Change of physical fractions of soil organic matter in different aggregate sizes in a natural forest soil after conversion to paddy rice field and determination of the most sensitive and resistant fractions to land use change

Abstract
Background and objectives: Human activities such as deforestation, conversion of natural ecosystems to farming, biomass burning, tillage and cultivation, grazing, fertilizer use have a significant and negative impact on soil organic carbon pool. In particular, the conversion of natural ecosystems to agriculture reduces soil organic carbon pools. Understanding the spatial distribution of organic carbon in soil is particularly important for understanding the role of soil in the global carbon system. Therefore, the purpose of the present study was to investigate the changes of total carbon and its components in different sizes of aggregates of a forest soil after conversion to paddy.
Material and Methods: For this study, Poplar Research Station in Guilan province was selected. Soil samples were collected from two uses of forest and adjacent paddy fields at two depths (0-20 and 20-40 cm). In the soil samples, aggregates of different sizes were separated and the amount of organic carbon in these aggregates was measured. Organic carbon was then fractionated into different sizes of these aggregates in both land use.
Results: Investigation of organic carbon distribution in relation to aggregate size in two land uses showed that with increasing aggregate size, organic carbon content increased so that organic carbon content in Populus natural forest and rice fields were 4.5 and 3.3 times higher in aggregates larger than 2 mm (larger aggregates) than smaller aggregates 0.053 mm, respectively. The amount of organic carbon in the aggregates decreased with the change from natural forest to paddy fields. However, the decrease was greater in aggregates larger than 2 mm and lowest in aggregates (0.25–0.053 mm). Results of soil organic carbon fractionation showed that percentage of soil weight in fractions increased with increasing density in two land use. This situation was visible in two depths and in all the aggregate sizes. By increasing the density of fractions, the concentration of organic carbon in the fractions decreased. The organic carbon content of the fractions as a percentage of organic carbon content of whole soil, in all the aggregate sizes, and in the fractions with density < 2 g cm-3 was smaller than those of fractions with density > 2 g cm-3. In all of the aggregates, with increasing depth, organic carbon content of the fractions with density < 2 g cm-3 as a percentage of organic carbon content of whole soil decreased while those of the fractions with density of > 2 g cm-3 increased. By increasing the density of fractions, the C/N ratio of the fractions decreased. In all of the aggregate, due to the change of land use from natural forests to rice field, organic carbon content of the fractions as a percentage of organic carbon content of whole soil, decreased in the fractions with density < 2 g cm-3 and increased in the fractions with density > 2 g cm-3 and this was more pronounced in aggregates of larger size. The most sensitive and resistant fractions to land use change were fraction 2g/cm3 respectively. The ratio of carbon to nitrogen decreased as a result of land use change from natural forest to the rice field.
Conclusion: The change of land use and long-term cultivation in virgin forests and its transformation in rice field has changed organic carbon content of the fractions. The amount of free and occluded particulate organic matter in the aggregate (light fraction), which plays a very important role in the nutrition of the crop, was greatly reduced. This reduces the soil's health and the potential for carbon sequestration in the soil.