| Other Abstract | This thesis explores the effect of landscape orientation on soil moisture, aboveground productivity, and soil C and N pools in alfalfa (Medicago sativa) grassland and fallow land within the semi-arid Loess Plateau in China. Four sloping sites with different landscape orientation were chosen for attained the aims during 2003–2005. The sites were Neu (north-east facing, upper slope), Nel (north-east facing, lower slope), Sem (south-east facing, middle slope), and Set (south-east facing, top slope). The study was also conducted to explore the ecological restoration effects of the conversion of cropland to forage legumes on soil characteristics in the semiarid Loess Plateau of China. Four types of treatments: (1) fallow (F); (2) alfalfa (Medicago sativa) forage legume (A); (3) sweetclover (Melilotus officinalis) forage legume followed by fallow (SF) and (4) erect milkvetch (Astragalus adsurgens) forage legume (E) were used to substitute spring wheat, the native crop in April 2003 to October 2005. The results indicated that：
After 3 years of alfalfa growth (measurements undertaken in April 2005), we found that the soil moisture contents within the 5 m soil layer at the Set, Neu, Sem, and Nel slopes were 427.51, 543.03, 560.01, and 662.71 mm, respectively. The total forage yields at the Set slope in 2005 exceeded those at Neu, Nel, and Sem slopes by 3.1, 29.5, and 19.7%, respectively, and the total organic carbon (TOC) content at the Nel slope was higher than that at the Set, Neu, and Sem slopes by 36.8, 23.9, and 38.9%, respectively. The light fraction organic carbon (LFOC) content at the Set slope was higher than that at the Neu, Sem and Nel slopes by 19.8, 22.9, and 27.4%, respectively. The observed trend of TN dynamics is similar to that of TOC. The mineral N content at the four sites was Neu ≈ Sem﹥ Set﹥ Nel. In other words, the lower slope (Nel) recorded the highest soil water, TOC, and TN contents, but the lowest aboveground biomass, LFOC, and AvaiN content; the opposite is true for the top slope (Set). We concluded that the conversion of cropland to alfalfa in a semi-arid agro-ecosystem has a positive effect on soil quality for all slope orientations and that the soil quality and stability decrease upslope.
After the farmland fallowed for 3 years, we found that the soil moisture, above-ground biomass, TOC content, TN content and LFOC content in these four slopes were all increased with time, while the mineral N content in these slopes was decre... |
