Mineral-associated Organic Matter Research Articles

A method for soil quality assessment in the metropolitan greenery: A comprehensive view of ecosystem services and soil functions

The soils in urban greenery considerably contribute to providing ecosystem services. However, appropriate tools to assess and manage urban soil quality under consideration of ecosystem services and soil functions are unavailable. In this study, we aimed to 1) provide detailed instructions for assessing a novel urban soil quality index (uSQI) and 2) propose the application of uSQI in urban soil quality management. The uSQI is the average of the scores for six soil functions. Each soil function was estimated by several measurable indicators that have high correlation with soil functions. The measurable soil indicators are bulk density, saturated hydraulic conductivity, litter-layer depth, mineral-associated organic matter, clay + silt content, inorganic nitrogen concentration, fluorescein diacetate hydrolytic activity, cation exchange capacity, concentrations of potentially toxic elements, and pH. The uSQI effectively identified the soils with low quality due to disturbances. The radar chart of six soil functions comprising uSQI could suggest the direction of management for urban stakeholders.•The uSQI represents soil functions necessary for urban greenery to provide ecosystem services.•The uSQI successfully identified the soils with low quality due to disturbances.

A Conceptual Framework on the Fate of Rhizodeposits in Forming Mineral-Associated Organic Matter or Encapsulating Into Microaggreagtes

Rhizodeposition, as transported from photosynthates and exudated in soils via fine roots, is the pivot linking above- and belowground carbon (C) cycling pathways. Meanwhile, rhizodeposit C serves as “currency” for plant nutrient acquisition because of its critical roles in priming soil microorganisms, maintaining plant-mycorrhizal symbionts, and elongating plant roots. Therefore, a conceptual framework integrating knowledge on the biogeochemical fate of rhizodeposit C can help understand plant nutrient economics and soil C sink function. However, it still remains a great challenge to efficiently delineate the dynamics of rhizodeposit C in soils. In the framework, we present the possible stabilization pathways of rhizodeposit C via formation of mineral-associated organic matter (MAOM) or encapsulation by microaggregates. We further propose that continuous and pulse 13CO2 labeling are powerful techniques to track the fate of rhizodeposit C and to quantify how much C could eventually be sequestrated in soils as the component of MAOM or microaggregates. This framework would provide future research possibilities to better optimize plant C allocation and productivity and preserve soil C stocks.

Soil Strength and Structural Stability Are Mediated by Soil Organic Matter Composition in Agricultural Expansion Areas of the Brazilian Cerrado Biome

A growing demand for resources has led to the expansion of agricultural areas worldwide. However, land conversion associated with poor soil management might lead to soil physical degradation. We investigated the effects of land conversion on soil physical properties in the Brazilian Cerrado region, under native Cerrado vegetation (NV)—pasture (PA) and NV—cropland (CL) conversion scenarios. Soil physical properties related to compaction, pore size distribution, and structure stability were assessed up to a 30 cm depth. Additionally, carbon levels of soil organic matter fractions (particulate and mineral-associated organic matter) were determined. Our results indicate that the compaction process equivalently reduced the soil porosity in PA and CL. However, soil penetration resistance was higher in PA (~2.5 MPa) than in CL (~1.5 MPa), as well as the stable mean weight diameter of soil aggregates. The highest total and labile organic carbon levels were observed in CL, while the lowest levels of total and labile organic carbon occurred in PA (smaller than in CL). These results suggest that the higher structural stability found in PA was mediated by the predominance of stabilized carbon (a decrease in the proportion of soil labile carbon), causing the gaining of soil strength under negligible soil volume variation (in comparison with CL). Our results suggest that the reduction in the soil porosity by compaction due to PA and CL uses can equivalently reduce macropore space and soil hydraulic functioning, and that soil carbon quality alterations (i.e., labile vs. stabilized fractions) are responsible for the gain in soil strength in long-term degraded PA areas. Future research should focus on understanding the magnitude in which soil organic matter controls soil physical attributes, such as soil strength in these expansion areas, and whether this gain in soil strength limits plant development and compromises productivity in the long term.

