Effect of stand age on the temporal dynamics of soil active carbon and nitrogen in Chinese cypress artificial forests

Stand age is a key factor for reforestation, and it has an important influence on the sustainable development of artificial forests. In order to reveal the temporal dynamics of soil active carbon (C) and nitrogen (N) in Chinese cypress artificial forests (CAFs) under different stand ages, soil samples were collected from five stands (25, 35, 45, 55, and 65 years old) in rocky mountainous of North China. The characterization of soil active C and N components, like soil microbial biomass C (MBC), microbial biomass N (MBN), dissolved organic C (DOC), dissolved organic N (DON), ammonium-N (NH₄⁺-N) and nitrate-N (NO₃⁻-N), as well as other soil properties were determined. The results showed that with the increase of stand age, the contents of soil MBC, MBN, DOC, DON, NH₄⁺-N and NO₃⁻-N tended to increase first and then decrease, and most of them reached the highest values in 45 years old. Soil active C and N components all reached peak values in June, and all of them showed the most significant variations in 0 ~ 10 cm soil layer (<i>P</i> &lt; 0.01). Correlation analysis revealed that soil active C and N components showed significantly positive correlations with soil total N, total C, the number of microorganism communities and soil water content, as well as litter biomass (<i>P</i> &lt; 0.01). Stepwise multiple regression analysis demonstrated that soil total N was the key controlling factor to soil active C and N components. In conclusion, with the increase of stand ages, soil active C and N components of CAFs did not accumulate all the time, indicating that stand age should be regarded as an important factor for the sustainable management of artificial forests.

林分年龄是人工造林的关键因子，对人工林的可持续发展具有重要影响。为揭示不同林分年龄下侧柏人工林（Chinese cypress artificial forests, CAFs）土壤活性碳（C）与氮（N）的时间动态变化，研究于华北石质山地选取5个林分年龄分别为25、35、45、55和65年生的样地采集土壤样品。测定了土壤微生物量碳（MBC）、微生物量氮（MBN）、溶解性有机碳（DOC）、溶解性有机氮（DON）、铵态氮（NH₄⁺-N）与硝态氮（NO₃⁻-N）等土壤活性碳氮组分，以及其他土壤理化性质。结果表明：随着林分年龄增长，土壤MBC、MBN、DOC、DON、NH₄⁺-N及NO₃⁻-N含量均呈先升高后降低的变化趋势，多数组分在45年生林分中达到峰值。土壤活性碳氮组分均在6月达到峰值，且在0~10 cm土层中变化最为显著（P < 0.01）。相关性分析显示，土壤活性碳氮组分与土壤全氮、全碳、微生物群落数量、土壤含水量及枯落物生物量均呈显著正相关（P < 0.01）。逐步多元回归分析表明，土壤全氮是调控土壤活性碳氮组分的关键影响因子。综上，侧柏人工林的土壤活性碳氮组分并未随林分年龄增加持续累积，这表明林分年龄应作为人工林可持续经营的重要考量因素。