中国科学院水利部水土保持研究所机构知识库

  东北黑土区作为我国重要的粮食生产基地之一，因近百年的过度垦殖和水土流失导致土壤碳氮含量显著降低，对我国粮食安全造成严重威胁。已有研究集中于开垦50年内单一土壤类型的碳氮含量变化，缺乏长时间序列侵蚀和耕作条件下土壤碳氮含量演变规律及侵蚀速率与碳氮、微生物群落结构的关系研究。因此，本文以黑龙江省嫩江县鹤山农场不同开垦时间黑土（未开垦及1906年、1956年、1965年、1986年、1996年开垦）和草甸土（未开垦及1952年、1986年、1997年、2001年开垦）为研究对象，在2006年和2019年分别采集各样地土壤样品，测定了土壤理化性质，并利用¹³⁷Cs和²¹⁰Pb_ex法估算了土壤侵蚀速率，采用高通量测序分析了土壤微生物群落结构和多样性，并整理了文献中黑土约18个开垦年限和草甸土约13个开垦年限的碳氮含量数据，在此基础上分析了两类土壤碳氮含量与开垦年限的相关性，研究了碳氮演变的驱动机制。主要研究结果如下：

（1）阐明了不同开垦年限黑土和草甸土有机碳、全氮含量演变规律

  未开垦黑土和草甸土表土有机碳含量分别为40.05～87.34 g kg^-1和80.28～129.74 g kg^-1。随开垦年限增加，两者都呈指数函数规律下降，表达式分别为y=22.99+34.64*exp(-0.04*x)和y=30.44+73.08*exp(-0.21*x)，其下降速率分别呈y=1.49*exp(-0.04*x)和y=15.49*exp(-0.21*x)的指数函数规律减小；分别在开垦110年和25年后趋于稳定，稳定时的黑土有机碳含量低于草甸土。近13年来，耕地黑土有机碳含量平均下降速率随开垦年限增加呈减小趋势；开垦67年耕地草甸土碳含量增加，可能与沉积作用有关。

  未开垦黑土、草甸土表土全氮含量分别为2.47～7.32 g kg^-1和5.26～10.88 g kg^-1。随开垦年限增加，两者都呈指数函数规律下降，表达式分别为y=1.88+2.81*exp(-0.04*x)和y=2.49+5.80*exp(-0.22*x)，其下降速率分别呈y=0.10*exp(-0.04*x)和y=1.29*exp(-0.22*x)的指数函数规律减小；分别在开垦130年和23年后趋于稳定，稳定时的黑土全氮含量低于草甸土。近13年来，耕地黑土和草甸土全氮含量平均变化速率随开垦年限增加没有明显变化规律。

（2）揭示了土壤侵蚀对黑土、草甸土碳氮含量的影响机制

  随开垦年限增加，除黑土开垦113年地块，黑土年均土壤侵蚀厚度、年均土壤侵蚀速率和总土壤侵蚀量均增加；草甸土年均土壤侵蚀厚度和总土壤侵蚀量随开垦年限增加而增加，但耕地年均土壤侵蚀速率接近；草甸土土壤侵蚀强度低于黑土。黑土和草甸土土壤有机碳、全氮含量与土壤侵蚀速率、土壤侵蚀量负相关；表层（0-15 cm）土壤碳氮含量随土壤侵蚀强度增加的下降速率大于0-30 cm土壤，土壤侵蚀对表层碳氮含量的影响大于亚表层（15-30 cm）；黑土碳氮含量随土壤侵蚀强度增加的下降速率小于草甸土，但黑土侵蚀速率大于草甸土，侵蚀后黑土土壤质量更差。

（3）建立了土壤微生物群落结构和多样性与黑土、草甸土碳氮含量的关系

  变形菌门、酸杆菌门、疣微菌门和放线菌门是不同开垦年限黑土和草甸土的主要优势菌门，共计分别占两类土壤总细菌丰度的68.66%～75.85%和71.47%～81.00%；开垦后各优势菌门相对丰度变化规律不同。黑土细菌丰度与土壤侵蚀量的相关程度高于与土壤侵蚀速率的相关程度，草甸土细菌丰度与土壤侵蚀速率的相关程度高于与土壤侵蚀量的相关程度。黑土物种丰富度和多样性随开垦年限的增加先升高后降低，草甸土的呈相反趋势；未开垦样地与开垦样地被聚类为两类；耕作和水力侵蚀导致土壤碳氮减少，而变形菌门的固氮作用利于黑土中硝态氮的增加，进而使土壤速效养分增加，限制了疣微菌门的生长。

  综上，开垦引起耕作侵蚀，与自然侵蚀共同作用降低黑土和草甸土有机碳、全氮含量，导致表层土壤碳氮含量随开垦年限增加呈指数下降，且对于侵蚀强度更大的黑土，土壤碳氮含量更难稳定；侵蚀作用下，土壤微生物群落组成发生改变，进而影响土壤碳氮含量。因此，保护黑土地需从减少土壤侵蚀做起。