Microscale spatial distribution and soil organic matter persistence in top and subsoil

The spatial distribution of organic substrates and microscale soil heterogeneity significantly influence organic matter (OM) persistence as constraints on OM accessibility to microorganisms. However, it is unclear how changes in OM spatial heterogeneity driven by factors such as soil depth affect the relative importance of substrate spatial distribution on OM persistence. This work evaluated the decomposition and persistence of 13C and 15N labeled water-extractable OM inputs over 50 days as either hotspot (i.e., pelleted in 1–2 mm-size pieces) or distributed (i.e., added as OM < 0.07 μm suspended in water) forms in topsoil (0–0.2 m) and subsoil (0.8–0.9 m) samples of an Andisol. We observed greater persistence of added C in the subsoil with distributed OM inputs relative to hotspot OM, indicated by a 17% reduction in cumulative mineralization of the added C and a 10% higher conversion to mineral-associated OM. A lower substrate availability potentially reduced mineralization due to OM dispersion throughout the soil. NanoSIMS (nanoscale secondary ion mass spectrometry) analysis identified organo-mineral associations on cross-sectioned aggregate interiors in the subsoil. On the other hand, in the topsoil, we did not observe significant differences in the persistence of OM, suggesting that the large amounts of particulate OM already present in the soil outweighed the influence of added OM spatial distribution. Here, we demonstrated under laboratory conditions that the spatial distribution of fresh OM input alone significantly affected the decomposition and persistence of OM inputs in the subsoil. On the other hand, spatial distribution seems to play a lower role in topsoils rich in particulate OM. The divergence in the influence of OM spatial distribution between the top and subsoil is likely driven by differences in soil mineralogy and OM composition.

Utilizing Soil Density Fractionation to Separate Distinct Soil Carbon Pools.

Soil organic matter (SOM) is a complicated mixture of different compounds that span the range from free, partially degraded plant components to more microbially altered compounds held in the soil aggregates to highly processed microbial by-products with strong associations with reactive soil minerals. Soil scientists have struggled to find ways to separate soil into fractions that are easily measurable and useful for soil carbon (C) modeling. Fractionating soil based on density is increasingly being used, and it is easy to perform and yields C pools based on the degree of association between the SOM and different minerals; thus, soil density fractionation can help to characterize the SOM and identify SOM stabilization mechanisms. However, the reported soil density fractionation protocols vary significantly, making the results from different studies and ecosystems hard to compare. Here, we describe a robust density fractionation procedure that separates particulate and mineral-associated organic matter and explain the benefits and drawbacks of separating soil into two, three, or more density fractions. Such fractions often differ in their chemical and mineral composition, turnover time, and degree of microbial processing, as well as the degree of mineral stabilization.

Litter quality and living roots affected the formation of new mineral-associated organic carbon but did not affect total mineral-associated organic carbon in a short-term incubation

Storing carbon (C) in soil organic matter (SOM) is critical for mitigating climate change as well as for providing food security under extreme weather events. Mineral-associated organic matter (MAOM), which is the organic matter attached to silt and clay, has a longer residence time than the light, sand-sized particulate organic matter (POM). Hence, increasing the C in this fraction is thought to be important for long-term C storage. However, the influence of litter quality and living plants on new MAOM formation and MAOM turnover is unclear. We hypothesized that lower C:N litter would form MAOM more efficiently and result in more MOAM than higher C:N litter because lower C:N litter favors biomass production of microbes, which are thought to be an important precursor of MAOM. In addition, we hypothesized that living plants would amplify the effect of litter quality on MAOM formation and turnover because roots growing with low C:N litter would be less N-stressed and rely less on MAOM turnover to access N than roots growing in high C:N litter. We conducted a greenhouse study where soil was mixed with four different plant litter treatments: a control without litter, soybean (Glycine max) with C:N = 21, buckwheat (Fagopyrum esculentum) with C:N = 35, and sudex (Sorghum bicolor [L.] Moench × S. sudanense [P.] Staph.) with C:N = 65. The four litter treatments were incubated with and without living oats (Avena sativa). The incubation was maintained for 40 days, and over this period soybean and buckwheat lost 89 % of their initial dry matter while sudex lost 69 % of its initial dry matter. Contrary to our expectation, the litter with the higher C:N (sudex) had the greatest proportion of degraded C transferred to MAOM (i.e., the litter MAOM-C transfer efficiency). However, the litter MAOM-C transfer efficiency was not associated with the total MAOM-C at the end of the incubation. The presence of living plants increased the amount of MAOM-C derived from soybean and sudex litter, but it did not increase the total MAOM-C. Thus, our results demonstrate that high C:N litter can efficiently form new MAOM-C, which can be enhanced by the presence of living plants, but the greater MAOM-C formation may be accompanied by a loss of pre-existing MAOM-C.

