Rice systems in Asia have intensified rapidly in the past 30 years, and significant areas of irrigated lowland rice are now supporting two or three rice crops per year. Our objective was to compare the chemical composition of soil organic matter (SOM) from four fields with different histories of rice cropping intensity and soil submergence: (i) a single-crop rainfed, dryland rice system without soil submergence, (ii) an irrigated rice and soybean rotation, and irrigated (iii) double-or (iv) triplecrop rice systems in which soil remains submerged during much of the year. In all four soils, extracted mobile humic acid (MHA) and calcium humate (CaHA) fractions were of modem age by I4C-dating, and represented about 20% of total N and organic C. The MHA was enriched in N and hydrolysable amino acids (AA) compared with CaHA in all soils. With increased frequency of irrigated rice cropping, however, there was a large increase in phenolic content of SOM. We speculate that slower lignin decomposition caused by deficiency of 0 2 in submerged soil leads to incorporation of phenolic moieties into young SOM fractions. The increased phenolic character of these fractions may influence N cycling and the N supplying capacity of lowland soils supporting two or three annual crops of irrigated rice.

We have collected solid‐state 13C nuclear magnetic resonance (NMR) data from the published literature (76 papers) and from our own results on 311 whole soils, physical fractions (25 clay‐, 43 silt‐, and 52 sand‐size fractions) and chemical extracts (208 humic and 66 fulvic acids). Our purpose was to see whether a comprehensive analysis of data on >300 soils that ranged in organic C content from 0.42 to 53.9% would show any universal influence of management practice on the chemical composition of soil organic matter (SOM). The relative abundance of functional groups was calculated for the following chemical shift regions: 0–50 ppm (alkyls), 50–110 ppm (O‐alkyls), 110–160 ppm (aromatics), and 160–200 ppm (carbonyls). There was a remarkable similarity between all soils with respect to the distribution of different forms of C despite the wide range of land use (arable,grassland, uncultivated, forest), climate (from tropical rainforest to tundra), cropping practice, fertilizer or manure application, and the different spectrometer characteristics and experimental conditions used. Functional groups in whole soils were always in the same abundance order despite the generally wide proportion range: O‐alkyls (a mean of 45% of the spectrum, increasing with soil C content), followed by alkyls (mean 25%), aromatics (mean 20%), and finally carbonyls (mean 10%, decreasing with soil C content). Humic and fulvic acids contained much smaller proportions of O‐alkyls than whole soils (means of 26%). Clay‐size fractions were the most different from whole soils, being more aliphatic (+8%). Sand‐size fractions generally gave very similar results to whole soils.

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SUMMARY The different forms of phosphorus (P) in 0.5 m sodium hydroxide extracts of soils from long‐term field experiments at Rothamsted were characterized by 31P‐nuclear magnetic resonance spectroscopy (NMR). The extract from an old grassland soil (pH 4.6) from a plot of the Park Grass Continuous Hay Experiment that had received no fertilizer or lime for at least 125 years contained the following forms of P: inorganic orthophosphate (22% of the extracted P), orthophosphate monoesters (49%), orthophosphate diesters (14%), phosphonates (3%), pyrophosphate (4%) and two unidentified forms of P (7%). The soil extract from a Park Grass plot given inorganic phosphate fertilizer (35 kg P ha−1) annually for 121 years contained the same forms of P and, in addition, a small amount of polyphosphate. There was also evidence of an increase in the orthophosphate monoester fraction. Another old grassland soil, of pH 6.1, contained more total and organic P than Park Grass but the extract contained fewer forms of P: inorganic orthophosphate (14% of the extracted P), orthophosphate monoesters (39%), orthophosphate diesters (34%) and an unidentified form (13%). An area of this grassland that had been ploughed up 20 years previously, and kept bare since, contained less organic P. The extract contained less of the phosphate diesters but the more stable monoesters remained relatively unchanged.

Summary The water retention characteristic provides the traditional data set for the derivation of a soil's pore‐size distribution. However, the technique employed to achieve this requires that assumptions be made about the way pores interconnect. We explore an alternative approach based on stray field nuclear magnetic resonance (STRAFI‐NMR) to probe the water‐filled pores of both saturated and unsaturated soils, which does not require information relating to pore connectivity. We report the relative size distributions of water‐occupied pores in saturated and unsaturated samples of two sets of glass beads of known particle size, two sands, and three soils (a silty loam, a sandy loam and a loamy sand), using measurements of the NMR T1 proton relaxation time of water. The T1 values are linearly related to pore size and consequently measured T1 distributions provide a measure of the pore‐size distribution. For both the sands and the glass beads at saturation the T1 distributions are unimodal, and the samples with small particle sizes show a shift to small T1 values indicating smaller voids relative to the samples with larger particles. Different matric potentials were used to reveal how the water‐occupied pore‐size distribution changes during drainage. These changes are inconsistent with, and demonstrate the inadequacies of, the commonly employed parallel‐capillary tube model of a soil pore space. We find that not all pores of the same size drain at the same matric potential. Further, we observe that the T1 distribution is shifted to smaller values beyond the distribution at saturation. This shift is explained by a change in the weighted average of the relaxation rates as the proportion of water in the centre of water‐filled pores decreases. This is evidence for the presence of pendular structures resulting from incomplete drainage of pores. For the soils the results are similar except that at saturation the T1 distributions are bimodal or asymmetrical, indicative of inter‐aggregate and intra‐aggregate pore spaces. We conclude that the NMR method provides a characterization of the water‐filled pore space which complements that derived from the water retention characteristic and which can provide insight into the way pore connectivity impacts on drainage.

