Background Benggang is vividly called "ecological ulcer" in the south red soil area. The purpose of this study is to clarify the nutrient circulation and limitation of Benggang's plant and soil environment in the extremely degraded ecosystem.

Methods Taking 3 Benggangs in Huangnikeng, Zhuotian town, Changting county of Fujian province representing 3 different active situations (active, semi-stable, and stable) as the study cases, and set up 3 different parts (catchment slope, collapse wall, and colluvial body) in each Benggang as sampling sites, the leaves, stems and roots of plants and the topsoil (0~10 cm) were separately collected 3 times in each site. Then the laboratory experiments were carried out. Differences among the active situations of Benggangs were examined by one-way analysis of variance. The relationship between plant and soil was analyzed using Pearson's correlation coefficients.

Results 1) There were no significant differences in the C, N, P content and ecological stoichiometric ratio among different active conditions except the N content and C:N of plant stem, the P content and C:P, and N:P of plant root. 2) The contents of C, N, P and ecological stoichiometric ratios among different plants were significantly different(P < 0.05), and the contents of leave were higher than those of stem and root in the same plant. 3) With the improvement of Benggang's stability, the organic C, total N contents of surface soil were gradually increasing. The contents of organic C were from 0.650±0.098 g/kg to 4.700±0.808 g/kg, and the contents of total N were from 0.205±0.009 g/kg to 0.570±0.080 g/kg. The soil C, N, P contents and ecological stoichiometric ratios of the stable Benggang significantly differed among the 3 different active situations(P < 0.05). The stable Benggang's soil organic C, total N and total P contents were the highest in the 3 Benggangs. But they were only up to 4.70 mg/g, 0.57 mg/g, 0.07 mg/g, which were far below the national level 11.12 mg/g, 1.06 mg/g, 0.65 mg/g. 4) The correlations of C, N and P contents and their stoichiometric ratios between plants and soil surface were not very relevant (P>0.05) except some individual indicators. For example, there were significant negative correlations between leave's P content and soil's organic C and total N content. There were significant positive correlations between stem's N content and soil's organic C content(P < 0.05).

Conclusions Different active situations of Benggang have important influences on soil nutrient. With the improvement of the stability, soil fertility can be improved but only be in the low soil fertility. The results prove that Benggang is the extremely degraded ecosystem and the ecological stoichiometric characteristics of plants are not mainly determined by soil nutrient conditions, but mainly affected by the characteristics of the plants, which show the adaptability of the plants to the extremely degraded ecosystem environment.