Residual nitrogen from slurry and mineral fertiliser two years after application: Fractionation and plant availability

Fertilisation with animal manure has a long-standing tradition as a way to close nutrient cycles on farms. However, the fate of nitrogen (N) from manure in the soil beyond the year of application remains poorly understood. The aim of this research was to understand the residual N fertiliser value of mineral fertiliser (Min) and cattle slurry (Slu) in relation to the partitioning of N from the fertilisers in different soil physical fractions. To this end, we characterised the fate of 15N-labelled Min and Slu in soil organic matter (SOM) physical fractions two years after field application and related it to plant uptake of residual N. A secondary objective was to compare two fractionation methods with respect to accuracy and easiness, namely a combined density and particle size fractionation, separating five fractions, and a simple particle size fractionation, separating two fractions. All fractions were analysed for 15N abundance. The residual N fertiliser value was determined as the uptake of 15N by ryegrass (Lolium multiflorum) during 6 weeks. Furthermore, we deduced the source of 15N taken up by the plants from changes in the percentage of N derived from labelled fertilisers in each of the SOM physical fractions obtained by simple size fractionation before and after the pot experiment. Two years after application, most 15N was found in the fractions <20 μm, for both fractionation methods, i.e. in the mineral-associated organic matter (MAOM). The 15N recovery in these fractions in percent of the quantity of 15N originally applied as fertiliser tended to be higher for Slu (density-size fractionation: 19.8 ± 6.6%; simple size fractionation: 25.1 ± 6.6%) than for Min (density-size fractionation: 12.0 ± 2.1%; simple size fractionation: 16.8 ± 2.6%). Irrespective of the fertiliser type, about 2% of applied 15N was available to ryegrass plants two years after application. Out of these 2%, most 15N in the plants originated from MAOM, suggesting that the availability of the N stored in MAOM could be larger than previously thought. This finding is in line with the emerging view that MAOM is a dynamic fraction that plays an important role in the N cycle.

Labile carbon input and temperature effects on soil organic matter turnover in subtropical forests

The objective of this study was to assess the effects of labile organic carbon (LOC) input on the dynamic turnover of soil organic matter (SOM) with temperature in subtropical forests. Soil samples were collected from three subtropical forests: an evergreen broadleaf forest, a mixed pine and broadleaf forest, and a pine forest. To simulate LOC inputs, soil samples were supplemented with 13C-glucose at levels of 0, 0.2, 0.6, and 1.2 mg C g−1 soil. The soils with the LOC treatments were incubated for six weeks at temperatures of 15, 25, and 35 °C. Results showed that the increase in LOC inputs reduced the priming effects of SOM decomposition significantly, whereas native SOM mineralization increased with temperature. The preferential utilization of LOC by microorganisms reduced SOM decomposition, resulting in net SOC increases with LOC inputs. In the temperature range, the net increases in SOC with the LOC inputs were −0.19 to 0.56 mg C g−1 soil. Moreover, the LOC inputs aided microbial anabolism and promoted the formation of new mineral-associated organic matter and the aggregation of particulate organic matter, resulting in carbon sequestration. Among the forest soils, the soil with the highest organic carbon content, total nitrogen, and carbon to nitrogen ratio (i.e., the mixed forest soil) had the lowest priming effects and was the most conducive to net carbon increase. The priming and carbon sequestration caused by the LOC inputs were not sensitive to temperature changes. Our findings suggest that the increase in the supply of LOC has the potential to alleviate SOM decomposition and increase carbon storage in subtropical forest soils.

