Long-term Bare Fallow Research Articles

Soil crusting: New insight from synchrotron 2D micro X-ray diffraction mapping of clay-particle orientation and mineralogy

Our understanding of soil crusting and its detrimental consequences is mainly based on the analysis of morphological changes occurring at the soil surface. Such changes are usually assessed at spatial scales ranging from a few millimeters to that of the field. The inner-reorganisation within the topsoil during soil crusting primarily involves the redistribution of clay particles. Their mineralogy and arrangement remain however delicate to determine by the sole use of microscope observations. To overcome these limitations, we developed an innovative methodology based on synchrotron 2D micro X-ray diffraction (2D-µXRD) to map relevant parameters related to the organisation of all types of soil clay minerals, i.e. relative diffracted intensities, degree of preferential orientation, mean deviation angle from the surface. Our methodological strategy was first validated on a laboratory-made model sample, composed of various pure clay phases with clear preferential orientations. Then, a vertical section of soil crust from long-term bare fallow (LTBF) experiment of the France's National Research Institute for Agriculture, Food and Environment (INRAE, Versailles, France) was studied. Micromorphological features of the soil crust were first described by using conventional scanning electron microscopy. Synchrotron 2D-µXRD analyses were then successfully applied on the same region of interest. The in-depth analysis provided by the synchrotron 2D-µXRD approach revealed the presence of clay microlayers that were not observed on SEM images. Additionally, clay mineralogy as well as preferred clay-particle orientation and overall alignment highlighted specific quantitative structural signatures. These signatures involved the discrimination between clay microlayers associated to the settling of clay particles in microdepressions and clayey microaggregates resulting from transportation of pre-existing soil crusts. This new technique has led to re-interpretation of the deposition mechanisms in the natural soil investigated here and opens wide perspectives for in-depth detailed characterization of clay mineralogy in soil crusts.

Variations in the chemical structure and Carbon-13 natural abundance in water-stable macro- and microaggregates in Haplic Chernozem under the contrasting land use variants

Water-stable macro- (WSAma) and free microaggregates (WSAmi) were isolated from the 2-1mm air-dry macroaggregates from the surface horizons of Haplic Chernozem in contrasting variants of land use: the steppe and the bare fallow. The 13C NMR data and the 13C natural abundance of the Occluded organic matter (OM) (LFoc) and Clay within WSAs in the steppe obviously indicate a lower degree of microbiological processing of OM within WSAmi as compared with WSAma. This is reflected in lower degrees of decomposition (DI) and aromaticity (ARI) of OM and the C/N ratio, as well as lower 13C enrichment. This implies that the "labile" part of OM within WSAmi (LFoc and Clay, which are components of microaggregates within water-stable aggregates (mWSAs)) is more physically protected compared to that within WSAma. However, the heavier total δ13C signature of OM within WSAmi indicates its greater degree of microbiological processing compared to that within WSAma. This seems contrary to the concept of greater physical protection of OM within microaggregates as compared to macroaggregates. It was revealed that the heavier total δ13C signature of OM within WSAmi (greater degree of microbiological processing) is determined by the "oldest" OM located in the inter-aggregate space of WSAs, which is concentrated in the Residue fraction (Res). Due to its quantitative dominance, the Residue fraction is crucial for the total δ13C signature of WSAs. Negative changes in the quality of OM under the long-term bare fallow (52-yr) were reflected in a sharp increase in the integral indices of the chemical structure (DI, ARI), as well as the hydrophobicity index (HI) in all studied OM pools. It was accompanied by their 13C enrichment in the bare fallow compared to the steppe. Free microaggregates (WSAmi) are fragments of disintegrated macroaggregates (WSAma). We found no evidence of their formation within macroaggregates.

Effects of Long-Term Bare Fallow During the Winter-Wheat Growth Season on the Soil Chemical Properties, Fungal Community Composition, and the Occurrence of Maize Fungal Diseases in North China.

