Effect of integrated application of green manure and chemical fertilizer on soil carbon and nitrogen, weed control, yield and quality of sugar beet

Continuous exploitation of soil results in organic matter and nutrient elements loss, and excessive chemical fertilizer application. For sugar beet, due to its higher nutritional demands, application of organic fertilizer is more important. Solitary or integrated application of green manures is a step towards reducing chemical fertilizer application. In this study, two species of green pea and Lathyrus sativus L. were planted as green manure and their dry matter accumulation and characteristics such as nitrogen and carbon content besides their effects on soil properties were evaluated compared with control. Then, effects of solitary or integrated applications of the abovementioned legumes with reduced levels of nitrogen fertilizer (80 and 160 kg ha-1 from urea source) versus 240 kg ha-1 urea as common rate and no fertilizer application as control, in total 8 treatments, were evaluated on sugar beet during 2012-14. Results showed that green manures had significant differences in terms of produced biomass, nitrogen and carbon. They changed nitrogen, carbon and carbon to nitrogen ratio of the soil in 50 days after application. Green manures showed optimum effects on weed density and biomass reduction. The highest root yield (47 t ha-1) and biomass (56.8 t ha-1) were achieved in green pea manure plus160 kg N ha-1 which was higher than chemical fertilizer (41.2 t ha-1). The highest sugar content (20.3%) was obtained under low N application in integrated green pea treatments. The highest rate of extraction coefficient of sugar was obtained in integrated green pea treatments and solitary application of green manure (about 18%), while the lowest extraction coefficient of sugar (16%) was obtained in nitrogen fertilizer treatment as well as integrated application of Lathyrus sativus + N. The highest white sugar yield (8 t ha-1) was obtained in green pea plus 160 Kg ha-1 N which showed 20% increase compared with control. In other integrated treatments, in spite of 1/3 to 2/3 reduction in N application compared with control, white sugar yield had no significant reduction. The lowest impurities rate including amino-nitrogen, K, and Na, and the lowest molasses sugar were obtained in green manure treatments or integrated application with low N levels. In conclusion, green manure application reduced chemical fertilizers usage while maintained yield, quality and product safety.