| The soil fertility has great significance on food safety and the sustainable development of agriculture. Crop yield increases with soil fertility, and more root and secretion are presented with high crop biomass, and then promote the increase of soil carbon and fertility. Fertilization is the key factor of influencing crop yield. Therefore, it is necessary to find the critical fertilization point for keeping the soil fertility of productive farmland and developing sustainably.A field experiment was conducted to study the effect of different fertilization application rates and maize-wheat growing on soil microbial biomass carbon and nitrogen, soil organic matter and soil respiration in typical farmland of huanghuaihai plain. The affects of different fertilization application rates on maize-wheat growth and yield were discussed. Meanwhile, a PVC experiment was conducted to analyze the differences of maize-wheat root biomass in different fertilizer levels. The carbon balance of farmland system under different fertilizer levels was estimated. The main results were as follows:(1) The analysis of the influence of different fertilizer levels on the growth and yield of maize and wheat showed that the index of yield for maize and wheat had the synchronous responses to different fertilization levels, namely the height, dry matter accumulation, yield and its constitute factors presented similar trend with the fertilization application increasing, which both increased first and then decreased. The nitrogen fertilization critical value was 150 kg/hm2. According to estimate, the organic carbon of maize and wheat aboveground biomass was 5299.25～7219.35 kgC/hm2 and 3872.14-6869.25 kgC/hm2, respectively.(2) Using PVC tube test studied the influence of different fertilizer levels on the root dry weight of maize and wheat, the results showed that the maximum of root biomass appeared in maize tasseling stage and wheat heading stage. Their values change range were 7.65～18.77 g/strains and 7.38-14.04 g/tube, respectively. The rooting biomass in the soil layer decreased rapidly downward the surface. Rooting dry weight of maize and wheat mainly concentrated in 0-20 cm soil layer and contributed the proportion to 0 to 100 cm total rooting weight were 55.4～70.1% and 57.6～78.6%, respectively. The rooting dry weight presented similar to the fertilization application increasing, which both increased first and then decreased. The nitrogen fertilization critical value was at 150 kg/hm2. The field rooting content of maize and wheat season in all treatments were estimated in 803.25～1970.33 kg/hm2 and 1512.34～2922.08 kg/hm2, respectively.(3)The analysis of the influence of different fertilizer levels and maize-wheat rotation on soil respiration showed that the soil respiration rate in bare soil and maize, wheat growing plot presented similar to the fertilization application increasing, which both increased first and then decreased. The nitrogen fertilization critical value of bare soil and maize, wheat growing plot were 70 kg/hm2 and 150 kg/hm2, respectively. The changes of soil respiration rate under the maize, wheat planting condition had close relationship with their growing seasons. The soil respiration rate at maize growing period followed the single-peak curve changes. The maximum value appeared at tasseling-flowering stage, which was 948.56 mg CO2/(m2·h). This value in wheat growing season presented "S" shape curves changes, the minimum and maximum appeared in over-wintering stage and filling stage respectively. Its value were 17.41 mg CO2/(m2·h) and 572.3 mg CO2/(m2·h). According to estimate, the total soil respiration in maize season was more than that in wheat season, while the corresponding bare soil was less than wheat season; The contribution rate of maize and wheat root respiration in growing seasons to the soil total breathing quantity was 67.3～77.6% and 44.5～67.3%, respectively. Preliminary estimation of the carbon balance of farmland ecosystem of maize and wheat throughout the growing season showed that the farmland CO2 release to soil-plant system in maize and wheat growing seasons were all "sinks" of atmospheric CO2, and their different fertilizer levels corresponded different levels of "sinks".(4) The changes of soil microbial biomass carbon and nitrogen were studied, the results showed that:soil microbial biomass carbon and nitrogen in bare soil and maize, wheat growing plot presented similar trend with the fertilization application increasing, which both increased first and then decreased. The nitrogen fertilization critical value of bare soil and maize, wheat growing plot were 70 kg/hm2 and 150 kg/hm2, respectively. The content of soil microbial biomass carbon and nitrogen in bare soil (wheat) had an extremely significant correlation with temperature. The correlation coefficient was as high as 0.9. Maize and wheat growth period had significant influence on soil microbial biomass carbon and nitrogen, the content of soil microbial biomass carbon and nitrogen were lowest at jointing stage of maize and wheat. The highest was at maize tasseling and wheat heading. (5) Soil organic matter was analyzed from June,2009 to June,2010. The results indicated that:The content of soil organic matter in topsoil (0-20 cm) was affected largely by fertilizer levels, the content of soil organic matter in bare soil decreased slightly with the fertilization application increasing, while that in maize-wheat growing soil increased slightly. The content of soil organic matter decreased with soil depth, and it mostly distributed in 0-20 cm soil layer of bare and maize, wheat growing plot, which was the largest and contributed 30.7%(average in bare soil) and 32.8%(average in maize-wheat growing soil) to total soil organic matter from 0 to 100 cm in soil profile. The C:N of soil was 17.16～26.20 (0-20 cm soil layer) and 14.21～21.54 (20-40 cm soil layer), which benefit of soil humification and organic nitrogen mineralization. |
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