[Intensive Citrus Cultivation Suppresses Soil Phosphorus Cycling Microbial Activity].

Soil microbes are key drivers in regulating the phosphorus cycle. Elucidating the microbial mineralization process of soil phosphorus-solubilizing bacteria is of great significance for improving nutrient uptake and yield of crops. This study investigated the mechanism by which citrus cultivation affects the soil microbial acquisition strategy for phosphorus by measuring the abundance of the phoD gene, microbial community diversity and structure, and soil phosphorus fractions in the soils of citrus orchards and adjacent natural forests. The results showed that citrus cultivation could lead to a decrease in soil pH and an accumulation of available phosphorus in the soil, with a content as high as 112 mg·kg-1, which was significantly higher than that of natural forests (3.7 mg·kg-1). Citrus cultivation also affected the soil phosphorus fractions, with citrus soil having higher levels of soluble phosphorus (CaCl2-P), citrate-extractable phosphorus (Citrate-P), and mineral-bound phosphorus (HCl-P). The phosphorus fractions of natural forest soils were significantly lower than those of citrus soils, whereas the phoD gene abundance and alkaline phosphatase activity were significantly higher in natural forest soils than in citrus soils. High-throughput sequencing results showed that the Shannon diversity index of phosphate-solubilizing bacteria in citrus soils was 4.61, which was significantly lower than that of natural forests (5.35). The microbial community structure in natural forests was also different from that of citrus soils. In addition, the microbial community composition of phosphate-solubilizing bacteria in citrus soils was also different from that of natural forests, with the relative abundance of Proteobacteria being lower in natural forest soils than in citrus soils. Therefore, citrus cultivation led to a shift of soil microbial acquisition strategy for phosphorus, with external phosphorus addition being the main strategy in citrus soils, whereas microbial mineralization of organic phosphorus was the main strategy in natural forest soils to meet their growth requirements.

[oil microbial biomass and enzyme activities among different artificial forests in Ziwuling, Northwest China.] / å­åˆå²­äººå·¥æž—åœŸå£¤å¾®ç”Ÿç‰©ç”Ÿç‰©é‡åŠé ¶æ´»æ€§.

Decades of ecological restoration on the Loess Plateau has achieved significant on-site benefits to reduce soil erosion and improve soil quality, with remarkable off-site effects of reducing sediment delivery to Yellow River. However, regional forest community succession is still far from being adequately developed. The Ziwuling forest region and its highly developed forest community, as an advanced eco-zone, can lend practical experience to other regions on the Loess Plateau and help to identify the most suitable tree species for a better regional restoration in the future. With the aim to systematically understand the potential effects of typical local tree species to soil properties, three typical and well-established artificial forests in the Ziwuling region, Robinia pseudoacacia, Pinus tabuliformis and Platycladus orientalis were investigated in this study, with the climax community Quercus wutaishanica as a reference. All the four forest type had comparable stand age (25 years). Soil samples from 0-20 cm layers were collected from those four plantations. The soil microbial biomass (carbon and nitrogen), soil enzyme (invertase, urease and alkaline phosphatase) activities and their correlations were measured and analyzed. The results showed that: 1) soil invertase activity ranged from 16.94 to 64.49 mg·g-1·24 h-1, the soil urease activity from 0.15 to 0.26 mg·g-1·24 h-1, and the alkaline phosphatase activity from 0.65 to 1.23 mg·g-1·24 h-1. The activities of those three enzymes were significantly higher in the P. orientalis soil that in the R. pseudoacacia and P. tabuliformis soils. The geometric average values in the P. orientalis soil were even greater than that in the Q. wutaishanica soil. 2) The soil microbial biomass carbon and nitrogen varied from 247.37 to 529.84 mg·kg-1 and 41.48 to 77.91 mg·kg-1, respectively. Both of them were significantly greater in the P. orientalis soil than that in the R. pseudoacacia and P. tabu-liformis soils. Even though the soil microbial biomass carbon in the P. orientalis soil remained lower than that in the Q. wutaishanica soil, its soil microbial biomass nitrogen was greater than in the Q. wutaishanica soil. 3) The dissolved organic carbon and nitrogen in the P. tabuliformis soil were much greater than that in other species, even greater than their own soil microbial biomass carbon and nitrogen. Such a result indicated that dissolved organic matter might play a more important role in providing plant available nutrients than microbial biomass in the P. tabuliformis soil. 4) The microbial biomass carbon and nitrogen were significantly positively correlated with the total organic carbon and the total nitrogen, particularly for the R. pseudoacacia and P. tabuliformis soils. There were significantly positive relationships between the soil invertase activity, urease activity and alkaline phosphatase activity, and their soil organic carbon, total nitrogen and total phosphorus contents. 5) Based on the results of principal component analysis, we concluded that the artificial forests types had obvious effects on soil microbial carbon and nitrogen, soil organic carbon, total nitrogen, the ratio of carbon to phosphorus, the ratio of nitrogen to phosphorus and urease activity. Overall, our findings suggested that P. orientalis is better than R. pseudoacacia and P. tabuliformis in term of improving soil properties in the south forest zone on the Chinese Loess Plateau.

