Physical soil architectural traits are functionally linked to carbon decomposition and bacterial diversity
Aggregates play a key role in protecting soil organic carbon (SOC) from microbial decomposition. The objectives of this study were to investigate the influence of pore geometry on the organic carbon decomposition rate and bacterial diversity in both macro- (250-2000 μm) and micro-aggregates (53-250 μm) using field samples. Four sites of contrasting land use on Alfisols (i.e. native pasture, crop/pasture rotation, woodland) were investigated. 3D Pore geometry of the micro-aggregates and macro-aggregates were examined by X-ray computed tomography (μCT). The occluded particulate organic carbon (oPOC) of aggregates was measured by size and density fractionation methods. Microaggregates had 54% less μCT observed porosity but 64% more oPOC compared with macro-aggregates. In addition, the pore connectivity in micro-aggregates was lower than macro-aggregates. Despite both lower μCT observed porosity and pore connectivity in micro-aggregates, the organic carbon decomposition rate constant (Ksoc) was similar in both aggregate size ranges. Structural equation modelling showed a strong positive relationship of the concentration of oPOC with bacterial diversity in aggregates. We use these findings to propose a conceptual model that illustrates the dynamic links between substrate, bacterial diversity, and pore geometry that suggests a structural explanation for differences in bacterial diversity across aggregate sizes.
The relationships between land uses, soil management practices, and soil carbon fractions in South Eastern Australia
This project aimed to identify land uses and soil management practices that have significant associations with soil organic carbon (SOC) stocks (0-0.3 m) in New South Wales (NSW), Australia. The work presented in this paper is based on a one-off survey targeting key land uses and management practices of eastern NSW. Because of the nature of the work, the land uses and management combinations surveyed in different soils and climatic conditions were significantly unbalanced, and separately analyzing associations after breaking the dataset into different land uses may lead to significant increases in Type errors. Therefore, redundancy analysis (RDA) was undertaken to explore the association between explanatory variables (i.e., land uses, soil management, soil properties and environmental variables) and the variation in stocks (mass per unit area) of particulate organic carbon (POC), humic organic carbon (HOC) and resistant organic carbon (ROC) across 780 sites in eastern NSW, south eastern Australia. Results indicated that soil properties, land uses, soil management and environmental variables together could explain 52% of total variation in stocks of the SOC fractions. Specifically soil properties and environmental variables explained 42.8%, whereas land uses and management practices together explained 9.2% of the total variation in SOC fractions. A forward selection RDA was also undertaken considering soil properties and environmental variables as covariates to assess the statistical significance of land uses and management practices on stocks of POC, HOC and ROC. We found that pasture had significant positive associations on stocks of carbon fractions. Among the soil properties and environmental variables rainfall, longitude and elevation had a significant positive influence while pH and bulk density had a significantly negative influence on the HOC, POC and ROC stocks. Using a novel multivariate technique, the current work identified the land uses and soil management that had significant impact on carbon stocks in soil after accounting for influences soil properties and environmental variables.
Impact of carbon farming practices on soil carbon in northern New South Wales
This study sought to quantify the influence of 'carbon farming' practices on soil carbon stocks, in comparison with conventional grazing and cropping, in northern New South Wales. The study had two components: assessment of impacts of organic amendments on soil carbon and biological indicators in croplands on Vertosols of the Liverpool Plains; and assessment of the impact of grazing management on soil carbon in Chromosols of the Northern Tablelands. The organic amendment sites identified for the survey had been treated with manures, composts, or microbial treatments, while the conventional management sites had received only chemical fertilisers. The rotational grazing sites had been managed so that grazing was restricted to short periods of several days, followed by long rest periods (generally several months) governed by pasture growth. These were compared with sites that were grazed continuously. No differences in total soil carbon stock, or soil carbon fractions, were observed between sites treated with organic amendments and those treated with chemical fertiliser. There was some evidence of increased soil carbon stock under rotational compared with continuous grazing, but the difference was not statistically significant. Similarly, double-stranded DNA (dsDNA) stocks were not significantly different in either of the management contrasts, but tended to show higher values in organic treatments and rotational grazing. The enzymatic activities of β-glucosidase and leucine-aminopeptidase were significantly higher in rotational than continuous grazing but statistically similar for the cropping site treatments. Relative abundance and community structure, measured on a subset of the cropping sites, showed a higher bacteria : fungi ratio and provided evidence that microbial process rates were significantly higher in chemically fertilised sites than organic amendment sites, suggesting enhanced mineralisation of organic matter under conventional management. The higher enzyme activity and indication of greater efficiency of microbial populations on carbon farming sites suggests a greater potential to build soil carbon under these practices. Further research is required to investigate whether the indicative trends observed reflect real effects of management.
