This study aimed to determine the impact of land use on organic carbon (OC) pools of soils with contrasting native organic matter (OM) content. Surface (0–15 cm) soils of four land uses (cropland, orchard, grassland, and fallow) were collected from four agroecological zones (AEZs) of Bangladesh with different OM content (AEZ-7: very low, −3: low, −9: medium, and −5: high). Bulk soils were physically fractionated into particulate and mineral associated OM (POM and MOM: >53 and <53 µm, respectively). Both bulk and fractionated soils were analyzed for OC and nitrogen (N). Among the land uses, undisturbed soils (grassland and fallow land) had significantly higher total OC (0.44–1.79%) than disturbed soils (orchard and cropland) (0.39–1.67%) in all AEZs. The distribution of OC and N in POM and MOM fractions was significantly different among land uses and also varied with native OM content. In all AEZs, cropland soils showed the lowest POM-C content (0.40–1.41%), whereas the orchard soils showed the highest values (0.71–1.91%). The MOM-C was highest (0.81–1.91%) in fallow land and lowest (0.53–1.51%) in orchard, and cropland had a moderate amount (0.70–1.61%). In croplands, distribution of a considerable amount of OC in the MOM pool was noticeable. These findings reveal that total OC in soils can be decreased with cultivation but does not inevitably indicate the loss of OC storage in the stable pool. Carbon storage potential of soils with both high- and low-native OM contents can be increased via proper land use and managements.
Nodule senescence is a complex developmental process during which essential nutrients are recycled. In order to understand the regulatory mechanism, transcript-profiling analysis during nodule senescence was performed in the Lotus japonicus-Mesorhizobium loti symbiosis. Microarray data showed significantly up-regulated expressions in 641 genes out of a total of 20,165 genes during nodule senescence, and down-regulated expressions were observed in 416 genes. These up-regulated genes during senescence were related to cell wall/membrane/envelope biogenesis and extracellular structures. Down-regulated genes were mainly responsible for defense mechanisms. We classified senescence up-regulated genes in two clusters. Genes in cluster 1 were induced at senescence specific stage and those in cluster 2 were induced from nitrogen fixation stage and expressed until nodule senescence. The genes in cluster 1 included typical marker for senescence like gene for heat shock protein. Four hundred sixteen down-regulated genes during nodule senescence were also classified in two clusters, cluster 3 and cluster 4. These genes corresponded to metabolisms for amino acid and plant hormones which are necessary for growth and cell division during nodule development and nitrogen fixation. These results provide the comprehensive data source for investigation of molecular mechanisms underlying nodule senescence in Lotus japonicusMesorhizobium loti symbiosis.
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