Soil Nitrogen Supply Feature And Its Relationship With Sugarbeet(Beta Vulgaris L.) Carbon Metabolism

Black soil is a main soil type for sugar beet(Beta vulgaris L.) cultivation in NE China, its most significant feature is high organic matter and deep soil layer. But now the black soil area in NE China is suffering from soil and water erosion, black soil degradation and other problems. One of the most prominent manifestations for its degradation is the decrease in the ability for black soil to regulate and control nitrogen nutrients, finally resulting in the decrease in nitrogen use efficiency and soil nitrogen supply ability. Therefore, how to efficiently regulate and control nitrogen in accordance with soil nitrogen supply features so as to achieve high yield, high sugar and good quality of sugar beet is the problem to need to be solved urgently at present. In this paper, organic nitrogen components and its supply features of black soil, dynamic changes in the migration and transformation of different forms of soil nitrogen in the growing season of sugar beet from 2010 to 2014 by field and laboratory experiments were studied in the main producing area of sugar beet. The effect of temperature and moisture on organic nitrogen mineralization, migration and transformation was explored through laboratory experiments. Meanwhile, fertilizer nitrogen’s migration and transformation in soil organic nitrogen pool by 15N tracer technique, their response to the changes in environmental factors were discussed. The influence of different types of nitrogen on carbon metabolism of sugar beet in growing season by pot experiment was analyzed. Therefore, this study was of great significance in rational fertilization, improvement of nitrogen supply ability and sugar beet quality by physiological regulation and control, and the enrichment of nitrogen nutrient theory for sugar beet. The main findings are as follows:1. Mineralization features of soil organic nitrogenThe main component of organic nitrogen in black soil in the main producing area is acid-hydrolysable nitrogen, and every component content and proportion was ranking in amino acid nitrogen>ammoniacal nitrogen>unknown nitrogen>amino sugar nitrogen. The content and proportion of soil acid-hydrolysable organic nitrogen increased after applying nitrogen treatment(NPK), the increased range in amino acid nitrogen and amino sugar nitrogen was highest among them.Nitrate nitrogen was dominated in soil mineralized nitrogen in the growing season of sugar beet, the cumulative mineralized nitrogen in the soil increased exponentially, the nitrogen supply ability of soil was enhanced after nitrogen fertilizers was applied. The first-order reaction kinetic equation (two pool model) during both stages could well describe the mineralization process of organic nitrogen in black soil.The application of nitrogen fertilizers can both increase soil nitrogen mineralization potential (No) and facilitate the formation of mineralized organic nitrogen (N1), and could improve the quality of organic nitrogen in black soil. Soil nitrogen mineralization potential (N0) was closely related to each component of organic nitrogen, amino acid nitrogen was a major contributor to soil mineralization potential. Temperature and moisture were important factors affecting soil organic nitrogen mineralization. The single effect of temperature or moisture on soil nitrogen mineralization rate was greater than their interactive effect. The temperature 30℃ and the moisture 30% were suitable for the mineralization of organic nitrogen in black soil.2. Temporal and spatial distribution features of different forms of soil nitrogenApplying Nitrogen fertilizer treatment(NPK) in the seedling stage of sugar beet can increase the content of ammonium nitrogen in 0-30 cm soil, and promote root’s absorption of nitrate nitrogen in 0-70cm soil in the sugar growth period (July). It can increase the content of ammonia nitrogen and amino acid nitrogen in 0-30cm soil in the growing season of sugar beet, in the meantime, the residual of non-acid hydrolysable nitrogen appears in deep soil layer in the late growth stage of sugar beet (July-September). Except amino sugar nitrogen and amino acid nitrogen, the change in the content of other constituents of soil organic nitrogen over time conformed to the distribution rule of cubic curve equation. In the growing process of sugar beet, ammonia nitrogen and amino acid nitrogen in the constituents of soil organic nitrogen contributed more to inorganic nitrogen.In the seedling stage of sugar beet, the residual chemical fertilizer nitrogen was mainly located in 0-50cm soil layer, mainly bonded to the soil organic nitrogen pool in the form of ammonia nitrogen, amino acid nitrogen and unknown acid hydrolysable nitrogen, but mainly in the form of unknown acid hydrolysable nitrogen and non-acid hydrolysable nitrogen in 50-70cm soil layer; In the harvest period