Ningxia Irrigation Silting Amount Of Soil Carbon Stock Time Series Evolution Research

Soil organic carbon stocks is the biggest of terrestrial ecosystem. It plays an important role in mediating global climate change caused by increased greenhouse gas just like CO2, et al. Meanwhile, the stocks is in a case of dynamics because it affected by natural factors or human activities. Among the factors which have effects on soil organic carbon pool changes, irrigation is considered to be one of the important measures that safeguard agriculture normal production and improve soil organic carbon pool. In the northwest region of China, irrigated agricultural has a long history, but also due to flow through the region of the river often contain large amounts of sediment, and to exert a strong influence on characters of irrigated farmland soils. Because the river sediment in the fields of continuous deposition, plus cultivated in farming land has formed an Anthrosols soil:Irrigated-silted soils, with some certain thickness of silted soil layer. Irrigated-silted soils was the most concentrated distribution, thicker and with obvious irrigation time series, in Ningxia Irrigation Zone, China.To reveal the long-term effect on soil organic carbon change and concentration with irrigation by Yellow River water, and the special mechanism of irrigation with sediment laden river water to increase soil organic carbon pool, the Ningxia Irrigation Zone was selected as a study area for the present study. Totally48sampling profiles were studied under different irrigation time series for5types of soils, in2009. Distributing5types of soil in different irrigation time sequence, determined soil organic carbon content and density in profile distribution and change characteristic, and with no irrigation farming of natural soil as control soil compared to estimate the rate of soil organic carbon sequestrated, analysis of different irrigation farming time that irrigation silted layer’s soil organic carbon storage change. This study also collected in1980Second National Soil Census in107soil profiles (0-100cm) and65surface samples (0-20cm), as well as the formula fertilization by soil testing143surface samples data, through the comparative analysis of the irrigation area of Yellow River in recent30years, in Ningxia, to reveal the variation characteristics of soil organic carbon change in profile deepth and the main driving factor of the irrigated area. The main conclusions as following:(1) Irrigation and cultivation are the important driving forces of variation of soil organic carbon density (SOCD). Results show that the effect of irrigation and cultivation on carbon sequestration varied with the soil. The same in irrigation and cultivation duration, the five types of soils followed a decreasing order of Irrigated-silted soils> Tidal soils>Recent deposited soils> Aeolian sandy soil> Sierozem soil in SOCD. Irrigation and cultivation increased SOCD to a varying degree in these soils, which followed Aeolian sandy soil> Recent deposited soils>Irrigated-silted soils> Sierozem soil> Tidal soil. The differences among the soils in effect of irrigation and cultivation on carbon sequestration are significant in the Ningxia Irrigation Zone.(2) The results showed that the differences of SOC content and density were significant (P<0.05) under different durations of irrigation and soil types. The longer duration of irrigation, the greater SOC content and density were found in surface layer. The similar result was found in SOC density of100cm depth. And the deeper soil layers were affected by irrigation under longer durations of irrigation from the Yellow River.(3) The SOC stocks is24.13Tg in Ningxia Irrigation Zone. Compared with the non-irrigated and non-cultivated control soils with same thickness, soil organic carbon stock increased with16.83Tg. Account for87.8%of the increment (11.43Tg C) of soil organic carbon stocks stored in Irrigation silted soil layer. Significant correlations were found between the SOC content increment of the surface (0-20cm), SOC density of the profiles (0-100cm), and irrigation years (P<0.05), especially between the SOC density increment of the ISS layer and irrigation years (P<0.01). Soil organic carbon stocks was greatly influenced by Long-term irrigation with sediment laden Yellow River water, especially in the ISS layer. Irrigated-silted soils plays an important role in soil carbon sequestration in Ningxia Irrigated Zone, China.(4) Soil organic carbon density in surface layer increased from1.63kg·m-2to2.19kg-m-2, increased1.2%in average (P<0.01), from1980to2009. Soil organic carbon density in profile (0～100cm) increased from5.77kg-m-2to6.05kg·m-2, increased0.16%in average in year (P<0.05). Soil organic carbon content and density have strong relationship with durations of irrigation. The rate of SOC increased by0.056kg C m-2·a-1, and the increment of soil organic carbon density of Irrigated-silted soils is greater than the other four types of soil in Ningxia Irrigation Zone.(5)Within the three carbon pool, the percentage of slow carbon pool is the biggest, followed by passive and active carbon pool. The percentages are lower than10%for the irrigated soils in Ningxia Irrigation Zone. This indicated that the soil organic carbon pool is relatively stable in Ningxia Irrigation Zone. (6) Soil types is an dominant factor to soil organic carbon stocks. The amount of chemical fertilizer per unit area, yield of crops per unit area and crops types are the more important factors to increase soil organic carbon content in the irrigated area. As an important factor that SOC density increased remarkably due to irrigation from the Yellow River. And the effect is the most prominent on soil organic carbon density change.