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Structure and transformation processes of soil organic matter (SOM) are extremely complex, but advancing our knowledge on SOM cycling is a prerequisite for a sustainable soil management. To get a better insight to this issue, we determined the vertical distribution of lignin, carbohydrates, and amino sugars in bulk soils and NaOH-extracts using wet chemical techniques. These results were compared with those obtained by solution [sup.13]C nuclear magnetic resonance (NMR) spectroscopy after alkaline extraction. Soil samples were taken under a primary forest, a secondary forest, a 20-year-old Pinus kesiya (Royle ex Gordon) reforestation established following 15 years of cultivation, and a cabbage cultivation site in northern Thailand. Significantly lower contents of organic C and N at the cabbage cultivation and reforestation sites indicated that the replacement of forests by arable land at the reforestation and cabbage cultivation sites about 30 years ago resulted in enhanced breakdown of SOM. This means that after 20 years of Pinus growth, reforestation did not lead to a significant build-up of organic matter in the mineral soil. With increasing soil depth the sites showed comparable decreases in soil organic matter, exhibiting a typical pattern of decomposition expressed by a higher degree of side chain oxidation, increasing carboxyl functionality, and a decrease of lignin-derived phenols and aromatic compounds. Microbial contribution to SOM was determined using the carbohydrate and amino sugar biomarker approach. The amino sugars were predominantly of fungal origin in the organic layer. In the mineral soil, bacterial amino sugars dominated and the relative contribution of amino sugars to SOM increased with depth. Comparison of results from wet chemical analyses and of liquid-state [sup.13]C NMR signatures requires that alkaline-extractable organic matter is representative for bulk soil organic matter. This seemed to apply to lignin-derived phenols and amino sugars but not to neutral sugars and uronic acids. Significant correlations were found for lignin-derived phenols with phenolic C (R = 0.74, P < 0.01) for the bulk forest site samples and amino sugars with O-alkyl C (R = 0.93, P < 0.001) for the mineral soil horizons, whereas the carbohydrate contents did not show any clear correlation. Therefore, we concluded that most of the phenolic C signal intensity might be attributed to lignin, and the enrichment of O-alkyl C with depth may be a result of bacterial resynthesis with a significant contribution of amino sugars. Additional keywords: soil organic matter, land use, alkaline extracts, bulk soil., Introduction Soil organic matter (SOM) encompasses plant, animal, and microbial residues in all stages of decomposition and it is often closely associated with inorganic soil components. The cycling of SOM [...]

We used natural [sup.13]C abundance coupled with particle-size fractionation to evaluate soil organic carbon (SOC) dynamics following deforestation and subsequent cultivation in the subhumid Ethiopia highlands. Surface soil (0-10 cm), leaf, root, and litter samples were collected from natural forest and fields cultivated for 25 yr (Wushwush) and from Podocarpus dominated natural forest and 30 yr cultivated fields (Munesa) and C, N and [delta][sup.13]C signature were measured. Total SOC declined by 55% (32.0 Mg [ha.sup-1]) at Wushwush and by 63% (40.2 Mg [ha.sup.-1]) at Munesa following cultivation, while losses of N amounted to 52% (2.8 Mg [ha.sup.-1]) and 60% (3.1 Mg [ha.sup-1]) at the two sites, respectively. [delta][sup.13]C values of bulk soils of natural forests at Wushwush (-24.3 [per thousand]) and Munesa (-23.4 [per thousand]) were significantly lower than those from the corresponding cultivated fields (-19.9 [per thousand], Wushwush and -15.5 [per thousand], Munesa). Deforestation and continuous cultivation at Wushwush and Munesa resulted in depletion of 80 and 96% of the initial forest-derived SOC in sand, while 73 and 85% of [C.sub.3] SOC was lost from silt fraction of the two sites, respectively. These results suggest that SOC in sand was a very labile component of SOM and is a more sensitive indicator to changes in soil C storage in response to land use changes. However, the substantial amount of forest-derived SOC lost from silt indicates that SOM associated with silt was also quite susceptible to management changes, and that at least in the soils under study represents a moderately labile SOM pool, which is generally not the case in temperate soils. Forest-derived SOC in clay declined by 48 and 61% at Wushwush and Munesa, respectively, suggesting that clay retained [C.sub.3] derived SOC more effectively and that SOM bound to clay was more stable than SOM associated with sand and silt fractions.