[Effects of the Farmland-to-Forest/Grassland Conversion Program on the Soil Bacterial Community in the Loess Hilly Region].

This study investigated the effects of the program aimed at converting farmland into forest or grassland on the soil bacterial diversity in the Loess Hilly region. Corn land was selected as the experimental control, and Caragana intermedia land and Stipa bungeana land were selected as the experimental lands. Soil from three different land use types were selected as subjects. The soil bacterial communities were analyzed using a high throughput sequencing technique (MiSeq). The sequence region was 16S rRNA V3-V4 variable region. The α diversity, community composition, and relative abundances of the soil bacterial groups were analyzed, in order to explore the effects of soil physical and chemical properties on the bacterial community structure. The results showed that the structure and diversity of the microbial communities differed under the different land use types. At the phylum level, the dominant phyla were Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi, Gemmatimonadetes, and Firmicutes. At the class level, α-Proteobacteria, Actinobacteria, Acidobacteria, ß-Proteobacteria, Thermoleophilia, Gemmatimonadetes, Bacilli, and δ-Proteobacteria were predominant. Compared to the corn land, Proteobacteria and Actinobacteria were the dominant bacteria in the Caragana intermedia land, while Actinobacteria and Acidobacteria were the dominant bacteria in the Stipa bungeana land. Soil bacterial diversity was the highest in the shrub land. RDA analysis revealed that soil organic matters and total nitrogen were the most influential environmental factors. It was shown thus that the program of converting farmland into forest or grassland has significantly improved the soil fertility and environmental conditions, and the composition of the soil bacterial community has also been appreciably changed.

[Ecological stoichiometry of leaf and litter of herbaceous plants in loess hilly and gully regions, China]. / 黄土丘陵区草本植物叶片与枯落物生态化学计量学特征.

This study was conducted in the forest, forest-steppe and steppe vegetation zones along the Yanhe River Basin, where the leaf and litter samples from four dominant herbaceous plants including Lespedeza davurica, Stipa bungeana, Artemisia sacrorum, Artemisia giraldii were taken. By measuring the concentrations of carbon (C), nitrogen (N), phosphorus (P) and potassium (K), we measured the concentrations and their ratios to explore the limit and resorption of nutrient in the herbaceous plants. The results showed that the leaf N/P of four herbaceous plants was all lower than 14, suggesting their growth was mainly limited by N content. Except for L. davurica, the mean nutrient resorption efficiency of K, N and P in the other three plants was 79.9%, 48.7% and 32.5%, respectively. The higher nutrient resorption efficiency and K concentration in the leaf were beneficial for soil water competition of A. sacrorum and A. giraldii. The litter C/N in A. sacrorum was significantly lower than that in S. bungeana and A. giraldii, which was easy to decompose to benefit the nutrient recycling. This resulted in the wide distribution of A. sacrorum in the three vegetation areas.

[Soil Bacterial Communities Under Different Vegetation Types in the Loess Plateau].

In this study, we collected soil samples from four different arborcommunities and four herb communities, which represented two vegetation ecosystems in the northwest of the Loess Plateau. Our objectives were to determine the diversity of soil bacterial communities and the affecting factors with the method of 454 high-throughput pyrosequencing technology. The results showed that the structures of the microbial communities differed in terms of both the predominant phylum and the relative abundance of each phylum. At the phylum level, the dominant phyla were Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi and Planctomycetes. At the class level, Actinobacteria, α-Proteobacteria, Acidobacteria, ß-Proteobacteria and Planctomycetacia were predominant. The relative abundance of Actinobacteria in grass vegetation ecosystem was more abundant than that in forest vegetation ecosystem. Proteobacteria in forest vegetation ecosystem were more abundant. Soil pH was significantly correlated with the relative abundance of Actinobacteria. Soil organic matter,soil total N and soil total P were the key factors affecting soil Proteobacteria. These results will provide useful information for the diversity of soil bacteria and guide the practice of vegetation restoration in the Loess Plateau.

