Mechanisms On Soil Nitrogen Transformations As Affected By Integrated Use Of Inorganic And Organic Fertilizers In A Rice-Wheat Cropping System

Both rice and wheat are the most important staple food crops in the world. In South Asia, most rice and wheat are grown in rice-wheat rotation system (R—W). R—W cropping has been widely practiced along the Yangtze River Basin in China with a total area of 13 million hectares. Therefore, it is critical for sustained production of R—W cropping to ensure the food security for China, and even for the whole world.The sustained grain production in R—W system depends mostly on better nutrient management satisifying crop demands both minimizing loss and maximizing the use efficiency. The integrated use of inorganic and organic N fertilizers (IOF) is a promising approach to achieve this requirement. However, previous studies were mainly concentrated on its biological effect in the R—W system; few report was on transformation and distribution of fertilizers nitrogen (N) among different soil N pools and plant parts. Moreover, little literature could be found on the interactive effects between organic and inorganic N fertilizers. It is helpful in investigating transformation mechanisms and interaction effects of different fertilizer N types for high utilization efficiency.Pot experiments were employed in present study with five treatments, namely, inorganic fertilizer N (IF), organic fertilizer N (OF), integrated inorganic fertilizer and organic fertilizer (IOF) with different 15N labelling at organic fertilizer N (IOF1) and inorganic fertilizer N (IOF2) and no fertilizer N as control (CK). Two soils with contrast textures (clay loam and clay) were selected, and 15N-labelled ammonium sulphate and rabbit feces were used as IF and OF sources, respectively. To investigate the availability of the soil residual N for subsequent crop, fertilizer N was applied only to rice (O. sativa L. cv. Wuyujing 7) season or only to winter wheat (Triticum aestivum L.cv Yangmai 6) season in R—W rotations and recorded as R—W system or W—R system accordingly.In the case that N was applied to only rice season in the R—W system, results for the current rice season showed that 1) grain yields were similar among IF and IOF treatments, 2) In two soils, clay loam and clay, NUE was lower in the IOF treament than in the IF treatment by more than 20% and 25%, respectively. In comparison with the treatment IF or OF alone, organic fertilizer N decreased the plant uptake of inorganic fertilizer 15N while inorganic fertilizer N increased the uptake of organic fertilizer 15N in grain and plants in the IOF treatments,3) for the treatment IOF, organic fertilizer N increased soil residual N and MBN pools of inorganic fertilizer 15N and thus reduced its loss by 12% in the clay loam and 28% in the clay, while inorganic fertilizer N increased N accumulation of organic fertilizer 15N fertilizer in rice grains in both clay loam and clay, and 4) fertilizer 15N recovered by plant was mainly (53% to 78%) allocated to rice grain and organic fertilizer N was the main contributor to grain N increase for the IOF treatment.Results for the subsequent winter wheat season in the R—W system showed that 1) treatment IOF produced the highest grain yields and treatment OF also gave similarly high grain yield in the clay,2) in comparison with the treatment IF, IOF treatment resulted in higher soil mineral N and MBN at tillering, jointing and heading stages of winter wheat,3) the increased soil mineral N in the IOF treatment was mainly derived from organic and inorganic fertilizer 15N for the clay loam, while for the clay, inorganic fertilizer 15N was the main contributor. The increased MBN in the IOF treatment was mainly derived from inorganic fertilizer 15N,4) IOF treament increased NUE compared with IF with 173% increase in the clay. In the IOF treatmet, organic fertilizer N increased the recovery of inorganic fertilizer 15N in soil and plant, while inorgaic fertilizer N increased the recovery of organic fertilizer 15N in the clay only, and 5) more than 80% of fertilizer 15N recovered by plant was accumulated in grain part and for the IOF treatment, organic fertilizer N increased the distribution rate of inorganic fertilizer 15N in vegetative organs of winter wheat.In the case that N was applied to only winter wheat season in the W—R system, results showed that 1) treatments IF and IOF produced the highest grain yields without significant difference,2) for the IOF treatment, inorganic fertilizer N enhanced N uptake of organic fertilizer 15N by grain and plant parts in the clay loam and clay to 514% and 133.3% for grain and 216% and 100% for plant, respectively whereas decreased uptake of inorganic fertilizer 15N by 30% for grain and 15% for plant, respectively,3) for the IOF treatment, inorganic fertilizer N increased plant uptake of organic fertilizer 15N while organic fertilizer N increased the soil residual N and MBN pools of inorganic fertilizer 15N, and 4) compared with OF, IOF treatment increased NUE by about 114% and 85% in the clay loam and clay, respectively. Compared with the IF or OF treatment, IOF treatment increased the accumulation of both inorganic and organic fertilizer 15N in vegetative organs in the clay loam, while for the clay, increased only inorganic fertilizer 15N in vegetative organs and organic fertilizer 15N in reproductive organs.For the subsequent rice season in the W—R system, results showed that 1) IOF treatment gave higher grain yields than IF treatment for both soils,2) compared with the IF, IOF maintained higher soil mineral N content. IOF treatment increased inorganic fertilizer 15N derived soil mineral N at heading stage in the clay loam and at tillering, jointing and heading stages in the clay. In addition, IOF treatment increased organic fertilizer 15N derived MBN at tillering and jointing stages in the clay, and inorganic fertilizer 15N at tillering, jointing and heading stages in both soils,3) organic fertilizer N increased the distribution rate of soil residual inorganic fertilizer 15N in reproductive organs, whereas inorganic fertilizer N decreased the distribution rate of soil residual organic fertilizer 15N in reproductive organs by about 20% but increased in vegetative organs, and 4) IOF treatment increased NUE of inorganic fertilizer N relative to IF treatment. In comparison with the IF or OF, organic fertilizer N increased plant uptake and soil residual of inorganic fertilizer 15N. Similarly, inorganic fertilizer N increased soil retention rate of organic fertilizer 15N. As a result, the losses of inorganic and organic fertilizer 15N were decreased by more than 15%.N utilization efficiency in the two consecutive crops was compared between the R—W and W—R systems. Results demonstrated that 1) W—R system yielded higher plant biomass than R—W system, without significant difference in grain yields except that W—R gave higher grain yield than R—W for OF treatment by 81% in the clay loam and 23% in the clay, than R—W system, without significant difference in grain yields except that W—R gave higher grain yield than R—W for OF treatment by 81% in the clay loam and 23% in the clay,2) W—R system had higher fertilizer N use efficiency (NUE) for OF treatment than R—W system both in clay loam and clay, and 3) NUE, fertilizer 15N use efficiency (15NUE), and fertilizer 15N loss rate (15NLR) under the same treatment within the two soils differed between R—W and W—R, suggesting that soil textures, fertilization methods and its application season have a significant effect on the transformation and fate of fertilizer N in the rice—wheat system.In summary, IOF approch produced similarly high grain yield for rice and winter wheat at reduced cost of inorganic fertilizer. During fertilizer application season IOF treatment increased the NUE of organic fertilizer N, while in the succeeding season increased the NUE of inorganic fertilizer N. In the IOF treament organic fertilizer N increased the immobilization of inorganic fertilizer 15N in soil MBN pool for the first crop season which will gradually release during the succeeding crop season. Moreover, IOF increased the recovery of fertilizer N in plant-soil system and thus decreased N loss to the environment. In the rice—wheat system, soil textures, fertilization methods and its application season have a significant effect on the transformation and fate of fertilizer N, thus all factors should be considered synthetically in the fertilization managements above a system level.