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

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 (NH4+)-N-15 and (NO3-)-N-15 (equivalent to 150 mg (15)Nm(-2)) to the soils of a meadow steppe and a typical steppe in northern China immediately after snowmelt, then quantified the retention dynamics of (NH4+)-N-15 and (NO3-)-N-15 in soils, microbes, and plants over the subsequent growing season. Approximately 70% of the added N-15 tracers were initially retained within the soil-microbe-plant systems in both temperate grasslands. In early spring, much N-15 was immobilized in soils and microbes, while little had been taken up by plants. During the subsequent growing season, approximately 45% of the N-15 was rapidly lost by the soils and microbes, but plant N-15 acquisition gradually increased. Although soils and plants retained similar levels of (NH4+)-N-15 and (NO3-)-N-15 during the growing season, soil microbes retained more (NH4+)-N-15 than (NO3-)-N-15. Different plant taxa had distinct N-15 acquisition capacities: perennial grasses and forbs accumulated the N-15 tracers rapidly, while annuals did not. Perennial grasses were effective immobilizers of the (NH4+)-N-15, whereas forbs were effective immobilizers of the (NO3-)-N-15. 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 (NH4+)-N-15 and (NO3-)-N-15 differed within the soil-microbe-plant systems. (C) 2020 The Authors. Published by Elsevier B.V.