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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]

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]