The Effects of Nitrogen Fertilizer on Soil Microbial Communities Under Conventional and Conservation Agricultural Managements in a Tropical Clay-Rich Ultisol

Abstract

The response of microbial communities to N fertilizer varies depending on environmental and management-related factors. However, few studies have compared the effects of N fertilization on microbial communities under different agricultural management. We thus compared the biomass and composition of microbial communities in N-fertilized and unfertilized long-term (25 years) fields managed under conventional tillage with crop residue removal (CTR) and no-tillage with crop residue mulch (NTM) systems. Soil samples were taken in January 2011 and July 2012 at a depth of 0 to 10 cm, and phospholipid fatty acid (PLFA) profiles were measured. Fungal abundance was lower in N-fertilized soils than in unfertilized soils under both management systems at both sampling times. Bacterial abundance was lower in N-fertilized soils than in unfertilized soils under the CTR system, whereas no difference was observed between N-fertilized and unfertilized soils under the NTM system in January 2011. However, in July 2012, bacterial abundance was lower in N-fertilized soils than in unfertilized soils under both management systems. Principal component analysis of the PLFA profiles showed that N fertilizer had a significant effect on the PLFA composition under the CTR system but had less impact under the NTM system. These results suggest that (i) fungal biomass is negatively affected by N fertilizer regardless of management system; (ii) bacterial response to N fertilizer differs temporally between management systems, reflecting temporal changes in soil C content; and (iii) the NTM system mitigates N effects on the composition of soil microbial communities.