生命科学学院知识库

为了探讨半干旱黄土高原典型区不同土地利用对土壤质量的影响， 我们选取了三种典型的土地利用类型，即不同种植年限的紫花苜蓿人工草地、不同演替阶段的撂荒地、不同种植年限的柠条人工灌木林地，对它们的土壤有机碳、轻组有机碳、微生物体碳、土壤氮素、磷素、地上生物量以及土壤水分动态进行了系统的研究。获得了以下主要结果：
在不同种植年限的紫花苜蓿（Medicago sativa L.）人工草地（生长 2 年、3年、7年、13 年、19 年、23 年）中，除23 年生苜蓿草地为坡地外，其它均为平坦的川地。2年生苜蓿草地第一年与豌豆混播（其前茬也为豌豆），第二年为纯的苜蓿草地，其它5个草地均为单播。3、7、13与19年生苜蓿草地的前茬为春小麦，23 年生苜蓿草地在播种前为天然草地。所有苜蓿草地均不施肥，每年刈割2 次。土壤含水量和水势随着苜蓿生长年限的增加而降低，土壤干层也随生长年限的增加而加剧。苜蓿干草产量则随生长年限呈曲线形式变化（y=-38.186x2+841.86x +1154.7, R2=0.4188**,P<0.01），由此得出苜蓿生长 11 年时产量最高。表层土壤（0-20cm）的轻组有机碳（LFOC）、微生物体碳（MBC）和氮（MBN）均随生长年限的增加而增加。当苜蓿生长 13 年后，土壤 MBC 增长非常缓慢，基本达到了稳定。当苜蓿生长 13 年以内时，土壤 SOC、土壤全 P（STP）、速效 P（AvaiP）和碳氮比（C/N）均随生长年限的增加而呈逐渐降低趋势，苜蓿生长 13 年之后，这些指标又逐渐增加。SOC 和 STP、AvaiP、土壤全碳 (STC，包括有机碳和无机碳)、全氮（STN）之间显著正相关（R 值分别0.627**, 0.691**, 0.497*, 0.546*）。 MBC 和 LFOC 与苜蓿的生长年限显著正相关（R分别为 0.873***和 0.521*）。 MBC 和土壤无机碳（SIC）之间也显著正相关。 LFOC、MBC、 MBC/SOC、 LFOC/SOC 相互之间显著正相关。

对5块不同演替阶段的撂荒地（农田撂荒 1年、3年、4年、5年、9年，坡度分ii别为 10-15o、5-10o、0-5o、10-15o、0o）和两块从未开垦的天然草地（坡度均为 20-25o，一块经常放牧，一块未放牧）及一块常规农田的比较研究表明，2m 土层内土壤含水量随撂荒演替的进程而逐渐降低。土壤容重有随着撂荒年限而逐渐升高的趋势，常规农田最低，未放牧的天然草地最高。而 SOC LFOC、MBC和MBN 在农田撂荒后，随演替年限的增加而增加，SOC、LFOC、MBC和MBN 与撂荒年限之间的相关系数R 分别为 0.609*、0.679**、0.842***和 0.893***。4年、9年撂荒地 0-20cm、20-60cm、60-100cm 土层的 SOC含量和0-20cm 土层每公顷的 SOC 量显著高于两块天然草地；5年撂荒地 0-20cm 土层的 SOC 也高于天然草地。 4 年、 9 年撂荒地 0-20cm 土层的LFOC 与天然草地类似，而 5 块撂荒地 0-20cm 土层的 MBC、 MBN 均低于天然草地。以 5 块撂荒地的坡度和撂荒年限作为 2 个自变量进行二元回归发现， MBC、 LFOC 和LFOC/SOC 含量均与坡度呈显著负相关，而只有 MBC 与撂荒年限呈显著正相关，表明坡面的水土流失对土壤活性有机碳的负面影响高于演替年限的正面影响。 7月份的地上生物量随演替年限的增加而增加，并且显著促进土壤 LFOC、 MBC、 MBN、LFOC/SOC 和 MBC/SOC 的提高（R=0.574**、0.625**、0.536*、0.619**和 0.451*）。总之，随撂荒演替年限的增加，撂荒地的植被覆盖度增加，可以维持相对较高的土壤活性有机质和微生物生物量。梯田和低坡度（0-5o）农田的撂荒，能够有效地恢复土壤 SOC、 LFOC 和土壤养分，维持较高的微生物活性。本地区陡坡地和一些不同退化程度农田的撂荒是保持土壤质量的一项有效措施。
对不同种植年限的柠条（Caragana korshinskii）灌木林地（生长 8 年、 19 年、42年。其中8年、19 年生柠条为水平阶种植，修筑水平阶之前坡度与天然草地一致；42 年生柠条为坡式种植，坡度为 10-15o）和天然草地（荒地，坡度为 20-25o）的研究发现，土壤含水量随柠条种植年限的增加而逐渐降低，土壤干层的程度也不断加剧，春夏秋三个季节内 2-5m 土层的土壤含水量都和柠条的生长年限之间呈极显著的负相关关系。三个季节 19 年、42 年生柠条样地 0.6-2m 土层的土壤含水量均低于天然草地，而 8 年生柠条样地则与天然草地接近， 42 年生柠条样地 0-20cm 的土壤含水量在所有样地中最高。地上生物量与生长年限之间呈极显著的正相关关系（R=0.911***,P<0.001）。三块不同种植年限的柠条灌木林地 0-20cm 土层的轻组土壤（dry matter oflight fraction）、 LFOC、 MBC 和 STC 均低于天然草地。 19 年和 42 年生柠条样地表层土壤 0-20cm 的 SOC 几乎与原生天然草地一致，说明随柠条生长年限的增加，土壤SOC 可逐渐恢复到原来的水平。 19 年生柠条样地 0-20cm、 20-60cm、 60-100cm 三个iii土层的 STN，42 年生柠条样地 20-60cm、60-100cm土层的 STN 以及这两块样地 0-20cm土层的 MBN 均高于天然草地，说明作为豆科植物的的柠条灌木林地的土壤氮素水平得到了改善，可以为植物与微生物的生长和活动提供更多的速效氮。 0-20cm 土层的SOC、 LFOC、 MBC 含量相互之间显著的正相关， 0-20cm 土层的 SOC、 LFOC、 MBC和 MBN 与轻组土壤含量之间也显著正相关。表明随柠条生长年限的增加，植物残体的输入增加，土壤有机碳逐渐增加。
对三种土地利用类型的土壤有机碳的比较发现， 撂荒地（包括撂荒地和天然草地）倾向于含有较高的 SOC、 STC、 LFOC 和 MBC，表明与苜蓿草地和柠条林地相比，撂荒地和天然草地能够汇聚更多的 C 到土壤中。柠条灌木林地表层 0-20cm 的 SOC、STC、 LFOC、 MBC 和轻组土壤的含量也高于苜蓿草地。柠条灌木林地和苜蓿草地的表层 0-20cm 土壤的 MBN 含量显著高于撂荒地，表明豆科植物能够为土壤微生物提供更多的速效氮。

