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Land‐use change has driven soil carbon stock losses in ecosystems worldwide. Implementing agricultural crops and exploiting forest resources trigger the breakdown of soil aggregates, thus exposing organic matter to microbial decomposition and enhancing carbon dioxide emissions, especially in biomes more susceptible to climate extremes as in the tropical semiarid regions. This study was based on the hypothesis that the undisturbed soil from the dry forest (Caatinga biome under natural revegetation in Brazilian semiarid) would have an improvement in the mass of macroaggregates and recover more than 50% of the soil C stock within 10 years. Thus, a field experiment was conducted to investigate soils from the Caatinga biome under native vegetation, "cowpea cropping" for over 30 years, and soil under natural revegetation for over 10 years, after conventional soil cultivation of maize and cowpea, to determine soil and soil‐aggregates carbon stocks and to estimate the recovery rate of these stocks. The proportional mass of aggregates of different sizes and the total stock of particulate organic carbon (POC) were also quantified. The results showed that soil under preserved native vegetation of dry forest Caatinga biome had higher total soil C stock (50.9 Mg ha−1) than that under cowpea cropping (23.2 Mg ha−1) and natural revegetation (45.1 Mg ha−1). The proportional mass of large macroaggregates was higher in soil under native vegetation for all depths. However, soil under cowpea cropping had lower C stocks in macroaggregates, and recovered roughly 63% of the original C stocks, while revegetation recovered 78% of the stock in 10 years. Although the conventional management system for cowpea monoculture aggravated losses in soil carbon stock by more than 50% of the original C stocks, dry forest under natural revegetation recovered 79% of this stock and almost 100% of POC stock in 10 years (~12 Mg ha−1). Furthermore, soil under undisturbed Caatinga dry forest achieved C stock levels equivalent to that of the global average range for semiarid tropical environments. The high recovery rate of C stock in forest soil under natural revegetation indicates the resilience potential of organisms responsible for structural protection of aggregates and the encapsulated soil organic matter content. [ABSTRACT FROM AUTHOR]

As soils have a recognized capacity of carbon storage, the understanding of the mechanisms responsible for soil organic matter (SOM) stabilization are of extreme importance. Yet, our knowledge about the factors and processes controlling the amounts of stabilized versus mineralized SOM is still remarkably limited. We examined the decomposition of different plant inputs in a top- (0 – 0.2 m) and subsoil (0.8 – 0.9 m) of an Andosol. We performed an incubation experiment during 60 days using as amendments 13C isotope labelled plant leaves (Salix spp.) and dissolved organic carbon (DOC) extracted after incubation of leaves (pelleted – 1 mm or liquid - < 0.7 µm). Soil respiration was measured with a δ13C CO2 Isotope Analyzer. After the incubation period, the soils were fractionated by density using a sodium polytungstate solution of 1.8 g cm-3 into two fractions: heavy (> 1.8 g cm-3) and light (< 1.8 g cm-3). The added 13C amount was then measured by Isotope-ratio mass spectrometry (IRMS). This experiment allowed us to observe distinctive SOM stabilization and mineralization patterns in both soil layers depending on the added material. The DOC, whether pelleted or liquid, was significantly more respired than the leaves in both soil layers. In the subsoil but not the topsoil, we observed a significant difference between the respiration of the different DOC forms (pelleted and liquid). Since both DOC forms vary by location (distributed vs point source) and size (pelleted vs liquid), the mineralization results point at several explanations: (i) smaller particles are more efficiently stabilized in subsoil layers by mineral interactions than larger structures due to their greater surface area, (ii) more dilute material is more efficiently adsorbed as concentrated organic matter exceeds sorption capacity (saturation concept), or (iii) more concentrated organic matter is more easily mineralized than more dilute because microorganisms are able to specialize to metabolize the substrate. As the leaves and pelleted DOC were both point sources of SOM, the differences in their mineralization rates indicates a significant influence of material solubility. We also observed distinct patterns of SOM stabilization in top- and subsoil. In the subsoil, a greater proportion of the added DOC (pelleted and liquid) than the leaves was found in the heavy fraction, which is commonly interpreted as stabilized C on clay surfaces and less accessible to microorganisms. This may indicate that mineral association explained differences in CO2 evolution. In the topsoil, added C remained in light fractions typically interpreted as largely unassociated with minerals. However, presence in such light fraction in the topsoil must still have reduced mineralization. Higher proportions of added leaves and pelleted DOC remained in the topsoil mineral fraction than of the liquid DOC, which indicates that point sources of C provided higher stabilization into soil minerals. These differences clearly illustrate distinct mechanisms for SOM protection depending on the evaluated soil layer and amendment added. We demonstrated that SOM stabilization in top and subsoil is significantly affected by the spatial distribution and the solubility of the amendment. [ABSTRACT FROM AUTHOR]

Decaying plant material is the major source of soil organic carbon (SOC). Nevertheless, it still remains unclear how the biochemical composition of litter is affecting the formation of SOM. Thus, we aimed to disentangle the effect of plant litter composition on C transference into differently stabilized soil fractions. We did this by individually incubating 13C enriched Eucalyptus spp. organs (bark, leaves, twigs and roots) in topsoil (0–20 cm) of a sandy-clay loam (Haplic Ferralsol) from Paula Candido – Brazil (20º 52' S, 42º 58' E). Additionally, a soil sample without plant residue addition was incubated and used as a control. The samples were incubated at 80% of their water-holding capacity at 25 ºC. The molecular composition of the incubated plant organ material was characterized by solid-state 13C-CP/MAS-NMR spectroscopy. At the end of the incubation experiment (after 200 days), aliquots were taken and the SOM was physically fractionated using a combined density-particle size separation method. The total C content and the relative abundance of 13C (δ 13C) of each soil organic matter fraction were measured by IRMS, whereas the litter-C contribution for each SOM fraction was assessed using a two-end member isotope mixing model. No significant differences were observed for total SOC contents among the different treatments. Conversely, lower total SOC contents were observed for the control treatment without residue addition, indicating mineralization of inherited SOM along the incubation period. The partitioning of litter-derived C into SOM fractions and CO2 indicates that leaf had higher proportion of C transferred to the total SOC, followed by roots, bark and twigs. The amount of carbon transferred to the mineral associated organic matter (MAOM) was as follows: leaves > bark > twigs > roots. Contrastingly, for the particulate organic matter (POM) the order was as follows: roots > bark > twigs = leaves. Contrasting NMR spectra demonstrate the significant effect of the biochemical composition of each plant organ in the fate of C in soil. Carbon from leaves was preferentially allocated to the MAOM fraction, and was more enriched in carboxyl and alkyl groups. Twigs, bark and roots, which were more enriched in O-Alkyl group, were preferentially respired or allocated into the POM fraction. [ABSTRACT FROM AUTHOR]