Soil Soil Organic Matter Research Articles

Spatial variation of the soil bacterial community in major apple producing regions of China.

In China, apple production areas are largely from the coastal to inland areas and across varied climate zones. However, the relationship among soil micro-organisms, environmental factors and fruit quality has not been clearly confirmed in orchards. Here we attempted to identify the variation of soil bacteria in the main apple producing regions and reveal the relationship among climatic factor, soil properties, soil bacterial community and fruit quality. Sixty soil samples were collected from six main apple producing areas in China. We examined the soil bacteria using bacterial 16S rRNA gene amplicon profiling. The results show that the soil bacterial diversity of apple orchards varied from the Bohai Bay Region to the Loess Plateau Region. Proteobacteria, Acidobacteria and Actinobacteria were the predominant taxa at the phylum level for all six areas. In the Bohai Bay and the Loess Plateau region, which are the two largest apple producing areas, Proteobacteria and Actinobacteria had the highest relative abundance, respectively. Furthermore, soil bacterial diversity showed positive correlation with the mean annual temperature (MAT), soil organic matter (SOM) and pH. Excluding a direct effect on the apple fruit quality, MAT exerted an indirect influence through soil SOM and pH to alter the relative abundance of dominant taxa and shift the bacterial diversity, which affects the apple fruit titratable acids and soluble solids. Geographic variables underlie apple orchard soil bacterial communities vary according to spatial scale. Environmental factors exert an indirect effect on apple fruit quality via shaping soil bacterial community. This study provides a list of bacteria associated with environmental factors and the ecological attributes of their interactions in apple orchards, which will improve our ability to promote soil bacterial functional capabilities in order to reduce the fertilizer input and enhance the fruit quality.

Riparian wetland properties counter the effect of land-use change on soil carbon stocks after rainforest conversion to plantations

Progressive conversion of tropical rainforests to agricultural monocultures in South East Asia increasingly affects landscape types such as riparian areas. The impacts of conversions on soil organic matter (SOM) vary with changing landforms. However, this was often not accounted for in previous studies where SOM in soils in riparian areas was combined with SOM from well-drained adjacent slopes. Because riparian areas have a high carbon (C) storage potential, our objectives were i) to assess their C stocks after conversion to rubber and oil palm plantations in Sumatra (Indonesia) and ii) to compare the impacts of land use conversion on C stocks between riparian and well-drained areas. Average soil C stock losses from the top 30 cm were about 14% and 4% following conversion of riparian forest to rubber and oil palm plantations, respectively, indicating a high resistance of C to mineralization. C losses from well-drained areas were twice as high as from riparian areas after the respective conversion. δ13C values from riparian areas showed clear heterogeneity down soil profiles that was explained i) by alternating oxic and anoxic conditions, leading to reduced SOM and litter decomposition in riparian areas and ii) by mineral sediments and organic matter deposition and accumulation by erosion from adjacent slopes covered by plantations. We conclude that riparian areas are more resilient in terms of soil C storage towards land-use change than well-drained areas because of sediment deposition and reduced oxygen availability. On this basis, we developed a conceptual model of the effects of land-use change and various ecotone characteristics on SOM mineralization in the top- and subsoil of riparian areas.

Effects of different particle sizes on the spectral prediction of soil organic matter

Soil organic matter (SOM) is an important criterion for soil quality. The rapid determination of SOM can provide basic data support for the implementation of precision agriculture. Although the spectral reflectance has been used to rapidly obtain the SOM content, the method is not convenient to comparisons between different research results because the selection pretreatment criteria are not the same for soil samples. In this study, soil samples were prepared with six different standards of <1 mm, 0.5–1 mm, 0.25–0.5 mm, 0.15–0.25 mm, 0.088–0.15 mm and <0.088 mm. The reflectance of the different soil samples were compared, and the correlation between the reflectance of different soil samples and SOM were analyzed, as were the modeling and precision of the predictions of SOM. The results show that the soil spectral reflectance and the correlations between the uniform samples and SOM increased with decreasing particle size. Among the six kinds of soil particles, <0.088 mm had the highest correlation with SOM, and the reciprocal correlation coefficient reached 0.84. In the process of single-variable and multivariate modeling, the mixed samples of <1 mm and <0.088 mm had a better prediction effect than the uniform sample. Although the prediction result of <1 mm was slightly worse than that of <0.088 mm, the modeling results were the same (coefficient of determination (R2) of 0.78), and the validation set also showed a satisfactory predictive ability (R2val = 0.77). The soil samples of <1 mm, regardless of the ease of comparison with other soil preparation standards or the ability to predict SOM showed a high correlation and stable predictive ability. Therefore, when measuring the soil spectrum, <1 mm would be ideal for the pre-treatment of soil samples.

