Responses of nitrogen and phosphorus resorption of understory plants to microscale soil nutrient hetero-geneity in Chinese fir plantation.

We compared the interspecific differences in leaf nutrient resorption of two dominant understory species (Lophatherum gracile and Oplimenus unulatifolius), and analyzed the correlations between the intraspecific efficiency of leaf nutrient resorption and nutrient properties of soil and leaves in Chinese fir plantation. The results showed high soil nutrient heterogeneity in Chinese fir plantation. Soil inorganic nitrogen content and available phosphorus content varied from 8.58 to 65.29 mg·kg-1 and from 2.43 to 15.20 mg·kg-1 in the Chinese fir plantation, respectively. The soil inorganic nitrogen content in O. undulatifolius community was 1.4 times higher than that in L. gra-cile community, but there was no significant difference in soil available phosphorus content between the two communities. Both leaf nitrogen and phosphorus resorption efficiency of O. unulatifolius was significantly lower than that of L. gracile under the three measurement bases of leaf dry weight, leaf area, and lignin content. Resorption efficiency in L. gracile community expressed on leaf dry weight was lower than that expressed on leaf area and lignin content, while resorption efficiency expressed on leaf area was the lowest in O. unulatifolius community. The intraspecific resorption efficiency was significantly correlated with leaf nutrient contents, but was less correlated with soil nutrient content, and only the nitrogen resorption efficiency of L. gracile had significant positive correlation with soil inorganic nitrogen content. The results indicated that there was significant difference in the leaf nutrient resorption efficiency between the two understory species. Soil nutrient heterogeneity exerted a weak effect on the intraspecific nutrient resorption, which might be attributed to high soil nutrient availability and potential disturbance from canopy litter in Chinese fir plantation.

Effects of acid rain and root exclusion on soil organic carbon in Cunninghamia lanceolata and Michelia macclurei plantations.

Acid rain is one of most serious environment problems in China. The types of acid rain have gradually transformed from sulfuric acid rain (SAR) to mixed acid rain (MAR) and nitric acid rain (NAR) in recent years. Roots are one source of soil organic carbon and play an important role in the formation of soil aggregates. However, the changes in acid rain type and the effect of root removal on soil organic carbon in forest ecosystems are poorly understood. In this study, we removed roots and simulated acid rain with different types (SO42-:NO3- ratio of 4:1, 1:1, and 1:4) for three years in Cunninghamia lanceolata (CP) and Michelia macclurei (MP) plantations to analyze the changes of soil organic carbon and physical properties and to measure the size and mean weight diameter (MWD) of aggregates. Results showed that root removal in C. lanceolata and M. macclurei remarkably reduced soil organic carbon pool by 16.7 % and 21.5%, and soil recalcitrant carbon by 13.5% and 20.0%, respectively. Root removal substantially decreased the MWD and proportion and organic carbon content of soil macroaggregates in M. macclurei, but not in C. lanceolata. Acid rain did not affect soil organic carbon pool and soil aggregate structures. Our results indicated that roots promote soil organic carbon stabilization and that their contribution to the stability of soil organic carbon varies with forest types. Moreover, soil organic carbon stabilization is not affected by different types of acid rain in the short term.

Effects of Chinese fir planting and phosphorus addition on soil microbial biomass and extracellular enzyme activities.

Plants can alter soil microbial biomass and extracellular enzyme activities related with carbon (C), nitrogen (N), and phosphorus (P), through litter and root exudates, with consequences on soil carbon, nitrogen and phosphorus (P) cycling. However, it is not well known how the changes in soil phosphorus availability affect the relationships between plants and soil microorganisms. In this study, a factorial experiment was conducted to investigate the effects of Chinese fir (Cunninghamia lanceolata) planting and different levels of P addition (0, 1.95, 3.9, 7.8 and 15.6 g P·m-2·a-1) on soil microbial biomass and extracellular enzyme activities. The results showed that planting Chinese fir planting significantly altered soil microbial biomass and C- and N- and P-related extracellular enzyme activities, but the effects were dependent on P addition levels. Without P addition, Chinese fir planting significantly reduced soil nutrient availability and pH, which led to the aggravation of P limitation and lower soil microbial biomass. P addition relieved P limitation, and reduced soil acid phosphatase (ACP) activities by 30.0%, 30.5%, 35.3% and 47.1% with the increasing P addition level (1.95, 3.9, 7.8 and 15.6 g P·m-2·a-1). Under three P addition levels (1.95, 3.9 and 7.8 g P·m-2·a-1), the negative effects of Chinese fir planting on soil microbial growth were alleviated. Under the high P addition level (15.6 g P·m-2·a-1), the negative effects of Chinese fir planting on soil microbial growth occurred again due to soil N limitation. Taken together, Chinese fir planting and soil P availability generally affected soil microbial biomass and extracellular enzyme activities, and changed P limitation.

Thinning effect on photosynthesis depends on needle ages in a Chinese fir (Cunninghamia lanceolata) plantation.

Canopies in evergreen coniferous plantations often consist of various-aged needles. However, the effect of needle age on the photosynthetic responses to thinning remains ambiguous. Photosynthetic responses of different-aged needles to thinning were investigated in a Chinese fir (Cunninghamia lanceolata) plantation. A dual isotope approach [simultaneous measurements of stable carbon (δ13C) and oxygen (δ18O) isotopes] was employed to distinguish between biochemical and stomatal limitations to photosynthesis. Our results showed that increases in net photosynthesis rates upon thinning only occurred in the current-year and one-year-old needles, and not in the two- to four-year-old needles. The increased δ13C and declined δ18O in current year needles of trees from thinned stands indicated that both the photosynthetic capacity and stomatal conductance resulted in increasing photosynthesis. In one-year-old needles of trees from thinned stands, an increased needle δ13C and a constant needle δ18O were observed, indicating the photosynthetic capacity rather than stomatal conductance contributed to the increasing photosynthesis. The higher water-soluble nitrogen content in current-year and one-year-old needles in thinned trees also supported that the photosynthetic capacity plays an important role in the enhancement of photosynthesis. In contrast, the δ13C, δ18O and water-soluble nitrogen in the two- to four-year-old needles were not significantly different between the control and thinned trees. Thus, the thinning effect on photosynthesis depends on needle age in a Chinese fir plantation. Our results highlight that the different responses of different-aged needles to thinning have to be taken into account for understanding and modelling ecosystem responses to management, especially under the expected environmental changes in future.

[Changes of non-structural carbohydrates and its impact factors in trees: a review].

Non-structural carbohydrates (NSCs) are an important energy source for the metabolism of plants. The size of the NSC pool is likely to mirror the overall carbon supply status and its dynamics strongly influences physiological processes in plants. In order to predict the response and adaptation of trees to climate change, this review summarized the current understanding of NSC pool in trees, and mainly focused on its seasonal and spatial variation for analyzing the relationships between environmental factors and NSC allocation. Moreover, the response and adaptation strategies of NSC pool in trees to climate change were also discussed. Finally, some suggestions were proposed for the potential study orientation of NSC pool in trees in future climate conditions.