Fertilizer selection influences soil microbial communities in alluvial and peat soils of the tropics

Abstract Fertilizer choices can alter soil microbial communities. The objective of this study was to determine the effects of urea, chicken manure, and Azolla fertilizers on soil microbial communities in alluvial and peat soils in West Kalimantan, Indonesia. Five nitrogen (N) fertilizer treatments were applied to spinach ( Amaranthus tricolor) grown on both soil types: control (0 kg N ha −1 ), urea (23 kg N ha −1 ), Azolla applied at the same urea‐N rate ( Azolla ‐U, 23 kg N ha −1 ), chicken manure (108 kg N ha −1 ), and Azolla applied at the same manure‐N rate ( Azolla ‐M, 108 kg N ha −1 ). Microbial community structure was determined from freeze‐dried soil samples (0–20 cm deep) by the ester‐linked fatty acid methyl ester (EL‐FAME) method and analyzed by gas chromatography–mass spectrometry. Treatment effects were dependent upon soil type. In peat soil, fertilizer treatment influenced the abundance of Gram‐positive bacteria, whereas in alluvial soil, fertilizer affected the population of Gram‐negative bacteria. In alluvial soil, Azolla ‐M increased EL‐FAMEs for total microbial biomass, Gram‐negative bacteria, and arbuscular mycorrhizal fungi. On the other hand, in the peat soil, Azolla ‐M had a significant effect on altering the microbial community structure toward greater fungal biomass, while urea increased the number of Gram‐positive bacteria and actinomycetes. Azolla ‐M altered the Gram‐positive:Gram‐negative ratio and reduced the Stress 2 ratio in the peat soil, but the manure and the lower application of Azolla ( Azolla ‐U) did not. Azolla ‐M may diminish stress encountered by the microbial community from unfavorable environmental conditions.