Afforestation greatly alters soil nitrogen (N) dynamics and soil microbial community, but the response of soil N dynamics to microbial activity, particularly to that of N-fixing microbes, remains unclear. In the present study, soil samples were collected from areas with a chronosequence of 17, 27 and 42 years of Robinia pseudoacacia L (RP17a, RP27a, and RP42a) and a sloped farmland (FL) as a control. The nifH gene was sequenced with the Illumina MiSeq platform to analyze soil microbial diversity and composition, and soil N fractions (TN, AN, NN, MBN, DON) were estimated. The soil N stocks, N fractions, and diversity of N-fixing microorganisms increased with the chronosequence of R. pseudoacacia. Beta diversity, analyzed using nonmetric multidimensional scaling, revealed that afforestation effectively improved soil microbial communities. The profiles of the N-fixing microorganism communities at sites 17a, 27a and 42a were clearly separated from the farmland, and soil N-fixing microbial community compositions generally changed in the farmland from Actinobacteria-dominated to Proteobacteria- dominated with stand ages. Redundancy analysis revealed that soil N fractions were positively correlated with the main species, particularly for AN, which had a 34.7% explanatory space in axis one and was significantly correlated with the Proteobacteria at the phylum level. Results implied the evolutionary tendency of the dominant microbial groups, and demonstrated the variation of the N fractions and some other physical-chemistry properties along the chronosequence of R. pseudoacacia, and that AN was the most sensitive N fraction to the N-fixing microorganism composition. Thus, our results provide evidence that the variation of soil compositions of N-fixing microbial communities is linked to the level of N fractions, especially the content of AN.

Variations of soil nitrogen-fixing microorganism communities and nitrogen fractions in a Robinia pseudoacacia chronosequence on the Loess Plateau of China

Stoichiometric ratios of carbon (C), nitrogen (N), and phosphorus (P) are considered indicators of nutrient status and ultimate ecosystem health. A detailed investigation of these elements in the leaves, branches, forest layer vegetation and soil, depending on stand age, was carried out. We investigated the effects of stand age (9-, 18-, 28-, and 48-year) on the aboveground plant parts (leaf, branch, herb, shrub, plant litter) and belowground pools (soil, roots) of P. massoniana plantations. The CNP stoichiometry of trees was affected by stand age. Mean N content in the aboveground parts in the nine-yr stand was greater than the other stands (18-, 28-, 48-yr), which decreased with increasing stand age. As stands aged, the nutrient demands of the plantations increased as well as their N:P ratios in soil. C content in the soil ranged from 30 to 105, the total N was 0.06 to 1.6, and the total P content ranged from 3.3–6.4 g kg−1. Soil C, N and P contents were greatly influenced by both stand age and soil depth, because surface soil sequester C and N more actively compared to deeper horizons, and more nutrients are released to the topsoil by the plant litter layer. Similarly, the ratios of other layers had a similar pattern as CNP because more nutrients were taken up by the plantations, decreasing nutrient supply in the deeper soil horizons. The green leaves N:P ratios (16) indicate limited growth of P. massoniana, as the range for global nutrient limitation for woody plants oscillated between 14–16, indicating N and P limitation. Young stands were observed to have greater P content and P resorption efficiency (56.9%–67.3%), with lower C:P and N:P ratios (704.4; 14.8). We conclude that with stand development, the nutrient demands of the plantations also increase, and soil N:P stoichiometry shows that these improve soil quality.

/pdf/ecological-stoichiometry-in-pinus-massoniana-l-plantation-28rtefdj.pdf

Ecological Stoichiometry in Pinus massoniana L. Plantation: Increasing Nutrient Limitation in a 48-Year Chronosequence

Soil microorganisms promote the recovery of contaminated soil by influencing the cyclic transformation of various substances. In this study, we investigated the impact of mercury pollution on the structure, composition, and main populations of soil microbial communities using a high-throughput sequencing method and observed that mercury pollution significantly influenced the diversity, structure, and distribution pattern of microbial communities. Furthermore, during mercury pollution, the Shannon and Chao indices decreased for the bacterial communities and increased for the fungal communities. Mercury pollution mainly reduced the relative abundances of Proteobacteria (16.2–30.6%), Actinomycetes (24.7–40.8%), and other dominant bacterial phyla. The relative abundance of Ascomycota decreased by 17.4% and 16.7% in alkaline and neutral soils, respectively, whereas the relative abundance of unclassified_k_Fungi increased by 26.1% and 28.6%, respectively. In acidic soil, Ascomycota increased by 106.3% and unclassified_k_Fungi decreased by 71.2%. The results of redundancy and correlation analyses suggested that soil microbial diversity was significantly correlated with soil properties such as pH, cation exchange capacity, soil organic carbon, and total nitrogen (p < 0.05) under different treatments. Our findings highlight the impact of Hg pollution on soil microbial communities, thereby providing a theoretical foundation for the bioremediation of soil Hg pollution.

/pdf/effects-of-mercury-contamination-on-microbial-diversity-of-3drc831n.pdf

Effects of Mercury Contamination on Microbial Diversity of Different Kinds of Soil

Sloping farmland is prevalent in hilly red soil areas of South China. Improper tillage patterns induce decreased soil organic matter, soil aggregate breakdown, and nutrient imbalance, thereby restricting crop production. However, the stoichiometric characteristics could reflect the nutrient availability which was mostly studied on bulk soil. The stoichiometric characteristics of soil aggregates with multiple functions in farmlands has rarely been studied. The study was to reveal the impact of tillage patterns on the size distribution, nutrient levels, and stoichiometric ratios of soil aggregates after 20 years’ cultivation. Soil samples of 0–20 cm and 20–40 cm from five tillage patterns, bare-land control (BL), longitudinal-ridge tillage (LR), conventional tillage + straw mulching (CS), cross-ridge tillage (CR), and longitudinal-ridge tillage + hedgerows (LH) were collected. The elemental content (C, N and P) and soil aggregate size distribution were determined, and the stoichiometric ratios were subsequently calculated. Through our analysis and study, it was found that the nutrient content of >2 mm soil aggregates in all plots was the highest. In the hedgerow plots, >2 mm water-stable soil aggregate content was increased. Therefore, LH plots have the highest content of organic matter and nutrients. After 20 years of cultivation, stoichiometric ratio of each plot showed different changes on soil aggregates at different levels. the C:N, C:P, and N:P ratios are lower than the national average of cultivated land. Among of them, the stoichiometric ratio in the LH plot is closer to the mean and showed better water-stable aggregate enhancement. Therefore, longitudinal-ridge tillage + hedgerows can be recommended as a cultivation measure. This study provides a reference for determining appropriate tillage measures, balancing nutrient ratios, and implementing rational fertilization.

https://www.mdpi.com/2071-1050/13/2/890/pdf

Nutrient and Stoichiometric Characteristics of Aggregates in a Sloping Farmland Area under Different Tillage Practices

The Influence of Plants on the Migration and Transformation of Nitrogen in Plant-Soil Systems: a Review

Plant stoichiometry characteristics and relationships with soil nutrients in Robinia pseudoacacia communities of different planting ages

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Plant stoichiometry characteristics and relationships with soil nutrients in Robinia pseudoacacia communities of different planting ages