| With the intensification of global change, mitigation of greenhouse gas emissions is imperative. Terrestrial ecosystem carbon cycle is the hot issue of global change research. Agricultural ecosystems are the most active part of terrestrial ecosystems. How to take effective measures to achieve the reduction in farmland ecosystem carbon sequestration has become one of the issues currently solved. Conservation tillage with straw mulching and ridge planting as a mainstay is an important approach to control soil erosion, to improve soil fertility, to guard against seasonal drought and to increase crop productivity in purple hilly regions in southwest China. The research on rules of carbon cycle and mechanism of carbon sink in farmland ecosystems under conservation tillage is of important theoretical significance and practical values for responding to global climate changes, improving agro-ecological environment and promoting sustainable land productivity. In this paper, data in field experiments during two production year had been measured, and farmland ecosystem carbon source/sink model was combed for study on laws of carbon budget in the triple intercropping system of wheat/corn/soybean in purple hilly regions of southwest China. There were six treatments including T (traditional tillage), R (ridge tillage), TS (traditional tillage+straw mulching), RS (ridge tillage+straw mulching), TSD (traditional tillage+straw mulching+decomposing inoculants), and RSD (ridge tillage+straw mulching+decomposing inoculants), which were replicated for three times. Form carbon sequestration of crop, soil carbon emissions and soil carbon storage point of view on the effects of three different crops and different farming practices to the carbon cycle. The contribution of atmospheric carbon dioxide from the soil-crop system was estimated in crop growing season. Then considering carbon emissions caused by agricultural inputs, carbon sequestration, carbon emission and net carbon sink was comprehensive evaluated in dry-farmland system under conservation tillage. The main results were as follows:1. The total carbon sequestration of crop was wheat<corn<soybeans, that in the entire system’s total annual carbon sequestration rate was20.29%,21.29%and58.42%. The performance of carbon sequestration rate was different at different growth stages of each crop, which showing an increase-decrease-and then increase-a further reduction of the "bimodal" trend. The peak position varies by crop. The rate of carbon sequestration became positive to negative when reaching maturity. Effect of different tillage methods on crop carbon sequestration rate was not significant, but there was a significant amount of influence in carbon sequestration. The total annual carbon sequestration of crop was39312.250,4375.201,47948.284,47049.809,46518.365and49512.140kg· hm-2in T, R, TS, RS, TSD and RSD treatments, respectively. Compared with the control, the total annual carbon sequestration of crop in other treatments was increased by11.29%,21.97%,19.68%,18.33and25.95%, respectively. As a result, ridge tillage and straw mulching treatments were in favor of more carbon fixation.2. The daily average soil respiration rate of each crop was wheat<corn<soybeans, that accounted for the total soil respiration rates was16.17%-16.34%,30.48%-33.64%and15.28%-16.35%. Under different tillage practice, soil respiration rate in wheat-soybean (w-s) belt was characterized by T<R<TS<TSD<RS<RSD, ridge tillage significantly increased soil respiration rate. Soil respiration rate in space-corn-space (k-c-k) belt was characterized by R<T<RS <TS<RSD<TSD. Compared with conventional tillage, in some extent straw mulching treatment enhanced soil respiration rate, but the ridge tillage had different effects because of the impact of the effect of different crops and different manifestations that is in wheat-soybean belt ridge tillage increased soil respiration rate but in corn belt was opposite.The experiment also shows that soil temperature and soil moisture content were the two major factors affecting soil respiration, explaining28.9%-53.7%and13.7%-42%of its seasonal variation. Regression analysis of the relationship between soil respiration and soil hydrothermal factors reveals that exponential equation (Rs=0.3lle0.095T) fits well the relationship between soil respiration and soil temperature at10cm in soil depth and the temperature sensitivity index (Q10) of soil respiration was2.25-2.69. However, the relationship between soil respiration and soil moisture content can be best described with a parabolic function. The threshold of soil moisture soil respiration responds to was14.94%. The dominant groups of soil animals were Collembola and Acarina, which were correlated with soil respiration to some extent. The correlation was high under the treatments of T and R, being ranged within0.669-0.921, whereas there was no remarkable correlation under the other treatments. The dominant groups of soil animals in maize field were Collembola, Acarina and Diptera. Compared to the control, ridge tillage reduced the number of soil animals, but straw mulching increased it and improved the index of soil animal diversity. This indicated that soil tillage