On the North China Plain, one of the most water-deficient regions in China, bare fallow has been implemented over a large-scale area to conserve water during the growth season of water-intensive winter wheat since 2015. However, the effects of this bare fallow on fungal community and the occurrence of crop diseases are poorly understood. Here we measured soil chemical properties, fungal community composition, and the occurrence of crop diseases after 15 years of long-term fallow (continuous maize or soybean) and non-fallow (maize-wheat rotation; soybean-wheat rotation) cropping systems. Bare fallow during the winter-wheat growth season significantly decreased soil organic matter, available nitrogen, and phosphorus. It also changed the composition of soil fungal communities, i.e., increased relative abundances of some potentially pathogenic species of Lectera, Fusarium, and Volutella but decreased beneficial Cladorrhium and Schizothecium. Meanwhile, the epidemic tendency of maize diseases changed correspondingly: the disease index of southern corn leaf blight and maize brown spot increased, but the incidence of stalk rot decreased compared with the non-fallow system. Soybean diseases were very mild regardless of the cropping system during the total experimental period. Network analysis demonstrated that the soil fungal diversity associated with maize diseases was affected by the decreased soil organic matter and available nitrogen and phosphorus. Our results suggest that bare fallow in the winter-wheat season affected the soil chemical properties, fungal community, and the occurrence of maize fungal diseases.

Changes in the hydrophobic-hydrophilic properties of the organic matter of the chernozems of the Kamennaya Steppe

Soil samples and physical size-density fractions isolated from them (silt particle size less than 1 µm, light fraction (LF) with a density of less than 2 g/cm3 and a fraction of the residue) of ordinary chernozem were studied in three contrasting variants of the experimental fields of the Kamennaya Steppe agrolandscape of the Voronezh region: mowed steppe, long-term permanent bare fallow and permanent corn – the main differences of which are in tillage (cultivated and not cultivated lands) and in the supply/absence of plant residues and root secretions. The LF content changes in the series: “mowed steppe” > “permanent corn” > “permanent bare fallow”, which corresponds to the direction of changes in the total carbon content of the soil and a decrease in the value of the contact angle of wetting (CA) of the surface of the solid phase of the studied chernozems. The determination of the total C and N content revealed the change in the qualitative and quantitative composition of the size-density fractions for different land use cases. Chromatographic fractionation of alkaline extractions of humus substances (HS) of chernozem samples and size-density fractions revealed an increase in the degree of hydrophilicity of HS while simultaneously increasing the hydrophobicity of the solid phase surface and the carbon content in the soil. HS of LF of the “mowed steppe” turned out to be by 63% more hydrophilic than HS of LF of “permanent bare fallow” and by 47% more hydrophilic than HS of LF of “permanent corn”. While the hydrophilicity of the HS silt differed by 16 and 27%, respectively. The hydrophilicity of the HS of the original soil in the plot of the “mowed steppe” was by 41% higher than the hydrophilicity of the HS in the soil in the plot of “permanent bare fallow” and by 24% higher than in the soil of the plot of “permanent corn”. In addition, changes in the hydrophilicity of HS of size-density fractions are more intense than the HS of the soil, so the change in the degree of hydrophilicity of HS of size-density fractions is an indicator of soil degradation under different agrogenic pressue.

Investigating soil carbon diversity by combining the MAXimum ENTropy principle with the Q model

Soil carbon diversity can be an important property for the stability of soil carbon. A problem is the lack of techniques for measuring this diversity. I suggest here the use of a combination of a general statistical principle, MAXimum ENTropy (MaxEnt), and a mechanistic model of organic matter decomposition, the Q model. The Q model provides the temporal development of the average carbon quality of litter and amount of soil organic C, which can be applied in a MaxEnt calculation to obtain a distribution of soil C over qualities. This distribution may not be the actual distribution but it is the most probable one. This distribution can be used to calculate aggregate properties for the total of soil C. I will use this distribution to calculate the temporal development of the variance in C quality as an expression of C diversity. The general tendency is that the variance declines with time of decomposition. Six long-term bare fallow (LTBF) from different climatic and management conditions were used to investigate which system properties are most important for the temporal development of the variance. The initial quality of the litter forming soil C is the dominant property. Chemical shifts in NMR spectra were tested as a possible way of measuring the variance in C quality.

