Karst ecosystems in China comprise approximately 5.8% of land surface, 23.5% of which is covered by grassland. Massive deforestation and agricultural expansion caused by human activities have resulted in serious land degradation, causing loss of various soil nitrogen (N) in karst grassland.

External N input (N fixation from leguminous plants, N deposition, and N fertilization) can increase N supply and alleviate N stress. Diazotrophs are the important functional microbes for N cycling and sustaining soil quality. However, how leguminous N input and N fertilization affect diazotrophs in both growing and non-growing seasons remains poorly understood.

Researchers from the Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences (CAS), Guangxi Zhuang Autonomous Region, southwest China investigated the changes of diazotrophs abundance, diversity, and community composition under different N inputs in the natural and artificial (forage grass cultivation) grasslands.

In the natural grassland, the nutrient addition experiment (control, N addition, phosphorus (P) addition, and N plus P addition) and the introduction of leguminous shrubs (low, and high densities of Amorpha fruticosa and Indigofera atropurpurea) were established. In artificial grasslands, the introduction of leguminous shrubs (A. fruticosa) and N fertilization were applied.

Researchers found that in both natural grassland and cultivated forage grass field, the diazotroph community composition, at the genus level, was dominated by Glomus. Diazotroph abundance was significantly lower in the N input and high-density I. atropurpurea treatments than in the control plots in July.

These results suggest that I. atropurpurea at a high planting density and N fertilization could inhibit diazotroph growth by increasing ammonium N and nitrate N, which was not conducive to positive succession.

Besides, diazotroph diversity and community compositions were more sensitive to P addition and leguminous shrub than to N addition, and P availability played an important role in regulating N2 fixation by increasing diazotroph diversity in karst grasslands.

Furthermore, both N fertilization (500 kg N ha-1 yr-1) and legumes could reduce the interspecific competition among diazotroph species by providing greater N availability in the forage grass.

The lesser complexity of the co-occurrence network in A. fruticosa plots with or without N fertilization, compared to those in N fertilization plots, suggests that planting legumes may reduce the necessity of cooperation among diazotroph groups when the nutrient availability would increase through root exudates or N fixing.

The study was supported by the National Key Research and Development Program, the National Key Basic Research Program of China, and the National Natural Science Foundation of China.

The study entitled "Phosphorus but not nitrogen addition significantly changes diazotroph diversity and community composition in typical karst grassland soil", "Responses of soil diazotrophs to legume species and density in a karst grassland, southwest China", and "Nitrogen fertilizer and Amorpha fruticosa leguminous shrub diversely affect the diazotroph communities in an artificial forage grassland" was published in Agriculture, Ecosystems and Environment and Science of the Total Environment, respectively. Details can be found at https://www.sciencedirect.com/science/article/pii/S0167880920301729

Contact: WANG Kelin

E-mail: kelin@isa.ac.cn

Institute of Subtropical Agriculture, Chinese Academy of Sciences