Field-moist Soil Research Articles

Reversible strengthening behaviour of subsurface layers in sandy soils – understanding variable response to strategic deep tillage

Deep sandy soils cover more than 10 M ha of Australia's southern dryland cropping lands. Farming these soils is inherently challenging, and yield gaps are often wide due to multiple cropping constraints. Physical constraints that limit rooting depth are commonplace and often attributed to traffic-induced soil compaction. However, recent observations draw attention to a reversible hardsetting behaviour in response to soil water content. This hardsetting phenomenon is poorly understood and it may explain inconsistent crop responses to deep ripping in sandy soils. We examined the dynamic behaviour of soil strength under different soil water contents in deep ripped and unripped sands from four South Australian sites. We analyzed the elemental, mineralogical, chemical and physical compositions to differentiate the nature of hardened subsurface layers. Soil compaction and cementation were ruled out due to the soil's bulk density (<1.6 g cm−3) encountered and the rapid slaking that occurred when a hardened pan was submerged in water. The hardsetting behaviour was confirmed by the abrupt change in soil strength in response to the soil water content, measured using intact samples in a laboratory-based penetrometer. This significant increase in soil strength was also observed for soils collected from the ripped plots and for repacked soil samples, with penetration resistance ranging from 0.4 to 5.7 MPa depending on soil content. In contrast to our hypotheses, the hardsetting layer could not be distinguished from the layers above or below it by any analytical method. Given soil strength is typically measured on field moist soil, and standard chemical analysis did not differentiate hardsetting behaviour, alternative methods of diagnosis are needed to understand where deep tillage is likely to have the desired long-term effect. Identifying the nature of potential binding agents responsible for increased strength upon drying would be valuable. Our results confirmed reversible hardsetting in deep sandy soils is driven by small changes in soil water content, from 0.08 g g−1 to 0.05 g g−1 upon drying. Given that rapid re-consolidation of the soil matrix after expensive deep ripping interventions are reported, there is a strong imperative to improve diagnosis, and to identify if soil ameliorants could improve longevity by disrupting hardsetting behaviour.

Nitric Acid and Organic Acid Mixture Show Less Deviation in Extractability of Potassium Between Field-Moist and Air-Dried Soil

ABSTRACT Air-drying of soils for testing can cause fixation/release of potassium (K), showing a K status different from that of the field. We compared different extractants to find one with the least deviation in K-extractability between field-moist and air-dried soils. We chose two treatments (No-K and 150% of recommended K fertilization) of a field experiment (maize-wheat) for the study. Field-moist soil (0–15 cm) was collected at 3, 4, 5, 6, 7, 8, 23, 24, 25, and 26 days after the irrigation (DAI) from each replicated plot at the flowering stage of the 2018–19 wheat crop. Soil K, before and after air-drying, was extracted with 1 M ammonium acetate (NH4OAc), 0.1 M nitric acid (HNO3), 0.01 M calcium chloride (CaCl2), and 0.01 M organic acid mixture (OAM, mixture of 0.0025 M each of citric acid, malic acid, tartaric acid, and oxalic acid). Air-drying significantly (P < .05) affected the soil K for all methods except HNO3 in plots without K fertilization. However, air-drying’s effect on K release was more pronounced in No-K soils with high initial moisture content (>10%). Averaged over K-fertilization and initial soil moisture, the K-extractability using HNO3 and OAM showed the least deviation (9.6% for both) by air-drying. It was significantly (P < .05) lower than NH4OAc or CaCl2, which showed as high as 21.1–23.0% deviation due to air-drying. Hence, one should use 0.1 M HNO3 or 0.01 M OAM instead of 1 M NH4OAc to get an available K status closer to that in the field.

