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Spatial and Temporal Variability of Ammonium in Flooded Rice Fields

The International Rice Research Institute, P.O. Box 933, Manila 1099, Philippines

J. L. Gaunt and I. F. Grant

Natural Resources Institute, Central Avenue, Chatham Maritime, Chatham, Kent, UK

* Corresponding author ( a.dobermann{at}cgnet.com).

ABSTRACT

Monitoring and modeling the dynamics of N in flooded rice fields require reliable methods for both sampling and analysis. Destructive soil sampling and centrifugation and in situ sampling of soil solution using microporous polymer tubes (rhizon soil solution samplers; RSSS) were used to study the dynamics and variability of NH⁺₄ in two field experiments. Both methods gave similar results in unfertilized treatments, whereas significant differences were measured in treatments with N application. The RSSS were suited for measuring point dynamics or microvariation of nutrients in the root zone, whereas soil-core sampling was a better method to obtain quantitative N data integrated across a particular depth range. We observed a highly significant linear relationship between the logs of soil exchangeable and solution NH⁺₄ only until panicle initiation (PI). Microvariability accounted for a large proportion of the within-field variation and was significantly reduced by bulking of soil cores. Coefficients of variation were higher in the fertilized field compared with an unfertilized field and were changing throughout the growth period of rice, with the highest variability between PI and flowering. Sampling to obtain representative mean values at the plot or field scale can be optimized with regard to temporal changes in the spatial distribution patterns of NH⁺₄. Before transplanting (TP) and at the end of the growth period, less samples are needed and can be taken as composite samples of the whole puddled soil layer. Between TP and flowering, more samples are needed and should be taken in several depth ranges.

This research was supported by the Federal Ministry for Economic Cooperation (BMZ) through the German Agency for Technical Cooperation (GTZ) and the Overseas Development Administration (ODA).

Received for publication October 11, 1993.

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