Aggregate hierarchy, a fundamental soil feature controlling various physical and biogeochemical processes, is well-studied for soils dominated by crystalline minerals but not for the soils rich in ...poorly-crystalline or short-range-order (SRO) minerals. We examined the presence and nature of aggregate hierarchy in the surface horizon of an Andisol which is characterized by high concentrations of organic matter and SRO minerals (esp. allophane/imogolite). Several pretreatments were tested to achieve maximum dispersion and the particle-size fractions isolated (<0.2 up to >53μm) were characterized by SEM, XRD, and elemental analyses. Mass distribution as well as physical and chemical characteristics among the isolated size fractions were strongly influenced by the dispersion level, indicating the importance to find optimal dispersion level. Upon minimum dispersion (wet sieving), over 95% of total C was present as >53μm aggregates and the C concentration and C:N ratio remained constant across all size fractions. Even after mechanical shaking, >53μm and 2–53μm fractions accounted for 37% and 41% of total C, respectively. Maximum dispersion was achieved only after sodium saturation pretreatment followed by the sonication at the energy level 5–10 folds higher than normally required for non-volcanic soils (5kJmL−1). While dissolving minor levels of organic matter and metals, the maximum dispersion treatment liberated large amounts of small particles (<2μm) that were enriched in N-rich organic matter, SRO mineral, and organo-metallic complex and that accounted for 48% of total mass, 63% of C, N, and 72 to 91% of extractable phases of Al, Si, and Fe by pyrophosphate and acid-oxalate. The step-wise breakdown of macro- and micro-aggregates with concurrent liberation of micron- to submicron-size particles upon the increased levels of dispersion, therefore, represents clear evidence of aggregate hierarchy on this soil type. Due to their abundance and chemistry, the small particles (<0.2 and 0.2–2.0μm fractions) likely acted as strong binding agents to form both micro- and macro-aggregates. Significant positive correlation of organic matter concentration and C:N ratio against the extractable metal concentrations was found when samples of all size fractions and dispersion treatments were combined. We proposed a simple conceptual model for Andisol aggregate hierarchy to explain the observed variation in the abundance and chemistry of isolated fractions.
•Strong evidence of aggregate hierarchy in Andisol was shown for the first time.•Clay-sized particles accounted for roughly half of total C and soil solid volume.•Amorphous clay, N-rich OM and organo-metal complex were enriched in these particles.•These organo-mineral particles contributed to both micro- and macro-aggregation.•A conceptual model of aggregate hierarchy for Andisol was proposed.
Strong correlations between iron oxides (FeOx) and organic matter (OM) in soils have implied the importance of the former in stabilizing the latter. One mechanism thought to be important in this ...stabilization is sorption. We tested this possibility by reductively dissolving FeOx in a wide variety of soils and measuring the organic carbon (OC) that was solubilized. The OC dissolved from non-FeOx phases via anion exchange was corrected for by parallel control extractions. The resultant pool, reductively soluble OC, made up a minor amount of total soil OC in all but one of these soils, indicating that simple sorption reactions do not stabilize the bulk of soil OC in most mineral soils. OC:Fe ratios in the extracts from 2/3 of these soils were less than 0.22 (wt/wt), consistent with a sorbed state for this OC and showing that OC sorption by FeOx in these soils is limited by the amount of FeOx. The remaining soils had low pH and high OM concentrations; their higher OC:Fe ratios indicate inclusion of precipitated organo-Fe complexes in the extracts, which are likely only partially extracted by our method. The high volumetric ratios of OM to FeOx found in correlations between them from the literature are inconsistent with a dominant sorption control and point instead to stabilization to other mechanisms such as organo-Fe complexes or ternary associations among FeOx, OM and other minerals.
Clay-size minerals play important roles in terrestrial biogeochemistry and atmospheric physics, but their data have been only partially compiled at global scale. We present a global dataset of ...clay-size minerals in the topsoil and subsoil at different spatial resolutions. The data of soil clay and its mineralogical composition were gathered through a literature survey and aggregated by soil orders of the Soil Taxonomy for each of the ten groups: gibbsite, kaolinite, illite/mica, smectite, vermiculite, chlorite, iron oxide, quartz, non-crystalline, and others. Using a global soil map, a global dataset of soil clay-size mineral distribution was developed at resolutions of 2' to 2° grid cells. The data uncertainty associated with data variability and assumption was evaluated using a Monte Carlo method, and validity of the clay-size mineral distribution obtained in this study was examined by comparing with other datasets. The global soil clay data offer spatially explicit studies on terrestrial biogeochemical cycles, dust emission to the atmosphere, and other interdisciplinary earth sciences.
