Soil Organic Carbon and pH Dominate the Effects of Nitrogen Addition on Soil Microarthropods in a Poplar Plantation in Coastal Eastern China

Soil biodiversity and fuction have been altered by the increasing levels of nitrogen as a result of fertilization and atmospheric deposition. Although soil microarthropods are a crucial component of soil biodiversity and play a key role in a diverse range of soil functions, our understanding of the mechanisms by which N addition affects them remains limited. Using a long-term nitrogen addition experiment (2012-2016) in poplar plantations (Populus deltoides L. CL`35') located along the coast of Yellow Sea Forest Park in northern Jiangsu, eastern China (32 degrees 52' N and 120 degrees 49' E), where the soil was entisols, we examined the response of soil microarthropods across three soil depths (0-15 cm, 15-25 cm, 25-40 cm) to five N input levels (0, 5, 10, 15, 30 g N m(-2) year(-1)) over four seasons. We found that the number of microarthropods per unit area initially grew and then dropped as more nitrogen was added to soils. Soil organic carbon (positive correlation, R-2 = 0.53) and pH (negative correlation, R-2 = 0.19) were the two dominant factors driving the effects of nitrogen addition on soil microarthropod densities at all soil depths. These results suggest that nitrogen input enhances the density of soil microarthropods via the increase in fresh organic matter input. However, the increase in organic matter may be offset by an indirect increase in acidity under high levels of N addition, providing one possible explanation for the reduced density of microarthropods in heavily fertilized soils.