Characteristics Of The Cool Of Soil Organtic Carbon And Soil Organtic Nitrogen On The Loess Plateau From Sortn To North

Loess Plateau is one of the main ecologically fragile areas in China, and a variety of adverse factors constrain soil organic carbon and nitrogen accumulation of this area. Thus, knowledge of the spatial distribution of soil carbon and nitrogen in this region and their relationship has the important theory significance for a better understanding of soil carbon and nitrogen cycle in the region. The soil organic carbon and nitrogen , soil microbial biomass nitrogen and their relationship of three types of soils on the Loess Plateau collected from north to south (Ust-Sandiic Entisols at Shenmu , Los-Orthic-Entisol at Yan'an and Eum-Orthic Anthrosol at Yangling) were investigated to determine how they were affected by location and soil depth (0-200 cm) on the Loess Plateau. The main results of our research are shown as follows:1. Clarify the spatial distribution of soil organic carbon and nitrogen content of the main soil types on Loess Plateau. The results showed that the contents of total soil nitrogen and soil organic carbon differed significantly with soil types and soil depth. In the same depth, the contents of soil total nitrogen and soil organic carbon decreased significantly from south to north on the Loess Plateau, and the values were highest in Eum Orthic Anthrosols, intermediate in Los Orthic Entisols and lowest in Ust Sandic Entisols. The contents of soil total nitrogen and soil organic carbon in the profiles exsisted a certain thickness layer of soil organic carbon nitrogen enrichment within 0-60cm, decreased remarkably within 10-60cm, and remained low and stable within 60-200cm. Ortherwise, the contents of soil total nitrogen and soil organic carbon in the ploughing layer on the Loess Plateau were lower than the average values of the whole country. The total reserves of soil organic carbon peaked in Eum Orthic Anthrosols (102.23t/hm~2), followed by Los Orthic Entisols (67.78t/hm~2) and lowest in Ust Sandic Entisols (27.07 t/hm~2). The soil organic carbon storage in 100-200cm of the above three types of soil were 65%, 74% and 58% of carbon storage in 0-100cm respectively. The soil nitrogen storage decreased in the order Eum Orthic Anthrosols (10.23t/hm~2)> Los Orthic Entisols (7.40t/hm~2)> Ust Sandic Entisols (4.17t/hm~2), and the soil nitrogen storage in 100-200cm of the above three types of soil were 62%, 59% and 62% of carbon storage in 0-100cm respectively. Therefore, we should take into account of the reserves of lower 100cm soil as we evaluate the soil carbon and nitrogen storage on Loess Plateau. 2. The soil fixed ammonium was investigated to assess the influences of soil location and depth on its distribution on the Loess Plateau. The results indicated that fixed ammonium was the main component of soil nitrogen storage, and the traditional understanding of the status of fixed ammonium in total nitrogen should be reevaluated. The content of soil fixed ammonium decreased from south to north on the Loess Plateau, the distribution was well-proportioned among profile, and the content of soil fixed ammonium had no obvious change in the profiles (P>0.05). The percentage of fixed ammonium in total nitrogen increased with soil depth and then kept stable, resulting from the distribution of soil particle which was coarser from south to north. The result of the fractal dimension of soil aggregates the well-proportioned distribution among profile: the fractal dimension of soil aggregates in the same layer and similar land use decreased from south to north. However, the fractal dimension of soil aggregates in the same location and different soil layer had no significant difference. There was positive relationship between the contents of physical clays (<0.01 mm) and soil fixed ammonium (r=0.5422), indicating that physical clay was the primary carrier of soil fixed ammonium. This characteristic was an inevitable result of Loess Plateau formation and supported the eolian theory of loess parent material. The percentage of fixed ammonium in soil total nitrogen was 61% in Ust-Sandiic Entisols, 48% in Los-Orthic-Entisol and 41% in Eum-Orthic Anthrosol, indicating that the traditional understanding of the status of fixed ammonium in total nitrogen should be reevaluated.3. The contents of soil organic nitrogen and its components differed significantly with soil location and soil profile on the Loess Plateau. The results showed that the contents of total hydrolysable N,ammonia N,amino sugar-N and amino acid-N in the same layer and land use all declined from south to north. The relative content of ammonia N, amino acid-N and hydrolysable unknown N (HUN Fraction) were higher compare to that of amino sugar-N. All the four fractions in the profiles decreased remarkably within 0-40 cm, decreased slightly within 40-80 cm and remained at the lowest level below 80 cm. The contents of the components except for amino sugar-N within 0-40 cm in grassland were higher than that in field; however, the variability became slighter with the profiles. The percentage of the components in total hydrolysable N differed with soil type: amino acid-N(38.2%)>ammonia N(32.3%)>HUN Fraction(24.9%)>amino sugar-N(2.5%)in