Ecosystem property and function can be considerably altered by changes in agricultural land use. Despite the expansion of abandoned fields in temperate humid areas worldwide, little is known about ...how ecosystem C stock and its partitioning change with secondary succession, especially under the invasion of non-native, fast-growing plants. Using a paired-plot approach, we investigated how plant community type (native vs. non-native, the latter represented by Solidago altissima) influences temporal changes in ecosystem C pools in abandoned paddy fields. Thirty-seven plots of control (paddy) and abandoned (2–37 years of succession) pair having similar soil types under the humid temperate climate of Japan were selected for this study. We detected a net loss of ecosystem C in the first 15.2 years, followed by a gradual C gain. Apparent maximal C gain (up to 10 Mg C ha⁻¹) was observed approximately 30 years after abandonment, based on a smooth spline model. The initial C loss was largely attributed to decomposition in mineral-associated soil C, owing presumably to the rapid decay of the C accumulated under temporally anaerobic paddy. While no detectable difference was found in the ecosystem C stock between plots dominated by S. altissima and native plants, the S. altissima-dominated plots showed significantly lower litter C stock. We therefore conclude that, despite the rapid growth and high aboveground biomass of S. altissima, its invasion did not increase ecosystem C storage in the studied region; these findings highlight the absence of ecological benefits associated with S. altissima invasion.
Drought is a major risk in global agriculture. Building-up soil organic carbon (SOC) enhances soil fertility and efficient use of rainwater, which can increase drought tolerance in food production. ...SOC management demonstrates its benefit at various locations and is a promising means to achieve food security and climate mitigation at once. However, no global assessment of its potential and co-benefits gained from SOC enhancement has been presented. Here we evaluated the extent to which SOC build-up could reduce agricultural drought risk. Using statistical analysis of spatially-explicit global crop and soil datasets, we find that relatively small enhancement in topsoil (0-30 cm) organic carbon content (OC
) could increase drought tolerance of the food production systems operating over 70% of the global harvested area (particularly drylands). By closing the gap between current and upper limit of tolerance levels through SOC addition of 4.87 GtC at the global scale, farmer's economic output in drought years would increase by ~16%. This level of SOC increase has co-benefit of reducing global decadal mean temperature warming by 0.011 °C. Our findings highlight that progress towards multiple development goals can be leveraged by SOC enhancement in carbon (C)-poor soils in drier regions around the world.
Identifying the soil properties that control soil organic carbon (SOC) storage is a critical step to monitor and manage SOC at regional and global scales. While clay content has been commonly used to ...explain SOC variation and to model SOC dynamics, recent studies point to the significant control of extractable metal phases such as pyrophosphate extractable Al (Al
p
) for a range of soil types. Phosphate adsorption coefficient (PAC), soil's capacity to adsorb phosphate, is known to show strong positive correlation with Al
p
. The soil database maintained by the national farmland fixed-point survey contains basic soil attributes including SOC and PAC and covers approximately 20,000 sites that belong to Andosols and Lowland soil great groups, representing roughly 70% of the arable land in Japan (MAT range from 5.3 to 22.3 °C). Using the database, we found that PAC showed more significant positive correlation with SOC compared to clay and silt-clay contents for both Lowland soils and Andosols. The OC:Al
p
ratio was estimated from the slope of SOC:PAC regression line for all soil groups and great groups ranged from 3.35 to 10.5 (mol:mol), which was within the range shown in previous studies on volcanic and spodic soils. The ANCOVA test further revealed that the SOC-PAC relationship was significantly affected by soil temperature regime in both soil group and soil great group levels (both p < 0.0001). All four soil groups within Lowland soils showed a similar temperature-dependent trend in the SOC:PAC ratio with little variation among the groups. The soil groups within Andosols, however, had no clear temperature trend, suggesting the influence of other factors that remain to be elucidated in future. We concluded that PAC was the more effective indicator of SOC than clay content for the major arable soils in Japan and that the PAC-SOC relationship was partly controlled by soil temperature and soil group. Further study to assess the role of Al
p
and the contribution of other factors such as land use and management type is important as major portions of the SOC variation was not explained by PAC.
Beyond clay Rasmussen, Craig; Heckman, Katherine; Wieder, William R. ...
Biogeochemistry,
02/2018, Letnik:
137, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial ...carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO₂ to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.