Quantifying mineral-associated organic matter in wetlands as an indicator of the degree of soil carbon protection

As atmospheric carbon (C) concentrations increase, so too has interest in understanding the mechanisms that preserve C within the soil organic matter (SOM). Mineral-associated organic matter (MAOM) is one pool of SOM recently shown to protect soil organic C from mineralization. However, most MAOM research has been in agriculture and forest mineral soils. Given the magnitude of soil C stored in wetlands, this study sought to determine the abundance of MAOM in wetlands. A standard method for quantifying MAOM in terrestrial soils was tested, modified, and optimized with three unique wetland soils. Using a physical fractionation method followed by a density fractionation, it was concluded that field moist soils and a dispersant were necessary for fractionating wetland soils (0–50 cm). The Bayhead Swamp soil had the greatest total C (484.07 ± 1.25 mg C g soil -1), but the smallest MAOM pool (0.63 %); most of the C was in the light fraction as particulate organic matter (POM, >98 %). The sandy-peat Cypress Dome soil also lacked MAOM (1.09 %) and was dominated by POM (66.26 %). The silt-rich Brackish Marsh had the least total C (94.37 ± 1.67 mg C g soil -1), but the largest fraction of protected C (24.60 % MAOM). Nitrogen (N) followed similar trends to C, except in the Brackish Marsh where nearly half the total N was MAOM. This research represents the first step in evaluating the role of MAOM in C persistence in organic-rich wetland soils; future work should consider C physical fraction, in addition to C quantity.

Evaluation of Alkaline Hydrolyzable Organic Nitrogen as an Index of Nitrogen Mineralization Potential of Some Coastal Savannah Soils of Ghana

Numerous biological and chemical methods have been proposed over the years for estimating the nitrogen (N) mineralization capacity of soils; however, none of them has found general use in soil fertility testing. The efficacy of a recently proposed alkaline hydrolysis method for assessing N availability in soils compared with the standard long-term incubation technique for determining potentially available N was evaluated. The nitrogen mineralization of 12 surface soils incubated under aerobic conditions at 25 °C for 26 weeks was determined. Field-moist soils were direct-steam distilled with 1 M KOH or 1 M NaOH; the NH3 released was trapped in boric acid, and its concentration was determined successively every 5 min for 40 min. The cumulative N mineralized or hydrolyzed was fitted to the first-order exponential equation to determine the potentially mineralizable N (No) and an analogous “potentially hydrolyzable N (Nmax)” for the soils. The flush of CO2 (fCO2) following the rewetting and incubation of air-dried soils under aerobic conditions for 3 days was also determined. The results showed that the Nmax values differed considerably among the soils, indicating differences in the chemical nature and reactivity of the organic N content of the soils, and were significantly correlated with No and fCO2 values. The estimated Nmax and No values ranged from 105 to 371 mg N kg−1 and 121 to 292 mg kg−1, respectively. Based on the simple and inexpensive nature of the alkaline hydrolysis procedure, the reduction in the incubation time required to obtain No (months to minutes), and the strong association between Nmax and No, we concluded that Nmax is a good predictor of the biologically discrete and quantifiable labile pool of mineralizable soil organic N (ON), and the use of the alkaline hydrolyzable ON as a predictor of No merits consideration for routine use in soil testing laboratories for estimating the N-supplying capacity of soils.

Seasonal Net Nitrogen Mineralization in the Topsoil of Subsurface Drip Irrigated Fields

ABSTRACT Nitrogen (N) mineralized from soil organic matter during the growing season can be an important source of crop-available N. However, accurate estimates for subsurface drip irrigated fields are currently not available. The main objective of the study was to determine seasonal net N mineralization rates in subsurface drip irrigated fields in California’s Central Valley. Soil samples were collected regularly from the top 15 cm of eleven commercial fields during the growing season and subsamples were incubated at an optimal moisture content and at field moisture (soil moisture content at the time of sampling). Net N mineralization in field moist soil was on average 45% of the potential N mineralization. When corrected for daily soil temperature at the sites, net N mineralization during a 4-month period from April 15 to August 15 averaged 25.2 kg ha−1 (median = 20.7 kg ha−1), which corresponds to 10 to 20% of the N in the aboveground biomass of the crops grown in these fields. To achieve a high N use efficiency and limit the risk of nitrate leaching to the groundwater, net N mineralization from soil organic matter should be accounted for when planning N fertilizer applications.