Effects of Oil Pollution on Indigenous Bacterial Diversity and Community Structure of Soil in Fushun, Liaoning Province, China

Four hundred fifty four pyrosequencing was used to analyze the microbial community and diversity of oily farmland soil from Fushun, Liaoning Province, China. The results showed that a total of 114 Phyla, 262 Classes, 430 Orders, 685 Families, 869 Genera, and 294 Species were detected and summarized in the soil microbe. The petroleum hydrocarbons significantly changed the structure and diversity of soil microbial communities. Proteobacteria, Bacteroidetes, Actinobacteria, Acidobacteria, Gemmatimonadetes, Nitrospirae, Firmicutes, Verrucomicrobia, Elusimicrobia became the dominant flora in oil-contaminated areas. And the changes in the main bacterial flora in the soil were highly correlated with soil TPH (total petroleum hydrocarbon), pH and SOM (soil organic matter). The prediction of community metabolism function exhibited that 8 KEGG pathways were significantly enriched in petroleum contaminated soil, and the abundance of Proteobacteria, Moraxellaceae, Enterobacteriales and Sphingobacteriales significantly decreased under the stress of oil pollution. This research provided a theoretical and technical basis for the development and application of functional strains in the future.

Cover crop diversity improves multiple soil properties via altering root architectural traits

Cover crops may influence soil health and functioning. However, little is known about the role of belowground root architectural traits in linking cover crop diversity with rhizosphere soil ecosystem properties. We hypothesize that cover crop diversity may improve root traits, which in return, could influence its effects on essential indicators of soil physicochemical heterogeneity, such as the composition of soil aggregate-size classes and nutrients, and the soil organic matter (SOM) and soil organic carbon (SOC) contents. We studied the impact of plant diversity on root traits and soil properties. The four soil aggregate-size classes, such as large macro- (>2000 μm), small macro- (<2000-500 μm), meso- (<500-250 μm), and micro-aggregates (<250 μm) were separated by the dry sieving. Root traits such as surface area (cm2) and length (cm) were quantified by image analysis using Winrhizo. The soil nutrient, SOM, and SOC contents were determined by standard methods. We used Fox's (2005) method for partitioning the selection and complementarity effects on plant productivity. Plant diversity improved productivity, root architectural traits, root coverage, the composition of soil aggregate-size classes and nutrients, SOM and SOC contents across the soil depth. It increased and decreased the relative abundance of meso- and micro-aggregates, respectively. Both root architectural traits and SOC contents positively correlated with several soil edaphic properties essential for physicochemical heterogeneity, health, and functioning. Our results suggest that competition among plant roots in species-rich than poor communities may improve rhizosphere soil carbon storage, the composition of soil aggregate-size classes, and nutrients.

Short-term temperature history affects mineralization of fresh litter and extant soil organic matter, irrespective of agricultural management

The influence of temperature on mineralization of plant litter and pre-existing soil organic matter (SOM) involves not only the prevailing temperature, but also how it has changed through time. However, little is known about how temperature variability through time influences mineralization processes. Here, we investigated how short-term temperature history affects the mineralization of SOM and plant litter in soils from different agricultural management systems. We used soils from a long-term experiment with conventional and organic management treatments to set up microcosms. The microcosms were exposed to eight days of contrasting temperature regimes (different mean temperatures and constant versus fluctuating temperatures). Microcosms were then returned to a common temperature of 16 °C, 13C-labelled plant litter was added to half of them, and CO2 efflux was measured over the following week. We found that SOM and litter mineralization were both sensitive to the temperature history, with lower mean temperatures during preliminary treatment associated with higher mineralization during the subsequent common-temperature incubation. This effect persisted through the week after temperature differences were removed. Different patterns of temperature fluctuation and agricultural management did not significantly affect mineralization during common-temperature incubation. The history sensitivity of litter mineralization, despite litter being added after temperature differences had ended, indicates that the temperature history effects may be driven by short-term microbial acclimation. We conclude that organic matter and litter mineralization, which are key processes in the carbon cycle, are sensitive to short-term temperature history. This suggests that future investigations of soil CO2 efflux may need to take recent weather effects into account.

