Search

Direct evidence-based approaches are vital to evaluating newly proposed theories on the persistence of soil organic carbon and establishing the contributions of abiotic and biotic controls. Our primary goal was to directly identify the mechanisms of organic carbon stabilization in native-state, free soil microaggregates without disrupting the aggregate microstructure using scanning transmission x-ray microscopy coupled with near edge x-ray absorption fine structure spectroscopy (STXM-NEXAFS). The influence of soil management practices on microaggregate associated-carbon was also assessed. Free, stable soil microaggregates were collected from a tropical agro-ecosystem in Cruz Alta, Brazil. The long-term experimental plots (>25 years) comparing two tillage systems: no-till and till with a complex crop rotation. Based on simultaneously collected multi-elemental associations and speciation, STXM-NEXAFS successfully provided submicron level information on organo-mineral associations. Simple organic carbon sources were found preserved within microaggregates; some still possessing original morphology, suggesting that their stabilization was not entirely governed by the substrate chemistry. Bulk analysis showed higher and younger organic carbon in microaggregates from no-till systems than tilled systems. These results provide direct submicron level evidence that the surrounding environment is involved in stabilizing organic carbon, thus favoring newly proposed concepts on the persistence of soil organic carbon.

Biological nitrogen fixation (BNF) in soybean [Glycine max (L.) Merr.] plays an important role in sustainable agriculture, reducing the limitations associated with other sources of N such as fertilizers and soils. Our major objective was to evaluate the weekly pattern of BNF in soybean influenced by nodule formation, using three different laboratory-cultured Bradyrhizobium strains. Plants were grown in 1-m polyvinyl chloride (PVC) columns for 17 week in a greenhouse and BNF was determined using an integrated approach by assessing nodule formation, stem ureide-N, and N partitioning in plant parts. Bradyrhizobium strains showed overall similar plant performance and N2-fixation capacity. During the beginning of flowering/full bloom (R1/R2) growth stages, nodule formation significantly increased and reached a maximum at pod-filling (R4) stage. Stem ureide-N was detected at early growth stages even with fewer small nodules, which significantly increased after the beginning of pod formation (R3). Peak N2-fixing rate (g N kg-1 d-1) started to decline after the onset of seed filling (R5.5). Relationship between BNF and nodule parameters (nodule number [R² = .65] and nodule weight [R² = .62]) suggested that they can be used as predictors of BNF. [ABSTRACT FROM AUTHOR]