Soil nitrogen forms and their coupling relationships during the growing season of a Larix principis-rupprechtii plantation at different growth stages and stand densities

Soil serves as an interface and medium for resource exchange (e.g., water, nutrients, and chemicals) between the environment and plants. The capacity of soil to retain multiple forms of nitrogen plays a vital role in their provision for plants. A comprehensive understanding of how forest stand density and seasons regulate the various forms of soil nitrogen in Larix principis-rupprechtii plantations of different ages is key for the optimal management of plantation ecosystems. For this study, nine plots with various density gradients in three subgroups of differently aged Larix principis-rupprechtii forest plantations were established, from which the soils were sampled every month during the growing season. The basic properties of the soil and total nitrogen, nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N), ammonium nitrogen (NH4-N), and microbial biomass nitrogen were measured in the laboratory, and the net ammonification rate (NAR) was calculated. The results indicated that as larch plantations matured (in China, it is generally believed that larch plantations are mature after 40 years), the soil also gradually matured. The soil nitrogen remains dynamic and stable after showing a positive accumulation over a certain time. A moderate forest stand density was found to be conducive to the sequestration of total soil nitrogen and the various forms thereof, in the understory of Larix principis-rupprechtii plantations of various ages. Further, certain forms of soil nitrogen are more sensitive to seasonal regulation, which can make them synergistically increase, or even break through the negative feedback inhibition. Although stand density has a regulatory effect, it cannot reverse this inhibition. The NAR possessed obvious seasonality and age phases; however, it had a weak regulatory response to stand density. During young stand ages and peak growing seasons, the soil of Larix principis-rupprechtii plantations maintained a high level of net soil ammonification efficacy.