| Soil respiration is the global carbon cycle of terrestrial ecosystem and it is the key to global climate change. Farmland ecosystem plays an important role in the terrestrial ecosystem.After straw returning, it can effectively reduce the crop straw burning and CO2 emissions. And after straw decomposition, large nutrient elements can be released into the soil, and maintain the balance of soil nutrients. Returning straw plays a positive role in farmland carbon sequestration and promotes the sustainable development of agriculture. This experiment used nylon net bags under two treatments including straw buried ridge line and covers the surface to study the decomposition regular and nutrient release patterns of soybean and maize straw returning to the soil. And maize-soybean-maize rotation field was taken as the research object, using closed static chamber method, seting up the experiment to investigate the soil CO2 emissions of farmland soil under 5 different tillage measures(MTS, NTS, MT, NT and CT) and analysis the relationship of soil CO2 emissions with soil temperature, moisture and straw returning. The difference of carbon content of straw returning and CO2 emissions was calculated and the carbon balance of the farmland soil was estimated. The result showed that:Straw decomposition rate variation of soybean and maize regularity showed that the decay is fastest in the first year, decomposition rate up to 38.31%~60.63%. The decomposition rate in the second year was 65.81%~86.27%, the decomposition rate in the third year was 81.96%~92.67%,the fastest decay period of straw returned is from May to August in the first year, but second years showed no regularity between months.The decomposition rate of buried ridge line of soybean straw returning were compared with the cover the surface respectively high 25.82%,20.46%, 9.50% in the first year, second years and third years; The decomposition rate of buried ridge line of maize straw returning were compared with the cover the surface respectively high 22.10%, 22.83%, 9.74% in the first year, second years and third years.The nutrient elements of straw returning of soybean and maize in release rate size are: K>C>P>N; The carbon, nitrogen, phosphorus, potassium accumulation release amount of the buried ridge line and cover the surface of soybean straw respectively was 66.43% and 58.31%, 32.64% and 30.32%, 48.36% and 41.67%, 89.72% and 72.06% in the first years; The carbon, nitrogen, phosphorus, potassium accumulation release amount of the buried ridge line and cover the surface of maize straw respectively was 57.57% and 36.87%, 44.04 % and 23.56%, 68.86% and 47.05%, 95.06% and 80.43% in the first years. The carbon, nitrogen, phosphorus, potassium accumulation release amount of the buried ridge line and cover the surface of soybean straw respectively was 92.44% and 83.16%, 87.40% and 77.66%, 93.49% and 87.52%, 99.07% and 94.48% in three years; The carbon, nitrogen, phosphorus, potassium accumulation release amount of the buried ridge line and cover the surface of maize straw respectively was 91.39% and 82.06%, 89.43% and 76.82%, 92.87% and 83.32%, 99.13% and 96.64% in three years. The cellulose decomposing rate of soybean straw is higher than that of hemicellulose, lignin; the hemicellulose decomposing rate of maize straw is higher than that of cellulose, lignin. The lignin content of soybean straw and corn straw has increased at first and then decreased in the decomposition process. After decomposition three years, lignin of soybean straw decomposition rate is higher than that of corn straw. Cellulose, hemicellulose, lignin decomposition rate of soybean and corn straw are reached 45.98%~71.55%, 50.12%~73.76%, 15.78%~35.55% respectively in the first year. Cellulose, hemicellulose, lignin decomposition rate of soybean and corn straw are reached 82.32%~95.42%, 85.85%~94.91%, 60.57%~91.38% respectively after three years.CO2 emission flux from farmland soil had obvious affected by seasonal changes. The minimum value of CO2 flux are appeared in the early observation stage(April), and observation stage(November). CO2 emission flux from farmland soil presented a single peak curve in summer(June, July, and August). The CO2 emissions of two years on average of maximum flux and average flux size of the order are: NTS>MTS>CT>MT>NT, returning straw could significantly increase soil CO2 emission.Varition of farmland soil temperature at different levels and CO2 flux of soil remained consistency, simulating the relationship of CO2 flux of soil and soil temperature through quadratic and exponential equations well. The coefficient of determination(R2) expressed 20cm>15cm>10cm>5cm, it showed 20 cm soil would better express the relationship between soil temperature and soil CO2 fluxes, and exponential equations would better express the relationship between soil temperature and soil CO2 fluxes than quadratic.In the experiment, the results of correlation analysis between volumetric water content of surface soil(5cm) and the CO2 emission flux show that there was significant different between the CO2 emission flux of MTS, NTS, CT and volumetric water content of surface soil, but no significant difference between MT, NT and volumetric water content of surface soil.Two-year cumulative increase in the amount of carbon of covered by minimum tillage(MTS) and no tillage with mulch(NTS) farmland soil was 4495 kg C·hm-2 and 4093 kg C·hm-2 repectively; Two-year cumulative reduce in the amount of carbon of minimum tillage(MT) and no tillage with mulch(NT) and plowing(CT) farmland soil was 801 kg C·hm-2, 611 kg C·hm-2and 1109 kg C·hm-2 repectively. The carbon sink ability of covered by minimum tillage(MTS) was the strongest and the average annual increase in the amount of carbon was 2248 C·hm-2. Conservation tillage could increase soil carbon sequestration, and plowing make against soil carbon accumulation. |
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