Soil pH rather than nutrients drive changes in microbial community following long-term fertilization in acidic Ultisols of southern China

Abstract

Long-term intensive cultivation and heavy fertilization improve the nutritional conditions in acidic Ultisols, but also sharply accelerate soil acidification. However, the impact of such dramatic environmental changes on soil microorganisms is unclear. The aims of this work were to investigate the responses of microbial community composition and metabolic function to long-term fertilization, and to determine the key factors that primarily drive microbial changes in acidic Ultisols. A long-term fertilization experiment under a winter wheat–summer maize rotation was established in 1990 in acidic Ultisols of southern China. Soils were collected from four treatments in June 2014: (1) non-fertilization control (CK); (2) only N fertilization (N); (3) N, P, and K fertilization (NPK); and (4) NPK plus manure (NPKM; 70% of total N obtained from manure). The amount of N used in all N treatments was 300 kg N ha−1 year−1. The soil pH, cation exchange capacity (CEC), soil organic carbon (SOC), total nitrogen (TN), phosphorus (TP) and potassium (TK), available nitrogen (AN), phosphorus (AP), and potassium (AK) were measured. Soil microbial community composition and metabolic function were determined by phospholipid fatty acids analysis (PLFA) and community-level physiological profile (CLPP) method, respectively. Compared with CK, NPKM significantly increased total PLFA biomass and average well color development (AWCD); NPK increased total PLFA biomass by 2.2 times, but its AWCD was not significantly different from CK, indicating that microbial metabolic efficiency in NPK decreased. N decreased total PLFA biomass by 27.9%, while almost completely inhibiting metabolic activity. NPKM maintained microbial functional diversity indexes at similar levels as CK, while NPK and N significantly decreased microbial functional diversity indexes. Redundancy analysis (RDA) revealed that soil microbial community composition and metabolic pattern were more stably maintained by application of manure compared to chemical fertilizers. Soil pH showed the primary effect on microbial community composition, metabolic activity, and functional diversity indexes. This research demonstrated that the negative effects of Ultisol acidification induced by long-term application of chemical N fertilizer on microorganisms overwhelmed the positive effects of soil nutrition improvement. The inhibiting effect of serious acidification on microbial metabolic function was stronger than that on community composition. Microorganisms live in a low active metabolic state to resist serious Ultisols acidification. Therefore, fertilization in acidic Ultisols should be based on the premise of preventing soil further acidification.