Nitrogen mineralization in paddy soils of the Taihu Region of China under anaerobic conditions: dynamics and model fitting
Introduction
As the country with the largest population, China takes its food supply as the most important matter. As population increases, there is little likelihood that an adequate food supply under the limited arable land resources can be maintained unless fertilizers are used (Zhu et al., 2000). In 1999, China consumed 2.18×107 mg N of chemical fertilizer (China Agricultural Yearbook, 1999), which accounted for one-fourth of the world's total. However, the N nutrient uptake efficiency by plants was very limited. In general, the plants used only 28–41% of the N fertilizer (Zhu and Wen, 1992). Most of the rest was lost or remained in soil, resulting in direct economic loss to farmers and exerting a negative impact on the atmospheric environment and water quality. In order to make more efficient use of N fertilizers, many kinds of methods and techniques have been used; for example, deciding on an appropriate kind of fertilizer and timing of application (Chu et al., 1978), modifying the application methods by deep placement (Cao and DeDatta, 1983), using slow-release N fertilizer Islam and Person, 1979, Sun et al., 1986 and so on. But the effects of all the methods and techniques were made on the same basis: understanding the N mineralized from soil. So an accurate prediction of the amount of inorganic N released from soil organic matter is essential for the development of farming practices that maximize N use efficiency and minimize adverse impacts of N on the environment (Wang et al., 2001).
In the complex nitrogen turnover in the soil–plant system, mineralization is a key process to be fully understood and taken into account when meeting the N demand of crops (Smith et al., 1977). This process has been studied extensively. In 1972, a one-component, first-order exponential model was used to simulate the dynamics of aerobic long-term mineralization (Stanford and Smith, 1972). Thereafter, other exponential models in different forms were used, such as the double exponential (two-pool) model (Molina, 1980, Richter et al., 1980, Richter et al., 1982, Aviva, 1983; Kazuyuki et al., 1985; Fisher and Parkey, 1987, Rasiah, 1995, Povira and Vallejo, 1997) and the special model Lindemann et al., 1988, Cabrera, 1993, Dou et al., 1995, Roelcke et al., 2002. In the last 30 years, many other models have been used, such as logarithmic models (Narteh and Sahrawat, 2000), parabolic models Broadbent, 1986, Marion and Black, 1987, hyperbolic models (Donald and Mohammas, 1998) and others. However, the exponential models have been used the most frequently. Mineralization is the process of decomposition of organic matter, mediated by soil microbes. Since soil microbial activity is determined by enzyme dynamics, which is often described by an exponential equation (Gao et al., 1984), mineralization should theoretically be described with an exponential equation.
However, all these models were usually used for the upland soils, and only few studies were made with flooded soils. In China, where paddy soils amount to 25% of the total arable land, the study of N mineralization in this type of soils has been an important work. Up until now, only the effective cumulated temperature model has been used to describe the mineralization of flooded soils Yoshino and Die, 1977, Cai et al., 1979, Zhu, 1982, Wang et al., 1983, Tao et al., 1993. This equation has not found international acceptance. Moreover, we found that in the common incubation experiments and analysis methods of nitrogen mineralization in paddy soils, the processes of coupled nitrification and denitrification were ignored, which makes us doubt the accuracy of the description of the mineralization dynamics and of this model as a whole.
The objectives of this study were (1) to study the process and characteristics of organic N mineralization of typical paddy soils with modified anaerobic incubation and (2) to compare the effective cumulated temperature equation with the commonly used exponential equations.
Section snippets
Materials and methods
The Taihu Region (30–31°N, 120–121°E) is one of China's most ancient agricultural regions with a rice cultivation history of several thousand years. It is also one of its most economically developed areas. The climate is warm and moist and has a long growing season. The annual precipitation is 1100–1400 mm, and the mean temperature is approximately 16 °C. The frost-free period is over 230 days. Current agricultural practice in this region is an intensive double-cropping system with irrigated
Results
The mineralized N was dominated by NH4+-N. Some NO3−-N was found on days 0 and 1 of incubation only. This initial NO3−-N amounted to 30% of inorganic N in all soils except for sample F, in which NO3−-N amounted to 60% of the inorganic N. This might have been due to the presence of fertilizer N in the soil at sampling. After 1 day, the NO3−-N decreased to 0, meaning the reduction of NO3−-N occurred quickly under the incubation conditions.
The cumulative nitrogen mineralization over incubation
Mineral N and mineralization rate
Among the six soils, sample S had the highest amount of mineral N, almost two times that of the other five samples. This phenomenon could be explained by the differences in the physical and chemical properties of the soils. There are many reports on the relationship between mineral capacity and soil properties. In general, the amount of N mineralized is correlated with total N, total C and microbial N Marion et al., 1981, Kafkafi et al., 1983, Dalal and Mayer, 1987, Ireneo et al., 1996,
Conclusions
Because only the effective cumulated temperature model has been used in flooded soil previously, we compared it to the special model in theory and practice. Firstly, the concept of effective cumulative temperature is based on the theory that mineralization is affected by temperature; it does, however, not reflect the essential characteristics of mineralization. The special model explains the process and characteristics of mineralization of two different types of organic matter, and it reflects
Acknowledgements
This research was funded by a grant (G19990118) from the National Natural Science Foundation of China. The comments to the manuscript by Dr. Marco Roelcke, TU Braunschweig, Germany, are gratefully acknowledged.
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