Integrated rice-fish system: methodologies for the quantification of nitrogen within the soil plant system
1994
Gaunt, J.L. | Neue, H.U. | Grant, I.F. (Natural Resources Inst., Catham Maritime, Catham, Kent (United Kingdom))
The measurement of labile N as soil exchangeable, soil solution and soil microbial biomass pools over time in the 0-15 cm puddled layer in intensively cultivated irrigated rice indicated that inorganic N was immobilized within the soil microbial biomass until flowering. This biomass pool size was constant after flowering, indicating that any subsequent crop N uptake was derived from fertilizer top dressing or mineralization of soil organic matter. Such measurements provide great insight into the mechanisms of N supply in the system. Given that the impact of fish during the crop cycle may be to charge both the quantities and characteristics of N supply, continuous monitoring of the N content in the crop for each management practice is needed. Non-destructive plant sampling to measure nitrogen concentration of leaves should be used. This can be done using a chlorophyll meter, correcting values for the specific leaf weight. Specific experiments to establish a mechanistic understanding of the seasonal N supply should be linked with studies of long impact of rice-fish management. It is important to evaluate the performance of the system through measurements such as total factor productivity as well as specific components such as soil fertility. A key component determining the long-term N fertility of soils is soil organic matter. Changes in the quantity and nature of soil organic matter will inevitably affect soil fertility. A large proportion of total soil carbon is relatively inert and thus its measurement is a poor indicator of soil fertility. Much research has identified the soil microbial biomass as a labile organic matter pool that is sensitive to impacts due to management practice. When the soil system is at a steady state, soil microbial biomass will form a constant proportion of total carbon (C). Where soil organic matter is accumulating or declining, this relationship with total C breaks down, however the relationship between biomass and labile C will remain constant
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