How do microaggregates stabilize soil organic matter?
Microaggregates play a key role to protect soil organic matter (SOM) from microbial decomposition. Several physical, physico-chemical and biochemical mechanisms have been proposed to describe the SOM stabilization in soil. However, no scientific consensus exists about a range of hypotheses. The aim of this review is to consolidate common notions and hypotheses on physical and physico-chemical protection mechanisms. The key notion of physical protection is exclusion of microbes and enzymes from microaggregate pores. Recent investigations showed higher microbial diversity and presence of accessible pore networks in microaggregates. The physico-chemical protection mechanism is more robust but monolayer or patchy adsorption of SOM onto clay surfaces requires further detailed research. The adsorption of SOM and exo-enzymes on pore walls and clay surfaces has been identified as a plausible concept of SOM stabilization.
Poorly crystalline iron and aluminium oxides contribute to the carbon saturation and sorption of dissolved organic carbon in the soil
Soil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (<53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (>15 g kg−1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil.
Soil organic carbon mineralization rates in aggregates under contrasting land uses
Measuring soil organic carbon (SOC) mineralization in macro-aggregates (250-2000 μm), micro-aggregates (250-53 μm) and the < 53 μm fraction helps to understand how spatial separation of SOC inside soil aggregates regulates its dynamics. We hypothesized that (i) compared with macro-aggregates SOC mineralization rate of micro-aggregates would be slower, (ii) adsorption of SOC on < 53 μm fraction decreases the SOC mineralization rate, and (iii) land use has a significant influence on SOC decomposition rate. To test these hypotheses we collected topsoil from Dermosol (Acrisols in FAO Soil Classification) sites under three contrasting land uses namely native pasture (NP), crop-pasture rotation (CP) and woodland (WL).
High water availability in drought tolerant crops is driven by root engineering of the soil micro-habitat
Improving our understanding of drought tolerance of crops is essential in light of future predicted changes in rainfall, decreased groundwater availability, and increasing temperatures. With a focus on above ground traits, significant improvements in drought tolerance of plants has occurred. With such gains plateauing, we have sought to quantify the belowground functional interactions between plant roots and soil in relation to drought tolerance. Using physical, chemical and biological approaches, we compared drought tolerant and sensitive model plants to demonstrate that a tolerant plant alters both the surrounding pore geometry and the relative abundance of bacteria and upregulates the development of a slow wetting rhizosheath, which increases water uptake under drought conditions. We propose that such rhizosheath traits can be targeted to modify the biophysical properties of the rhizosheath to access water in drought conditions.
Characterization of Soil Organic Matter in Aggregates and Size-Density Fractions by Solid State 13C CPMAS NMR Spectroscopy
Understanding the changes in soil organic matter (SOM) composition during aggregate formation is crucial to explain the stabilization of SOM in aggregates. The objectives of this study were to investigate (i) the composition of SOM associated with different aggregates and size-density fractions and (ii) the role of selective preservation in determining the composition of organic matter in aggregate and size-density fractions. Surface soil samples were collected from an Alfisol on the Northern Tablelands of NSW, Australia, with contrasting land uses of native pasture, crop-pasture rotation and woodland. Solid-state 13C cross-polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy was used to determine the SOM composition in macroaggregates (250-2000 μm), microaggregates (53-250 μm), and <53-μm fraction. The chemical composition of light fraction (LF), coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM), and mineral-associated soil organic matter (mSOM) were also determined. The major constituent of SOM of aggregate size fractions was O-alkyl carbon, which represented 44-57% of the total signal acquired, whereas alkyl carbon contributed 16-27%. There was a progressive increase in alkyl carbon content with decrease in aggregate size. Results suggest that SOM associated with the <53-μm fraction was at a more advanced stage of decomposition than that of macroaggregates and microaggregates. The LF and cPOM were dominated by O-alkyl carbon while alkyl carbon content was high in fPOM and mSOM. Interestingly, the relative change in O-alkyl, alkyl, and aromatic carbon between aggregates and SOM fractions revealed that microbial synthesis and decomposition of organic matter along with selective preservation of alkyl and aromatic carbon play significant roles in determining the composition of organic matter in aggregates.
