Study On Characteristics Of Carbon And Nitrogen Of Plant Litters During The Ecology Reconstruction Of The Loess Plateau In Soil

As the product of photosynthesis of plants, plant litter is a major way for nutrients returning to an ecosystem. And litter decomposition rate affects directly on the accumulation of litter, and on the nutrient availability in soil, as well as on the formation and accumulation of organic matter and the physical and chemical properties of soil. Litter decomposition is the primary means of both nutrient cycling and energy conversion in terrestrial ecosystems, and plays an important role in maintaining soil fertility, the availability of nutrients to plant.The Loess Plateau is the one of the most serious areas of land degradation and soil erosion not only in China, but also in the world. Therefore, Chinese government has initated a big ecological project in late 1990s to reestablish the ecology in this region by returning of croplands to forests and grasslands. In recent years, there were a lot of researches to study the relationship between ecological environment and vegetation restoration. However, these researches focused on the reduction of runoff and sediment, nutrient loss and soil properties during vegetation restoring. And there were few researches to study the decomposition and their effects on soil properties during litter decomposing.In this research, both incubation and field experiment were used to study the chemical elements and decomposition of different species litters and their effects on soil microbial biomass carbon and nitrogen and mineral nitrogen in order to understand carbon and nitrogen transformation in soil during litter decomposing. The main conclusions were as follows:1. Plant litters were sampled from Shenmu in the Loess Plateau, and the contents of soluble organic carbon (SOC) in two sizes litters (2 mm and 1 cm length) was extracted with two extractants (distilled water and 0.01 mol·L^-1 CaCl₂). And a 7-day incubation experiment was conducted to compare the biodegradability of soluble organic carbon of the different plant litters. The result showed that the contents of SOC in the different plant species ranged from 4.21 g·kg^-1 to 156.82 g·kg^-1, and the ratios of SOC to total carbon (SOC/TC) of the plant litters were in range of 0.99% and 32.84%. And the average of grass litter was the lowest. After 7-day of incubation, biodegradation rates of SOC in different plant litters ranged from 34.7% to 80.6%, and the average of grass litters was the lowest. The UV280 absorbance and humification index (HIXem) at the end of incubation were significantly higher than that that start of incubation, and the UV280 absorbance and HIXem were significantly negatively correlation with the bio-degradation rate of soluble organic carbon in plant litter.2. A laboratory experiment was conducted to determine the content of soil microbial biomass carbon and nitrogen (SMBC, SMBN) and mineral about both different species litters and nitrogen forms ((NH₄⁺-N and NO₃^--N). In general, the addition of the sole plant litter significantly increased the contents of SMBC and SMBN. The increasing rates of SMBC and SMBN were higher when the plant litters with low C/N ratios (e.g., A. adsurgens, C. Korshinskii,) than those with a higher C/N ratio (R. pseudoacacia, Salix psammophila, Stipa bungeana). The dual addition of plant litter and N fertilizer, either as NH₄⁺-N or NO₃^--N, also increased the contents of SMBC and SMBN, and the increasing rate was significant higher under the NH₄⁺-N treatment, but not under the NH₄⁺-N treatment, compared to the sole plant litter treatment. Our results thus indicated that microorganisms in this Losses soil may prefer to use NH₄⁺-N rather than NO₃^--N. In addition, the addition of plant litters with higher C/N ratios increased the immobilization of mineral N in the soil, and there was a positively significant relationship between the immobilized N and the C/N ratio in the plant litters.3. An incubation experiment was conducted to study the effects of addition of different mixtures of litters of plant species on contents of SMBC, SMBN and mineral nitrogen in soil. The results showed that the addition of either one single or two or three mixed plant litters to the soil significantly increased the contents of SMBC and SMBN. Overall, both the averaged SMBC and SMBN during incubation were highest in the treatments with three-plant-species litters, next were the treatments with two-plant-species litters, and then the treatments with single plant species litter. In contrast to the SMBC and SMBN, contents of mineral nitrogen in soil showed a reverse pattern among the three litter treatments: three-plant-species litter > two-plant-species litter > single plant species litter addition. Meanwhile, there was a significantly positive relationship between the SMBC and SMBN and the C/N ratio in one-plant-species or the two-species litters, but not in the three-plant-species litter. These results indicate that SMBC and SMBN contents were affected by a range of factors, including incubation conditions (temperature, soil water holding capacity, incubation time period, etc.) and the chemical properties of the litter (C and N content, organic compounds, etc.), particularly when the mixed three-plant-species litters were added to soil.4. The different mixures of six plant litters were put into litter bags and buried into 15 cm depth of soil in Yangling to study the litter decomposition in soil. The result showed that it took 0.79～1.43 years for 50% decomposition of the different litter treatments, and 3.43～6.19 years for 95% decomposition. The order of average litter decomposition rates were three-plant-species litter > two-plant-species litter > single plant species litter. At the end of field experiment, the average release rate of total carbon and nitrogen of three-plant-species litters was higher than that under two-plant-species litters; and the single plant species litter was the lowest. The content of soluble organic carbon and total soluble nitrogen and the ratio of the soluble organic carbon/total carbon and total soluble nitrogen/total nitrogen were variable in the first two months; and decreased significantly in the following four months, and then decreased slowly. At the end of experiment, the order of the content of soluble organic carbon and total soluble nitrogen and the ratios of the soluble organic carbon/total carbon and total soluble nitrogen/total nitrogen were single plant species litter > two-plant-species litter > three-plant-species litter.5. The different mixures of six plant litters were put into litter bags and buried into 15 cm depth of soil in Shenmu to study the litter decomposition in soil. The results showed that the average mass loss rate of three-plant-species litter was higher than that of two-plant-species litter; and the lowest was single-species litter. At the end of experiment, the order of the release rate of total carbon and nitrogen was single plant species litter > two-plant-species litter > three-plant-species litter. However, the order of soluble organic carbon was two-plant-species litter > three-plant-species litter > single plant species litter; and the order of total soluble nitrogen was three-plant-species litter > two-plant-species litter > single plant species litter. In addition, there was a close relationship between the mass loss rate and soluble organic carbon of litter.Therefore, we suggest increasing the more species of plants during the vegetation restoration in order to maintain the ecological system sustainable and healthy in the gully and valley region in the Loess Plateau.