Insights from size fractions to interpret the erosion-driven variations in soil organic carbon on black soil sloping farmland, Northeast China

Declines of soil organic carbon (SOC) and degradation of soil function are caused by long-term soil erosion and cultivation. Although the variations of bulk SOC have been extensively studied, the driving mechanisms of variations in SOC fractions, i.e., carbon in coarse particulate, fine particulate, and mineral-associated organic matter (CPOM-C, FPOM-C, and MAOM-C), and their relationship with soil erosion remain unclear on sloping farmland. In this study, two areas of farmland with different slope types (convex vs. straight) and reclamation periods (50 years vs. ≥100 years) in the black soil region of Northeastern China were chosen to explore the roles of soil erosion on the variations of SOC and their size fractions based on the 137Cs tracer technique. The results showed that the average net erosion rate of the straight slope was 2810.09 t km-2a-1 and the convex slope was 3801.71 t km-2a-1 with higher spatial variability. When compared with uncultivated land, SOC contents of convex and straight slopes have decreased by 13.58% and 35.18%, with a reduced rate of 0.089 g kg-1 a-1 and 0.116 g kg-1 a-1, respectively. Although the CPOM-C fraction showed the most significant relative decreases (over 73.24%), the absolute declines of MAOM-C were more significant, especially in the straight slope with a more extended cultivation period. The contributions of erosion to the SOC decreases on convex and straight slopes were 69.54% and 13.09%, respectively. Erosion exerted a significant influence on finer particles such as FPOM-C and MAOM-C. With increasing cumulative erosion, the absolute magnitude of each SOC fraction in soils decreased. But the proportion of MAOM-C increased, while FPOM-C and CPOM-C decreased. Overall, our findings demonstrated that soil erosion not only induced the loss of SOC fractions but also transformed the proportional distribution of their fractions, and then affected the functions of SOC. Moreover, the effects of tillage (i.e., an increase in microbial activity) on straight slopes are not negligible in farmland that has experienced long-term cultivation but relatively little erosion. Apart from erosion reduction in areas of severe erosion, appropriate fertilization for farmland with a long history of cultivation should also be adapted to achieve sustainable development of black soil resources.

Native multispecies and fast-growing forest root biomass increase C and N stocks in a reclaimed bauxite mining area.

This study is aimed at evaluating C and N stocks in fractions of soil organic matter (SOM) in an area of bauxite mining under recovery with tree species. We have analyzed the long-term recovery of C and N stocks of organic matter fractions from five types of forest cover (Eucalyptus, Anadenanthera peregrina, mixed plantation of 16 native species, a mined area without vegetation cover as a control site, and a natural forest cover as a reference site). The total organic C (TOC) and N (TN) stocks and also organic matter fractions, particulate organic matter (POM), mineral-associated organic matter (MAOM), microbial biomass (MB), and labile C (LC), were determined, as well as the C/N ratio and the carbon management index (CMI). Although the stocks of TOC and LC, CMI, and MB did not differ between the types of forest cover in the 0-60 cm layer, they were lower than the values in the native forest. Forest cover increases the stocks of TOC, LC, MB, and CMI in the area of bauxite mining compared to the control site. In addition, we found that the TOC C and TN stocks and also SOM fractions (LC, C-MAOM, C-POM, N-MAOM, and N-POM) are positively correlated (r ≥ 0.71 for all cases) with volume of roots larger than 2 mm. Therefore, Eucalyptus, A. peregrina, and a mixed of 16 native trees contribute for restoring stocks of soil C and N following bauxite mining in the Brazilian Atlantic Forest.

Latitudinal patterns of particulate and mineral-associated organic matter down the soil profile in drylands