[Ecological stoichiometry characteristics of leaf-litter-soil in different plantations on the Loess Plateau, China]. / 黄土高原不同人工林叶片-凋落叶-土壤生态化学计量特征.

In order to research into the influence of Grain to Green Project in Ziwuling forest region, this paper took three typical plantations which were Robinia pseudoacacia plantation, Pinus tabuliformis plantation, and Platycladus orientalis plantation in the Ziwuling forest region of Shaanxi Province as research objects and analyzed the carbon, nitrogen and phosphorus contents of leaf, litter and soil among the three plantations. The results showed that the contents of C, N and P in the three plantations were in order of leaf > litter > soil, the contents of N and P in leaf of R. pseu-doacacia plantation were significantly higher than that of P. tabuliformis plantation and P. orientalis. Leaf N:P was 12.21, 5.36 and 6.09 in R. pseudoacacia plantation, P. tabuliformis plantation and P. orientalis plantation, respectively. It was indicated that the three species were all subject to N deficiency. C:N and C:P showed the trend of litter > leaf > soil, and N:P demonstrated the trend of leaf > litter > soil. There were highly significant positive relationships in C:N between leaf and litter in P. tabuliformis plantation. N and P in the leaf development process of R. pseu-doacacia plantation were proportionally absorbed, and proportionally remained in the litter after N and P resorption. R. pseudoacacia was the better plantation species than P. tabuliformis and P. orientalis in the south fo-rest zone on the Loess Plateau.

[Effects of stem and leaf decomposition in typical herbs on soil enzyme activity and microbial diversity in the south Ningxia loess hilly region of Northwest China]. / å®å—å±±åŒºå ¸åž‹è‰æœ¬æ¤ç‰©èŒŽå¶åˆ†è§£å¯¹åœŸå£¤é ¶æ´»æ€§åŠå¾®ç”Ÿç‰©å¤šæ ·æ€§çš„å½±å“.

With the method of litter bags, the characteristics of soil enzyme activities, soil microbial diversity at later stage of decomposition, and the relationships between soil enzyme activity and initial soil property were investigated in the process of stem and leaf decomposition of three typical herbs, i.e., Stipa bungeana, Artemisia sacrorum and Thymus mongolicus in the south Ningxia loess hilly region, Northwest China. The results showed that soil enzyme activity increased under different treatments after 480 d during stem and leaf decomposition. Soil sucrose activity (32.40 mg·g-1·24 h-1) and alkaline phosphatase activity (1.99 mg·g-1·24 h-1) were the highest in S. bungeana treatment. Soil urease activity (2.66 mg·g-1·24 h-1) was the highest in T. mongolicus treatment, and soil cellulase activity (1.42 mg·g-1·72 h-1) was the highest in A. sacrorum treatment. Soil cellulose activity at later stage of decomposition had significant positive correlation with initial microbial biomass carbon of soil. Soil cellulose activity at later stage of decomposition had significant negative correlation with initial nitrate nitrogen content of soil. Ace index, Chao index and Shannon index of soil bacteria and fungi in plant tissue addition treatments were higher than in the control. However, Simpson index was opposed. The stem and leaf decomposition significantly promoted the abundance and diversity of soil bacteria and fungi, accelerated the decomposition rate of stems and leaves, and promoted the cycle and transformation of soil nutrients.

[Characteristics of soil organic carbon and enzyme activities in soil aggregates under different vegetation zones on the Loess Plateau].

In order to explore the distribution characteristics of organic carbon of different forms and the active enzymes in soil aggregates with different particle sizes, soil samples were chosen from forest zone, forest-grass zone and grass zone in the Yanhe watershed of Loess Plateau to study the content of organic carbon, easily oxidized carbon, and humus carbon, and the activities of cellulase, ß-D-glucosidase, sucrose, urease and peroxidase, as well as the relations between the soil aggregates carbon and its components with the active soil enzymes were also analyzed. It was showed that the content of organic carbon and its components were in order of forest zone > grass zone > forest-grass zone, and the contents of three forms of organic carbon were the highest in the diameter group of 0.25-2 mm. The content of organic carbon and its components, as well as the activities of soil enzymes were higher in the soil layer of 0-10 cm than those in the 10-20 cm soil layer of different vegetation zones. The activities of cellulase, ß-D-glucosidase, sucrose and urease were in order of forest zone > grass zone > forest-grass zone. The peroxidase activity was in order of forest zone > forest-grass zone > grass zone. The activities of various soil enzymes increased with the decreasing soil particle diameter in the three vegetation zones. The activities of cellulose, peroxidase, sucrose and urease had significant positive correlations with the contents of various forms of organic carbon in the soil aggregates.