Multidecadal persistence of organic matter in soils: multiscale investigations down to the submicron scale

Abstract. Minerals, particularly clay-sized minerals, protect soil organic matter (SOM) from decomposition by microorganisms. Here we report the characterization of SOM and the associated minerals over decades of biodegradation, in a French long-term bare fallow (LTBF) experiment started in 1928. The amounts of carbon (C) and nitrogen (N) in the study area declined over time for six fractions (sand, coarse silt, fine silt, coarse clays, intermediate clays, and fine clays). The C:N ratios of SOM associated with silt fractions remained constant, whereas the ratios significantly decreased in clays, reaching very low values in intermediate and fine clays (C:N < 5) after 8 decades of LTBF conditions. X-ray absorption spectroscopy revealed the following: (i) bulk-scale SOM chemical speciation remained almost constant; (ii) submicron particulate OM was present in coarse clays, even after 79 years of LTBF conditions; and (iii) illite particles became progressively SOM-free with time, whereas mixed-layer illite/smectite and smectites were always associated with OM throughout the bare fallow treatment. In summary, these results suggest that clay-sized minerals preferentially protect N-rich SOM and that smectites and mixed-layer illite/smectite seem to protect associated OM more effectively than pure illites.

A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils

Abstract. Changes in global soil carbon stocks have considerable potential to influence the course of future climate change. However, a portion of soil organic carbon (SOC) has a very long residence time (> 100 years) and may not contribute significantly to terrestrial greenhouse gas emissions during the next century. The size of this persistent SOC reservoir is presumed to be large. Consequently, it is a key parameter required for the initialization of SOC dynamics in ecosystem and Earth system models, but there is considerable uncertainty in the methods used to quantify it. Thermal analysis methods provide cost-effective information on SOC thermal stability that has been shown to be qualitatively related to SOC biogeochemical stability. The objective of this work was to build the first quantitative model of the size of the centennially persistent SOC pool based on thermal analysis. We used a unique set of 118 archived soil samples from four agronomic experiments in northwestern Europe with long-term bare fallow and non-bare fallow treatments (e.g., manure amendment, cropland and grassland) as a sample set for which estimating the size of the centennially persistent SOC pool is relatively straightforward. At each experimental site, we estimated the average concentration of centennially persistent SOC and its uncertainty by applying a Bayesian curve-fitting method to the observed declining SOC concentration over the duration of the long-term bare fallow treatment. Overall, the estimated concentrations of centennially persistent SOC ranged from 5 to 11 g C kg−1 of soil (lowest and highest boundaries of four 95 % confidence intervals). Then, by dividing the site-specific concentrations of persistent SOC by the total SOC concentration, we could estimate the proportion of centennially persistent SOC in the 118 archived soil samples and the associated uncertainty. The proportion of centennially persistent SOC ranged from 0.14 (standard deviation of 0.01) to 1 (standard deviation of 0.15). Samples were subjected to thermal analysis by Rock-Eval 6 that generated a series of 30 parameters reflecting their SOC thermal stability and bulk chemistry. We trained a nonparametric machine-learning algorithm (random forests multivariate regression model) to predict the proportion of centennially persistent SOC in new soils using Rock-Eval 6 thermal parameters as predictors. We evaluated the model predictive performance with two different strategies. We first used a calibration set (n = 88) and a validation set (n = 30) with soils from all sites. Second, to test the sensitivity of the model to pedoclimate, we built a calibration set with soil samples from three out of the four sites (n = 84). The multivariate regression model accurately predicted the proportion of centennially persistent SOC in the validation set composed of soils from all sites (R2 = 0.92, RMSEP = 0.07, n = 30). The uncertainty of the model predictions was quantified by a Monte Carlo approach that produced conservative 95 % prediction intervals across the validation set. The predictive performance of the model decreased when predicting the proportion of centennially persistent SOC in soils from one fully independent site with a different pedoclimate, yet the mean error of prediction only slightly increased (R2 = 0.53, RMSEP = 0.10, n = 34). This model based on Rock-Eval 6 thermal analysis can thus be used to predict the proportion of centennially persistent SOC with known uncertainty in new soil samples from different pedoclimates, at least for sites that have similar Rock-Eval 6 thermal characteristics to those included in the calibration set. Our study reinforces the evidence that there is a link between the thermal and biogeochemical stability of soil organic matter and demonstrates that Rock-Eval 6 thermal analysis can be used to quantify the size of the centennially persistent organic carbon pool in temperate soils.

