Microbial functional diversity, metabolic quotient, and invertase activity of a sandy loam soil as affected by long-term application of organic amendment and mineral fertilizer

Organic and inorganic fertilizers are used primarily to increase nutrient availability to plants. Monitoring balanced versus unbalanced fertilization effects on soil microbes could improve our understanding of soil biochemical processes and thus help us to develop sound management strategies. The objective of this study was to investigate the effects of long-term fertilization regimes on soil microbial community functional diversity, metabolic activity, and metabolic quotient and to find out the main factors that influence these parameters. A long-term fertilization experiment established in a sandy loam soil at northern China has received continuous fertilization treatments for more than 20 years, including control, mineral fertilizers of NK, PK, NP, and NPK, organic amendment (OA), and half organic amendment plus half mineral fertilizer (1/2 OM). Top soil samples (0-15 cm) from four individual plots per treatment were collected for the analysis of chemical properties and microbial parameters. Microbial biomass C was analyzed using the fumigation-extraction method. Invertase activity and basal respiration were determined based on incubation method. Then, the microbial metabolic quotient was calculated as the ratio of basal respiration to microbial biomass C. To this end, microbial functional diversity was evaluated using the community level physiological profile method by Biolog Eco-microplate. Higher microbial biomass C, invertase activity, and basal respiration, but lower microbial metabolic quotient, were observed in P-fertilized soils, and OA had significantly greater (P < 0.05) impacts on the biomass, activity, and quotient compared with mineral fertilizers. Both the sole-carbon-source utilization activity and the functional diversity of soil microbial community were significantly increased (P < 0.05) by balanced fertilization (NPK, OA, or 1/2 OM), and species richness of community and relative abundance of the most common species in the K-deficient (NP) treatment were also significantly increased (P < 0.05). Principal component analysis and redundancy analysis showed that both organic and mineral fertilizers could affect microbial parameters by increasing soil organic C contents, and P was the key factor to increase soil microbial diversity and soil fertility. Long-term balanced fertilization greatly increased soil microbial biomass, functional diversity, and invertase activity and played an important role in decreasing soil microbial metabolic quotient, while P could be considered as the key factor to control soil microbial diversity as well as soil fertility. With regard to the different effects of OA and mineral fertilizer on soil organic C contents and root exudates, combined application of mineral and organic fertilizers is recommended in the region.