C Sequestration And Soil C Balance Under Long-Term Fertilization On Arid-Highland Of The Loess Plateau

Soil organic carbon (SOC) is the key component of the C cycle in the terrestrial ecosystem. The input of soil organic matter to soil, SOC accumulation, and CO₂ emission are the main components of soil C cycle. It is a win-win strategy to increase SOC accumulation and reduce CO₂ emission for sustainable agricultural development and the decreased CO₂ concentration. The Loess Plateau, one of the areas with the most serious soil erosion in the word, is a typical rainfed agricultural area and an ecological fragile zone. Since 1980s, fertilizer has been the main way to increase crop yield and soil nutrients, but there is a little knowledge about the mechanisms of SOC cycle process. Based on long-term fertilization experiments in the arid-highland of the Loess Plateau, crop C sequestration, SOC accumulation, and CO₂ emission were studied and the effects of fertilization on the process were discussed.The long-term experimental site in the arid-highland of the Loess Plateau is located in Changwu County, Shaanxi Province, China (N 35°12′, E 107°40′). Eleven treatments were tested, specifically, 5 treatments with the same phosphorus level of 39 kgP·hm^-2 for the long-term fertilizer experiment (N₀, N₄₅, N₉₀, N₁₃₅, and N₁₈₀) and 6 treatments of fallow (F), unfertilized (CK), N treatment (N), N and P in combination (NP), manure (M) alone, and N, P, and M in combination (NPM) for the long-term rotation fertilization experiment. The crop used for the long-term experiment is winter wheat.Results are as follows:Fertilization in the period from 1984 to 2007 significantly improved SOC content and SOC accumulation within 20 cm of soil surface. SOC content was stabilized at the level of 6.5 g·kg-1 in the treatment without N fertilizer. Contrasted to the N0, SOC contents within 20 cm of soil surface for the treatments of N45, N90, N135, and N180 were increased by 1.3, 1.8, 2.1, and 2.2 t·hm^-2 with the increased annual rates being 57, 78, 90, and 96 kgC·hm^-2·a^-1, respectively. SOC was increased by 1.2 kg·hm^-2 for 1kgN·hm^-2 input. SOC content in the surface layer was increased from 6.5 g·kg-1 in 1984 to 10.8 and 11.0 g·kg-1 in 2007 in the M and NPM treatments and meanwhile the accumulated SOC was 8.6 and 9.5 t·hm^-2 with the increased rates of 373 and 413 kg·hm^-2, respectively. In the period from 1984 to 2007, SOC content in F treatment was slightly decreased from 6.5 to 5.9 g·kg-1 and SOC was decreased by 1.09 t·hm^-2. SOC was mainly accumulated in > 5 mm water stable aggregates during the 23 years. Contrasted to the unfertilized treatment, the improvement of aboveground C sequestration capacity of crop may supply more organic matter (stubble carbon) to the soil subject to fertilization, which may further improve the SOC accumulated as well. Contrasted to the N(015.8μmolCO₂·m^-2·s^-1), photosynthetic rates were increased by 11.4% in the N45, 17.1% in the N₉₀ (18.5μmolCO₂·m^-2·s^-1), 21.5% in the N135(19.2μmolCO₂·m^-2·s^-1), and 22.8% in the N₁₈₀(19.4μmolCO₂·m^-2·s^-1). Contrasted to the N₀ (2169 kgC·hm^-2), aboveground biomasses at manure stage in the N₄₅, N₉₀, N₁₃₅, and N₁₈₀ were increased by 100%, 142%, 167%, and 183%, respectively. Meanwhile, C inputs in the N₄₅, N₉₀, N₁₃₅, and N₁₈₀ were increased by 100%, 142%, 167%, and 183%, respectively. Photosynthetic rates in the treatments of M and NPM were increased by 19.6% and 29.6%, respectively, and the amounts of aboveground C accumulation were increased by 167% and 184%, respectively. C input to soil was significantly increased. There was a significant linear relationship between SOC content and C input amount, which can be described by the equation of Y=2.3393x+5.2747 (r2=0.8746 and n=11). Fertilizer can improve aboveground C accumulation and C input to soil and thus enhanced the SOC sequestrated and SOC storage in the soils. In the rotation experiment, C input of 1 t may increase SOC storage by about 0.23 t.Fertilization can enhance CO₂ emission. Annual average soil respiration rate was found to be 1.19μmolCO₂·m^-2·s^-1 in the control treatment. After adding N fertilizer at the level of 90 kg·N·hm^-2, soil respiration changed little. For N fertilizer at the level of 180 kg·N·hm^-2, soil respiration rate was less than those in the N₉₀ and N₁₃₅. Meanwhile, the relationship between N rate and cumulative CO₂ emission had the form of parabola with the annual emissions of 1608, 1993, 2483, 2585, and 2354gCO₂·m^-2·a^-1. Contrasted to the control treatment with the soil respiration of 1.50μmolCO₂·m^-2·s^-1, soil respiration in the M and NPM increased by 61% and 63%, with the annual emission of 3777 and 3821gCO₂·m^-2·a^-1, respectively.Although fertilization may improve CO₂ emission, aboveground C accumulation increment was more than the increment of CO₂ emission under the same fertilization in contrast to the N0. Aboveground C accumulation or C input to soil in the N45, N90, N135, and N180 were increased by 95%, 124%, 173%, and 162%, respectively, but CO₂ emission increased by 24%, 54%, 58%, and 53%, respectively, comapred to the N₀. When N fertilizer exceeded 90 kgN·hm^-2, CO₂ emission was not increased significantly. When it exceeded 135 kg N·hm^-2, CO₂ emission even decreased, which proves that N fertilizer can improve the SOC sequestratation.CO₂ emission was not increased obviously fro the N fertilizer based on manure fertilizer, but aboveground C accumulation was increased significantly by 54%, which may be the important mechanism of SOC accumulation under fertilizer and manure in combination.Meanwhile, CO₂ emission in fallow was analyzed for providing the basis for estimating carbon dioxide emission under fallow on the Loess Plateau. The results show that rate of CO₂ emission in fallow was significantly lower than that in tilled soil without plants and input to soil. Meanwhile, SOC accumulation was lower than emission with the trend of SOC decreasing. Soil respiration in fallow was affected by temperature and soil moisture and the connection can be showed by a two-factor model. The two-factor model, R = ae^bΤW^c(a, b, and c were constants), can well describe the relationship between soil respiration rate, soil temperature, and soil water in 5, 10, 15, 20, 40, and 60cm depth with a R² between 0.74 and 0.85. Soil respiration rate had a significant positive correlation with DOC and MBC (P<0.05), while it had highly significant negative correlation with soil carbon-nitrogen ratio (C/N) (P<0.01).