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.

Differential response of plant water consumption to rainwater uptake for dominant tree species in the semiarid Loess Plateau

Whether uptake of rainwater can increase plant water consumption in response to rainfall pulses requires investigation to evaluate the plant adaptability, especially in water limited regions where rainwater is the only replenishable soil water source. In this study, the water sources from rainwater and three soil layers, predawn (Ψpd), midday (Ψm) and gradient (Ψpd−Ψm) of leaf water potential, and water consumption in response to rainfall pulses were analyzed for two dominant tree species, Hippophae rhamnoides and Populus davidiana, in pure and mixed plantations during the growing period (June–September). In pure plantations, the relative response of daily normalized sap flow (SFR) was significantly affected by rainwater uptake proportion (RUP) and Ψpd−Ψm for H. rhamnoides, and was only significantly influenced by Ψpd−Ψm for P. davidiana (P < 0.05). Meanwhile, the large Ψpd−Ψm was consistent with high SFR for H. rhamnoides, and the small Ψpd−Ψm was consistent with the low SFR for P. davidiana, in response to rainfall pulses. Therefore, H. rhamnoides and P. davidiana exhibited sensitive and insensitive responses to rainfall pulses, respectively. Furthermore, mixed afforestation significantly enhanced RUP, SFR, and reduced the water source proportion from the deep soil layer (100–200 cm) for both species (P < 0.05). The SFR was significantly influenced by RUP and Ψpd−Ψm for both species in the mixed plantation. Lower Ψm and higher Ψpd were adopted by H. rhamnoides and P. davidiana in mixed plantation, respectively, to enlarge Ψpd−Ψm, enhance rainwater uptake, and decrease water source competition from the deep soil layer. These results indicate that mixed afforestation enhanced the influence of rainwater uptake to water consumption after rainfall pulse, regardless of sensitivity to rainfall pulses. This study provides insights into suitable plantation species selection and management considering the link between rainwater uptake and consumption in water limited regions.