characterized in the 0-20 cm profile soil.A carbon-13(C^(13))nuclear magnetic resonance(^(13)C-NMR)spectroscopy was used to analyze the spectral pattern of the organic carbon in the rhizosphere soil.The results showed that the organic carbon content of rhizosphere soil was higher than that in the bulk soil for most treatments.About 18.3%and 26.7%of exogenous organic carbon contents were supplemented in the MN and M2N2 treatments,respectively.The ^(13)C-NMR spectroscopy showed that there were a higher proportion of O-alkyl C and the lower proportion of aromatic C than those in the bulk soil,indicating that the soil organic carbon structures were changed by the rhizosphere effect.Furthermore,the organic carbon content of rhizosphere soil increased significantly by fertilization,especially the highest organic carbon content under M2N2 treatments,compared with the CK.The organic carbon contents increased by 3.21 and 1.93 g/kg in the 0-20 cm profile soil of M2N2 treatments,respectively,compared with the N2 and M2.There was also an increased proportion of alkyl C,aromatic C,the ratios of alkyl C to O-alkyl C,and the ratios of aromatic C to total C in rhizosphere soil under the M2 and M2N2 treatments.There was an increase in the proportion of alkyl C,O-alkyl C,and the ratios of alkyl C to O-alkyl C under the MN treatment,where the formation of aggregates was promoted for the stability of soil particle structure.Nevertheless,there was a decrease in the proportion of alkyl C,aromatic C,and O-alkyl C in the rhizosphere soil under the N2 treatment,particularly for the less stability of aggregates,which did not benefit the soil carbon sequestration.Correspondingly,the MN/M2N2 treatment can be widely expected to significantly promote the organic carbon content of rhizosphere soil,the alkyl C,the ratios of alkyl C to O-alkyl C in the rhizosphere soil,and the O-alkyl C in the bulk soil,particularly for the formation of aggregates and the stability of soil particle structure in the bulk soil.Anyway,the mixed application of horse manure and nitrogen fertilizer can greatly contribute to enhancing the soil’s organic carbon retention.The excessive application of nitrogen fertilizer(N2)can reduce the alkyl C,aromatic C in the rhizosphere soil,and the O-alkyl C in the bulk soil,leading to a threat to the environment from the less stability of aggregates once after the autumn harvest.At the same time,the ^(13)C-NMR spectrum combined with the semi-quantitative analysis can be widely used to comprehensively characterize the structures of functional groups,and further better understand the stability mechanism of organic carbon in the rhizosphere soil.