近100年植被破坏侵蚀环境下土壤质量退化过程的定量评价

Quantifying soil quality degradation over 100 years after deforestation under erosional environments

以黄土高原子午岭地区近100a不同开垦年限位于墚坡的农地为研究对象,通过相关分析并结合所选指标的有效性、全面性和敏感性分析,从6个土壤物理指标、6个土壤化学指标、8个土壤生物指标,共20个土壤性质指标中,筛选出有机碳、真菌数量、碱性磷酸酶活性、蔗糖酶活性、毛管孔隙度、物理性粘粒、粗粉粒和水稳性团聚体平均重量直径(MWD)等8个土壤性质指标,作为土壤质量退化评价指标体系。通过主成分分析,计算土壤质量综合指数(SQI),定量评价近100a植被破坏侵蚀环境下的土壤质量退化过程。结果表明,林地SQI在0—20和20—40cm土层分别为0.909和0.427;林地开垦耕种100a期间,SQI随开垦年限的增加呈快速下降的趋势,尤其是在开垦耕种初期4a内,SQI年均下降速率在0—20和20—40cm土层分别达0.073和0.028;同林地相比,SQI分别下降了32.3%和26.2%。此后随着开垦耕种年限的增加下降速率逐渐减小。开垦耕种12a后,0—20和20—40cm土层的SQI分别为0.549和0.235,分别下降39.6%和45.0%;4—12a期间的SQI下降速率分别为开垦初期的10.9%和35.7%。开垦耕种43a后,SQI在0—20和20—40cm土层分别为0.333和0.144,分别较林地下降64.4%和66.3%,12—43a期间的SQI下降速率分别为开垦初期的9.6%和10.7%;开垦耕种100a期间,SQI在0—20和20—40cm土层分别为0.140和0.068,分别下降84.6%和84.1%;43—100a期间的SQI下降速率分别为开垦初期的4.1%和3.6%。同时,SQI年均下降幅度在0—20cm土层高于在20—40cm土层;且随开垦耕种年限的增加,SQI在0—20cm和20—40cm上下两土层之间的差异趋于减少,说明加速侵蚀是研究区土壤质量下降的主要原因。

Soil erosion and soil quality degradation following deforestation are major processes responsible for ecoenvironmental deterioration on the loess Plateau. However, there is little information available on soil quality degradation caused by deforestation and increased soil erosion. To quantify the degradation rates, we selected eight croplands on southern slopes with different years of cultivation within 100 years in the Ziwuling area as a case study. Using correlation analysis in conjunction with effectiveness, completeness, and sensitivity of all 20 indictors, we selected eight indictors, i. e. , soil organic carbon, fungi amount, alkaline phosphatase activity, invertase activity, capillary porosity, coarse clay, coarse silt, and mean weight diameter of water-stable aggregate（MWD） for use in the evaluation system. A quantitative Soil Quality Index （SQI） comprised of these 8 indicators was developed using a principal component analysis. Results showed that SQI in the 0 - 20 and 20 - 40 cm soil depths of the forestlands were 0. 909 and 0. 427. The SOI decreased in the top 40 cm layer as cultivation years increased, especially during the first four years of cultivation with annual reduction rates of 0. 073 and 0. 028 in the first （0 --20 cm） and second （20 --40 cm） depths respectively. Compared with the corresponding forestlands, SQI during the initial four years of cultivation decreased by 26.2% and 32.3% in the I^st and 2^nd depths, respectively. After 12 years of cultivation, SO1 in the 1^st and 2^nd depths were 0. 594 and 0. 235, decreased by 39.6% and 45.0% , respectively. Reduction rates in the 1 st and 2^nd depths during 4 -- 12 years of cultivation were 10.9% and 35.7% relative to the initial four years of cultivation. After 43 years of cultivation, SQI in the 1^st, and 2^rd depths were 0. 333 and 0. 144 and reduced by 64.4% and 66.3% , respectively, compared with the forestlands. Reduction rates in the 1^st and 2nd depths during 12 --43 years of cultivation were 9.6% and 10.7% relative to the initial four years of cultivation. During 100 years of cultivation, SQI in the two depths were 0. 140 and 0. 068 and reduced by 84.6% and 84.10% , respectively. Reduction rates in the two depths during 100 years of cultivation were 4.1% and 3.6% relative to the initial four years of cultivation. Meanwhile, the reduction rates of SQI in the lS＇ depth were greater than those in the 2^nd depth, and the differences of SO1 between the two layers decreased as the cultivation years increased, which implied that accelerated erosion could be a main cause for soil quality degradation in the region.