Nitrite is an important precursor of many environmentally hazardous compounds (e.g., nitrate, nitrous oxide, and nitrous acid). However, its dynamics in the soil environment are not yet fully ...understood. The NtraceNitrite tool has been successful in analyzing 15N tracing data. Here, based on a 15N tracing experiment (under aerobic condition) where either the nitrite, the nitrate, or the ammonium pool was labelled, we developed an extended model (NO2Trace), which was featured by the addition of coupled nitrate reduction and nitrite re-oxidation and the separation of the nitrate pool in two sub-pools. With 5 additional parameters optimized, NO2Trace was able to achieve a superior fit to the data, as compared to the NtraceNitrite tool. The additional features might offer a suitable explanation for the isotopic composition of nitrate produced via nitrification in terrestrial ecosystems. Our results carry two important implications: (i) a key assumption of the classical isotope pool dilution technique (i.e., no reflux of tracer) for estimating gross nitrate fluxes is violated, leading to considerable underestimations (22–99% in the datasets tested); (ii) re-oxidation can dominate the consumption (∼75%) of nitrite derived from nitrate reduction, indicating the potential of this process as a target for nitrogen retention mechanism against gaseous nitrogen losses (through nitrite reduction). The additional features of the extended model show a tighter cycle between soil nitrite and nitrate than considered previously and provide a more comprehensive description of soil nitrite transformations. This study also highlights that more work is needed to develop methods capable of separating process- and pathways-specific nitrate and nitrite pools.
•Soil NO2− dynamics were traced using triplet 15N-labelled NO2−, NH4+, and NO3−.•We developed a novel process-based model for simulating soil NO2− dynamics.•The new model fits better to our and a published dataset than the existing NtraceNitrite.•Our results revealed a tighter cycle between NO2− and NO3− than considered previously.•Classical isotopic pool dilution risks underestimating gross NO3− fluxes.
Organic matter (OM) can be protected from abiotic and biotic breakdown via its association with iron (Fe) and aluminum (Al) in both terrestrial and aquatic ecosystems. We hypothesized that the ...natural variation in sediment redox and pH regime govern how the two metals interact with OM in near-surface mineral sediments of 40 kettle holes of varying hydroperiod in NE Germany. Sediments were separated by density, and Fe and Al present in the high-density fractions (> 1.6 g cm
−3
, HF) were quantified by pyrophosphate (PP) and dithionite-citrate (DC) extractions. The OM in HF was analyzed for δ
13
C and δ
15
N values before and after the extractions to assess the possible origin and its degradation state. While only < 20% of OM in HF were dissolved by PP and DC extractions, OC in HF showed significant positive correlation with the extractable Fe and Al. DC-extractable Fe was predominantly present as low-crystallinity phase, and mostly overlapped with PP-extractable and, for the selected samples examined, with acid-oxalate and ascorbic acid extractable Fe pool. Across the HF samples, high levels of Al
PP
and Al
DC
contents corresponded to sediments having lower pH and OM with lower δ
15
N and higher C:N ratio. In contrast, the sediments with higher Fe
DC
contents corresponded to shorter hydroperiod and showed OM with higher δ
15
N and lower C:N ratio. These results support the idea that reactive monomeric Al preferentially binds with organic ligands derived from less-decomposed OM under more acidic and anoxic conditions, whereas low-crystallinity Fe formed under more oxic conditions tend to bind with more microbially-processed OM.
Understanding the controls on the amount and persistence of soil organic carbon (C) is essential for predicting its sensitivity to global change. The response may depend on whether C is unprotected, ...isolated within aggregates, or protected from decomposition by mineral associations. Here, we present a global synthesis of the relative influence of environmental factors on soil organic C partitioning among pools, abundance in each pool (mg C g−1 soil), and persistence (as approximated by radiocarbon abundance) in relatively unprotected particulate and protected mineral‐bound pools. We show that C within particulate and mineral‐associated pools consistently differed from one another in degree of persistence and relationship to environmental factors. Soil depth was the best predictor of C abundance and persistence, though it accounted for more variance in persistence. Persistence of all C pools decreased with increasing mean annual temperature (MAT) throughout the soil profile, whereas persistence increased with increasing wetness index (MAP/PET) in subsurface soils (30–176 cm). The relationship of C abundance (mg C g−1 soil) to climate varied among pools and with depth. Mineral‐associated C in surface soils (<30 cm) increased more strongly with increasing wetness index than the free particulate C, but both pools showed attenuated responses to the wetness index at depth. Overall, these relationships suggest a strong influence of climate on soil C properties, and a potential loss of soil C from protected pools in areas with decreasing wetness. Relative persistence and abundance of C pools varied significantly among land cover types and soil parent material lithologies. This variability in each pool's relationship to environmental factors suggests that not all soil organic C is equally vulnerable to global change. Therefore, projections of future soil organic C based on patterns and responses of bulk soil organic C may be misleading.
In the first global meta‐analysis to examine both radiocarbon and C concentrations among different soil C pools, we found that three critical carbon pools (free particulate, occluded particulate, and mineral associated) respond differently to climate. Moisture had an almost equal influence as temperature on C persistence and abundance, highlighting the need for climate change studies focused on moisture manipulations. The strong variation in pool characteristics and their relationship to environmental factors indicates that we need to go beyond bulk soil carbon measurements to understand and model the responses of soil organic carbon to global change; it is critical to evaluate distinct pools as response variables.
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