Competition between shrubs and grasses in a shrub-encroached temperate grassland of North China: implications from the nitrogen acquisition

In recent decades, large areas of temperate grasslands in arid regions worldwide have been increasingly encroached upon by shrubs. This encroachment has intensified the competition for resources, particularly nitrogen (N), a critical element for plant growth, between the newly established shrubs and the surrounding grasses in temperate grasslands. Despite this, it remains unclear whether shrubs and grasses adopt distinct strategies for N acquisition and how these strategies may contribute to competition between them. Additionally, the role of soil microbes in regulating the N competition between shrubs and grasses also remains unclear. To address these gaps, we conducted an in situ ¹⁵N labeling 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^- across soil depths. However, in the subsoil (10–30 cm) at the upper slope, shrubs displayed significantly higher total N uptake compared to grasses. 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 only greater at both upper and lower slopes. Moreover, the R_S/G in the subsoil or overall soil depth was markedly higher at the upper than lower slope.

The competition for N between shrubs and grasses also regulated by soil microbes, with higher ¹⁵N recovery in soil microbes (R_M) than plants (R_S or R_G). The ratio of N uptake by grasses to soil microbes (R_G/M) was higher in the topsoil, and varied across N forms and slope locations. Structural equation model (SEM) reveals that location changes strongly affect plant-soil interactions, influencing the R_S/G. Increased soil depth lowered soil organic matter (SOM), soil microbial biomass N (MBN), and soil water content (SWC), but increased shrub root biomass (SRB). Lower slopes have associated higher MBN and SWC, but less SRB. SWC enhanced MBN, which reduced SRB. SOM lowered R_S/G, whereas SRB increased it. The competition for nitrogen (N) between shrubs and grasses was more intense in the subsoil and particularly pronounced at the upper slope. These findings provided valuable insights into the competitions between shrubs and grasses for N, as well as the role of soil microbial regulation in temperate grasslands of North China undergoing shrub encroachment, highlighting the influence of soil depths and slope locations.