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It is known that crop residue input has a strong impact on the quantity of soil organic matter (SOM). However, its influence on SOM quality is often unknown. The effect of organic matter (OM) input on soil organic carbon (SOC) and functional SOC pools was investigated in a Chinese long-term double-cropping experiment with rice. It could be demonstrated that the C content of hot-water soil extracts (C) may serve as an indicator of the decomposable SOC pool. A particle size-density fractionation was used to separate SOC pools with different interactions with the mineral soil matrix. The hot-water C extractability of different particle size-density fractions, together with their δC isotopic signatures, was applied to differentiate stability and potential degradability of SOC pools. The main findings were: first, the input of OM (rice straw + animal manure + green manure) to the unfertilized (Nil) and fertilized (NPK) treatments resulted in a 43-64% increase of SOC over 14 years. Second, during the same period the C pool declined by 25-32% in plots without OM input. Organic amendments reduced the decrease in the C pool by about 50%. Hot-water-extractable carbon (C) (C) of bulk soil could serve as an indicator of decomposable SOC pools with the potential for nutrient supply during decomposition. Third, the importance and capacity of smaller-sized clay particles (CF1 <1 µm) for C storage was proved by large C enrichment factors (E). Fourthly, functional C pools in different size-density fractions could be clearly separated from each other according to their stability, by relating the C isotopic abundance of C pools to the hot-water-extractable component ( δC*). [ABSTRACT FROM AUTHOR]

Presents information on the measurement of methane and nitrous oxide flux in a flooded rice soil. Global importance of flooded rice to the atmosphere; Measurement of fluxed from flooded rice.

Measurements of profiles of ferrous and ferric iron and pH in blocks of reduced soil in contact with planar layers of rice (Oryza sativa L.) roots are reported. Initially 11-d-old plants were kept in contact with the soil for up to 12 d. Over this period, substantial quantities of iron were transferred towards the root plane, producing a well-defined zone of ferric hydroxide accumulation. The pH in this zone fell by more than two units. The profiles changed with time. The decrease in pH was in part due to protons generated in ferrous iron oxidation, and in part due to protons released from the roots to balance excess intake of cations over anions, N being taken up chiefly as NH4+. But the decrease in pH was less than expected from the net acid production in these two processes, possibly because of proton consumption in CO2 uptake by the roots. Because of the pH-dependence of soil acidity diffusion, the two sources of acidity greatly reinforce each other. Some implications for nutrient and toxin dynamics are discussed. [ABSTRACT FROM AUTHOR]

Methane (CH4) is an important greenhouse gas. Flooded rice fields (paddies) are a significant source of atmospheric CH4; estimates of the annual emission from paddies range from less than 20 to 100 million Tg, with best estimates of 50 1 20 Tg. The emission is the net result of opposing bacterial processes; production inanaerobic microenvironments, and consumption and oxidation in aerobic microenvironments, both of which occur sequentially and concurrently in flooded rice soils. With current technologies, CH4 emission from rice fields will increase as production increases. Over the next 25 years rice production will have to increase by 65% from the present 460 Mt/y to 760 Mt/y in 2020. The current understanding of the processes controlling CH4 fluxes, rice growth and riceproduction is sufficient to develop mitigation technologies. Promising candidates are changes in water management, rice cultivars, fertilization, and cultural practices. A significant reduction of CH4 emission from rice fields, at the same time that rice production and productivity increase at the farm level, is feasible, although the regions where particular practices can be applied, and the trade-offs that are possible, have still to be identified. [ABSTRACT FROM AUTHOR]