Competition between shrubs and grasses in a shrub-encroached temperate grassland: Implications from nitrogen acquisition

In recent decades, shrub encroachment in arid temperate grasslands globally has heightened resources competition especially for nitrogen (N), a vital element for plant growth, between encroaching shrubs and native grasses. Despite this, it remains unclear whether shrubs and grasses adopt distinct N acquisition strategies and how these might impact their competitive dynamics, with the regulatory role of soil microbes in this competition also being poorly understood. We conducted an in-situ(15)N labelling experiment in a shrub-encroached temperate grassland with significant slope variations of North China. The study aimed to investigate the competition for N acquisition among shrubs, grasses, and soil microbes. The results revealed that both shrubs and grasses preferred to absorb NO3- (> 60% of total N uptake) across soil depths. However, in the subsoil (10-30 cm) at the upper slope, shrubs displayed significantly 2.71 times higher total N uptake compared to grasses due to their deeper roots. The ratio of N uptake by shrubs to grasses (R-S/G) for different N forms was consistently higher in the subsoil, and that for total N uptake of subsoil was greater at both upper (1.54 times) and lower (0.52 times) slopes. Moreover, the R-S/G in the subsoil or overall soil depth was markedly higher at upper than lower slope. The competition for N between shrubs and grasses also regulated by soil microbes, with higher N-15 recovery in soil microbes (R-M) than plants (R-S or R-G). Structural equation model (SEM) revealed that location changes directly and indirectly regulated R-S/G through plant-soil feedbacks. Increased soil depth lowered soil organic matter (SOM), soil microbial biomass N (MBN), and soil water content (SWC), but increased the root biomass of shrubs. Lower slopes have associated higher MBN and SWC, but less root biomass of shrubs. SWC enhanced MBN, which reduced the root biomass of shrubs. SOM lowered R-S/G, whereas the root biomass of shrubs increased it. The mechanistic framework explained that the competition for N between shrubs and grasses was more intense in the subsoil and particularly pronounced at the upper slope in the temperate shrub encroached grasslands of North China.