As one of the most important grain production bases in China, the black soil region in Northeast China has been seriously threatened by the significant reduction of soil organic carbon (SOC) and total nitrogen (TN) due to excessive reclamation and soil erosion in the past 100 years. Previous studies have focused on the changes of SOC and TN content of a single soil type within 50 years of reclamation. However, there are few studies on the evolution of SOC and TN, as well as the relationship among soil erosion rate, carbon, nitrogen and microbial community, especially over long periods of reclamation. To assess this, the black soil (uncultivated and reclaimed in 1906, 1956, 1965, 1986, 1996) and meadow soil (uncultivated and reclaimed in 1952, 1986, 1997, 2001) with different reclamation time of Heshan Farm in Nenjiang County, Heilongjiang Province were taken as the research object, the soil samplings of each site were sampled in 2006 and 2019, and the soil physical and chemical properties were determined, and the soil erosion rate was estimated using ¹³⁷Cs and ²¹⁰Pb_ex methods, as well as soil microbial community structure and diversity were analyzed by high-throughput sequencing. Also, the data on soil organic carbon and total nitrogen content of about 18 reclamation time for black soil and 13 reclamation time for meadow soil were compiled from the literature. On this basis, the correlation between soil carbon, nitrogen content and reclamation years of the two types of soil was carried out, and the driving mechanism of the changes in carbon and nitrogen content was studied. The main findings are as follows:

(1) The evolution of soil organic carbon and total nitrogen content in black soil and meadow soil with different reclamation years was clarified

The uncultivated surface SOC contents of black soil and meadow soil were between 40.05-87.34 g kg^-1 and 80.28-129.74 g kg^-1, respectively. With the increase of reclamation years, both decrease exponentially, and the functions were y=22.99+34.64*exp(-0.04*x) and y=30.44+73.08*exp(-0.21*x), respectively, as well as the decline rate of SOC content decreased with the increase of reclamation years in an exponential function of y=1.49*exp(-0.04*x) and y=15.49*exp(-0.21*x), respectively. With the SOC content of black soil and meadow soil tended to be stable after 110 and 25 years of reclamation, respectively, the SOC content of black soil was lower than that of meadow soil. In the past 13 years, the average decline rate of SOC content in black soil of cultivated land showed a decreasing trend with the increase of reclamation years; the SOC content of cultivated meadow soil increased in 67 years of reclamation, which may be related to sedimentation.

The uncultivated surface TN contents of black soil and meadow soil were between 2.47-7.32 g kg^-1 and 5.26-10.88 g kg^-1, respectively. With the increase of reclamation years, both decrease exponentially, and the functions were y=1.88+2.81*exp(-0.04*x) and y=2.49+5.80*exp(-0.22*x), respectively, as well as the decline rate of TN content decreased with the increase of reclamation years in an exponential function of y=0.10*exp(-0.04*x) and y=1.29*exp(-0.22*x), respectively. With the TN content of black soil and meadow soil tended to be stable after 130 and 23 years of reclamation, respectively, the TN content of black soil was lower than that of meadow soil. In the past 13 years, the average change rate of TN content in cultivated black soil and meadow soil had no obvious regularity with the increase of reclamation years.

(2) The influence mechanism of soil erosion on SOC and TN content in black soil and meadow soil was elucidated

With the increase of reclamation years, the average annual soil erosion thickness, average annual soil erosion rate and total soil erosion amount of black soil increased, except for 113 years of black soil reclamation; the average annual soil erosion thickness and total soil erosion amount of meadow soil increased with the increase of reclamation years, but the average annual soil erosion rate of cultivated land was similar. The soil erosion intensity of meadow soil was lower than that of black soil. The SOC and TN content of black soil and meadow soil were negatively correlated with soil erosion rate and soil erosion amount. The decline rate of SOC and TN content in surface layer (0-15 cm) of black soil and meadow soil with the increase of soil erosion intensity was greater than that in 0-30 cm soil, and soil erosion had a greater impact on surface layer SOC and TN content than subsurface layer (15-30 cm). The decline rate of black soil carbon and nitrogen content with the increase of soil erosion intensity was less than that of meadow soil. However, since the erosion rate of black soil was higher than that of meadow soil, the quality of black soil became worse after erosion.

(3) The relationships between soil microbial community structure and diversity and the SOC, TN content of black soil and meadow soil were established

Proteobacteria, Acidobacteria, Verrucomicrobia and Actinobacteria were the main dominant phyla of black soil and meadow soil with different reclamation years, which together accounted for 68.66%-75.85% and 71.47%-81.00% of the total bacterial abundance of the two soils, respectively. The changes of relative abundance of each dominant bacteria phylum were different after reclamation. For black soil, the correlation degree between bacterial abundance and soil erosion amount was higher than its correlation degree with soil erosion rate, and for meadow soil, the correlation degree between the bacterial abundance and the soil erosion rate was higher than the correlation degree with erosion amount. The species richness and diversity of black soil first increased and then decreased with the increase of reclamation years, while that of meadow soil shown the opposite trend. Uncultivated plots and reclaimed plots were clustered into two categories. Tillage and water erosion lead to the reduction of SOC and TN, and the nitrogen fixation function of Proteobacteria was conducive to the increase of nitrate nitrogen in black soil, and then soil available nutrients increased, which restricted the growth of Verrucomicrobia.

In conclusion, tillage erosion caused by reclamation, together with natural erosion, reduced the contents of SOC and TN in black soil and meadow soil, resulting in an exponential decrease of surface SOC and TN content with the increase of reclamation years. For black soil with greater erosion intensity, SOC and TN content were more difficult to stabilize. Soil microbial community composition was changed under erosion, which affected soil carbon and nitrogen content. Therefore, the protection of black soil needs to start from reducing soil erosion.