Three land uses: seeded alfalfa (Medicago sativa L.) with different growth years,abandoned fields after cropland with series succession proceeding and Caragana korshinskii shrubs with different growth years, from Loess Plateau of China, were used to evaluate the effect of land uses on soil organic carbon (SOC), light fraction of organic C (LFOC), soil biomass microbial C (MBC) and N (MBN), soil P and N elements,aboveground biomass and soil water content, for a purpose of providing insights into the development of sustainable agriculture systems and vegetation restoration and construction on the Loess Plateau. The results as shown follows:
In the different growing years of artifical alfalfa (Medicago sativa L.) grasslands (2-,3-, 7-, 13-, 19-, 23-year alfalfa grassland), 23-year alfalfa grassland is a sloping field on a
hillside, and the other treatments are all flat fields in a valley. In these six grasslands, the 2-year alfalfa was growing in a mixture with pea in the seeding year and pea was in the rotation the year before seeding, and was pure alfalfa grassland since the second year; the other treatments were all monocropped with wheat in rotation before seeding, except the 23-year alfalfa was native grassland before seeding. All the grasslands were unfertilized and mowed twice one year. The soil water content, soil water potential decreased and the
desiccated layers grew with the number of growing years of alfalfa. Correlationship between forage yield and increasing years was curvilinear (y =-38.186x2+841.86x +1154.7,R2 = 0.4188**, P<0.01), therefore the highest forage yield was achieved when alfalfa has been growing for 11 year. Soil LFOC, MBC and MBN all increased with the number of growing years. When alfalfa has been growing for more than 13 years, the soil MBC increased slowly, suggesting that MBC value is likely to reach a constant level. SOC, soil total P (STP), available P  (AvaiP) and the ratio of SOC to soil total N (C/N) all tended to decrease monotonically with the growing years of alfalfa up to 13 year, and then increased.SOC was significantly positive correlated with STP, AvaiP, soil total C (STC, including SOC and inorganic carbon SIC), soil total N (STN) (R = 0.627**, 0.691**, 0.497*, 0.546*,respectively). MBC and LFOC was significantly positive correlated with the number of growing years of alfalfa (R = 0.873***and 0.521*, respectively). Significant positive correlation exists between MBC and SIC. LFOC, MBC, LFOC/SOC and MBC/SOC were all significantly positive correlated with each other.
