Distribution of nutrients in soils is highly heterogeneous (nutrient patchiness) due to natural and anthropogenic factors. In agricultural soils, this inherent patchiness is increased by fertilizers application. As a fertilizer granule dissolves, its salts diffuse outwards and water move inwards. This creates local, transient gradients in ionic concentrations and moisture contents. In the nutrient patch, soil physico-chemical properties, microbial community and activity, nutrient transformation and availability, fertilizer behavior, and root proliferation might change. However, apart from root proliferation, little is known about others. The soil microbial community is probably the most important functional component of the soil biota. Soil microorganisms play a key role in the energy flows, nutrient transformations and element cycles in the environment. Microbial biomass itself is the essential source and sink of nutrients for the whole terrestrial ecosystem, supporting the soil fertility. They also make an important contribution to humus production and soil structure stabilization. However, the sensitivity of microbes to environmental changes determines which is affected first, and many processes related to microbial activities are also changed. It is, therefore necessary to investigate the microbial communities in order to understand other changes in nutrient patches. Phospholipid fatty acids (PLFA) analyses have been used to describe microbial communities in soil. This method is based the fact that PLFA are the major constituents of membranes of all living cells and different subsets of a community have different PLFA pattern. Some specific PLFAs can be used as indicators of microorganisms.
For example, PLFA i14∶0, (a15∶0) are specific to Gram-positive bacteria, and cy17∶0, cy19∶0 are specific to Gram-negative bacteria. Fungal PLFA are (18∶2ω6,)9、(18∶1ω9,) and 10Me18∶0 is suitable for actinomycetes. The current study aimed to: (1) investigate the transformation of nitrogen forms and changes of their concentrations in the microzones diffused by urea and (2) whether microbial community structure changed along urea diffusion through PLFA method. The experiment was conducted at the constant temperature room (28℃), and a special designed container was used. The results showed that, after 7d culture, the highest concentrations of NH~+_4、NO~-_2、NO~-_3 were in the No.7 microzone (7cm away from the fertilizer) and No.8 microzone (8cm away from the fertilizer). NO~-_2 that was the primary ion existed in soil. Principal component analysis (PCA) of PLFA data indicated there was a gradual change in PLFA pattern for different nutrient concentrations; this could be seen in either the first or the second principal component. As for special marked PLFA, high nutrients resulted in a increase in the fungal PLFA 18∶2ω6,9 and 18∶1ω9, which increased 173% and 47.2% in the No.7 than in the No.20 microzone, respectively; while decreases were found in the bacterial PLFA a15∶0, cy19∶0. Actinomycete PLFA 10Me18∶0 slightly changed.