Understanding the controls on particulate organic matter (POM) and mineral-associated organic matter (MAOM) accumulation is essential in order to accurately predict carbon-climate feedbacks and for ecosystem management. However, how POM and MAOM fractions vary across latitudinal gradients in dryland ecosystems, and what drives this variation remains unclear. In this study, we sampled soils from 100 natural shrubland sites across a latitudinal gradient (23°N to 32°N) in the dry valleys of southwestern China to identify latitudinal patterns of POM and MAOM fractions at four soil depths (0–10 cm, 10–20 cm, 20–30 cm, and 30–50 cm). We found that both C and N fractions in POM and MAOM exhibited a binomial trend with latitude at all depths, whereas the N fractions in POM and MAOM exhibited a pronounced increase at higher latitudes in subsurface soils (20–30 cm and 30–50 cm). Variation in C fractions (i.e., particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)) were mainly explained by the independent effect of soil properties, with the most important explanatory factor being exchangeable calcium. In contrast, variation in N fractions (i.e., particulate organic nitrogen (PON) and mineral-associated organic nitrogen (MAON)) were largely explained by interactions between soil , climate and vegetation properties, with the most important driver being mean annual temperature and mean annual precipitation for PON, exchangeable calcium for MAON at 0–10 cm and 10–20 cm depths, and mean annual temperature for MAON at 20–30 cm and 30–50 cm soil depths. Interestingly, climate was more important in explaining variation in C and N fractions in the subsurface soils, and climate and vegetation properties had higher predictive power for soil N fractions than C fractions in the subsurface soils. These results advance our understanding of the role of soil properties on SOM accumulation in dryland ecosystems. Our study also highlights the important impact of climate and vegetation properties in explaining the spatial variation of SOM in subsurface soils, which should be incorporated into large-scale assessments to better predict SOM dynamics.

Effects of exogenous organic/inorganic nitrogen addition on carbon pool distribution and transformation in grassland soil

AimsIncreases in nitrogen (N) deposition may significantly affect the organic carbon (OC) cycle in soil. The inconsistent findings of the influence of added N on soil OC pools highlight the need of quantifying responses of the OC pool distribution to N addition. Moreover, the influence of N addition with a mixture of organic and inorganic N on OC pool distribution and stabilization in grassland soil remains unclear. MethodsWe carried out a five-year field experiment with adding N to examine the effects of different types of N addition on soil OC pool distribution and transformation in a meadow steppe in Inner Mongolia. We applied N in the ratios of inorganic N (IN) and organic N (ON) at 10:0 (N1), 7:3 (N2), 5:5 (N3), 3:7 (N4), 0:10 (N5), and 0:0 (CK), respectively. We measured OC content in bulk soil, particulate organic matter (POM), and mineral-associated organic matter (MAOM) fractions. Additionally, a short-term soil incubation was conducted to assess potential OC mineralization. ResultsOur study showed no significant effect on soil organic carbon content of different ratios of IN/ON addition. N addition reduced microbial biomass C/N ratio, the fraction of mineral-associated organic matter, cumulative CO2 emission, and microbial metabolic quotient. Compared with ON addition alone, IN addition alone showed a stronger effect on the C in different soil fractions and soil OC mineralization. The particulate organic matter (POM) fraction was more sensitive to N addition than the mineral-associated organic matter (MAOM) fraction. ConclusionsOur results suggest that the contribution of N in organic and inorganic forms affecting OC pool distribution with different turnover rates should be considered when assessing the effects of N addition types on soil OC processes in grassland.

Plant litter chemistry controls coarse‐textured soil carbon dynamics

Abstract As soils store more carbon (C) than the Earth's atmosphere and terrestrial biomass together, the balance between soil C uptake in the form of soil organic matter (SOC) and release as CO2 upon its decomposition is a critical determinant in the global C cycle regulating our planet's climate. Although plant litter is the predominant source of C fuelling both soil C build‐up and losses, the issue of how litter chemistry influences this balance remains unresolved. As a contribution to solving that issue, we traced the fate of C during near‐complete decomposition of 13C‐labelled leaf and root litters from 12 plant species in a coarse‐textured soil. We separated the soil organic carbon into mineral‐associated organic matter (MAOM) and particulate organic matter (POM) pools, and investigated how 14 litter chemical traits affected novel SOC formation and native SOC mineralization (i.e. the priming effect) in these soil fractions. We observed an overall net increase in SOC due to the addition of litter, which was stronger for root than for leaf litters. The presumed stable MAOM‐C pool underwent both substantial stabilization and mineralization, whereas the presumably less stable POM‐C pool showed substantial stabilization and reduced mineralization. Overall, the initial increase in soil C mineralization was fully counterbalanced by a later decrease in native soil C mineralization. POM‐C formation as well as MAOM‐C formation and mineralization were positively related to the initial litter lignin concentration and negatively to that of the nitrogen leachates, whereas the opposite was observed for POM‐C mineralization. Synthesis. Our results highlight the importance of litter chemical traits for SOC formation, and stabilization, destabilization and mineralization. In our coarse‐textured soil, the amount of MAOM‐C did not change despite large C fluxes through this pool. The litter chemical traits that drove these processes differed from those frequently reported for fine‐textured soils far from mineral‐associated C saturation. To account for these discrepancies, we propose an integrative perspective in which litter quality and soil texture interactively control soil C fluxes by modulating several SOC stabilization and destabilization mechanisms. Irrespective, our results open new critical perspectives for managing soil C pools globally.