A detailed discussion of the quantitative nature of 13C CPMAS NMR spectra as applied to solid samples, such as soil, is given. In particular, the influence of the cross-polarization (CP) time constant (TCH), the relaxation time constant of protons in the rotating frame ( T l p~) and the contact time (t,) in the CPMAS experiment are considered. Three distinct quantitation regimes are numerically identified according to sample parameters TcH and TlpH, and the experimental choice of tc: (i) quantitation obtainable from a single CPMAS spectrum; (ii) quantitation obtainable from a series of CPMAS spectra; and (iii) quantitation not possible using CPMAS. Strategies for the measurement of sample parameters TCH and TlpH are reviewed. When quantitation is not possible using CPMAS it is necessary to regress to the direct polarization (DP) of 13C nuclei. The sensitivity problems of DPMAS are discussed, as too are general factors that affect the quantitation of 13C data such as spinning sidebands. More specifically in relation to soil samples, the effects on quantitation arising from the presence of paramagnetics and the actual methods for the measurement of signal intensities are covered.

C-nuclear magnetic resonance (NMR) spectra taken using magic-angle spinning (MAS), cross polarization (CP) and with total suppression of side bands (TOSS) are reported for soils from two long-term field experiments. One set of soils was from the Broadbalk Experiment at Rothamsted, UK (monoculture of winter wheat since 1843) and the other was from the Lermarken site of the Askov Long-Term Experiment on Animal Manure and Mineral Fertilizers (arable rotation since 1894). At both sites soil samples were taken from three fertilizer treatments: nil, inorganic fertilizers, animal manure. Spectra were obtained from whole soil samples and from the size fractions clay ( < 2 pm), silt (2-20 pm) and, in some cases, sand (20-2000 pm).Comparison of the total strengths of the 13C-NMR signal for each size separate in relation to its total organic C content shows that clay, particularly, contains large percentages of C not detected by NMR because of the large magnetic susceptibilities of the soil minerals. It is proposed that the observed signals come from the more labile pools of soil organic matter (SOM), on the presumption that these pools are less closely associated with soil minerals and iron oxides and are likely to be less protected from microbial or enzymic decomposition.For both Rothamsted and Askov, functional groups in the 45-1 10 ppm region (N-and 0-alkyls) dominate in the spectra for whole soils, with aromatics (1 10-160 ppm) and alkyls (0-45 ppm) signals being the next prominent. In the Askov whole soil samples I3C-NMR revealed no differences between nil, inorganic fertilizer and animal manure treatments but in the Rothamsted whole soil there were some small differences.Clay and silt fractions from Askov contain more alkyls and less aromatics than those from Rothamsted. For both sites clay in enriched in alkyls and depleted in aromatics relative to silt. Clay from Askov, but not Rothamsted, contains more N-alkyls (45-65 ppm) and less acetals (90-1 10 ppni) than silt. 0-alkyls (65-90 ppm) account for more than 20% of the total signal in clay and silt from both sites. Fertilization regimes have not significantly affected the chemical composition of SOM associated with clay-and silt-sized fractions in the soils at either site. We conclude that the chemical composition of SOM is determined primarily by the interaction between the organisms responsible for decomposition and the mineral soil matrix rather than the nature of substrate input.

El 4NS13C N.m.r. data are presented for the series of complexes (CO)s-,ML,, (where M = Mo or W ; L = alkylphosphine, alkyl phosphite, bisdiphenylphosphinoethane, bisdiphenylphosphinomethane. and n = 1 or 2) and for the corresponding free ligands. Chemical shifts and coupling constants for the co-ordinated carbon monoxide and ligand carbons are compared with results from other spectroscopic techniques. The chemical shift of co-ordinated carbon monoxide is found to reflect the charge donor ability of L. The phosphorus to carbon monoxide coupling constants vary in a similar way to phosphorus-phosphorus coupling constants in this class of compounds and a dependence of lJ M-'TO on s-electron density a t the metal atom is suggested.THE Group VIA metal carbonyls and their substituted carbon monoxide are sensitive to changes in its stereoderivatives have been the subject of numerous bonding chemistry and electronic environment .7-Q We have studies using 31P n.rn.r.,ls2 i.r.: and U.V. spectros~opies.~~~ examined a range of phosphine and phosphite deriv-The recent developments in instrumentation now atives of molybdenum and tungsten carbonyls and make this class of compounds amenable to study by compare our results with those of other f3C n.m.r. spectroscopy. Recent reports on the 13CCarbonyl l3C-Shifk-There are four obvious trends n.m.r. of transition-metal carbonyl complexes have in 6CO (see Table 1): (1) SCO increases (i.e. there is a shown that the =C chemical shifts, 6C0, in co-ordinated downfield shift) if CO is replaced by any ligand L (L = 1 E. G. Conam., 1971, 1078. 8 B. E. Mann, unpublished results. B (a) 0. A. Gansow, B. Y . Kimura, G. R. Dobson, and R. A Brown, J . Amev. Chem. SOC., 1971, 93, 6922; (b) 0. A. Ganson-, D. A. Schexnayder, and B. Y . Kimura, J . Amev. Chem. SOC., 1972, 94, 3406. 5713.