The dynamics of soil organic C pools, soil fertility and water content of five abandoned fields with various years of succession (1-, 3-, 4-, 5- and 9-year abandoned fields after cropland termination with slope gradient of 10-15o, 5-10o, 0-5o, 10-15o and 0o,respectively) as well as the dynamics in two original grasslands (with slope gradient of 20-25o, one with grazed and trampled frequently, and the other without grazed and trampled) that have never been cultivated were investigated from the Loess Plateau of China. In addition, the dynamics of SOC pools, soil fertility of one conventional cropland was also determined. Soil water content distribution in 0-200 cm soil profiles gradually decreased with succession proceeded. Soil bulk density tended to increasing with abandoned succession process, and soil bulk density in the conventional cropland was the lowest and the original grasslands without grazed and trampled the highest among all the fields. SOC, LFOC, MBC and MBN enhanced after cropland abandonment and increased with abandoned field succession proceeding (The correlation coefficients (R) of SOC,LFOC, MBC and MBN with abandoned years were 0.609*, 0.679**, 0.842*** and 0.893***, respectively). SOC content at 0-20cm, 20-60cm, 60-100cm soil layers and carbon mass at 0-20cm layer in the 4-, 9-years old abandoned fields as well as that at 0-20cm layer in the 5-years old abandoned field was higher than that in the two original grasslands. LFOC at 0-20cm layer in the 4- and 9-year old abandoned fields was similar to that in the original grasslands, whereas, MBC and MBN at 0-20cm soil layer in the five abandoned fields was lower than that in the original grasslands. Multiple regression analysis from the five abandoned fields showed that slope gradient had significantly negatively influence on MBC, LFOC and LFOC/SOC, whereas succession years only had significantly positively influence on MBC, indicating that soil erosion on slope gradient had more negative effect on labile soil organic matter than the positive effect of years of succession in this semi-arid Loess Plateau area. The aboveground biomass increased in July with succession proceeding and was significantly positively correlated with soil LFOC, MBC, MBN, LFOC/SOC ratio and MBC/SOC ratio (R = 0.574**, 0.625**, 0.536*,0.619** and 0.451*, respectively). In conclusion, with the succession of abandoned fields soil vegetation coverage increased, a relatively high content of labile organic carbon and soil microbial biomass can be maintained. Abandoned fields with level terrace or 0-5o slope gradient can effectively restore soil SOC, LFOC and nutrients, sustaining high microbial activity. Converting both steeper gradient and some degraded level croplands to abandoned field is an efficient way in soil conservation in this semi-arid ecosystem.
Various soil carbon pools, dry matter of light fraction, soil fertility and soil water content were studied from three Caragana korshinskii shrublands (8-, 19-year shrub planted at level terrace about 1m width, interlacing with natural grassland about 1.5 m width on hillside with a gradient of 25-30o; 42-year shrub planted at a hillside with a gradient of about 15o without inter-shrub belt. Vegetation before these was original natural grasslands) and one original natural grassland (with a gradient of 25-30o), in a typical loess hilly region on the Loess Plateau of China. The results revealed that soil water decreased and the desiccated layers grew with the increasing years of Caragana korshinskii shrubs growth. The mean soil water content at 2-5m layer in three seasons was significantly negatively correlated with increasing growth years of shrubs. The mean soil water content at 0.6-2m layer in 19-, 42-years old shrubs was lower than that in original grassland, and that in 8-years old shrub was similar to original grassland. The 42-years old shrub had the highest soil water content at 0-20cm layer among all the fields. And there was significant correlation occurred between aboveground biomass and growth years (R =0.911***,P<0.001). The Caragana korshinskii shrubs tend to has lower mean value of dry matter of light fraction, LFOC, MBC and STC than that in original natural grasslands. SOC at 0-20cm depth in 19- and 42-year shrubs was similar to that in Original grassland,indicating that SOC at this layer can restored to the original level with the growth years of shrubs. STN content at 0-20cm, 20-60cm, 60-100cm soil layer in the 19-year shrub and at 20-60cm and 60-100cm in the 42-year shrub, and MBN at 0-20cm layer in these two sites was higher than that in original grassland, indicating that the N-fixation Caragana korshinskii can provide more available N to soil microorganisms than the no-legume dominant grasslands and maintain higher N level in soils.SOC,LFOC and MBC were significantly correlated with each other. Dry matter of LF was significantly correlated with SOC, LFOC, MBC and MBN, indicating that high soil LF can maintain high microbe activities and SOC pools in soils.
Thre comparation of SOC and its labile organic matter at these three land uses indicated that abandoned fields (including 5 abandoned fields and two organic grasslands)after termination of agricultural activities tend to have the highest mean value of SOC, STC, LFOC and MBC, indicating that natural vegetation can sequester more carbon in soils and more friendly to the eco-systems than alfalfa grasslands and Caragana korshinskii shrublands.Mean value of SOC,STC,LFOC,MBC and dry matter of LF in Caragana korshinskii shrublands was also highervii than that in alfalfa grasslands. Whereas, MBN content at 0-20cm siol layer in Caragana korshinskii shrublands and alfalfa grasslands was higher than that in abandoned fields,revealing that the N-fixed plant can maintain higher available N to microbial organisms.