Oxygen availability regulates the quality of soil dissolved organic matter by mediating microbial metabolism and iron oxidation

Dissolved organic matter (DOM) plays a vital role in biogeochemical processes and in determining the responses of soil organic matter (SOM) to global change. Although the quantity of soil DOM has been inventoried across diverse spatio-temporal scales, the underlying mechanisms accounting for variability in DOM dynamics remain unclear especially in upland ecosystems. Here, a gradient of SOM storage across 12 croplands in northeast China was used to understand links between DOM dynamics, microbial metabolism, and abiotic conditions. We assessed the composition, biodegradability, and key biodegradable components of DOM. In addition, SOM and mineral-associated organic matter (MAOM) composition, soil enzyme activities, oxygen availability, soil texture, and iron (Fe), Fe-bound organic matter, and nutrient concentrations were quantified to clarify the drivers of DOM quality (composition and biodegradability). The proportion of biodegradable DOM increased exponentially with decreasing initial DOM concentration due to larger fractions of depolymerized DOM that was rich in small-molecular phenols and proteinaceous components. Unexpectedly, the composition of DOM was decoupled from that of SOM or MAOM, but significantly related to enzymatic properties. These results indicate that microbial metabolism exhibited a dominant role in DOM generation. As DOM concentration declined, increased soil oxygen availability regulated DOM composition and enhanced its biodegradability mainly through mediating microbial metabolism and Fe oxidation. The oxygen-induced oxidation of Fe(II) to Fe(III) removed complex DOM compounds with large molecular weight. Moreover, increased oxygen availability stimulated oxidase-catalyzed depolymerization of aromatic substances, and promoted production of protein-like DOM components due to lower enzymatic C/N acquisition ratio. As global changes in temperature and moisture will have large impacts on soil oxygen availability, the role of oxygen in regulating DOM dynamics highlights the importance of integrating soil oxygen supply with microbial metabolism and Fe redox status to improve model predictions of soil carbon under climate change.

Natural revegetation over ∼ 160 years alters carbon and nitrogen sequestration and stabilization in soil organic matter on the Loess Plateau of China

Natural revegetation has been reported to play a very active role in ecosystem carbon (C) and nitrogen (N) sinks in degenerated ecosystems. However, the responses of C and N sequestration and stabilization in soil organic matter (SOM) to natural revegetation remain inadequately understood. In this study, we analyzed C and N contents and δ13C and δ15N values of SOM, free light fraction (FLF), intra-aggregate particulate organic matter (IPOM) and mineral-associated organic matter (MAOM) along ∼ 160 years of natural revegetation on the Loess Plateau of China. The results showed that natural revegetation significantly (P < 0.05) enhanced soil C and N contents in FLF, IPOM, MAOM, SOM in the surface soil (0–20 cm) during the later stages of revegetation, which exhibited smaller impact on the deeper soil (20–60 cm). Natural revegetation significantly increased distribution proportions of C and N in FLF (i.e., 18.00–36.00%, 8.46–22.57%, respectively), and that in IPOM (i.e., from 11.60 to 25.38%, 10.89–26.92%, respectively), while it decreased that in MAOM (i.e., from 56.25 to 37.00%, 67.69–50.24%, respectively) in the surface soil, thereby altering C and N stabilization in SOM (0–20 cm). The climax Quercus liaotungensis forest exhibited the highest C distribution proportion in FLF and IPOM, and the lowest C distribution proportion in MAOM in the surface soil. The δ13C and δ15N were enriched with the decomposition of SOM and soil profile depths at each revegetation stage. The highest δ13C and δ15N values (0–60 cm) and the lowest C:N ratio (0–20 cm) of SOM, FLF, and IPOM were found in the farmland. In conclusion, ∼160 years of natural revegetation substantially promoted C and N sequestration in SOM, whereas altered C and N stabilization in SOM of the surface soil through shifting C and N towards more non-protected and pure physically protected SOM fractions rather than the most stable MAOM. Meanwhile, soil δ13C and δ15N in SOM and its fraction were changed along with natural revegetation. The most enriched soil δ15N in the farmland implied that soil N cycle in the farmland was more open to N losses relative to the other revegetation stages.

