Ricefields

Davey, R.B., M.V. Meisch, and F.L. Carter. 1976. Toxicity of five ricefield pesticides to the mosquitofish, Gambusia affinis, and green sunfish, Lepomis cyanellus, under laboratory and field conditions. Environ. Entomol. 5 1053-1056. 

Ricefields exclude upland rice. The physical area is calculated from the sum of irrigated rice (51 Mha of which 47% is double- or triple-cropped with rice and 33% under rice-wheat rotation), rainfed lowland rice (54 Mha) and deepwater rice (4 Mha). The riceland of China is taken to be all temperate. 

Equation (2.15) can be used to calculate the relative importance of mass flow and diffusion under conditions in ricefields. From the discussion above, rates of... 

Figure 3.2 Diurnal changes in pH and concentrations of carbonate species in the flood-water in a ricefield (Mikkelsen et al., 1978). Reproduced by permission of Soil Sci. Soc. Am.

The corresponding changes in alkalinity are H-17/106 = - -0.16molc per mol C fixed for NO3 nutrition and —15/106 = —0.14molc per mol C fixed for NH4" nutrition. More significant changes in the alkalinity of ricefield floodwater are... 

Figure 3.3 Calculated diurnal changes in the pH and concentrations of carbonate species in ricefield floodwater for sinusoidally varying [H2CO3 ] with (a) [Aik] = lOmM, (b) [Aik] = 0.5 mM. The free CO2 concentrations are in mgL to be consistent with Figure 3.2...

An important practical problem in ricefields is the loss of N fertilizer through volatilization of NH3 from the floodwater. Loss of NH3 is sensitive to the pH of the floodwater, and hence is intimately linked to the dynamics of dissolved CO2 (Bouldin and Alimago, 1976). To quantify this it is necessary to consider the simultaneous transfers of CO2 and NH3 across the air-water interface and their coupling through acid-base reactions. There is an equation of type (3.33) for the flux of NH3 across the still air layer and, as for the dissolved CO2 and carbonate species, the flux across the still water layer is... 

These general features of NOs reduction in submerged rice soils are bom out by field observations. Buresh et al. (1993b) found that from 60 to 75 % of N-labelled NOs" applied on the surface of flooded ricefields was lost by denitrification over 2-3 weeks, as measured by the not recovered in the soil, floodwater and plants. The recovery of (N2 + N20)- N in chambers placed over the floodwater was less than the estimated denitrification loss because gas bubbles became entrapped in the soil. More N2 + N2O was recovered when the chambers were placed over the rice plants showing that some of the gas escaped through the plants. The not lost by denitrification was presumed to have... 

Figure 5.6 shows profiles of NO3 concentration measured with microsensors in soil cores taken from ricefields by Revsbech and co-workers (Liesack et al., 2000). During illumination of the cores, O2 generated in the floodwater penetrated to a depth of 2-3 mm and a clear peak of was apparent, produced in... 

Figure 5.6 Concentration profiles of NO3 in soil cores from a ricefield, illuminated and not illuminated (Liesack et al., 2000). Reproduced with permission from Elsevier Science...

Figure 5.7 Concentration profiles of O2 and CH4 in soil cores from a ricefield (a) illuminated and (b) not illuminated (Damgaard et al, 1998). Reproduced by permission of American Society for Microbiology...

The subsoil at greater depths may be aerobic in well-drained soils with a perched water table owing to an impermeable layer—such as the traffic pan in ricefields or anaerobic in soils fhaf are poorly drained fhroughouf. If may provide significanf quantifies of nufrienfs to planfs growing in the soil if their roots can reach them. 

Figure 5.8 Temperatures in ricefields over a year, IRRI, Laguna, Philippines. Values are means over a month air temperature is maximum water and soil temperatures were taken at 1400 h (Roger, 1996). Reproduced by permission of IRRI...

Information on the productivity of the floodwater is scarce. Data compiled for ricefields by Roger (1996) give an average of 0.351 dry wt ha of cyanobacteria. 

There has been much research aimed at developing N2 fixing systems for greater yield levels. For practical reasons, such systems cannot entirely substimte for mineral N fertilizers, and there is the difficult dilemma that N2 fixation within the ricefield is inhibited by additions of mineral N. 

Figure 5.11 Effects of tubificids on concentrations of N species, P and Fe in the floodwater of unplanted microplots in ricefields. Species B. sowerbyi, density 1000 (Kikuchi and Kurihara, 1982). Reprodnced with kind permission of Klnwer Academic Publishers...

The resnlts broadly tally with experimental findings for rice. The maximum length of primary root required to sustain a plant depends on soil conditions and planting density. Typically the depth to the plongh pan in a puddled ricefield is less than 2 dm, and a typical spacing between plant hills is 25 cm x 25 cm... 

Note that the above conclusions refer to uptake of soil N by the main body of the rice root system in the anoxic soil beneath the soil-floodwater interface. Uptake of fertilizer N broadcast into ricefield floodwater and absorbed by roots in the floodwater or soil near the floodwater is not likely to be limited by root uptake properties or transport (Kirk and Solivas, 1997). 

Greenland (1997) has compiled realistic average annual nutrient balances for wetland ricefields pre- and post-1960 from probable inputs and outputs. Inputs come from rainfall, R, irrigation and floodwater, F, sediments, S, nitrogen fixation, N, and manures and fertilizers, M. Outputs are due to crop removals in... 

Table 7.1 Acid-base changes in nitrogen transformations in ricefields...

At the opposite end of the fertility scale from ricefields are peat bogs in pluvial landscapes. Nutrient inputs come almost entirely from rainfall, and the nutrient reserves in the organic matter buffering the soil solution are small (Moore and Bellamy, 1974). The chemistry of peat bogs is therefore precarious and changes in the composition of the rainfall can have a large effect on the composition of the soil solution. 

---