Influence of texture on organic matter distribution and quality and nitrogen and sulphur status in semiarid Pampean grassland soils of Argentina

Texture, among several other factors, may play an important role in controlling the distribution and activity of soil microbial biomass and, in turn, the size and quality of soil organic matter (SOM) pools, and the related dynamics of N and S in soil. Little information is available in the current literature on the occurrence and extent of these textural effects in semiarid soils of Argentinean Pampas. Separation of the primary SOM pools by particle-size fractionation were expected to contribute to elucidate how different SOM fractions could be affected by texture in Pampean soils. The main objective of this study was to evaluate the amount, quality and distribution of SOM fractions, either physically separated, i.e., particulate and mineral-associated SOM, or chemically separated, i.e. the humic acid (HA) and fulvic acid (FA) fractions of SOM, in the whole soil and fine fractions (silt and clay) of variously textured soils from semiarid grasslands in the Argentinean Pampas region. Furthermore, the influence of texture was investigated on the N and S status and distribution, also in relation to SOM fractions. The content of any SOM fraction showed a highly positive correlation with the fine fraction content of the parent soil. The amounts of N and S in particulate SOM were not affected by soil textural differences, whereas they decreased in the fine fractions and in HAs with increasing content of soil fine fractions. Fine-textured soils contained an amount of total N and S, possibly mobilized from SOM components of the fine fraction including HAs, which was larger than that occurring in coarse-textured soils. The HAs in the fine fractions of fine-textured soils featured a degree of aromatic polycondensation, a level of conjugated chromophores and a humification degree larger, and a molecular heterogeneity smaller than those of HAs isolated from the fine fractions of coarse-textured soils. An opposite trend was exhibited by the corresponding FAs. In conclusion, soil texture appeared to have an important impact on the amount, distribution and chemical properties of the various SOM components, and especially the HA and FA fractions, in Pampean grassland soils. As well, S and N mobility and availability in these soils was markedly influenced by textural differences.