The quality of soil organic matter, accessed by 13C solid state nuclear magnetic resonance, is just as important as its content concerning pesticide sorption

The global increase of food production has been achieved mainly through the intensive use of inputs such as pesticides. Once released to the soil, sorption (which could be represented by Freundlich solid-water distribution coefficients – KF) and degradation are two governing processes that determine the distribution and persistence of pesticides in the environment. In spite of the huge dataset, the only apparent generalisation is the high correlation between KF and soil organic matter (SOM) content. However, in this work no correlation was observed between KF and organic C content (OC) and so the obtained KOC (KF normalised by OC) spread out in a wide range: 1100 to 11,400 mL g−1 for abamectin; and 30–150 mL g−1 for atrazine, both ranges corroborate with data from literature. These high variabilities indicate that other soil components or SOM quality strongly interfere in the pesticide sorption in addition to SOM content. Seeking to estimate the influence of SOM quality in the abamectin and atrazine KOC values, the humic acids, a fraction of the SOM, was analysed by 13C nuclear magnetic resonance spectroscopy (13C NMR) and Principal Component (PC) Regression. The first PC of 13C NMR spectra presented negative loadings for aliphatic compounds and positive loadings for aryl C, typical of partially oxidised pyrogenic C. Their scores showed strong correlation with the abamectin KOC values (R2 = 0.91, p < 5 10−8) and weaker with atrazine KOC (R2 = 0.63, p < 0.0001), in addition to a smaller standardised slope: 1.01 for abamectin and 0.76 for atrazine. These results could be explained by the higher hydrophobicity of abamectin, being thus more prone to interact with the polycondensed aryl groups from the pyrogenic C. It is also important to highlight that humic acids are useful proxies for understanding the paramount interaction of SOM with pesticides.

Biochar is an effective amendment to remediate Cd-contaminated soils—a meta-analysis

Cd immobilization can be affected by many factors, among which the soil pH, soil organic matter (SOM), and amendment types are the most critical factors that have been considered in many previous studies. The objective of this study was to find the amendments with best performance for Cd remediation in soils with different range of soil pH and SOM. The effects of soil pH, SOM, and amendment types on soil Cd fractions and Cd uptake by rice were investigated by meta-analysis performed on MetaWin software, based on relevant data of 513 observations from 51 published papers that studied the soil fields in China from 2000 to 2020. Cd immobilization was more responsive to amendments when soil pH ≤ 6.0 as compared with those with pH higher than 6.0, which is probably because most amendments increased the soil pH and thus decreased the Cd activity. The Cd immobilization was most effective with the presence of 20–30 g/kg SOM; excessive SOM activated Cd in soil. Synthetic material appeared to be the most effective amendment on immobilization of the exchangeable fraction of Cd in soil by 42.5% inhibition rate. Biochar showed the strongest inhibition effect on the Cd uptake by rice (65.2% inhibition rate), while organic material exhibited the lowest immobilization effect. We demonstrated that amendments showed best performance under conditions of soil pH ≤ 6.0 and SOM of 20–30 g/kg, and biochar is an effective amendment to remediate Cd-contaminated soils. The findings enable us to remediate Cd contamination on farmland more effectively and efficiently according to the local conditions.

Mercury distribution in the surface soil of China is potentially driven by precipitation, vegetation cover and organic matter

BackgroundUnderstanding the mechanism of Hg accumulation in soil, which is a net Hg sink, at a national scale is important for protecting the environment and improving food safety. The mercury (Hg) distribution in surface soil in China is quite uneven, with relatively high concentrations in southeastern China and low concentrations in northwestern China. The reason for this distribution is inconclusive, especially at the continental scale. In this study, the relative contributions of the key impact factors, including dry and wet deposition, soil organic matter (SOM) and solar radiation to soil Hg, were evaluated.ResultsWet and dry deposition associated with precipitation and vegetation cover and emissions influenced by SOM are key factors controlling Hg distribution in surface soil. In southeastern China, high levels of wet deposition associated with the South Asia monsoon and dry deposition, enhanced by vegetation canopies, together with low levels of emissions caused by highly vegetated surfaces and solar radiation, are responsible for the high Hg levels in soil (> 0.08 mg/kg). In northeastern China, moderate levels of wet Hg deposition, high levels of dry deposition via throughfall and litterfall, low emissions due to weak solar radiation and high levels of SOM are responsible for high Hg accumulation in soil. In northwestern China, low levels of wet deposition, together with high emissions levels, low vegetation cover (bare soil) and SOM and strong solar radiation, contributed to the low Hg level in the surface soil (< 0.03 mg/kg).ConclusionsWe suggest that wet deposition derived from the Asian monsoon, dry deposition linked to vegetated surfaces and Hg emissions associated with vegetation cover, SOM and solar radiation play key roles in the soil Hg level in China. In other terrestrial environments worldwide, especially in regions with significantly high levels of wet deposition and high amounts of vegetation cover and soil SOM, high Hg concentrations may exist in the surface soil.