Centennial Fertilization-Induced Soil Processes Control Trace Metal Dynamics. Lessons from a Long-Term Bare Fallow Experiment

Long-term bare fallow (LTBF) experiments with historical sample archives offer unique opportunities to study long-term impacts of anthropogenic activities on mineral soil fractions. In natural agro- and ecosystems, such impacts are often masked by organic matter due to its buffering action and rapid turnover. The 42-plot LTBF trial of INRA (Institut National de la Recherche Agronomique) started in Versailles (France) in 1928 to assess the impacts of prolonged application of fertilizers and amendments on the composition and properties of loamy soils. Here, we established geochemical budgets of major and trace elements on surface samples from 1929 and 2014 for four groups of treatments relevant for developed soil processes. We considered accompanying effects of soil compaction or decompaction due to changing physicochemical conditions over 85 years. Element losses from the surface horizon were quantified via fertilization-induced or -amplified soil processes: clay leaching favored by Na- or K-based fertilization, and lixiviation of major and trace elements in acidic or alkaline soil conditions. Enhanced mineral weathering was shown for acidified and nonamended plots. Conclusions on trace metal migration were confirmed by selected analyses on subsurface horizons. Additional information was provided on specific element inputs via fertilizers and/or diffuse inputs via atmospheric deposition.

Carbon saturation drives spatial patterns of soil organic matter losses under long-term bare fallow

Spatial controls of soil organic carbon (SOC) turnover are not well understood. We hypothesized that spatial patterns of SOC turnover are related to carbon (C) saturation rather than to the size of measurable SOC-pools such as particulate organic matter (POM), determined as SOC in particle-size fractions. Therefore, we repeatedly grid-sampled a field after one, three, seven, and eleven years under bare fallow management, which revealed a spatial gradient from high to low degrees of C saturation. We measured the contents of SOC and the contents of SOC in coarse sand-size (2000–250μm, POM1), fine sand-size (250–53μm, POM2), silt-size (53–20μm, POM3), and fine silt to clay-size fractions (nonPOM, <20μm), calculated the degree of C saturation from textural properties and nonPOM contents, and related these parameters to SOC losses. In the first year of bare fallow, the soil contained on average 12.1g SOC kg−1, of which 0.6gkg−1, 1.7gkg−1, and 2.1gkg−1 were lost after three, seven, and eleven years of bare fallow, respectively. The SOC losses within eleven years were spatially variable and varied between 1% and 46% relative to the initial SOC content. In support of our hypothesis, SOC losses were largest at subsites with largest degrees of C saturation (R2=0.83). Although the POM fractions declined most drastically, they only comprised 4 to 9% of bulk SOC, and they did neither correlate with nor explain spatial patterns of SOC losses. We conclude that the concept of C saturation is superior to conventional physical fractionation approaches for predicting spatio-temporal patterns of SOC turnover at sites with a high degree of C saturation.

Evidence that stable C is as vulnerable to priming effect as is more labile C in soil

A significant fraction of soil organic carbon, named stable organic carbon (C) pool, has residence times longer than centuries and its vulnerability to land use or climatic changes is virtually unknown. Long-term bare fallows offer a unique opportunity to isolate the stable organic pool of soils and study its properties. We investigated the vulnerability of the stable organic C pool to fresh organic matter inputs by comparing the mineralization in a long-term bare fallow soil with that of an adjacent arable soil, containing stable C as well as more labile C. For this, we amended or not the soil samples with two different 13C-labelled fresh organic matter (straw or cellulose). In all cases we found a positive priming effect (i.e. an increased mineralization of soil organic carbon) when fresh organic matter was added. By comparing the results obtained on both soils, we estimated that half of the “primed” C in the arable soil due to straw addition as fresh organic matter, originated from the stable C pool. Our results suggest that under such conditions, which frequently occur, the stable pool of soil organic matter may largely contribute to soil extra-CO2 emissions due to priming effect. Consequently, the C storage potential of this pool may be modified by changes in land use and/or biomass production that might change the priming of the mineralization of the stable pool of soil organic carbon.

