| Soil organic carbon(SOC) pool is one of the largest and most active carbon pool in terrestrial ecosystem, the small changes can also have an great impact on the entire ecosystem. As a part of terrestrial ecosystem, agricultural soil is the most active in global carbon storage, and its turnover rate is very rapid. Agricultural soil plays a crucial role in the global carbon balance under reasonable management practices, and agricultural soil has great carbon sequestration potential. Qinghai province is located in the high latitude region, the study on changes and influencing factors of the SOC pool in farmland ecosystem is little in this region.At the present study, we combined the second national soil survey data(1982), the recent(2011) sampling data, and the agricultural statistical data(1981 ~ 2012), by the GIS technology, mainly focus on 1) The characteristics of SOC storage in surface soil of farmland during 30 years in Ledu county in Qinghai, 2) The changes and relationship of crop yield and soil organic carbon density in Ledu county in Qinghai, 3) The changing characteristics of SOC during recent 30 years in Qinghai Province. In addition, we explored the factors of SOC changing in the study area. The main results are as follows:(1) SOC density(SOCD) at surface layer of cropland was 3.8 kg·m-2 and 2.8 kg·m-2 in 1982 and 2011 respectively. At a county scale, SOCD decreased by 26% during the period from 1982 to 2011, and it showed a decreasing tend in Northeast and South, as well as an increasing tend in Northwest in Ledu county. SOC storage at surface layer of cropland was 1.8×106 t in 1982 and 1.4×106 t in 2011, decreasing by 24% in recent 30 years. In all soil types, meadowl, kastanozem and chernozem showed carbon losing, the changing rates were-137.3 g C·m-2yr-1,-35.0 g C·m-2yr-1,-91.0 g C·m-2yr-1, then damp and sierozem showed carbon sequestration, the changing rates were 9.7 g C·m-2yr-1, 7.3 g C·m-2yr-1. Furthermore, the changing rates of SOCD in recent 30 years had a negative linear correlation with the SOCD in 1982(y=0.35-0.13x).(2) Wheat yields had a significant character in high, middle and low yield on the spatial distribution. From the 1980 s to 2001, wheat yields was stable in high yield area, increased from 2171 kg?hm-2 to 2450 kg?hm-2 in middle yield area, at an increase of 12.9%, respectively 1172 kg?hm-2 and 1393 kg?hm-2 in low yield area, at an increase of 18.9%. Accordingly, SOCD was the lowest in high yield area, and higher in middle and low yield area. From the 1980 s to the years after 2000, SOCD increased from 1.95 kg·m-2 to 2.20 kg·m-2 in high yield area, at an increase of 12.8%, decreased from 4.31 kg·m-2 to 3.25 kg·m-2 in middle yield area, at an decrease of 24.6%, decreased from 6.62 kg·m-2 to 3.26 kg·m-2 in low yield area, at an decrease of 50.8%. The structural factors, such as topography, land use patterns etc, effected the relationship between SOC and crop yields.(3) SOC of farmland surface was decreased as a whole in recent 30 years in Qinghai province, and had a drop of 21%. SOC increased in 5 counties(Huzhu, Gonghe, Datong, Xunhua and Ping,an) in recent 30 years, and had an increase of 35.6%, 25.1%, 8.2% and 0.9%. SOC decreased in Menyuan, Guide, Huangzhong, Guinan, Hualong, Minhe and Ledu, and the drop was 3.6%, 51.4%, 28.8%, 39.1%, 39.3%, 20.4% and 46.8%. |
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