N2 fixation is less sensitive to changes in soil water content than carbon and net nitrogen mineralization

Biogeochemical processes catalyzed by soil microorganisms depend on the soil water content (SWC). Yet, little is known about the sensitivity of non-symbiotic N2 fixation to changes in SWC in comparison to carbon (C) and net nitrogen (N) mineralization. Therefore, we determined the rates of N2 fixation, C mineralization, and net N mineralization in soils at five SWCs that were created by water addition to the field-moist soil (rewetting), resulting in SWCs of 20, 40, 60 and 80% of the soil water holding capacity (WHC) which correspond to 14, 28, 42, and 57% water-filled pore space (WFPS). The soils originated from an arid, semiarid, Mediterranean, and humid site located in the Coastal Cordillera of Chile. The N2 fixation rate was measured immediately after the adjustment of the SWC and additionally after a five-day pre-incubation period. We found that the rates of all three processes where higher in the rewetted soils compared to the field-moist soils, but the sensitivity to changes in SWC differed between the processes. N2 fixation tended not to increase with increasing SWC beyond 20% WHC. Furthermore, the N2 fixation rate increased faster after rewetting in the soils of the semiarid and Mediterranean zone than in the soil of the humid zone. In contrast, C mineralization reached the highest rate at 80% WHC in the soil of the humid zone and at 60% and 80% WHC in the soils of the other three climate zones. The net N mineralization rate was highest at 40% and 60% WHC in the soils of the semiarid and Mediterranean zone, and was significantly decreased at 80% WHC compared to 60% WHC. Our study shows, first, that N2 fixation is less sensitive to changes in SWC than C and net N mineralization and tends to be already at its maximum in a given soil at low SWC, i.e., 20% WHC. This is likely because the diffusion of N2 in soil does not increase with the SWC, in contrast to the diffusion of organic solutes which are the substrate of C and N mineralization. Second, our results indicate that the N2 fixation rate increases more quickly in response to rewetting of soils of the more arid climate zones than of the humid zone, which might indicate a microbial adaptation to the climate conditions. The results have important implications since they suggest that the N2 fixation rate is likely less affected than the C and net N mineralization rates by decreases in the SWC that might occur more frequently in the future due to climate change.

Reclaimed Water Use in Agriculture: Effects on Soil Chemical and Biological Properties in a Long-Term Irrigated Citrus Farm

In Mediterranean regions, the scarcity of freshwater for agricultural purposes is leading to the use of alternative water sources. This study aimed to evaluate the effects of long-term irrigation with reclaimed water on chemical and biological soil properties. On a mandarin tree orchard (Citrus clementina, cv. Orogrande), freshwater (FW) and tertiary reclaimed water (RW) were supplied for irrigation. In spring 2017, a soil sampling was carried out, collecting from each experimental plot four samples at 0–0.20 m depth. Chemical and biochemical soil properties were determined on air dried and sieved soil and on fresh and field-moist soil, respectively. The irrigation with reclaimed water significantly increased the soil water extractable organic carbon (WEOC), available P, Mg, and Na content, and the electrical conductivity (EC). Although not significant, the respiration rates and enzymatic activities were higher in RW treatment. The results of this research highlighted that the irrigation with reclaimed water, providing organic carbon and other nutrients, could have, in the long-term, beneficial effects on soil microorganism and their activities. In any case, especially in arid and semi-arid environments, a proper management of wastewater should be recommended to avoid soil degradation due to salt accumulation in the rootzone.

