Assessing the impact of soil aggregate size on mineralization of nitrogen in different soils, China

Soil aggregates play a critical role to support the development of soil nitrogen (N) pools, but the underlying mechanism by which soil aggregate size impacts N mineralization remains unclear. The objective of this study was to determine the effect of aggregate size in soil N mineralization through cultivation experiments of different type soils. The topsoil samples of ten typical profiles in different regions of China were collected, and a series of laboratory column incubation experiments were conducted to quantify the relationship between the distribution of soil aggregate size and the concentration and mass of mineral N (i.e., NH₄-N and NO₃-N) under N fertilizer addition. The results indicated that the weight of aggregate with different sizes was less affected by the intensity of fertilization, which was mainly influenced by the soil types. In most soils, the weight of soil aggregates of 1–0.5 mm accounted for the largest proportion of aggregates in the five categories (i.e., 5–2 mm, 2–1 mm, 1–0.5 mm, 0.5–0.25 mm, and < 0.25 mm). Moreover, the intermediate aggregates (1–0.5 mm and 0.5–0.25 mm) were detected to have the highest mineral N concentration and mass, and they were also demonstrated to be significantly (P < 0.05) correlated to the potential nitrogen mineralization (PNM) rates in most soils. Subsequently, the obtained PNM indices were classified into three levels: i.e., >65% (highly responsive), 45%–65% (moderately responsive), and < 45% (lowly responsive), and we proposed the preliminary strategies on fertilizer types, agricultural intensity, and crop selection for different soils. The outcome of this study provides new insights into the relationship between soil aggregate sizes and N mineralization abilities and will help in developing guidelines for fertilizer management.