We investigated the delta(15)N profile of N (extractable NH(4)(+), NO(3)(-), and organic N (EON)) in the soil of a N-saturated subtropical forest. The order of delta(15)N in the soil was EON > NH(4)(+) > NO(3)(-). Although the delta(15)N of EON had been expected to be similar to that of bulk soil N, it was higher than that of bulk soil N by 5 per thousand. The difference in delta(15)N between bulk soil N and EON (Delta(15)N(bulk-EON)) was correlated significantly with the soil C/N ratio. This correlation implies that carbon availability, which determines the balance between N assimilation and dissimilation of soil microbes, is responsible for the high delta(15)N of EON, as in the case of soil microbial biomass delta(15)N. A thorough delta(15)N survey of available N (NH(4)(+), NO(3)(-), and EON) in the soil profiles from the organic layer to 100 cm depth revealed that the delta(15)N of the available N forms did not fully overlap with the delta(15)N of plants. This mismatch in delta(15)N between that of available N and that of plants reflects apparent isotopic fractionation during N uptake by plants, emphasizing the high N availability in this N-saturated forest.

Vegetation plays a key role in water conservation in the southern Qilian Mountains (northwestern China), located in the upper reaches of the Heihe River. Nitrogen-fixing bacteria are crucial for the protection of the nitrogen supply for vegetation in the region. In the present study, nifH gene clone libraries were established to determine differences between the nitrogen-fixing bacterial communities of the Potentilla parvifolia shrubland and the Carex alrofusca meadow in the southern Qilian Mountains. All of the identified nitrogen-fixing bacterial clones belonged to the Proteobacteria. At the genus level, Azospirillum was only detected in the shrubland soil, while Thiocapsa, Derxia, Ectothiorhodospira, Mesorhizobium, Klebsiella, Ensifer, Methylocella and Pseudomonas were only detected in the meadow soil. The phylogenetic tree was divided into five lineages: lineages I, II and III mainly contained nifH sequences obtained from the meadow soils, while lineage IV was mainly composed of nifH sequences obtained from the shrubland soils. The Shannon–Wiener index of the nifH genes ranged from 1.5 to 2.8 and was higher in the meadow soils than in the shrubland soils. Based on these analyses of diversity and phylogeny, the plant species were hypothesised to influence N cycling by enhancing the fitness of certain nitrogen-fixing taxa. The number of nifH gene copies and colony-forming units (CFUs) of the cultured nitrogen-fixing bacteria were lower in the meadow soils than in the shrubland soils, ranging from 0.4 × 10⁷ to 6.9 × 10⁷ copies g⁻¹ soil and 0.97 × 10⁶ to 12.78 × 10⁶ g⁻¹ soil, respectively. Redundancy analysis (RDA) revealed that the diversity and number of the nifH gene copies were primarily correlated with aboveground biomass in the shrubland soil. In the meadow soil, nifH gene diversity was most affected by altitude, while copy number was most impacted by soil-available K. These results suggest that the nitrogen-fixing bacterial communities beneath Potentilla were different from those beneath Carex

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