Spectroscopic and chromatographic investigation of soil organic matter composition for different agrosystems from arid saline soils from Southeastern Tunisia

In Tunisia, several studies estimated the soil organic matter (SOM) stocks. However, little is known about SOM composition and dynamics especially in arid regions. Globally, arid soils of Tunisia are saline. This study aims to investigate the composition and stability of SOM in salt-affected soils under different vegetation covers in Southeastern Tunisia. Two depths are sampled in ten profiles from different localities. SOM composition was assessed by FTIR spectroscopy and gas chromatography-mass spectrometry (GC/MS). The results showed that, despite the differences of studied soils, SOM contents are low (< 20 g/kg) and decrease with soil depth. The lowest content of SOM is attributed to native soils which suggest that SOM in arid saline soils is mainly controlled by available litter input. FTIR spectroscopy revealed that SOM is predominated by polysaccharides, aliphatic C, and aromatic C. Polysaccharides and aliphatic C increased with soil depth for agricultural and native soils. However, aromatic C increased with soil depth for agricultural soils, while it decreased for native soils. Agricultural soils are enriched in aliphatic C and polysaccharides compared with native soils that had higher content of aromatic C. Thus, the chemical composition of SOM in arid saline soils is strongly influenced by litter input quality and decomposition rate of SOM. GC/MS analyses which showed the predominance of mid-chain n-alkanes confirm those finding. Because of their relative recalcitrance and/or their protection by association with soil minerals, polysaccharides, mid-chain n-alkanes, aliphatic C, and aromatic C may have crucial effect on SOM stabilization in arid saline soils.

Applications of physical methods in estimation of soil biota and soil organic matter

Soil biota is the living component of soil organic matter (SOM), and plays a key role in the decomposition of SOM. Both soil biota and SOM are indicators of soil fertility and soil quality. However, they both are sensitive to soil disturbance. Although researchers developed various technologies to detect soil biota and SOM, they are mostly destructive and cause disturbance to soil, which may not reflect the actual situation of soil biota and SOM. Therefore, here we mostly focused on the non-destructive physical methods for estimating soil biota and SOM and discussed their advantages and disadvantages. These methods include but not limited to acoustic detection, radio frequency identification, radioactive tagging, hyperspectral sensing and electron energy loss spectroscopy. In addition, we pointed out the current research problems and the potential research directions for applications of physical methods in estimation of soil biota and SOM.

Increasing the organic carbon stocks in mineral soils sequesters large amounts of phosphorus.

Despite the fact that phosphorus (P) is critical for plant biomass production in many ecosystems, the implications of soil organic carbon (OC) sequestration for the P cycle have hardly been discussed yet. Thus, the aims of this study are, first, to synthesize results about the relationship between C and P in soil organic matter (SOM) and organic matter inputs to soils, second, to review processes that affect the C:P ratio of SOM, and third, to discuss implications of OC storage in terrestrial ecosystems for P sequestration. The study shows that the storage of OC in mineral soils leads to the sequestration of large amounts of organic phosphorus (OP) since SOM in mineral soils is very rich in P. The reasons for the strong enrichment of OP with respect to OC in soils are the mineralization of OC and the formation of microbial necromass that is P-rich as well as the strong sorption of OP to mineral surfaces that prevents OP mineralization. In particular, the formation of mineral-associated SOM that is favorable for storing OC in soil over decadal to centennial timescales sequesters large amounts of OP. Storage of 1,000kg C in the clay size fraction in the topsoils of croplands sequesters 13.1kg P. In contrast, the OC:OP ratios of wood and of peatlands are much larger than the ones in cropland soils. Thus, storage of C in wood in peatlands sequesters much less P than the storage of OC in mineral soils. In order to increase the C stocks in terrestrial ecosystems and to lock up as little P as possible, it would be more reasonable to protect and restore peatlands and to produce and preserve wood than to store OC in mineral soils.

