Effects Of Irrigation-Nitrogen Coupling On Greenhouse Soil Nitrogen Supply And Immobilization

The regulation of irrigation and fertilization has always been an important part of the facility agriculture production. Therefore, scientific management of irrigation and fertilization is a necessary basis of productive and highly efficient facility agriculture. The important role of irrigation and nitrogen in the growth and development of crop along with the high losses and low utilization rate of nitrogen has made the relationships of irrigation and nitrogen occupy an important position in the study of the regulation of water and fertilizer. Therefore, the effective coordination of irrigation and nitrogen supply and improving water and fertilizer use efficiency have a great significance to achieve the sustainable development of facility agriculture. Thus, a plot experiment with completely randomized design of irrigation lower limit and nitrogen rate was conducted on a facility plot, which had continuously planted tomato for 3 years. The effects of different irrigation lower limit and nitrogen rate on the features of nitrogen mineralization and immobilization of topsoil during tomato growing period and the distribution and accumulation of soluble and immobilized nitrogen in the soil profile of fallow period was studied with the field in-situ incubation method. The obtained results are as follows:(1) Field in-situ mineralization studies have shown that, both the single factor and interaction of irrigation and nitrogen have significant effect on the maximum cumulative net nitrogen mineralization (P<0.05). Cumulative net nitrogen mineralization were highest under the combination of irrigation lower limit 25kPa-high nitrogen (525 N kg·hm-2) and 35kPa-high nitrogen (525 N kg·hm-2) as well as irrigation lower limit 45kPa-low nitrogen (75 N kg·hm-2), however, combination of high irrigation(25kPa) and high nitrogen (525 N kg·hm-2) and combination of low irrigation (45kPa)and low nitrogen (75 N kg·hm-2) are likely to cause tremendous loss of surface soil mineral nitrogen, while net mineralization nitrogen reduced as the increase of nitrogen at high irrigation lower limit(45kPa).(2) There was a large fluctuation of net nitrogen mineralization rate, mineral nitrogen and soluble organic nitrogen during the tomato growth period. Soluble organic nitrogen accounted for over fifty percent of total soluble nitrogen in average and thus was considered as the main form of greenhouse soil. Under irrigation lower limit of 25 and 35kPa, net mineralization rate increased with the increment of fertilizer, while showed an opposite trends under lower irrigation limit of 45kPa. Growth period, irrigation and fertilization as well as the interaction of the three factors significantly affected the soil mineral nitrogen content of surface layer.(3) During tomato growing period, fixed ammonium of topsoil reduced significantly with increased irrigation lower limit and showed an increasing trend with the increment of fertilizer. However, nitrogen amounts and the interaction of irrigation and nitrogen had no significant effects on fixed ammonium. Microbial biomass nitrogen content decreased with the enhancement of irrigation lower limit and was significantly affected by fertilization. The maximum microbial biomass nitrogen content was observed at medium nitrogen treatment.(4) The accumulation of mineral nitrogen in 0-50cm soil layer increased significantly with increased nitrogen amount and soluble organic nitrogen showed the same trend with no significant difference. FA content in the surface layer is higher than the subsurface and cumulative phenomenon occurred in 20-30cm. Both irrigation and fertilization had significant impact on the accumulation of FA in soil profile of 0-50cm. Microbial biomass nitrogen increased as irrigation increased showing no significant difference and was significantly impacted by fertilizer. Besides, appropriately adding fertilizer could exert priming effect on SMBN.(5) Correlation analysis showed that in fallow period, soil total nitrogen showed good correlation with soil fixed ammonium and soil soluble total nitrogen. FA and STSN showed a significant parabolic trend as the increment of total nitrogen. There was a significant positive correlation between FA and STSN (r=0.980**, P<0.01). Both soil mineral nitrogen and soil soluble organic nitrogen showed highly significant positive correlation with soil immobilization nitrogen.