皆伐油松人工林天然更新对土壤肥力和酶活性的影响

目的探讨油松人工林皆伐后天然更新成林的过程中对土壤肥力和酶活性的影响，分析植物群落演替过程中土壤质量的变化规律，为油松林的地力维护和可持续经营提供理论依据。方法采用演替时序法，在河北平泉地区选择立地条件一致的中龄林（32年）、近熟林（40年）、成熟林（53年），皆伐后不同时间（5年、10年、24年）的天然更新林及皆伐后8年撂荒地作为对照，分0 ~ 10 cm、10 ~ 20 cm土层测定土壤肥力和酶活性，并对2组指标进行典型相关分析。结果（1）从中龄林到成熟林土壤质量持续提升，其中在0 ~ 10 cm土层土壤含水量、全氮含量和蔗糖酶活性分别提高了161.5%、379.1%和181.3%，各指标在10 ~ 20 cm土层也有显著提升（P < 0.05）。（2）皆伐后5年天然更新林与成熟林相比土壤肥力和酶活性差异不显著（P > 0.05），到伐后24年土壤肥力和酶活性逐渐下降，与伐后5年相比，土壤毛管持水量、全氮含量和蔗糖酶活性在0 ~ 10 cm土层分别下降了24.4%、61.0%和28.8%，在10 ~ 20 cm土层各指标也均有明显下降（P < 0.05），但伐后24年土壤肥力和酶活性仍高于中龄林和近熟林。（3）天然更新林的土壤肥力和酶活性显著高于撂荒地（P < 0.05），天然更新林在0 ~ 10 cm土层土壤毛管持水量、有机质含量和脲酶活性相比撂荒地分别提高了43.7%、145.7%和116.6%，在10 ~ 20 cm土层各指标也显著高于撂荒地（P < 0.05）。（4）典型相关分析表明土壤酶活性与土壤肥力之间具有极显著相关性（P < 0.01），土壤肥力主要受土壤密度、毛管持水量、毛管孔隙度、有机质、全氮的影响，而土壤酶活性主要受脲酶和蔗糖酶的影响。土壤肥力和酶活性的典型变量得分均表现为成熟林 > 伐后5年 > 伐后10年 > 伐后24年 > 近熟林 > 撂荒地 > 中龄林。结论油松人工林成熟林阶段土壤质量明显改善，随着时间推移皆伐会造成一定程度的土壤肥力和酶活性下降，但在皆伐迹地上及时实现植被恢复会显著减轻下降的程度，且土壤质量明显好于同一发育阶段的人工林。皆伐油松人工林天然更新可以有效地缓解地力衰退的问题。 

Effects of natural regeneration on soil fertility and soil enzyme activities in Pinus tabuliformis plantations after clearcutting

ObjectiveLong maintenance of soil quality after clearcutting is a primary research content of sustainable forest management in plantation. This study aims to explore the effects of natural regeneration of Pinus tabuliformis on soil fertility and enzyme activities after clearcutting, in the meantime, analyze the variation of soil quality during succession, which has great significance for maintaining the site producivity of Pinus tabuliformis plantation. Method The variations of soil fertility and enzyme activities in 0?10 cm and 10?20 cm soil layers were investigated through chronosequence approach in the Pinus tabuliformis plantation (middle age forest of 32 years, near-mature forest of 40 years and mature forest of 53 years), natural regeneration forest after clearingcutting (5, 10, 24 years after clearcutting) and wasteland after cutting as a contrast to natural regeneration in Pingquan of Hebei Province. Canonical correlation analysis was conducted on soil fertility and enzyme activity indicators.Result (1) Soil fertility and enzyme activities were continuously improved from middle-aged forest to mature forest. Soil water content, total nitrogen content and invertase activity in 0?10 cm soil layer were increased by 161.5%, 379.1% and 181.3% respectively, and the indexes were also significantly increased in 10?20 cm soil layer (P < 0.05). (2) There was no significant difference in soil fertility and enzyme activity between 5 years natural regeneration forest after clearcutting and mature forest (P > 0.05). However, soil fertility and enzyme activities decreased gradually from 5 years after clearcutting. Compared 24 years with 5 years after clear cutting, soil capillary moisture capacity, total nitrogen content and invertase activity in 0?10 cm soil layer decreased by 24.4%, 61.0% and 28.8%, respectively, and the indexes in the 10?20 cm soil layer also decreased significantly (P < 0.05). However, soil fertility and enzyme activities in 24 years after cutting were still higher than middle-aged and near-mature forests. (3) The soil fertility and enzyme activities of natural regeneration forest were significantly higher than those of wasteland in 10 years after clear cutting (P < 0.05). Compared with wasteland, the soil capillary moisture capacity, organic matter content and urease activity of natural regeneration forest in 0?10 cm soil layer increased by 43.7%, 145.7% and 116.6%, respectively, the soil indexes in 10?20 cm soil layer were also significantly higher than those of wasteland as well (P < 0.05). (4) Canonical correlation analysis showed that there was an extremely significant correlation between soil fertility and enzyme activity (P < 0.01). Soil fertility was mainly affected by bulk density, capillary moisture capacity, capillary porosity, organic matter and total nitrogen, while soil enzyme activity was mainly affected by urease and invertase. The canonical variables of soil fertility and enzyme activity were as follows: mature forest > 5 years after cutting > 10 years after cutting > 24 years after cutting > near-mature forest > wasteland > middle-aged forest. Conclusion The soil quality of Pinus tabuliformis plantation was significantly improved at the mature forest stage. The certain extent declining of soil fertility and enzyme avtivity would be caused by clearcutting. However, timely vegetation restoration on clear cutting land will significantly reduce the degree of decline, and the soil quality was better than the plantation forest at the same developmental stage. Natural regeneration of Pinus tabuliformis plantation after clear cutting can effectively alleviate the problem of soil degradation.