Effects of Combined Application of Various Organic Materials and Chemical Fertilizer on Soil Nitrogen Formation and Greenhouse Gas Emission Under Equal Nitrogen Rates from Purple Soil
【Objective】In this study, the effects of agricultural organic materials combined with chemical fertilizer on soil nitrogen species (ammonium nitrogen, nitrate nitrogen, available nitrogen and total nitrogen) and greenhouse gases (carbon dioxide (CO_2), methane (CH_4) and nitrous oxide (N_2O)) emission were investigated to provide knowledge for chemical fertilizer reduction and friendly environment according to the same nitrogen rate addition. From the aspect of greenhouse gases mitigation, theoretical suggestions were also provided for dry land fertilization strategy. 【Method】 A vegetable rotation cropping experiment (lettuce-cabbage-chili rotation) with six treatments was carried out on purple soil in the field, and the treatments were control without any chemical fertilizer and organic material (CK), conventional fertilizing (F), straw returning with chemical fertilizing (SF), mushroom dregs returning with chemical fertilizing (MF), biochar returning with chemical fertilizing (BF), and cow dung returning with chemical fertilizing (CF). The variations of mineral nitrogen, available nitrogen and total nitrogen contents and the emissions of CO_2, CH_4 and N_2O from soil were monitored from November 2016 to September 2017.【Result】 In the condition of equal nitrogen rates, organic materials could change the formation of soil nitrogen. The contents of ammonium nitrogen increased after organic materials application in SF and MF treatments. CF treatment could increase the content of ammonium nitrogen during lettuce and cabbage growth seasons, and BF treatment increased the content of nitrate nitrogen and available nitrogen during chili growth season. Seasonal dependent CO_2, CH_4 and N_2O emission was found and high emission mainly occurred during summer. Fertilization and irrigation events also increased CO_2, CH_4 and N_2O emission. Compared with F treatment, N_2O emission reduced by 7.5% under BF treatment, but significantly increased by 233.5% under CF treatment. Combined application of organic materials and chemical fertilizer could increase the CO_2 emission. Significant increases of CO_2 emission were found under MF and CF treatments, which increased the mean CO_2 emission flux of 35.6% and 31.3% compared with F treatment, respectively. However, biochar returning delayed the emission peak of CO_2 and increased the CO_2 emission during summer. Negative CH_4 emission was found in most treatments, indicating a CH_4 adsorption by soil itself. Moreover, the emission of CH_4 fluctuated during the chili growth season and the highest CH_4 emission (668.7 mug·m~(-2)·h-1) appeared under BF treatment with the condition of high temperature and intensive precipitation. However, the mean CH_4 emission flux under SF, MF and BF increased significantly by 104.85%, 175.2% and 77.5%, respectively. Moreover, SF and MF treatment could promote CH_4 consumption and reduce mean CH_4 emission flux of -0.1 and -1.3 kg·hm~(-2), respectively. Compared with CK and F treatment, organic materials combined with chemical fertilizer application could increase the global warming potential (GWP) by 26.7%-52.4% and18.1%-42.0% respectively. The lowest GWP was found under SF treatment.【Conclusion】Combined application of organic materials and chemical fertilizer had different greenhouse gases emission characters and soil nitrogen speciation. Biochar and straw returning combined with chemical fertilizer could reduce the greenhouse gases emission and increase available nitrogen content. However, cow dung returning with chemical fertilizing increased greenhouse gases emission.