of sugar beet (September), the residual chemical fertilizer nitrogen apparently decreased in 0-50cm soil layer, but 5.29%～33.92% residual amount of chemical fertilizer nitrogen was still in 0-50 cm soil layer, resulting in a waste of nitrogen fertilizer resources, they largely were combined in organic nitrogen pool in the form of ammonia nitrogen and unknown acid hydrolysable nitrogen, while immobilized in organic nitrogen pool in the forms of non-acid hydrolysable nitrogen in 50-90 cm soil layer.Each constituent of soil organic nitrogen can be mutually transformed with chemical fertilizer nitrogen. Ammonia nitrogen had the largest direct contribution to the transformation of chemical fertilizer nitrogen in the constituent of soil organic nitrogen, followed by unknown acid hydrolysable nitrogen. In the case of shortage of soil nitrogen supply, chemical fertilizer nitrogen immobilized in amino nitrogen and amino sugar nitrogen can be transformed into absorbed nitrogen by plants through its mineralization. The transformation of chemical fertilizer nitrogen in the constituent of soil organic nitrogen between unknown acid hydrolysable nitrogen and non-acid hydrolysable ammonia nitrogen played a temporary accumulation role indirectly through ammonia nitrogen. In the harvest period, nitrogen-15N that can be absorbed by the plant was closely related to non-acid hydrolysable nitrogen-15N and amino acid nitrogen-15N, hence, as absorbable organic nitrogen source, amino acid nitrogen played an important role.3. Response of migration and transformation of different forms of soil nitrogen to changes in temperature and moistureFor no fertilizer application treatment(CK) soil, the content of NH4+-N and NO3--N in 0-90cm soil layer had insignificant difference with the changes in temperature and moisture, but for fertilizer application treatment (NPK), the migration distance of NH4+-N in the soil column mainly occurred within 0-15 cm soil layer and that of NO3--N mainly appeared in 0-50 cm soil layer with significant response to the changes in temperature and moisture.For no fertilizer application treatment(CK) soil, the transformation rate (mineralization rate, nitrification rate and ammonification rate) in 0-30 cm soil layer increased as temperature rose, while the mineralization rate and the nitrification rate decreased when the moisture increased. Significant increases in soil transformation rate in 30-90 cm soil layer occurred at the turning point of 20℃ as temperature rose. The nitrification rate changed significantly as moisture increased. For fertilizer application treatment(NPK) soil, the mineralization rate, nitrification rate and ammonification rate in 0-30 cm soil layer were significantly higher than those for no fertilizer application treatment(CK) soil. Under high moisture conditions, soil transformation rate increased as temperature rose. The mineralization rate and ammonification rate in 30-50 cm soil layer with low moisture had significant response to the change in temperature, and within the specified temperature range, the nitrification rate increased as moisture rose. The mineralization rate, nitrification rate and ammonification rate in 50-90 cm soil layer with high moisture first increased and then decreased as temperature rose. Within 5-20℃ range, the mineralization rate, nitrification rate and ammonification rate increased as moisture rose, the ammonification rate had an insignificant response to the change in moisture. Under different temperature and moisture conditions, the difference in the nitrogen transformation rate between fertilizer and no fertilizer application treatment (CK and NPK) soil was significant, and different degree of interactions occurred between various factors.4. Relationship between the form of soil nitrogen and carbon metabolism of sugar beetInorganic nitrogen application in the soil with a certain proportion of organic nitrogen can promoted the growth of sugar beet, can increase the dry matter accumulation of sugar beet above and below ground, and the chlorophyll content in the whole growth period, and can improve the photosynthetic capacity. After the growth period of sugar, if inorganic nitrogen with high proportion of organic nitrogen(67% of the total amount of nitrogen application) was applied in the soil, the beet root and leaf had the largest amount of accumulated nitrogen. This result indicated that amino acid nitrogen can also be a favorable nitrogen source for the growth of sugar beet. Amino acid nitrogen improved the activity of sucrose phosphate synthase (SPS), sucrose synthetase (SS), and the vitality of invertase of beet leaves, and maintained carbon and energy sources required by the growth of beet root. The content of soluble sugar and sucrose in the root increased significantly and that of reducing sugar decreased. These would promote the accumulation of sucrose in the root and increase the output of beet and sugar.