植被类型与坡位对喀斯特土壤氮转化速率的影响

土壤氮素转化对于植物氮素营养具有重要作用,尤其是对于受氮素限制的喀斯特退化生态系统。选取植被恢复过程中4种典型喀斯特植被类型(草丛、灌丛、次生林、原生林)和3个坡位(上、中、下坡位)表层土壤(0—15cm)为对象,利用室内培养的方法,研究不同植被类型和坡位下土壤氮素养分与氮转化速率(氮净矿化率、净硝化率和净氨化率)的特征及其影响因素。结果表明,植被类型对土壤硝态氮含量、无机氮含量、氮净矿化率、净硝化率和净氨化率均有显著影响(P0.01),即随着植被的正向演替(草丛—灌丛—次生林—原生林),土壤硝态氮含量、无机氮含量、土壤氮净矿化速率和净硝化速率整体上呈增加趋势,而坡位以及坡位与植被类型的交互作用对上述土壤氮素指标无显著影响(P0.05)。冗余分析结果表明凋落物氮含量、凋落物C∶N比和硝态氮含量对土壤氮转化速率有显著影响,其中凋落物氮含量是影响土壤氮转化速率的主要因子(F=35.634,P=0.002)。可见,尽管坡位影响喀斯特水土再分配过程,但植被类型决定的凋落物质量(如凋落物氮含量等)对喀斯特土壤氮素转化速率的作用更为重要。因此,在喀斯特退化生态系统植被恢复初期,应注重植被群落的优化配置(如引入豆科植物)和土壤质量的改善(如降低土壤C∶N),促进土壤氮素转化及氮素的有效供给。

Effects of vegetation type and slope position on soil nitrogen transformation rate in Karst regions

Soil net nitrogen mineralization and nitrification are critical processes that provide nitrogen nutrients for plant growth in degraded Karst ecosystems with limited nitrogen. However, little is known about these processes. In this study, soil samples derived from topsoil (0-15 cm) were collected from the upper, middle, and lower slope positions of four Karst ecosystems-tussock (T),shrub (S),secondary forest (SF) and primary forest (PF), which are a typical vegetation succession series in Karst areas. A soil incubation experiment was conducted to measure the soil nitrogen transformation rates, including net nitrogen mineralization rate, net nitrification rate, and net ammonification rate. The results showed that vegetation types significantly (P<0.01) affected soil NO₃^--N and inorganic nitrogen content, soil net nitrogen mineralization rate, net nitrification rate, and net ammonification rate. In particular, the content of soil NO₃^--N and inorganic N, soil net nitrogen mineralization rate, and net nitrification rate gradually increased along the vegetation succession. The slope position and the interaction between vegetation type and slope position had no significant effect on the monitored variables (P > 0.05). RDA analysis showed that the soil nitrogen transformation rate was primarily affected by the litter nitrogen content, litter C : N, and the content of soil NO₃^--N. However, the litter nitrogen was the main factor affecting the soil nitrogen transformation rate (F=35.634, P=0.002).Our results suggest that vegetation restoration has positive effects on Karst soil nitrogen transformation. The improvement of litter quality (e.g., litter nitrogen content) may be the key factor influencing the soil nitrogen transformation rates over Karst vegetation succession. Therefore, managing litter C: N (e.g., to introduce legumes) may be conducive to the restoration of degraded Karst ecosystems. These findings may serve as a scientific basis for further understanding of soil nitrogen cycling in Karst regions and help facilitate preservation and restoration activities in Karst regions.