Variation in leaf nitrogen and phosphorus stoichiometry in the nitrogen-fixing Chinese sea-buckthorn (Hippophae rhamnoides L. subsp. sinensis Rousi) across northern China

Vegetation and Topographic Factors Affecting SOM, SOC, and N Contents in a Mountainous Watershed in North China

This study aimed to reveal the main environmental factors affecting PH, SOM, SOC, TN, and AHN in mountainous areas of Beijing, using the Chaoguan Xigou watershed as the research object. The relationship among pH, SOM, SOC, TN, AHN, topographic factors, and vegetation factors was analyzed by correlation and redundancy analysis (RDA). The results showed that altitude was significantly positively correlated with vegetation types in the study area (p < 0.01). Menhinnick richness index (D), Shannon–Wiener diversity index (H), and Alatalo evenness index (E) ranged from 0.35–0.79, 0.86–1.73, and 0.39–0.7, respectively, indicating moderate variations. E was significantly positively correlated with stand type (p < 0.05), altitude (p < 0.05), and H (p < 0.01). The variation ranges of PH, SOM, SOC, TN, and AHN in soil were 5.78–7.13, 54.73–90.38 g/kg, 23.77–60.25 g/kg, 1.71–4.22 g/kg, and 95.64–223.26 mg/kg, respectively. All soil nutrient indexes had medium variation except for pH (weak variation). In this study, RDA results showed that altitude is the main environmental factor affecting the soil pH, SOM, SOC, TN, and AHN in this area and could explain 25.9% of the total variance. However, the effects of factors associated with altitude on pH, SOM, SOC, TN, and AHN need to be further studied.

Soil Carbon, Nitrogen, and Phosphorus Storages and Their Stoichiometry Due to Mixed Afforestation with Hippophae rhamnoides in the Loess Hilly Region, China

Mixed-species tree plantations have additional ecological benefits over single-species tree plantations, such as habitat restoration and increasing biodiversity. However, changes in the soil carbon, nitrogen, and phosphorus storages and stoichiometry after mixed afforestation with the N-fixing tree species under the “Grain for Green Project” in the Loess Plateau of China are not well understood. Typical restoration types, including the mixed plantations of Pinus tabuliformis with Hippophae rhamnoides (HrPt) and Robinia pseudoacacia with H. rhamnoides (HrRp), as well as the pure forests of P. tabuliformis (Pt) and R. pseudoacacia (Rp), were chosen to examine changes in the storages and stoichiometry of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in 0–100 cm soil layers. The results showed that compared with the corresponding pure forest, HrRp significantly increased the SOC content in the 0–20 cm soil layer and the SOC storage in the 0–100 cm layer, while HrPt significantly increased the SOC content in the 0–10 cm layer, but there was no significant difference for SOC storage in the 0–100 cm layer between Pt and HrPt. Similarly, HrRp significantly increased the TN content in the 0–10 cm layer and the TN storage in the 0–100 cm layer, but there was no significant difference in TN storage between Pt and HrPt. Furthermore, HrRp significantly increased the TP content in the 0–100 cm layer and TP storage was higher than that of Rp, while there were no significant differences in TP content and storage between Pt and HrPt. In the 0–10 cm soil layer, HrRp significantly reduced C:N and increased N:P, but HrPt significantly increased C:P. In addition, compared with the pure forest, the soil physical and chemical properties had a stronger control effect on the soil storages and stoichiometric ratios in the mixed forests. In summary, compared with P. tabuliformis, the introduction of N-fixing tree species into the R. pseudoacacia forest was more conducive to the accumulation of SOC, TN, and TP reserves and the improvement of the N and P utilization efficiency. These results have important implications for the restoration of degraded vegetation and scientific management of mixed plantations on the Loess Plateau and can provide basic data for the assessment of soil quality at the regional scale.

Decoupling of nutrient element cycles in soil and plants across an altitude gradient

Previous studies have examined the decoupling of C, N, and P under rapid changes in climate. While this may occur in different environment types, such climactic changes have been reported over short distances in mountainous terrain. We hypothesized that the decoupling of C, N, and P could also occur in response to increases in altitude. We sampled soil and plants from Mount Gongga, Sichuan Province, China. Soil C and N were not related to altitude, whereas soil P increased with altitude. Soil N did not change with mean annual temperature (MAT), mean annual precipitation (MAP), vegetation and soil types, whereas soil P varied with MAT and vegetation type. Plant C remained constant with increasing altitude; plant N exhibited a quadratic change trend along the altitude gradient, with a turning point at 2350 m above average sea level; and plant P decreased with altitude. MAP mostly accounted for the variation in plant P. MAT was responsible for the variation of plant N at elevations below 2350 m, whereas MAT and vegetation type were the dominant influential factors of plants growing above 2350 m. Thus, the decoupling of C, N, and P in both soil and plants was significantly affected by altitude.