石羊河下游退耕地土壤微生物变化及土壤酶活性

采用时空替代法,对石羊河下游不同年限(1,2,3,4,5,8,15,24、31 a)退耕地土壤微生物(细菌、真菌和放线菌)数量、生物量(碳、氮和磷)及土壤酶(过氧化氢酶、蔗糖酶、脲酶和磷酸酶)活性变化及三者的相关性进行了测定和分析。结果表明,在退耕1—31 a的9个样地样方中土壤三大类微生物数量以细菌最高,放线菌次之,真菌最低。总体来看,三大土壤微生物数量的加权平均值最大值均在退耕后的前8 a。土壤微生物生物量碳在退耕初期随着退耕年限的增加而减小,退耕4 a后逐渐增大,退耕24 a期间达到了加权平均值的最大,最后趋于稳定;土壤微生物生物量氮在退耕初期随着退耕年限的增加而增加,退耕4 a加权平均值的最大值出现,随后逐渐减小的趋势,并且不同退耕年限土壤微生物生物量氮差异显著;土壤微生物生物量磷在退耕初期随着退耕年限的增加而增加,退耕8 a前后加权平均值达到最大值,随后逐渐减小,最终趋于稳定。土壤酶活性总趋势随着退耕地自然演替时间的增加呈波动式下降。不同土壤层次(0—10 cm,10—20 cm,20—30 cm及30—40 cm),土壤微生物数量、生物量及土壤酶活性随土层深度显著降低,并且表层土壤微生物生物量及土壤酶活性占有较大比例。土壤微生物及土壤酶活性的变化是一个极其缓慢的互动过程,存在着互相回馈的响应,特别是真菌与放线菌、微生物量氮及蔗糖酶,放线菌与过氧化氢酶、蔗糖酶,微生物量碳与磷酸酶,微生物量氮与脲酶,微生物量磷与蔗糖酶均存在极显著的相关性。总体来看在退耕年限4—5 a前,有利于土壤发育,退耕后期土壤肥力呈下降的趋势。

Soil microbial and soil enzyme activity in a discontinued farmland by the Lower Shiyang River

To better understand the changes in soil conditions after agriculture use ceases, we measured and analyzed the changes in three components-soil microbial quantity, soil microbial biomass, and soil enzyme activity-in sample sites where farming had been discontinued for different periods (1, 2, 3, 4, 5, 8, 15, 24 and 31 years), in the lower Shiyang River area. By using the data, the correlations between the three components were also studied in 2012. The three components, respectively, contained 1) bacteria, fungi, and actinomycetes; 2) microbial biomass carbon, microbial biomass nitrogen, and microbial biomass phosphorus; and 3) catalase, sucrase, urease, and phosphatase. Synchronic space investigation was adopted instead of diachronic temporal investigation. Results showed that bacteria were the most abundant of the three soil microorganisms in the nine soil samples taken 31 years after farming ceased, followed by actinomycetes and fungi. Overall, the maximum weighted average of the three soil microbial quantities all occurred in the first 8 years after farming ceased. The quantity of soil microbial biomass carbon initially decreased with increasing time since farming ceased. It gradually increased starting in the 4^th year and reached the peak in the 24^th year, after which it became stable. The soil microbial biomass nitrogen appeared to increase with time. After the maximum weighted average in the 4^th year, there was a steady reduction with large significant differences in soil microbial biomass nitrogen among soil sites of various ages. The weighted average soil microbial biomass phosphorus increased at the same pace as nitrogen at the beginning of the soil restoration period. The biomass phosphorus level climaxed at the 8^th year, after which it tended to decline gradually before finally stabilizing. The general trend of soil enzyme activity decreased, but with fluctuations, with increasing soil succession age. Moreover, soil microbial quantity, soil microbial biomass, and soil enzyme activity declined dramatically in all restoration ages with soil depth from 0 to 10 cm, 10 to 20 cm, and 20-30 to 30-40 cm. In addition, the surface soil microbial biomass and soil enzyme activity were the largest among the four soil levels. The changes in soil microbial quantity, soil microbial biomass, and soil enzyme activity were extremely slow. In addition, an interactive process and a feedback response between the three components was observed. Highly significant correlations were found, especially between fungi and actinomycetes, microbial biomass nitrogen and sucrase, actinomycetes and catalase and sucrase, microbial biomass carbon and phosphatase, microbial biomass nitrogen and urease, and microbial biomass phosphorus and sucrose. To summarize, we noted an improvement in the soil conditions in the initial 4-5 years after ceasing agricultural practice, followed by a declining trend in soil fertility.