Aggregate hierarchy and carbon mineralization in two Oxisols of New South Wales, Australia
The conventional model of aggregate formation suggests a hierarchy where micro-aggregates with lower porosity and therefore reduced soil organic carbon (SOC) mineralization form inside macro-aggregates. This model has however been questioned for highly weathered Oxisols where inconclusive results regarding the presence of aggregate hierarchy have been obtained to date. We hypothesized that in Oxisols (i) an aggregate hierarchy would be present (ii) the porosity of micro-aggregates would be lower than that of macro-aggregates and (iii) pore geometry of aggregates would influence SOC mineralization. We collected topsoils from Oxisols in northern New South Wales, Australia from which macro-aggregates (>250 μm), micro-aggregates (53-250 μm) and <53 μm fractions were isolated from bulk soil by wet sieving. 3D images of macro- and micro-aggregates were produced using X-ray computed tomography (μCT) showing the presence of micro-aggregates inside macro-aggregates, which confirmed the presence of an aggregate hierarchy in the Oxisols studied. Macro-aggregates were more common and SOC in higher concentrations in forest systems compared with agricultural (the cultivation or pasture) land-uses, but aggregate geometry differed little between the land-uses studied. The porosity of macro-aggregates (4%) was significantly lower than micro-aggregates (5.5%). Despite the differences in pore geometry between macro- and micro-aggregates, SOC mineralized (SOC'min') during a 2-month incubation (at 25°C) was similar in macro- (3% of SOC concentration) and micro-aggregates (2.8% of SOC concentration). We conclude that although aggregate hierarchy exists in these soils and that aggregate geometry did differ between aggregate size classes, there was no evidence to support the porosity exclusion principle and the assumption that SOC is preferentially stabilized within micro-aggregates in these soils.
Mean Residence Time of Soil Organic Carbon in Aggregates Under Contrasting Land Uses Based on Radiocarbon Measurements
Radiocarbon is a useful tool for studying carbon dynamics in soil aggregates. The objective of the current study was to determine the mean residence time (MRT) of soil organic carbon (SOC) in macroaggregates and microaggregates under contrasting land uses. Contrasting land uses investigated at Alfisol (equivalent to Dermosol in Australian Soil Classification) sites were native pasture (NP), crop-pasture rotation (CP), and Eucalypt woodland (WL), whereas in Oxisol (Ferrosol in Australian Soil Classification) sites, land uses comprised improved pasture (IP), cropping (CR), and forest (FR). Soil aggregates were separated into macroaggregates (250-2000 μm) and microaggregates (53-250 μm) by wet-sieving, and their 14C signatures were determined by accelerator mass spectrometry (AMS). The 14C activity in both macro- and microaggregates was >100 pMC, indicating the presence of post-bomb carbon in the soil. The mean residence time (MRT) of SOC in macro- and microaggregates (MRTagg) was on average 68 yr longer in the Oxisol compared with that in the Alfisol. The MRTagg in microaggregates was 10 yr longer than that of macroaggregates in the Alfisol. However, the MRTagg in microaggregates was 50 yr shorter compared to macroaggregates in the Oxisol. The MRT of macro- and microaggregates can be separated into active, slow, and stable SOC pools. Among the 3 SOC pools, the MRT of the stable pool is of higher significance in terms of SOC stabilization in soil aggregates because of its longer MRT. However, isolation and direct MRT estimation of the stable SOC pool is difficult. The MRT of active and slow SOC pools associated with macro- and microaggregates was measured using a SOC mineralization experiment to estimate the MRT of the stable SOC pool under contrasting land uses by applying a mass balance criterion. The MRT of active (MRTA) and slow (MRTS) SOC pools in macro- and microaggregates varied between 1-50 days and 13-38 yr, respectively. The estimated MRT of the stable pool carbon (MRTP) in microaggregates was 897 yr longer compared to that of macroaggregates in the Alfisol. However, in the Oxisol, MRTP in microaggregates was 568 yr shorter than that of macroaggregates. Among the land uses, WL in Alfisol and CR in Oxisol had longer MRTagg and MRTP compared to other land uses.