The Use Of ¹⁵N Natural Abundance To Quantify Biology Nitrogen Fixation By Medicago Sativa Longdong

Lucerne is a high quality forage of major importance to the development of integrated agricultural systems in Qingyang prefecture, at the centre of the Chinese Loess Plateau. Biological nitrogen fixation in legumes is the most economically viable nitrogen source, thus identifying and quantifying biological nitrogen fixation of the local Longdong cultivar of lucerne is valuable to the development of local agricultural systems management and reduction of agricultural inputs. This study was conducted from July 2007 to June 2009 in the College of Pastoral Agriculture Science and Technology of Lanzhou University with the objective of ascertaining the constant (B value) of nitrogen-fixation of the Longdong cultivar. A pot experiment was conducted at a research station in Qingyang prefecture to measure the B value which is obtained from N2 in the atmosphere absorbed by lucerne that was grown in nitrogen free media matrix culture. We chose Longdong lucerne of two ages sown at 1998 and 2002 as simultaneous experimental subjects in field conditions in order to estimate the BNF. We used the 15N natural abundance method to quantify the two B values of the differently aged lucerne cultivars.9 continuous samples taken from 2007 to 2009 from Longdong Lucerne grown in a nitrogen-free sand culture and were measured by isotope mass spectrometry to determine 15N natural abundance levels. The results were as follows:1. The dynamic B value was successfully obtained, of which the mean value was-2.03‰. It provided the basis on quantifying biological nitrogen fixation in Longdong.2. The biological nitrogen fixation capacity in the younger lucerne sown in 2002 was 76.4% which was higher than the venerable lucerne sown in 1998 which had a 61.8% biological nitrogen fixation capacity. 3. For the first and second crop harvest of the 8-year lucerne in sections of 0-100cm soil depth, nitrate content was more than that of the 4-year being 100%and 50% higher respectively. In 2007, the basic trend of soil nitrate content from the differently aged lucerne was almost the same. The older lucerne contributed more nitrogen to the system than younger lucerne, however biomass of younger lucerne was higher than the old. Nitrogen uptaken by the plant caused N content declined. The old luncern accumulated more nitrogen than the younger one.4. Biological nitrogen fixation capacity in lucerne changes with the soil nitrogen content. When the soil nitrogen is sufficient, the ability of nitrogen fixation was found to decline.5. Nitrate Reductase Activity (NRA) of Longdong lucerne changes with soil nitrogen and soil nitrate in a certain range will promote NRA activity. The increase of ammonium activity inhibited NRA activity and thus NRA activity level may reflect the level of soil nitrate content.6. Compared to measuring the B value in the greenhouse under nitrogen-free sand culture, the 15N natural abundance method is still difficult to apply in the field. Thus the constant of nitrogen-fixation in perennial plants requires for further study.