The nitrogen cycle and factors affecting it in the belowground ecosystem

Nitrogen (N) is one of the main factors limiting primary productivity in terrestrial ecosystems due to its role in plant growth and development. In belowground ecosystems, most nitrogen exists in the form of complex polymers, but these forms of nitrogenous compounds cannot be absorbed and assimilated directly by plants. Hence, the ecological process from complex polymers to simple inorganic substances has always been a focus of research; this process is regulated and controlled by a series of complex biogeochemical reactions that occur belowground via symbiotic associations between plant roots and bacteria (e.g., biological nitrogen fixation). In this paper, we summarize the effects of soil biotic and abiotic factors on the belowground nitrogen cycle. Soil organisms promote the belowground nitrogen cycle through metabolism and release of various enzymes; Plant roots change the soil properties and secrete organic matter. The development of soil organisms is strongly restricted by abiotic factors, so the interaction between organisms and some abiotic factors also exerts important influences on the belowground nitrogen cycle. Currently, due to the high abundance of soil organism species, the complexity of nutrient circulation patterns, the limited understanding regarding the response of nature to global climate change, and the expense of molecular biotechnology, it is still difficult to define the whole nitrogen nutrition circulation network in the belowground ecosystem at the molecular level. This review summarizes our current understanding regarding the belowground nitrogen cycle and how it is regulated by biotic and abiotic factors.