| 【Objective】Combining organic fertilizer with chemical fertilizer is an effective approach to enhance crop yield and soil fertility, as well as reduce the environmental pollution under drip irrigation. With the rapid development of agricultural water-saving drip irrigation in recent 20 years, water saving fertigation technology seriously relied on good solubility of chemical fertilizer,, leading to a rapid decline of organic fertilizer application amounts year by year. Therefore, organic fertilizer should be paid more attention especially in the drip irrigation system. However, the mechanism of soil nitrogen transformation and the N2 O emission law was not clear while the organic fertilizer applied in the drip irrigated field. Thus, a continuous field experiment was carried out to study the impacts of substituting 20-40% of total NPK chemical fertilizer by 3000~6000 kg/hm2 organic fertilizer on cotton productivity, cotton N uptake, soil nitrogen transformation, and soil microbial biological effects. The N2 O emission in the drip irrigated soil was also studied in our paper. The objective was to provide the reliable reference for the applying of organic fertilizer combined with chemical fertilizer under drip irrigation.【Method】A filed experiment was carried out in 2013 and 2014 after a 2-years-long-term experiment. In the present research, the effects of substituting the chemical fertilizer by different ratios of organic fertilizer on nitrogen forms and transformation, cotton plant nutrient absorption, microbial biological character and N2 O emission were studied. The method of most probable number and enzymology was used in order to reveal the law of N2 O emission with organic fertilizer application. Gas chromatography method was also used.【Result】1) Organic fertilizer at different rates of 3000 kg/hm2 or 6000 kg/hm2 combined with NPK chemical fertilizer significantly increased(P<0.05) cotton biomass in the two years. Compared with CF, cotton biomass trailed with organic fertilizer increased by 18.5%~38.8% and 9.3%~42.8% at boll opening stage in 2013 and 2014, respectively. Compared with CF, organic fertilizer application treatment increased cotton yield by 6.9%~18.5% and 16.8%~33.9% in 2013 and 2014, respectively. An obvious enhancement of cotton N uptake was observed under organic manure combined with CF treatments. Compared with CF treatment, the amounts of N absorbed with organic manure increased by 4.9%~31.7% at boll opening stage in 2013. Moreover, cotton N use efficiencies(P<0.05) with manure combined with CF treatments was significant higher than CF treatment. With the increasing of organic manure application rates, soil water holding capacity(WHC) substantially increased, while soil p H and electrical conductivity were markedly decreased(P<0.05). 2) There had a significant improving of soil NH4+-N and NO3--N concentration(P<0.05) as well as the ratio of soil NH4+ to soil total N by organic fertilizer amendment. The highest value of soil mineral Nwas obtained with 60%CF+BF2 treatment. Soil total N, soil microbial C and microbial N were significantly increased by means of continuous organic fertilizer application(P<0.05), while the MBC/MBN was decreased. Moreover, bio-organic fertilizer treatment(CF+BF) had an obvious enhancing of the proportion of MBN/TN, in contrast to CF, soil TN content increased by 13.3%~24.5%(2013) and 8.2%~31.8%(2014); soil MBC and MBN increased by 16.4%~74.3% and 38.4%~171.6% with different treatments of organic supply in 2013 and 2014, respectively. Meanwhile, organic fertilizer combined with chemical fertilizer substantially improved soil nitrification potential and soil mineralization capacity. 3) Two years’ study showed the same result, organic fertilizer combined with chemical fertilizer substantially increased the activities of soil urease, soil nitrate reductase, soil nitrite reductase and hydroxylammonium reductase. The order was CF+BF > CF+OF; 60%CF+OF2/BF2 > 80%CF+OF1/BF1; meanwhile,soil nitrogen transformation bacteria(ammonifying bacteria, nitrifying bacteria, nitrite bacteria, denitrifying bacteria) were significantly increased by organic fertilizer or bio-organic fertilizer amendments, the same tendency was observed in microbial microorganism. In addition, there was a positive correlation between soil enzyme activity and microbial, with the increasing of fertilization years. 4) N2 O emission with fertilizer treatments was higher in contrast to no fertilizer treatment at the 3/4 days or 7/8 days after irrigation and fertilization. The order of N2 O emission at 3/4 days after irrigation and fertilization was: CF > 60%CF+OF > 60%CF+BF > CK, and significant difference was existed among treatments(P<0.01). However, treatments with organic fertilizer combined with chemical fertilizer increased N2 O emission in compared with chemical fertilizer supply at 3/4 days after irrigation and fertilization. The N2 O emission trend was same as this period when the irrigation fertilization was stopped. The highest total N2 O emission was found by 100% CF treatment in the cotton growing season. Compared with CK, the N2 O emission of 100% CF, 60%CF+OF and 60%CF+BF was increased by 111.5%, 3.75% and 8.37%, respectively.The N2 O emission factor was decreased by 1.39%, 73.8% with organic fertilizer application compared to chemical fertilizer supply. Meanwhile, There had significant correlations among the N2 O emission fluxes and soil temperatures in 5 cm-deep layer and soil moisture(P<0.01). Path analysis showed that the soil NO3--N content was more closely with N2 O emission than soil NH4+-N.【Conclusion】 1) Long-term organic fertilizer combined with chemical fertilizer could significantly increase irrigated cotton biomass in major time compared with chemical fertilizer. Organic fertilizer or bio-organic fertilizer application can markedly increase cotton yield. Meanwhile, an obvious cotton N uptake enhancement was observed under organic manure combined with CF treatments. Moreover, cotton N use efficiencies were significantly higher(P<0.05) by manure combined with CF treatments, and the increasing effect was more and more significant with the amount increasing of organic fertilizer. When compared two types of organic fertilizer, bio-organic fertilizer had enough advantage.2) Organic fertilizer at different rates of 3000 kg/hm2 or 6000 kg/hm2 combined with NPK chemical fertilizer significantly increased the soil total nitrogen, soil mineral nitrogen and soil microbial biomass C, N, and in contrary, microbial biomass C/N was significantly decreased. Organic fertilizer combined with chemical fertilizer substantially improved soil nitrification potential and soil mineralization capacity, soil physical and chemical properties were also improved. Organic fertilizer could significantly increase soil enzyme activities and soil microbial microorganism.Two years’ data have common tendency, the significantly correlation was observed among soil enzyme, soil microorganism and different types of soil nitrogen. 3) Organic fertilizer at rates of 6000 kg/hm2 combined with NPK chemical fertilizer significantly decreased N2 O flux in the fertigation cycle of irrigated cotton. Especially, the lowest treatment of N2 O flux occurred in bio-organic fertilizer combined chemical fertilizer. The soil NO3--N content was more closely with N2 O emission than soil NH4+-N under drip irrigation, the denitrification might be a major pathway of N2 O loss. |
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