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.