Effect of wetting on the determination of soil organic matter content using visible and near-infrared spectrometer

The assessment of soil organic matter (SOM) content by proximal sensing using Visible and Near-Infrared (VNIR) reflectance spectroscopy of field soil samples is complicated by interactions with various soil constituents and moisture content. This study examined a total of 486 archived agricultural soil samples, covering a wide range of soil reflectance characteristics and soil textures. Spectral reflectance was measured with a spectrometer for samples with wetness ranges from air-dry to near saturation. Prediction models for soil water and SOM content were then developed using partial least square regressions (PLSR) combined with various reflectance spectrum pre-processing methods (standard normal variate transformation and detrend, as well as spectral derivatives). Our results indicate that the PLSR model based on spectra measured on sufficiently wet soil samples had slightly better accuracy for SOM predictions than models based on air-dried samples. The mechanisms of wetting to increase prediction accuracy were also explored. Important reflectance wavelengths associated with organic functional groups such as aromatics, aliphatics, and amides were identified through analysis of their variable importance in projections (VIP). Robustness of the developed models was tested against other two independent datasets (comprised of sample numbers 126 and 99 each), achieving prediction accuracies of mean bias difference (MBD) = 0.02%, root mean square difference (RMSD) = 0.99%, Ratio of Performance to Inter-Quartile (RPIQ) = 2.90 and MBD = −0.23%, RMSD = 1.35%, RPIQ = 1.44, respectively. These findings suggest that developing SOM prediction PLSR models based on sufficiently wetted soil samples may be a viable approach, particularly when developing models for operational use in the field.

Effect of Inorganic Fertilizer on Soil Properties, Growth and Yield of Maize in Makurdi, Southern Guinea Savanna Zone of Nigeria

Field experiment was conducted with the objective to determine the effect of inorganic fertilizer on soil properties, growth and yield of maize at the Teaching and Research Farm, Federal University of Agriculture, Makurdi, Southern Guinea Savanna Zone of Nigeria, during 2017 and 2018 cropping seasons. The experimental treatments consisted of 0, 75, 150 and 300 kg ha-1 of NPK fertilizer laid out in a randomized complete block design (RCBD) and replicated thrice. Soil samples were collected from a plough layer of 0-15 cm before and after the experiment for each cropping season according to the treatments and were analyzed for particle size distribution (PSD), soil pH, organic carbon (OC), total nitrogen (TN), available phosphorus, exchangeable cations and cation exchange capacity (CEC). Maize plant (growth and yield) data collected were subjected to analysis of variance after which significant means were separated using least significant difference (LSD) at P &lt; 0.05. From the results, the effects of fertilizer treatments on soil properties investigated were significant. Application of fertilizer increased soil organic matter (SOM), TN and CEC in both cropping seasons. However, the SOM was low in all plots with no application of fertilizer. Exchangeable bases, base saturation and CEC reduced in 2017 in no fertilizer treatments but remarkably increased in 2018 due to application of 75, 150 and 300 kg ha-1. The 300 kgha-1 NPK fertilizer applications produced higher growth and yields of maize plant than remaining treatments. Similarly, soil properties (soil pH, SOM, TN and CEC) were also improved with application higher levels of mineral fertilizers for agricultural production.

Decadal shifts in soil pH and organic matter differ between land uses in contrasting regions in China

Soil organic matter (SOM) and pH are critical soil properties strongly linked to carbon storage, nutrient cycling and crop productivity. Land use is known to have a dominant impact on these key soil properties, but we often lack the ability to examine temporal trajectories across extensive spatial scales. Large-scale monitoring programmes provide the data to evaluate these longer-term changes, and under different climatic conditions. This study used data from Chinese soil surveys to examine changes in soil pH and SOM across different land uses (dry farmland, paddy fields, grassland, woodland, unused land), with surface soil (0–20 cm) collected in the periods 1985–90 (Survey 1; 890 samples) and 2006–10 (Survey 2; 5005 samples) from two contrasting areas. In the southern part of China the mean pH of paddy soils fell sharply over the two decades between surveys - from pH 5.81 to 5.19 (p < 0.001), while dry farmlands in the northern sampling area fell slightly (from pH 8.15 to 7.82; p < 0.001). The mean SOM content of dry farmland soil rose in both areas and the mean SOM of paddy fields in the southern area also rose (all p < 0.001). Woodland soil pH in the south showed an increase from 4.71 to 5.29 (p < 0.001) but no significant difference was measured in the woodlands of the northern area, although the trend increased. The SOM content of woodland top soils rose in the northern (p = 0.003) and southern (p < 0.001) study areas. The implications and potential causes of these changes over the two decade timespan between surveys are discussed and suggestions made as to how large scale soil sampling campaigns can be designed to monitor for changes and potential controlling factors.