Influence of Organic Amendments on Soil Carbon Sequestration Potential of Paddy Soils under Two Irrigation Regimes

Soil organic carbon (OC) is one of the most important soil components regulating soil quality, fertility and agronomic productivity as well as the global carbon (C) cycle. Soil acts as a sink for global C, which can be influenced by the water regime and organic matter (OM) management in field. The aim of this study is to evaluate the effect of the application of different organic amendments on C sequestration in paddy soils under contrasting irrigation regimes. A 4-month pot experiment was conducted under net house conditions and the treatments were composed of two organic amendments: rice straw (RS) and poultry manure (PM); four application rates of amendment: 0 g (control), 2.5 g, 5.0 g and 15.0 g kg−1 soil; and two irrigation regimes: (i) continuous waterlogging condition (CWL) and (ii) alternate wetting and drying (AWD). After the incubation period, soil samples were collected from the pot and isolated into labile (&gt;53 µm) and mineral-associated (&lt;53 µm) OM. Bulk (before and after incubation) and fractionated soil samples were analyzed for OC, total nitrogen (N), C:N ratio; and C sequestration percentage was calculated. Relatively higher amounts of soil OC were present in CWL condition (1.23%) than AWD (1.13%). The C sequestration potential also showed the similar trend (CWL: 47% &gt; AWD: 35%). This was explained by the induced aerobic condition in between the anerobic condition in AWD and the continuous anaerobic condition in CWL which resulted in a difference in OM decomposition. The mineral-associated OM fraction (&lt;53 µm) was higher in the CWL condition than AWD condition which also indicated the importance of the chemical stabilization of OC (OC bound to minerals) in the CWL condition. The application of PM led to a significant increase (45%) in C sequestration potential than RS (37%). This could be attributed to C:N ratio and probable biochemical composition of amendments which resulted in lower decomposability of PM than RS, and also in line with the higher distribution of OC in mineral-bound OM than labile fraction. The application of higher organic amendments did not increase OC content, and declined C sequestration potential in soils as the microbial activity presumably did not match with the amendment amount. Overall, C sequestration potential was higher with 5 g PM kg−1 soil application under CWL-irrigated paddy soil. The findings indicated the need to pay more attention to the selection of the proper type and rate of organic amendments for higher C sequestration in soil under a specific irrigation system for sustainable agriculture.

Shrub willow chips incorporated after potato harvest enhance soil properties in Prince Edward Island, Canada

Willow (Salix viminalis spp.) shrubs are being planted along riverbanks, on erodible and marginal farmland. Wood chips made from the woody biomass could improve the properties of light-textured soils with low organic matter content in potato-based systems. Willow chips were applied at 0, 20, 40, and 60 Mg ha−1 (fresh weight) as a soil amendment. Soil health parameters were evaluated after 12 months, followed by C and N contents in whole soil, particulate, and mineral-associated organic matter fractions after 18 months. Willow chip application increased soil aggregation, respiration, C and N contents in whole soil, and plant-available K.

Perennial grain Kernza® fields have higher particulate organic carbon at depth than annual grain fields

Conversion from annual to perennial grains such as intermediate wheatgrass Kernza® could sequester soil organic carbon (SOC). To date, no studies have quantified SOC under Kernza on working farms. We sampled three sites with paired fields under annual grains and converted to Kernza 5–17 years ago to 100 cm and compared their SOC stocks as distributed between mineral-associated organic matter (MAOM) and particulate organic matter (POM). POM-C was higher under Kernza cultivation but total and MAOM-C were similar. Our findings suggest that Kernza increases SOC at depth as POM. Further study is needed to assess whether this will result in long-term SOC sequestration.