Exploring Key Soil Parameters Relevant to Arsenic and Cadmium Accumulation in Rice Grain in Southern China

Paddy soils in some areas of southern China are contaminated by arsenic (As) and cadmium (Cd), threatening human health via the consumption of As- and/or Cd-tainted rice. To date, a quantitative understanding of how soil characteristics control As and Cd accumulation in rice grains under field conditions is still deficient. Based on 31 paired soil-grain samples collected in southern China, we statistically explored which soil parameter or parameter combination from various soil analyses best estimates As and Cd in rice. We found that CaCl2 extraction of field-moist soil collected at rice harvest provided the best estimation (R2adj = 0.47–0.60) for grain Cd followed by dry soil CaCl2 extraction (R2adj = 0.38–0.49), where CaCl2 extractable Cd from moist or dry soil was the dominant soil parameter. Compared to soil totals, parameters from neither dry soil ascorbate-citrate extraction nor anoxic soil incubation improved model performance for grain As (R2adj ≤ 0.44), despite their closer relevance to soil redox conditions during plant As uptake. A key role of soil-available sulfur in controlling grain As was suggested by our models. Our approach and results may help develop potential soil amendment strategies for decreasing As and/or Cd accumulation from soils.

A Comprehensive Method for Casein-based Assay of Soil Protease Activity

ABSTRACT Proteolytic enzymes play an important role in soil N cycling and are assayed by measuring protease activity. The casein substrate-based method of Ladd and Butler is commonly used to assay soil protease activities, but usage is constrained by several issues: methodological details in the original protocol are incomplete; controls for non-enzymatic activity are missing; and modifications of the assay have led to inconsistencies in how activities are measured. To clarify soil protease assays and improve their accuracy, we describe a comprehensive protocol in which field-moist soil is incubated with casein for 2 h at 50°C, and protease activity is determined by colorimetric measurement of the tyrosine produced after correction for soil- and substrate-only controls. The importance of such correction was investigated by a comparative study involving a single soil-substrate control as well as the dual-control approach. Soil-specific optimization of assay conditions was achieved by kinetic characterization of the maximum rate of activity (Vmax) and the substrate concentration required to obtain half of Vmax (Km). Soil protease activities were higher with a single control due to differential extraction of humic-phenolic substances, and for this reason, the dual-control approach is recommended. The procedure detailed herein stands to enhance the utility of casein-based assays of soil protease activities and improve the value of these assays for comparative applications.

Soil structural stability following decades of straw incorporation and use of ryegrass cover crops

Maintaining good soil structural stability is an important element in agricultural sustainability. Incorporation of cereal straw and use of cover crops improve soil structural stability but the long-term individual and interactive effects of these management practices is poorly understood. We examined the impact of four rates of straw incorporation in spring barley (0, 4, 8 and 12 Mg ha−1, annually) combined with a ryegrass cover crop undersown in the barley in spring. Soil was sampled after four decades of treatments in the Askov (Denmark) straw incorporation experiment situated on a sandy loam with 12% clay. We assessed clay dispersibility measured on two macro-aggregate size fractions (ClayDis 1–2 mm and ClayDis 8–16 mm), wet-stability of aggregates, and clay dispersibility of < 8 mm field-moist soil and aggregate strength. Soil structural stability and strength were related to soil organic carbon (SOC), SOC/Clay and root biomass. The SOC content increased with straw rate and when ryegrass cover crops were grown. Root biomass was marginally greater (P = 0.068), when cover crops were included. The soils receiving 8 and 12 Mg straw ha−1 had a significantly lower ClayDis 1–2 mm and ClayDis 8–16 mm than soil with straw removal. Inclusion of a ryegrass cover crop did not affect ClayDis 1–2 mm, but decreased ClayDis 8–16 mm marginally (P = 0.054) and decreased clay dispersibility of field-moist soil. The results suggest, that binding agents from cover crops such as roots increase stabilization of large macro-aggregates, while the increased stability due to straw incorporation was related to SOC irrespective of aggregate size.

