Dynamics Of The Aboveground Biomass And Soil Physical Properties In The Abandoned Land With Different Treatments In The Semi-arid Loess Plateau Of China

It was important way to improve the ecosystem after conversion of farmland for forest and grassland Regeneration. However, it is hard for us to select which species and how to restore the plant cover after the farmland was abandoned. To improve the ecosystem and develop sustainable agriculture, this experiment was designed. In April 2003, cropland was enclosed and three treatments were implemented:(1) natural regeneration (NR); (2) the perennial legume species alfalfa (Medicago sativa Linn.) (AF); and (3) the biennial legume species sweetclover (Melilotus officinalis Linn.) (SF).In NR, the aboveground biomass was unstable and significantly differed among years in the experiment. Beside these, the soil water content at depth of 0-500 cm increased significantly. Compared to 2003, the soil pH and soil bulk density in 0-20 cm and 20-40 cm decreased significantly, and the soil water-stable aggregates in >0.25 mm category increased significantly in 2008; Moreover, in the large soil aggregates (with diameter in>0.25 mm class) (LSA), middle soil aggregates (with diameter in 0.25-0.05 mm class) (MSA) and small soil aggregates (with diameter in <0.05 mm class)(SSA), the soil organic carbon (SOC) increased significantly, and the total nitrogen (TN) and total soil phosphorus (TP) were no significant differences; meanwhile, the soil available phosphorus (AP) decreased significantly in all aggregates classes in 2008. During the experiment, the content of SOC, light fraction organic carbon (LFOC) and TN increased, and a positive correlation was existed between SOC, LFOC and TN and the growth years (the values of R was:R=0.659**, R=0.939***, R=0.709**, respectively). During the experiment, the values of SOC to AP (C/AP) and soil microbial biomass carbon (MBC) increased, and the MBC was positively correlated with SOC (R=0.832*) and LFOC(R=0.839*), negatively correlated with AP (R=-0.826*).In SF, the aboveground biomass was stable except 2005 in the experiment. The soil water content in 0-500 cm increased significant in the experiment. During the experiment, the soil pH and bulk density in 0-20 cm and 20-40 cm decreased but the water-stable soil aggregates increased significantly. In all the soil aggregates classes, the SOC and TN increased. In the experiment, the SOC and TN increased and reached the maximum values in 2005 and then declined. The C/N ratios decreased significantly in 2005 then increased; the LFOC increased and was correlated with the growth years (R=0.702**); The values of soil C/AP increased. Meanwhile, the soil MBC increased significantly in the experiment, and a positive correlationship existed between MBC and SOC (R=0.838*, P<0.05), TN (R=0.831*, P<0.05).In AF, the aboveground biomass increased and a positive correlation-ship existed between the aboveground biomass and the growth year(R=0.817*) during the experiment; the soil water content in 0-500 cm decreased significantly, especially below 100 cm, a negative correlation-ship existed between the soil water content and growth year (R=-0.79***) and a permanent dry soil layer formed as the alfalfa lasted more than five years. During the experiment, the soil pH and bulk density in 0-20 cm and 20-40 cm decreased significantly; Additionally, the water-stable aggregates, soil aggregates in>0.25 mm class and SOC in LSA, MSA and SSA all increased significantly; AP in all soil aggregates categories decreased significantly in the experiment. During the experiment, there was a positive correlation between SOC, LFOC and the growth years (The values of R were:R=0.654**, R=0.920***, respectively); TN increased significantly and was positive correlated with the growth years and LFOC (The values of R were:R=0.576*, R=0.562*, respectively); the soil AP decreased significantly. In experiment, the C/AP and MBC increased significantly. Beside these, the MBC was positively correlated with SOC (R=0.949**) and TN (R=0.838*), and negatively correlated with AP (R=-0.986**).Compared to SF and AF, the soil water in NR at depth of 0-500 cm was higher, which was help to improve the deep soil water. The SOC and TN in NR and AF increased steadily and were higher than SF in April 2008. Above all, the aboveground biomass and soil physical properties had been improved after the legume species introduced into the cropland in the experiment. However, the AF extracted amount of soil water during its growth and formed a dry layer below 100 cm; the SF did not help to improve the soil nutrients. Therefore, the NR was the best way to improve the deep soil water and soil qualtiy, helped to develop the sustainable agriculture.