Nitrogen and water addition alter nitrogen uptake preferences of two dominant plant species in a typical Inner Mongolian steppe

Abstract

<p indent="0mm">The preferences for different nitrogen (N) forms by plants play an important role in determining the fate and dynamics of N in ecosystems. Nevertheless, it remains unclear whether and how resource addition influences N uptake preferences of plants. Here we investigated the effects of long-term <sc>(10 a)</sc> N and water addition on plant N uptake in a temperate semi-arid grassland. We used the ¹⁵N natural abundance technique to measure N content and isotope in soils and leaves of two dominant plant species, <italic>Stipa krylovii</italic> and <italic>Agropyron cristatum</italic>, and quantified the proportion of plant N derived from different N sources with an isotopic mixing model. We found that N addition, but not water addition, significantly increased soil nitrate and ammonium content, and also enhanced plant leaf N concentration. Soil nitrate accounted for 80% and ammonium for 20% contributing to plant N in non-treated controls. Compared to the controls, N addition reduced nitrate uptake of the two species, whereas water addition decreased nitrate uptake of <italic>S</italic>.<italic> krylovii </italic>but promoted the nitrate uptake of <italic>A</italic>.<italic> cristatum</italic>. Water interacted with N to significantly affect plant N uptake preference, indicating that the effect of water addition on plant N uptake varies with soil N availability. At the ambient N level, water addition enhanced ammonium utilization of <italic>S</italic>.<italic> krylovii</italic> but decreased that of <italic>A</italic>.<italic> cristatum</italic>. At the increased N level, water addition reduced ammonium uptake and enhanced nitrate uptake in both species. Our results demonstrate that the dominant plant species of the typical Inner Mongolian steppe mainly take up the relatively abundant nitrate rather than the scarce ammonium in this semiarid grassland. Plant preferences for different N forms changed with resource addition, suggesting that future N deposition and increases in nitrate availability may promote the growth of the dominant plant species in temperate grasslands, which may contribute to alleviating the global climate change.