Combination of efficient signal pre-processing and optimal band combination algorithm to predict soil organic matter through visible and near-infrared spectra.

Visible and near-infrared (Vis-NIR) spectroscopy is a promising alternative to replace soil physicochemical Analysis to quickly and effectively determine the content of soil organic matter (SOM). However, choosing appropriate pre-processing methods and effective data mining techniques is the essential step in Vis-NIR to improve the quality of spectral data and the accuracy of the model prediction. In this study, nine spectral pre-processing methods and optimal band combination algorithms were introduced to process the spectra and select sensitive spectral parameters. The purpose of this study is to determine the effective pre-processing method and explore the prediction potential of the optimal band combination algorithm. Two hundred thirty-three soil samples were gathered from northwestern Xinjiang, China, and the soil properties and reflectance spectra were measured in the laboratory. The spectra were subjected to nine pre-processing methods, e.g., Savitzky-Golay (SG) smoothing, discrete wavelet transformation (DWT), First (FD) and second (SD) derivatives, multiplicative scatter correction (MSC), standard normal variate and detrend (SNV-DT), continuum removal (CR), correction by the maximum reflectance (CMR) and pseudo-absorbance values and detrend (Abs-DT). The results indicate, the SG proved to be the most effective pre-processing method for SOM in saline soil. The Abs-DT, FD, SD, SNV-DT, MSC, CR, DWT, and CMR led to degrading the prediction performance. Furthermore, the use of SG before further processing can improve the prediction effect, although it is not obvious. The optimal band combination algorithm can derive spectral parameters that have a good correlation with SOM content. Prediction accuracy (RPIQ was 3.058 and 3.045 in independent and cross-validation respectively) and model complexity (latent variables were both 4) from spectral parameter combination were both better than that from full-spectrum data. In summary, the combination of SG and the optimal band combination algorithm can improve the prediction accuracy of SOM in saline soil.

Spatial stochastic model for predicting soil organic matter using remote sensing data

Accurate soil organic matter (SOM) estimation could provide critical information to understand soil organic carbon sequestration, soil fertility, and the global carbon cycle. A nearest-neighbourhood autoregressive moving average (NN-ARMA) modelling technique along with linear regression has been used to predict localized soil SOM variation based on topographical characteristics and vegetation indices in semi-arid region of Saudi Arabia. Seven topographic variables derived using DEM, and twelve vegetation indices obtained from Landsat 8 used in the model. The best NN-ARMA model showed seven significant variables explaining 96.4% of the total variation of SOM, whereas the best linear regression model could explain 78.8% of the total variation of SOM. The results showed that NN-ARMA model gave better results compared to the linear regression model. Our research gave a better understanding of the possibility of accurate estimation of SOM using the NN-ARMA approach using topographical characteristics and vegetation indices easily acquired by RS sensors.

Molecular level study of hot water extracted green tea buried in soils - a proxy for labile soil organic matter

Understanding the composition of soil organic matter (SOM) is vital to our understanding of how soils form, evolve and respond to external stimuli. The shear complexity of SOM, an inseparable mixture of thousands of compounds hinders the determination of structure-function relationships required to explore these processes on a molecular level. Litter bags and soil hot water extracts (HWE) have frequently been used to study the transformation of labile SOM, however these are still too complex to examine beyond compound classes. In this work, a much simpler mixture, HWE buried green tea, was investigated by Nuclear Magnetic Resonance (NMR) spectroscopy and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS), as a proxy for labile SOM. Changes induced by the burial over 90 days in a grassland, woodland and two peatland sites, one damaged by drainage and one undergoing restoration by drain-blocking, were analysed. Major differences between the extracts were observed on the level of compound classes, molecular formulae and specific molecules. The causes of these differences are discussed with reference to abiotic and biotic processes. Despite the vastly different detection limits of NMR and MS, chemometric analysis of the data yielded identical separation of the samples. These findings provide a basis for the molecular level interrogation of labile SOM and C-cycling processes in soils.