Ust-Sandiic Entisols;amino acid-N(36.0%)>ammonia N(35.6%)>HUN Fraction(23.1%)>amino sugar-N(3.1%)Los-Orthic-Entisol;and ammonia N(53.3%)>amino acid-N(29.0%)>HUN Fraction(14.9%)>amino sugar-N(2.8%)in Eum-Orthic Anthrosol. Indicating that amino acid-N and ammonia N were the major state of the soil organic N on the Loess Plateau.4. Explicate the content and distribution of the component of soil humus, we found that the ratio of soil humic acid and soil fulviic acid reduced from south to north on the Loess Plateau, but increased with the soil depth, indicating that soil humus became more stable with the soil layer. In this study, the percentages of soil humus in soil organic C were low and differed slightly with profile. The percentage was 32.1%, 29.9 % and 26.6 % within 0-40cm, 40-120cm and 120-200cm, respectively, for Eum-Orthic Anthrosol, 32.5 %, 32.3% and 33.4 % for Los-Orthic-Entisol, and 54.7%, 56.0% and 42.5% for Ust-Sandiic Entisols. These results indicated that the process of humification was low and the accumulation was limited, due to the dry climate on the Loess Plateau. The degree of humification was one of the indices evaluating the soil humus quality pros and cons. The results showed that degree of humification increased with soil layer, whereas decreased from south to north in the same soil layer. The ratio of soil humic acid and soil fulviic acid was 0.56, 0.55 and 0.65 within 0-40cm, 40-120cm and 120-200cm, respectively, for Eum-Orthic Anthrosol, 0.35, 0.49 and 0.64 for Los-Orthic-Entisol, and 0.20, 0.55 and 0.60 for Ust-Sandiic Entisols.5. Illustrate the distribution and relationship of soil microbial biomass carbon (MC) and light fraction organic carbon. The results showed that soil Mc and light fraction organic carbon revealed the similar rule of distribution with soil location and profile. Land use had significantly influence on the contents of the total soil N and soil organic carbon (SOC), and remarkable effect on the contents of soil MC and MN as well as light fraction organic carbon within 0-60cm. The contents of soil MC and MN, the ratio of MN and the total N, microbial entropy, and the content of light fraction organic carbon in grassland soil was slightly higher compared with farmland. The results indicated that the soil MC and MN, the ratio of soil MC and MN and soil organic nitrogen and carbon reflected the variation of soil quality more effectively. Soil light fraction organic carbon was significantly correlated with soil MC and MN. Soil light fraction organic carbon accounted 70.7% and 25.7% of soil microbial biomass carbon and nitrogen, whereas the percentage of heavy fraction organic carbon over soil microbial biomass carbon and nitrogen were 38.3% and 16.1%.The latter was lower than the former, indicating that the relationship for light fraction with microbes was closer, light fraction organic carbon was the main source of carbon and energy for soil microbes and microbial activity had remarkable influence on it. The results indicated that the variation of soil light fraction organic carbon reflected the variation of soil carbon storage more effectively.6. Through the analysis of the correlation among the soil organic carbon and nitrogen components we found that heavy fraction organic carbon and humic acid are important contributors for total hydrolysable N and ammonia N on the Loess Plateau typically region, but also reveals the humic acid and heavy fraction organic matter played an important role in the stability of the Loess Plateau of organic carbon and nitrogen. The results had important reference value in understanding the formation of the Loess Plateau farmland productivity. There was remarkable relationship between the contents of total N and soil fixed ammonium, as well as the soil organic nitrogen components (P<0.01). There was also remarkable or significant correlation between the content of soil organic nitrogen components excluding (P <0101). The contents of soil organic nitrogen components were highly correlated (p<0.01) or significantly correlated (p<0.05), but the correlation between amino sugar-N and HUN Fraction were not significant. The contents of soil organic N, MC, soil humic acid, soil fulviic acid l fraction organic carbon light and heavy fraction organic carbon were remarkably correlated (p<0.01). The percentage of soil fulviic acid and light fraction organic carbon over MC was 62.4% and 70.7%, respectively, indicating that the chemical composition of soil fulviic acid and the physical components of the light fraction organic carbon had most significantly impact on MC. The correlation analysis revealed that there were the significant correlations between soil total N, total hydrolysable N, ammonia N, amino acid-N and HUN fraction and the chemical and physical components of the SOC (P<0.01), except for amino sugar-N. And soil fixed ammonium had no direct relationship with the contents of MC and soil fulviic acid. In general, heavy fraction organic carbon and humic acid were important contributors to total hydrolysable N and ammonia N on the Loess Plateau typical region, but also revealed that the heavy fraction organic matter and humic acid played an important role in the stability of the organic carbon and nitrogen on the Loess Plateau.