# Comparison of Microbial Gene Diversity in Grassland Topsoil Depending on Soil Quality
###### Abstract  
Soil has multiple functions, including the provision of habitat to organisms, and most biological activities occur in the surface soil. Due to the negative effects of soil erosion, efforts for soil conservation are being made, including the development of a reliable index that can help assess soil quality. In this study, the physical and chemical properties and biological genes from grassland topsoil were analyzed, in order to identify surface soil organism markers that could be used as a soil quality index. Six spots of grassland topsoil were analyzed, one high-quality and five low-quality, based on a web-based soil quality assessment module. Consequently, eukaryotes and prokaryotes with different soil quality ratios were compared and examined. The following bacteria and archaea have the potential to be used in soil quality assessment: circulation of materials including nitrogen, _Nitrospin_ spp., _Candidatus_ Nitrososphaera, and _Candidatus_ Nitrosotalea; biological purification, _Geobacter_ spp.; pathogens, _Burkholderia_ spp., _Paraburkholderia_ spp., _Pseudomonas brassicacarum_, and _Rhizobacter_ spp.; antibiotic secretion, _Candidatus_ Udaeobacter; and material degradation _Steroidobacter_ spp. and _Rhodanobacter_ spp. This study provides primary data for identifying biological markers for soil quality evaluation. In the future, a wider variety of data need to be accumulated to develop a highly reliable index related to soil quality.  
soil quality index; surface soil organism marker; grassland topsoil 25 September 2021 13 October 2021 14 October 202  
soil loss is constantly increasing, due to an extreme climate [6]. Surface soil erosion could threaten the versatility of soil through the loss of ecological niches, and it could exacerbate global warming because of reduced carbon storage capacity and the aggravation of river water pollution, due to soil loss. As it takes a long time to rebuild the eroded surface soil, this is considered an important field in agriculture and environment sciences [4]. On the contrary, strict measures for soil conservation, such as the construction of an early warning system for the potential loss of soil versatility and the development of a reliable index to assess soil quality, have been implemented in developed countries such as those in Europe. Accordingly, a soil quality assessment model based on physical and chemical properties has been reported [7].  
However, the debate regarding the biological index for soil quality assessment is ongoing. Although it requires the discovery of organisms that could ideally explain core functions and elucidate the key roles of different species, it is difficult to identify specific keystone markers, owing to the large functional redundancy of the soil microbial community [3]. The application of molecular biology techniques based on next generation sequencing (NGS) allows the accumulation of baseline data on soil biodiversity [2; 8; 9; 10; 11; 12]. However, related research has focused on the agricultural environment linked to the production of high-quality crops, with relatively limited research on soil quality. In addition, agricultural soil may not be ideal for diversity research related to soil quality, as it could be affected by artificial factors, including specific agricultural crops and the fertilizers applied. In contrast, grassland occupies around 26-40% of the total land mass, contains the highest level of biomass on earth [13], and is not influenced by artificial factors like agricultural soil; hence, it is seemingly suitable for diversity research. However, because of limited baseline data and related research, in this study, we compared and analyzed NGS-based biological gene diversity according to soil quality in grassland topsoil, and examined potential biological markers related to soil quality.