Metabolic capacities of microorganisms from a long-term bare fallow

Soil degradation due to erosion or to soil organic matter (SOM) depletion is known to reduce the ecosystems services performed by soils. Amendments of organic matter are often proposed to restore the soil quality as they may increase water holding capacity, microbial activity and decrease erosion. In this work, we tested the ability of microbial communities from a soil very depleted in organic matter to mineralize fresh organic matter. We used a soil deprived of organic matter inputs since 1929 as a model of degraded soil. We amended it with 13C-cellulose or 13C-straw at two different levels (0.5 g C kg −1 of soil vs. 5 g C kg −1). The same treatments were performed on an arable soil as control. The soils were incubated at 20 °C during 209 days and the amount of CO 2 released and δ 13C–CO 2 were monitored during incubation. We measured the microbial biomass by fumigation extraction and determined the metabolic profiles of microorganisms by using Biolog ® ECOPlates. The straw and cellulose were mineralized without lag-time in both soils. It suggested that the bare fallow soil had kept its metabolic capacities to degrade straw and cellulose, but at a lower rate when compared to the arable control soil. The Shannon index ( H) calculated from Biolog® ECOPlates was very high in the two types of soils, which suggest that the two microbial communities were ubiquist. Moreover, the bare fallow microbial community was efficient to mineralize the amino acids and the mannitol in the Biolog® ECOPlates. Consequently, we assumed that the bare fallow soil microbial community had kept its metabolic capacities because of the microbial turnover. The dead microbial biomass might be an important substrate for the microbial community explaining why it may easily mineralize labile organic matter such as straw or cellulose.

Changes in clay-associated organic matter quality in a C depletion sequence as measured by differential thermal analyses

Land use changes result in significant decreases in soil organic matter stocks due to enhanced mineralization attributed to increased tillage, and due to decreased organic matter inputs. The current paradigm of soil organic matter dynamics suggests that decreasing organic matter stocks are also associated with shifts in organic matter quality to more resistant fractions as the more labile pools are decomposed. The objectives of the current study were to characterize changes in clay-associated organic matter quality in a soil C depletion sequence in terms of thermal properties, and to thermally characterize peroxide-resistant organic matter. Clay-sized fractions were isolated from a sequence of soils ranging from native forest to long-term bare fallow, and analyzed using thermogravimetry (TG) and differential scanning calorimetry (DSC) before and after hydrogen peroxide treatment. Differential scanning calorimetry traces had exothermic maxima near 285 and 333 °C, consistent with analyses of chemically extracted organic matter and humic substances reported in the literature. Peak fitting analyses showed that these maxima consisted of several hidden peaks, but their interpretation is problematic. The qualitative shifts in the thermal properties of clay-associated organic matter with differing land use observed in the DSC traces were quantified using cultivation and fallow treatment to forest ratios. Thermogravimetric mass loss ratios were greater in the thermally labile (180–310 °C) exothermic region than in the more thermally resistant (310–450 °C) exothermic region. Similarly, ratios of peak heights and areas for the fitted DSC peaks were higher for the 324 °C peak, compared to peaks at 257 and 284 °C. The higher ratios indicate that the more thermally resistant organic matter has been retained and the more thermally labile organic matter is lost. The observed shift in the distribution of organic matter from thermally labile to thermally resistant fractions from forest to long-term bare fallow clay samples is consistent with the current paradigm of decomposition consisting of a shift to more biologically resistant fractions with increasing time under cultivation and decreasing organic matter inputs. These results suggest that the thermal properties of clay-associated organic matter are related to their biological decomposability. Hydrogen peroxide treatment of clay samples removed approximately 87% of the initial organic C in all samples, and peroxide-resistant organic matter was found to be more thermally stable than the whole. However, thermal analyses of the peroxide-resistant fraction did not show any changes in thermal properties with decreases in total C content. Rather than isolating a biologically resistant pool of organic matter, peroxide-resistant organic matter may isolate an inert pool of soil organic matter with thermal properties consistent with the presence of black carbon, which remains unchanged by changes in land use.