Understanding the water sources and physiological responses to soil moisture pulses for plantation species, particularly in mixed plantations, are essential to assess the water use strategy and vegetation restoration in semiarid regions. We used hydrogen stable isotopes in plant and soil water to determine the potential water sources for Pinus tabuliformis and Hippophae rhamnoides in both pure and mixed plantations in the semiarid Chinese Loess Plateau during the vigorous growing season (June–August) in 2016. Stomatal conductance (gc), midday leaf water potential (Ψm) and photosynthetic rate (Pr) were measured concurrently to analyse the physiological response. The P. tabuliformis in the pure plantation depended largely on shallow and middle soil layers regardless of precipitation amount, permitting this species to maintain stable Ψm at the expense of Pr through stomatal control. In contrast, H. rhamnoides in the pure plantation shifted its water source from shallow to deep soil layers following decreases in precipitation, allowing this species to maintain stable gc and Pr at the expense of Ψm. Thus, P. tabuliformis and H. rhamnoides displayed isohydric and anisohydric behaviour, respectively. Additionally, both species in mixed plantations largely absorbed water from shallow soil layers and shifted to deep soil layers when precipitation decreased. Mixed afforestation significantly reduced (p < .05) Pr for P. tabuliformis and Ψm for H. rhamnoides. Although contrasting physiological responses were adopted by these species, the major proportion of water resources were competitively obtained from similar soil depths—indicating that their mixed afforestation requires further investigation.

As a major driving element of the structure and function of arid and semiarid ecosystems, rainfall is the essential factor limiting plant biological processes. To clarify the characteristics of transpiration and responses to summer rainfall, sap flow density (F d) of Pinus tabulaeformis and Hippophae rhamnoides was monitored using thermal dissipation probes. In addition, midday leaf water potential (ψm) and leaf stomatal conductance (G s) were also analyzed to determine water use strategies. The results indicated that the diurnal variation in the normalized F d values exhibited a single‐peak curve for P. tabulaeformis, while H. rhamnoides showed multiple peaks. The normalized F d for P. tabulaeformis remained relatively stable regardless of rainfall events. However, there was also a significant increase in the normalized F d for H. rhamnoides in response to rainfall in June and August (p < .05), although no significant differences were observed in July. The normalized F d values for P. tabulaeformis and H. rhamnoides fitted well with the derived variable of transpiration, an integrated index calculated from the vapor pressure deficit and solar radiation (R s), using an exponential saturation function. The differences in fitting coefficients suggested that H. rhamnoides showed more sensitivity to summer rainfall (p < .01) than P. tabulaeformis. Furthermore, during the study period, P. tabulaeformis reduced G s as soil water decreased, maintaining a relatively constant ψm; while H. rhamnoides allowed large fluctuations in ψm to maintain G s. Therefore, P. tabulaeformis and H. rhamnoides should be considered isohydric and anisohydric species, respectively. And more consideration should be taken for H. rhamnoides in the afforestation activities and the local plantation management under the context of the frequently seasonal drought in the loess hilly region.

The objective of the present study was to evaluate soil organic carbon stock and mineralization characteristics in forest ecosystems in subtropical China. We explored the factors underlying soil respiration and vegetation in 12 soils at different altitudes in Jiangxi province, Wuyi mountain national nature reserve. Soil organic carbon was applied to analysis field survey and indoor cultivation techniques. Our results showed uneven distribution of four perpendicular bands: meadow grassland, subalpine elfin, mixed coniferous and broad-leaved forests. Differences in total carbon (TC) and total nitrogen (TN) content in the soil were not significant. Soil pH was acidic. The crown density representing relative soil water content suggested fewer differences in low dissolved organic carbon (DOC) with altitude. The soil microbial biomass carbon (MBC) was the lowest in the meadow grassland soil, and in that of the mixed needle leaved forest. The NO3 --N levels decreased with altitude and the changes in NH4 + -N were not obvious. The respiratory rate of the meadow grassland soil was the lowest, and the respiratory rate of the subalpine elfin forest was high. Cumulative emissions without soil CO2 increased with altitude showing regular changes related to vegetation. In mixed needle the soil respiration was significantly stronger t han in the meadow grassland and broad-leaved forest soil. Carbon emissions produced by soil respiration and total carbon (TC) and C: N were positively correlated (P < 0.05), with no obvious correlation with MBC. The soluble and microbial carbon levels in the soils at different altitudes were positively but not significantly related. Soil pH and TC were negatively correlated with TN and NO 3 --N. The effect of soil total carbon, total nitrogen and crown density partially controlled soil respiration.

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