patterns had great impact on soil animals. Soil respiration was only related with the animals above soil surface. The more was the amount of animals being active above soil surface, the stronger was the soil respiration. In all the treatments, the correlation coefficient of R was the highest (r=1.000, p=0.017), followed by TS (r=0.915, p=0.029). However there was no remarkable relationship between soil animals and soil respiration under traditional tillage. The dominant groups of soil animals in the soybean farmland were Collembola, Acarina and Diptera. There was no significant relationship between soil respiration and soil animals captured by tullgren apparatus and pitfall traps method. The correlation coefficient under T was significant, being r=0.901, p=0.037, when added together two methods for soil animals.3. Soil aggregates on the response of different crops were different. It told that planting corn was beneficial to the formation of soil water-stable micro-aggregates. Soil aggregates and organic carbon content were influenced by tillage and planting crops. Straw mulching treatment significantly increased soil organic carbon content in local soil and each particle aggregate both in0-5cm and5-10cm soil layers. The annual change of aggregate indicated that traditional tillage increased water-stable micro-aggregates of organic carbon content, while straw mulching treatment significantly increased organic carbon content of local soil and each particle size aggregate in every soil layers. In the5-10cm soil layer the soil organic carbon content increased slightly, indicating that this soil layer greatly influenced by straw mulching treatment. By estimating the carbon sequestration potential discovered that soil carbon sequestration potential was greater in corn belt than that in wheat-soybean belt. Straw mulching could improve soil carbon sequestration potential and reasonable arrangement of crops also helped enhance the carbon sequestration capacity. It played an important role in soil carbon sequestration and emission reduction by selective planting crops in straw mulching treatment. It suggested that changes in response to TC, POC, DOC, ROC and MBC on tillage measures were consistent:there was no significant response to R treatment but greatly influenced by the straw mulching treatment. The proportion of POC in TC was35.74%-49.66%and there was significant positive correlation between the two. So POC can be used as a sensitive index to reflect the change of soil organic carbon. Water-soluble organic carbon in soil organic carbon accounted for about0.2%, the difference between the different treatments was not significant. So it had no sensitivity. Easily oxidized organic carbon in soil organic carbon in the proportion accounted for6.29%-8.20%. Although the proportion of ridge and straw mulching treatment help improve easily oxidized organic carbon in soil organic carbon, but the proportion in TOC was smaller in ridge tillage treatment than in straw mulching treatment. It was indicated that straw mulching treatment help stabilize the soil organic carbon.4. There was a closely relationship between the farmland ecosystem carbon source/sink system characteristics and setting boundaries and spatial and temporal scales. The results showed that the regardless of soil-crop system or the farmland ecosystems of consideration with human activities were expressed as carbon sinks. The proportion of root respiration in total soil respiration was different in different crops at different growth stages. With the crop growing the proportion was first increased and then decreased. The soil-crop system carbon source/sink law consistent performance in wheat, corn and soybean fields. It was carbon source in the initial crop growth and maturity stage but in the rest of the time it was carbon sink. The carbon sequestration in R, TS, RS, TSD and RSD treatments was higher than the control. As a result, the ridge and straw mulch-based conservation tillage can significantly improve farmland ecosystem soil-crop system carbon sink capacity, increasing as much as9.93%-30.67%. In the course of tillage from conventional tillage to conservation tillage of change, despite there was an increased in soil carbon accumulation in straw mulching, but it was also increased carbon emissions from investment in agricultural system. It was also considered the performance of atmospheric carbon dioxide "source." Ridge tillage treatment not only made the accumulation of carbon in the soil reduced but also increased carbon emissions of the investment in agricultural system, so it was performance as the carbon dioxide "source". It was proved that it was valid only for a period of time the cumulative effect of changes in carbon form farming practice changes. It would eventually reach a steady state which there was no carbon accumulation rate. The changes of soil carbon stock would occur in farming practices when it happend initial changes, while carbon emission was stable from agricultural inputs. Therefore it was great to improve farm to enhance carbon sequestration and emission reduction potential relying on farming management practices. |
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