Effects Of Landscape Positions And Methods Of Conversion Of Croplands To Grasslands On Soil Quality In The Semi-arid Loess Plateau, China

This thesis explores the effect of landscape orientation on soil moisture, abovegroundproductivity, and soil C and N pools in alfalfa (Medicago sativa) grassland and fallow landwithin the semi-arid Loess Plateau in China. Four sloping sites with different landscapeorientation 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-eastfacing, middle slope), and Set (south-east facing, top slope). The study was also conductedto explore the ecological restoration effects of the conversion of cropland to foragelegumes on soil characteristics in the semiarid Loess Plateau of China. Four types oftreatments: (1) fallow (F); (2) alfalfa (Medicago sativa) forage legume (A); (3) sweetclover(Melilotus officinalis) forage legume followed by fallow (SF) and (4) erect milkvetch(AstragaIus adsurgens) forage legume (E) were used to substitute spring wheat, the nativecrop in April 2003 to October 2005. The results indicated that:After 3 years of alfalfa growth (measurements undertaken in April 2005), we foundthat the soil moisture contents within the 5 m soil layer at the Set, Neu, Sere, and Nelslopes were 427.51,543.03, 560.01, and 662.71 mm, respectively. The total forage yieldsat the Set slope in 2005 exceeded those at Neu, Nel, and Sem slopes by 3.1, 29.5, and19.7%, respectively, and the total organic carbon (TOC) content at the Nel slope washigher 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 atthe Neu, Sem and Nel slopes by 19.8, 22.9, and 27.4%, respectively. The observed trend ofTN 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 toalfalfa 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-groundbiomass, TOC content, TN content and LFOC content in these four slopes were allincreased with time, while the mineral N content in these slopes was decreased with time.The above-ground biomass, TOC content, TN content and LFOC content in the top slopewere higher than those in the other slopes. It means that it will be an effective method toconversion the cropland to fallow for the better soil quality in a semi-arid Loess Plateau.Fallow can improve the soil quality of slopes in every direction, every position and everydegree.Dissimilarly to the situation about the difference in soil quality among differentlandscape position in conversion of cropland to alfalfa grassland, the soil quality has nosignificant difference in every landscape position when 3 years of conversion of croplandto fallow. So fallow must be a more friendly method for improving the soil quality indeteriorated slope than conversion cropland to alfaifa grassland.After the conversion of crops to various forage legumes for 3 years, the highest soilwater content in various soil layers was observed in F treatment during the wholeexperimental period, followed by SF treatments. The light fraction of organic C (LFOC)and increase of soil water content in SF was the highest. The aboveground biomass in SFwas lower than that in E but higher than that in A and F. The total organic carbon (TOC)content in F and SF was higher than that in the other two treatments. But the total nitrogen(TN) content in F and SF was lower than that in A and E. Over the entire experimentalperiod, the water use efficiency (WUE) of E was higher than that of A, SF and F by70.02%, 42.05% and 36.80%, respectively. In conclusion, SF proved to be a better patternfor soil productivity, soil quality and nutrient cycling under semiarid conditions.