Effects of decadal nitrogen addition on carbon and nitrogen stocks in different organic matter fractions of typical steppe soils

Recent frameworks have proposed that division of soil organic matter (SOM) into particulate and mineral-associated organic matter (POM and MAOM) can help us better understand SOM cycling and its responses to increasing atmospheric nitrogen (N) deposition. However, responses of these factions to N deposition with combination of their relative distribution across soil profile remain unclear. Here we determined total N and soil organic carbon (SOC) as POM and MAOM separately in soils at depths of 0–10, 30–40 and 70–100 cm after 10-year N addition (at rates of 50, 10, 2 and 0 g m−2 yr−1) in a typical steppe. We further calculated their stocks in POM, MAOM and bulk soil and detected their relationships with both physicochemical features and microbial properties. Nitrogen addition increased the stocks of SOC (POM: +23 %; MAOM: +11 %) and total N (POM: +27 %; MAOM: +10 %) in both POM and MAOM fractions in topsoil (0–10 cm), but increased only in MAOM in 30–40 cm (SOC: +24 %; total N: +24 %) and 70–100 cm (SOC: +15 %; total N: +13 %) soils. Moreover, the increasing effects were strengthened with increasing N addition rates. We found that the share of SOC and total N in the MAOM was slightly decreased by N addition in topsoil, but significantly increased in deeper soils. Soil physicochemical features exerted stronger controls than microbial properties in the distribution of SOC and total N in the two fractions regardless of soil depth. SOC and total N contents of MAOM were correlated negatively with soil pH across the soil profile, and were correlated positively with bulk soil total N, dissolved organic N and inorganic N. Our findings imply that more soil C would be stabilized as MAOM under increasing atmospheric N deposition, and therefore the C saturation level of MAOM should be a target for further studies and be considered in predicting SOM dynamics, especially in N-limited grassland ecosystems.

Plant biomass amendment regulates arbuscular mycorrhizal role in organic carbon and nitrogen sequestration in eco-engineered iron ore tailings

Arbuscular mycorrhizal (AM) symbiosis plays an important role in organic matter (OM) stabilization and aggregate formation in eco-engineered Fe-ore tailings technosol, but their role may be influenced by the properties of tailings technosol with different plant biomass amendment and water supply, which has not been addressed. The present study investigated if different plant biomass amendments-- Lucerne hay (LH) (low C: N ratio) and Sugarcane mulch (SM) (high C: N ratio) -- could influence the AM roles in organic carbon (OC) and nitrogen (N) sequestration in tailings under well-watered or water deficit conditions. Sorghum spp. Hybrid cv. Silk with or without AM fungal inoculation was grown in tailings amended with plant biomass (either LH or SM) for 20 weeks under glasshouse conditions. The OC and N distribution among aggregates of various sizes (based on physical fractionation) were examined in the tailings technosol with different treatments. To uncover the mechanisms of OC sequestration, the mineral composition, OC forms, and the Fe phases in organo-mineral association were determined by means of X-ray photoelectron spectroscopy (XPS), attenuated total reflection-Fourier transform infrared analysis (ATR-FTIR), quantitative X-ray diffraction (qXRD), and synchrotron-based Fe K edge X-ray absorption fine structure spectroscopy (Fe K edge XAFS) analysis. AM symbiosis stimulated OC and N sequestration in aggregates formed from the amended tailings, which was regulated by the amendment of plant biomass with different C: N ratios. In LH amended tailings, AM symbiosis enhanced OC sequestration in both particulate OM (POM) and mineral-associated OM (MOM) within macroaggregates. In comparison, AM symbiosis enhanced N sequestration in MOM of both macro- and micro- aggregates in the SM amended tailings. Water deficit generally diminished these AM effects. Furthermore, AM symbiosis enriched carboxyl-rich organics in association with Fe/Si-rich minerals within MOM fraction. These results imply that the AM role in OC and N sequestration in Fe ore tailings is regulated by amendment using plant biomass with different C:N ratio, which should be considered in the eco-engineered pedogenesis for sustainable mine site rehabilitation.