Retention of early-spring nitrogen in temperate grasslands: The dynamics of ammonium and nitrate nitrogen differ

Abstract

In nitrogen (N)-limited temperate regions, winter is an important period of N accumulation. The accumulated N is released during snowmelt and thawing, and the availability peaks in early spring. However, the early-spring dynamics of specific N forms (i.e., ammonium NH4+ and nitrate NO3−) in temperate grasslands are still not fully understood. Here, we added 15NH4+ and 15NO3− (equivalent to 150 mg 15N m−2) to the soils of a meadow steppe and a typical steppe in northern China immediately after snowmelt, then quantified the retention dynamics of 15NH4+ and 15NO3− in soils, microbes, and plants over the subsequent growing season. Approximately 70% of the added 15N tracers were initially retained within the soil−microbe−plant systems in both temperate grasslands. In early spring, much 15N was immobilized in soils and microbes, while little had been taken up by plants. During the subsequent growing season, approximately 45% of the 15N was rapidly lost by the soils and microbes, but plant 15N acquisition gradually increased. Although soils and plants retained similar levels of 15NH4+ and 15NO3− during the growing season, soil microbes retained more 15NH4+ than 15NO3−. Different plant taxa had distinct 15N acquisition capacities: perennial grasses and forbs accumulated the 15N tracers rapidly, while annuals did not. Perennial grasses were effective immobilizers of the 15NH4+, whereas forbs were effective immobilizers of the 15NO3−. These findings provided evidence of the substantial retention of early-spring N over the following growing season in temperate grasslands, regardless of the vegetation type and N form. However, it was clear that the dynamics of early-spring 15NH4+ and 15NO3− differed within the soil−microbe−plant systems.