Effect Of Cropland Management On Soil Carbon Stock And Modeling Study

Soil carbon pool is a predominant component of global carbon cycle, with high incoming and out coming carbon fluxes. As a kind of special soil, compared with natural soil, cropland is significantly disturbed by human activity, which large consequential carbon stock transfers with atmosphere. Long-time conventional tillage on a large scale causes significant losses of soil organic matter (SOM), which results in the rise of carbon dioxide emissions to atmosphere. However, in recent years, organic carbon in cropland has been increasing due to the spread of no-tillage techniques, and increasing the area where straw-returning is practiced, as well as the extensively use of cropland management tools such as fertilization and irrigation. This paper analyzes the change of soil organic carbon (SOC) under different cropland management regimes and estimates the carbon sequestration under cropland management, containing chemical fertilizer application (CF), organic fertilizer application (OF), chemical and organic fertilizer application (COF), straw returning (SR) and no-tillage (NT). This was performed by extensive experimental data collection. The conclusions are as follows:1. 98% (OF), 97% (COF), 95% (SR), 92% (NT) of experiments show that soil organic carbon increases with these cropland management patterns. The rate of annual SOC increase under no-tillage management is of 0～8%, and between 0 to 4% under other management regimes. Only 74% of the trails under chemical fertilizer application show a small SOC value increase. The results of each experiment was analyzed by Meta-analysis software, which showed that annual increase of SOC are of 0.832tC·ha-1·a-1(COF), 0.728tC·ha-1·a-1(NT), 0.613tC·ha-1·a-1(OF), 0.542tC·ha-1·a-1(SR)and 0.155tC·ha-1·a-1(CF).2. The value of annual SOC increase is 1.20tC·ha-1·a-1 in the Yellow-Huaihe-Haihe Rivers region under COF; The effect of CF and NT is significant in the Upper and middle reaches of the Yangtze River region, with an SOC increase of 0.23tC·ha-1·a-1 and 3.26tC·ha-1·a-1 respectively; The effect of OF and SR is significant in the Southwest region, and the SOC increases increases amount 1.39tC·ha-1·a-1 and 0.89tC·ha-1·a-1 respectively. In the Northeast region, the effect of all cropland management regimes to SOC rise is mild. The SOC increases for litter under OF, COF, SR and NT. However, SOC declines at 0.06tC·ha-1·a-1 under CF. There is significant coherence between annual experimental increase rate of SOC and cropland management factors of IPCC.3. The SOC increase in paddy field is higher than that in dry land under SR, NT and OF. The effects of CF and OCF are opposite, but the difference is small. The type of soil also influences absorption and transformation of carbon. The SOC increases of red soil, paddy soil and fluvial-aquatic soil are more than that of black soil, brown soil and desert soil under CF, OF, COF and SR.4. Reasonable cropland management makes the SOC increase, and the trend of increase presents one rule. The experiments are divided into five groups, containing 3 to 5 years duration, 6 to 10 years duration, 11 to 15 years duration, 16 to 20 years duration and more than 20 years duration, according to the length of experiments. With the time length of experiments increasing, the annual increase of SOC falls. The correlation degree between length of experiments and the annual increase of SOC is statistically significant. There is a negative relation between the annual growth rate of SOC and the initial value under cropland management except for NT. Carbon sequestrations with straw-returning and fertilizer use together with manure were elevated.5. The results of DNDC modeling show that the contens of carbon sequestrated under OF and OCF are large, both of which reach 1.78tC·ha-1·a-1 on average between 1981 and 2000. Next come the NT and SR management techniques, attaining 0.71tC·ha-1·a-1 and 0.47tC·ha-1·a-1 respectively. The effect of carbon sequestration under CF is the least significant, being of only 0.09tC·ha-1·a-1 only. The modeling result is similar with that estimated by references.