Soil nitrogen cycling is determined by the competition between mycorrhiza and ammonia-oxidizing prokaryotes.

Mycorrhizal fungi have considerable effects on soil carbon (C) storage, as they control the decomposition of soil organic matter (SOM), by modifying the amount of soil nitrogen (N) available for free-living microbes. Through their access to organic N, ectomycorrhizal (ECM) fungi compete with free-living soil microbes; this competition is thought to slow down SOM decomposition. However, arbuscular mycorrhizal (AM) fungi cannot decompose SOM, and therefore must wait for N to first be processed by free-living microbes. It is unclear what form of N the ECM fungi and free-living microbes compete for, or which microbial groups compete for N with ECM fungi. To investigate this, we focused on the N transformation steps (i.e., the degradation of high-molecular-weight organic matter, mineralization, and nitrification) and the microbes driving each step. Simple comparisons between AM forests and ECM forests are not sufficient to assert that mycorrhizal types would determine the N transformation steps in soil, because soil physiochemistry, which strongly affects N transformation steps, differs between the forests. We used an aridity gradient with large differences in soil moisture, pH, and SOM quantity and quality, to distinguish the mycorrhizal and physicochemical effects on N transformation. Soil samples (0-10cm depth) were collected from AM-symbiotic black locust forests under three aridity levels, and from ECM-symbiotic oak forests under two aridity levels. Soil physicochemical properties, extractable N dynamics and abundance, composition, and function of soil microbial communities were measured. In ECM forests, the ammonia-oxidizing prokaryotic abundance was low, whereas that of ECM fungi was high, resulting in lower nitrate N content than in AM forests. Since ECM forests did not have lower saprotrophic fungal abundance and prokaryotic decompositional activity than the AM forests, the hypothesis that ECM fungi could reduce SOM decay and ammonification by free-living microbes, might not hold in ECM forests. However, the limitation of ECM fungi on nitrate N production would result in a feedback that will accelerate plant dependence on these fungi, thereby raising soil C storage through an increase in the ECM biomass and plant C investment in soils.

Importance of soil interparticle forces and organic matter for aggregate stability in a temperate soil and a subtropical soil

Soil organic matter (SOM) and interparticle forces, including the electrostatic repulsive force (ERF), surface-hydration repulsive force (SHRF) and van der Waals attractive force (vDWAF), play crucial roles in aggregate stability. However, few studies investigated their coupled effects on aggregate stability in variably charged soils, in which soil particle interactions are more complex than in permanently charged soils due to the coexistence of positive and negative charges and the variable surface charge characteristics. Therefore, this study aims to: 1) investigate the combined effects of soil solution pH and the overlapping of electric double layers (EDLs) between positively charged Fe/Al (hydro)oxides and negatively charged SOM on the ERF and aggregate stability before (control) and after SOM removal and after straw incubation for 240 days; and 2) evaluate the importance of soil interparticle forces and SOM for aggregate stability in a permanently charged temperate soil (Vertisol) and a variably charged subtropical soil (Ultisol). Soil aggregate stability was determined by measuring the release of small particles (w(<d)%) after aggregate breakdown at different KCl concentrations (10−1 to 10−5 mol L−1). Soil solution pH was adjusted by varying KCl concentration. In both soils, the w(<d)% increased exponentially with the net force of the soil interparticle forces. However, in contrast to the Vertisol, the w(<d)% of the differently treated Ultisol showed almost no change before the KCl concentration decreased to 10−2 mol L−1 and continuous changes after the KCl concentration decreased to 10−2 mol L−1. In addition, the differences in the w(<d)% between the SOM removal treatment and the control and straw incubation treatments were much larger in the Ultisol than in the Vertisol. In the Ultisol, the ERF did exhibit little and continuous increase over the whole range of tested pH. When considering the impact of the overlapping of oppositely charged EDLs between Fe/Al (hydro)oxides and SOM, a much stronger reduction in ERF was observed in the control and straw incubation treatments and at lower KCl concentrations. Consequently, in both the Vertisol and Ultisol, aggregate stability is essentially controlled by the soil interparticle forces, whereas the effects of soil solution pH and the overlapping of oppositely charged EDLs between SOM and Fe/Al (hydro)oxides on the ERF must be considered in variably charged soils (i.e. Ultisol). Moreover, SOM can play a more important role in aggregate stability in subtropical soil than in temperate soil.