Physical fractionation, degradative wet-chemical analysis and liquid-state [sup.13]C nuclear magnetic resonance (NMR) spectroscopy were used to assess the impact of land use changes on the amount and structural composition of soil organic matter (SOM) in bulk soils and size separates in the subhumid highlands of southern Ethiopia. Soil samples (0-10 cm) were collected from natural forest, tea plantations, 25-yr cultivated fields at Wushwush (Paleudalf), Podocarpus-dominated natural forest, Cupressus plantations, and 30-yr cultivated fields at Munesa (Palehumults) sites. Forest clearing and continuous cultivation led to significant depletion (P < 0.05) of total soil organic C (SOC) (55% and 63%) and N (52% and 60%) in the surface soils, respectively. Compared with the cultivated fields, lower proportions of SOC (51 and 27%) and N (49 and 13%) were lost from the tea and Cupressus plantations, respectively. The largest depletion occurred from the labile SOM associated with the sand separates concurrent with higher oxidation states of lignin. However, substantial amounts of these organic substrates were also lost from the stable SOM fraction. Particularly, SOM, associated with the silt-size separates, decreased suggesting that the SOM in silt was quite susceptible to land use changes and represents a moderately labile SOM pool in the soils under study. Solution [sup.13]C NMR spectra revealed larger proportions of protonated and C- and O-substituted aryl-C in the silt than in clay-size separates. In contrast, O-alkyl-C structures were more prominent in the clay than in silt-size separates, coinciding with the lignin distribution obtained by wet-chemical analysis. Deforestation and subsequent agricultural management not only resulted in SOM depletion but also markedly altered the chemical composition of SOM in the subhumid highland ecosystems.

Sulfur, besides phosphorus, is crucial for the nutrition of plants on tropical soils. Its availability is closely related to the turnover of soil organic matter. To get a better insight into transformation of soil S forms during the decomposition of organic matter, we studied inorganic and organic S pools in bulk samples and alkaline extracts of soils under different land uses representative of the tropical highlands of northern Thailand. Samples were taken from a cabbage cultivation, a Pinus reforestation, a secondary forest, and a primary forest. Total S ranged from 483 ± 549 mg/kg in the subsoil to 1909 ± 376 mg/kg in the organic layers, which is relatively high for tropical soils. The major S component in soil was organic S, comprising 75-99% of total S. Organic S was significantly correlated with total S, organic C, and total N, indicating that there is a close relationship between C, N, and S cycling in soil. C-bonded S was the predominant form in the topsoils (35-99% of total S) but its presence decreased with soil depth. The maximum concentrations of ester S[O.sub.4]-S were found in the A horizons (128 ± 49 mg/kg), whereas the concentrations of inorganic S[O.sub.4]-S were small in all horizons. Compared with the forest site, the cabbage cultivation site was strongly depleted in S. C-bonded S was more depleted than ester S[O.sub.4]-S. A comparison of the S forms in NaOH extracts with S forms in bulk soil and C forms as indicated by [sup.13]C-NMR spectroscopy showed (i) that the extracts were very representative of soil organic S fractions and (ii) that ester S[O.sub.4]-S was mainly associated with O-substituted aliphatic C. In contrast, C-bonded S seemed to be connected to more-or-less all C binding types. Additional keywords: transformation of soil organic matter, sulfate., Introduction Maintenance of sufficient levels of sulfur (S) for plants and animals involves a complex biogeochemical cycle in which S is converted among various organic and inorganic forms. Nearly 95% [...]

Changes in the composition and amount of saccharides have been shown to reflect soil organic matter (SOM) dynamics. The effect of climate on soil monosaccharide pools was investigated in this study. Particle-size fractions were obtained from composite samples taken from the top 10 cm of soil at 18 native grassland sites along mean annual temperature (MAT) and precipitation (MAP) transects from Saskatoon, Canada to southern Texas, USA. Neutral and acidic sugars were determined in the bulk soil,