东北黑土区坡耕地水土流失危险程度评价

Potential hazard assessment of slope farmland soil erosion in black soil region of northeastern China

富含有机质的黑土层是东北黑土区成为我国重要商品粮基地的基础,目前面临水土流失使黑土层变薄和丧失的危险。水利部现行的SL718—2015《水土流失危险程度分级标准》主要针对植被恢复,是否适宜评价黑土区坡耕地水土流失危险程度?为此,本研究从水土流失危害黑土层的角度进行评价。在松嫩黑土区20.7万km2的范围内采集196个坡耕地样点的土壤剖面样品,利用中国土壤系统分类暗沃表层和均腐特性的定量指标,判断当前黑土层厚度。在此基础上,首先依据标准提出的抗蚀年限指标和植被恢复年限与坡度组合指标2种方法,判断了196个样点的水土流失危险程度;然后依据黑土成土速率,对抗蚀年限标准进行修订,利用修订后的标准重新判断水土流失危险程度。结果表明:抗蚀年限能反映水土流失导致的土层变薄或丧失的危险,但为维持作物正常生长,将临界土层厚度由10 cm改为20 cm对于维护耕地生产力更有意义。应用现行标准评价196个坡耕地水土流失危险程度时,用抗蚀年限指标评价的轻度以上危险程度样点只有13.8%,没有反映出水土流失危害黑土层的危险;用自然植被恢复年限与坡度组合指标评价的结果集中在轻度、中度和重度危险3个级别,轻度危险比例为84.7%,没有反映水土流失危害黑土层危险和地区差异。标准导致黑土区坡耕地水土流失危险程度被明显低估,不利于黑土资源的保护。为了维持黑土资源可持续利用,土壤流失速率应小于成土速率。以成土速率作为微度和轻度以上危险程度分界,提出抗蚀年限标准为:2000年以上是微度危险,>500~2000年是轻度危险,>100~500年是中度危险,>50~100年是重度危险,≤50年是极度危险。据此判断196个坡耕地样点中,轻度以上危险程度比例为92.3%。其中轻度、中度、重度和极度危险程度比例分别为36.7%、36.2%、5.1%和14.3%。该标准既体现了水土流失对黑土层的危害,又反映了区域分异特征,可为水土保持规划提供依据,对于维护黑土可持续生产力具有重要意义。

[Background]Soil depth with rich organic matter provides nutrients for grain production in the black soil region of northeastern China.However,at present,the thickness of the black soil layer in the black soil region of northeastern China is thinner or even lost.A large number of ephemeral gullies and permanent gullies have directly affected the sustainable use of black soil resources and national food security.Classification Standard for Potential Hazard of Soil Erosion(SL718—2015)issued by the Ministry of Water Resources(MWR)focusing on vegetation recovery may not be suitable for assessing slope farmland soil erosion risk.[Methods]Total 196 soil profile samples from slope farmland were collected in Songnen black soil region with area of 200000 km2,and top soil depth for each sample was identified using quantitative index of mollic epipedon and isohumo features in Chinese soil taxonomy.According to the standard SL718—2015,the erosion risk was assessed for 196 samples using single index of anti-erodibility duration and combined index of vegetation recovery duration and slope degree respectively.Furthermore,they were assessed again by using modified values of anti-erodibility duration for each class based the rate of black soil formation.[Results]The anti-erodibility duration index could be used for assessing the potential hazard of soil depth decreasing caused by soil erosion.However,to maintain the crop growth and farmland productivity,it was better to change the value of threshold soil depth for estimating anti-erodibility duration from 10 cm to 20 cm.According to the standard SL718—2015,only 13.8%of the total 196 samples demonstrated potential hazard by using the index of anti-erodibility duration,and all the samples were classified as light,medium,and heavy potential hazard by using combined index of vegetation recovery duration and slope degree,of which 84.7%was light.The potential hazard degree was underestimated and the standard was not suitable for assessing the soil erosion potential hazard for the slope farmland.[Conclusions]To maintain the sustainable productivity of the black soil,the soil erosion rate should be less than or equal to soil formation rate,which could be used as the critical value for identifying soil erosion risk.The modified threshold values of the anti-erodibility duration for each potential hazard class were suggested as 2000 years for not potential hazard,>500-2000 years for light,>100-500 years for medium,>50-100 years for heavy,and≤50 years for extreme potential hazard.Of total 196 samples,36.7%,36.2%,5.1%,and 14.3%were light,medium,heavy,and extreme potential hazard under modified index class threshold values,i.e.,the percentage over light potential hazard was 92.3%.