| The main objective of this study was to evaluate the effect of root exudates in three different crops (maize, soyb soybean, sugar beet) on soil carbon and nitrogen fraction and content, the number of microorganisms and enzymes activity. Soil sample was taken from black soil experimental field at Sugar Beet Reaserch Insititute of Chinese Academy of Agricultural Sciences in Harbin, Heilongjiang Province. Firstly, root exudates of maize, soybean, sugar beet were extracted by water. Then the extraction of root exudates was concentrated by rotary evaporation apparatus and the contents of carbon, nitrogen and their main fractions were analysed in root concentrated exudates. Finally, it was evaluated that the effect of root exudates on soil carbon and nitrogen fractions and content, the number of microorganism and enzyme activity.1 Aqueous extraction was available to the crop root exudates. Total carbon concentration was 554.415,292.448 and 347.766μg/mL and total nitrogen concentration was 98.249, 64.164 and 73.074μg/mL for maize, sugar beet, soybean root exudates, respectively. The secretion rate of total carbon was 9.1007,5.7722 and 5.6177μg/rfw·g·h, that of total nitrogen was 1.6127,1.2663 and 1.1807μg/rfw·g·h, for maize, sugar beet, soybean root exudates, respectively. The total content of organic acid was 0.02999 mg/rfw·g·h which contained mainly oxalic acid, malonic acid, malic acid, acetic acid, succinic acid in sugar beet root exudates. The composition of total composition of amino acid, whose content was 0.2573 mg/rfw·g·h, was Asp, Gln, Lsy amino acids, and the amount of sucrose was 0.01573 mg/rfw·g·h in sugar beet root exudates. Oxalic acid, malonate, tartaric acid, malic acid, acetic acid, fumaric acid, succinic acid were found, and total amount of organic acid 0.03865 mg/rfw·g·h; Asp, Glu, Ser, Arg, Trp, Phe, Leu amino acids were found, and total amount of amino acid 0.3628 mg/rfw·g·h; amount of sucrose 0.00697 mg/rfw·g·h were detected in maize root exudates. Oxalic acid, malonic acid, malic acid were found, and total amount of organic acid 0.01353 mg/rfw·g·h; Asp、As、Arg、ALa、Met、Val、Trp、Phe、Leu、Lsy amino acids were found, and total amount of amino acid 0.6013 mg/rfw·g·h; amount of sucrose 0.01620 mg/rfw·g·h were detected in soybean root exudates.2 After maize, soybeans and sugar beet root exudates were added to the soil, soil organic matter content did not substantially increase. The content of CO2 from soil respiration was obviously increased 23.4%、19.7% and 20.8% for maize, soybeans and sugar beet root exudates compared to the control, respectively. Microbial biomass C content of soil was apparently increased 43.6%、10.9% and 9.3%, and soluble organic C content increased 35.8%、34.4% and 58.5% for maize, soybeans and sugar beet root exudates compared to the control, respectively. Thus three crops root exudates added to the soil significantly improved soil active organic carbon content.3 The root exudates of maize, soybeans and sugar beet added to the soil did not substantially increase soil total N content. The content of microbial biomass N distinctly increased 180.1%,78.3% and 40.7%, that of soluble total N content increased 46.4%,39.5% and 12.3%, that of soluble inorganic N increased 1150.7%,238.8% and 113.8%, that of soluble organic N content of soil increased 7.5%、32.4% and 8.8%, and that of exchange inorganic N increased 19.8%、61.3% and 53.5% for maize, soybeans and sugar beet root exudates compared to the control, respectively. Thus crop root exudates improved soil nitrogen supply capacity, which of maize is the best, followed by of soybean, sugar beet.4 Root exudates of sugar beet, corn, soybean crops have increased microbial populations, the maximum amount increased was the amount of soil fungi, and the increase was similar in the number of degrees to the bacteria and actinomycetes in the soil.5 The influence of different crop root exudates on soil phosphatases, urease, invertase and catalase activity in soil were different. After added maize, soybean and sugar beet root exudates, soil phosphatase activity increased 3%, almost unchanged (down 1%) and reduced 7% respectively. After added soybean, sugar beet and maize root exudates, soil urase activity increased 9%,3% and 1% respectively. After added sugar beet, soybean and maize root exudates, soil invertase activity increased 37%,17.9% and 10.4% respectively. After added sugar beet, maize and soybean root exudates, soil catalase activity increased 10.6%,4.8% and reduced 5.9% respectively.The results demonstrated that sugar beet, maize, soybean root exudates were rich with amino acids, organic acids, sugar and other nutrients, increasing content of active C, N and quantity of bacterium, actinomycetes and fungi in soil, influencing soil phosphatases, urease, invertase and catalase activity. But the effect of sugar beet, maize, soybean root exudates on these were very different. Therefore, we should pay attention to the crop root exudates on soil fertility, making good crop rotation, thereby improving soil fertility and crop yields steadily. |
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