VNIR and MIR spectroscopy of PLFA-derived soil microbial properties and associated soil physicochemical characteristics in an experimental plant diversity gradient

Improving our understanding of the functions and processes of soil microbial communities and their interactions with the physicochemical soil environment requires large amounts of timely and cost-efficient soil data, which is difficult to obtain with routine laboratory-analytical methods. Soil spectroscopy with portable visible-to-near infrared (VNIR) and mid-infrared (MIR) instruments can fill this gap by facilitating the rapid acquisition of biotic and abiotic soil information.In this study, we evaluated the capabilities of VNIR and MIR spectroscopy to analyze soil physicochemical and microbial properties in a long-term grassland biodiversity-ecosystem functioning experiment. Soil samples were collected at the Jena Experiment (Jena, Germany) and measured with portable VNIR and MIR spectrometers in field-moist condition to determine their potential for on-site data collection and analysis. Reference data to calibrate spectroscopic models were acquired with routine analytical methods, including PLFA extractions of microbial biomarkers. We further collected reference VNIR and MIR data on pre-treated soils (dried and finely ground) to assess the anticipated impact of field measurements on spectroscopic calibrations.MIR spectra allowed more accurate estimates of soil physicochemical and microbial properties than VNIR data on pre-treated samples. For soils in field condition, MIR calibrations were more accurate for physicochemical properties, but VNIR data gave significantly better estimates of microbial properties. Combined VNIR/MIR estimates achieved the most accurate estimation results for all soil properties in each case.Soil physicochemical properties could be estimated from VNIR/MIR data with high accuracy (R2 = 0.72–0.99) on pre-treated soil samples, whereas the results for soil microbial properties were more moderate (R2 = 0.66–0.72). On field-moist soils, estimation accuracies decreased notably for organic and inorganic carbon (ΔRMSE = 52–72%), improved slightly for soil texture (ΔRMSE = 4–7%) and decreased slightly for microbial properties (ΔRMSE = 4–9%). The VNIR/MIR estimates derived from soils in field condition were sufficiently accurate to detect experimental plant treatment effects on organic carbon, as well as bacterial and fungal biomass. We further found that spectroscopic estimates of soil microbial properties were primarily enabled through indirect correlations with spectrally active soil constituents, i.e., associations between soil microbial properties and the physicochemical soil environment.Our findings highlight the capacity of VNIR and MIR spectroscopy to analyze the physicochemical soil environment, including potential on-site data collection and analysis on soils in field condition, and indicate that VNIR/MIR data can estimate soil microbial properties when soil physicochemical properties shape the distribution of soil microbial communities.

Drying-rewetting changes soil phosphorus status and enzymatically hydrolysable organic phosphorus fractions in the water-level fluctuation zone of Three Gorges reservoir

The water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) in China has been frequently subjected to drying and rewetting (DRW) since the construction of the Three Gorges Dam. The increased soil labile phosphorus (P) after DRW potentially increases the risk of the release of soil P from the WLFZ to the reservoir water. Our study explored the effects of DRW on the P status of WLFZ soils. In June 2018, surface soil (0–2 cm) samples were taken in the WLFZ at two altitudes of in a tributary (Pengxi River) of the TGR. The labile P and the soil properties were determined in both field-moist and air-dried soils, and the organic P fractions in the bicarbonate extracts were determined through enzymatic hydrolysis. In addition, the contributions of various sources to the increase of soil labile P after DRW were quantified by using the improved organic P fractions tracing method. The results showed that the contents of TP, OP, Olsen-TP, Olsen-IP, Olsen-OP and the proportion of bioavailable P in the field-moist soils at 145 m were significantly lower than those at 165 m. Inorganic P was the main contributor to the increase in P in the bicarbonate extractants following DRW. Pmic, OM and Feox were found to be the main factors influencing the increase in soil labile P after DRW. The organic P fractions, such as diester P, phytate-like P and unknown P, also increased significantly after DRW. The transformation of slowly cycling P contributed 78% and 51% to the increase in Olsen-TP at 145 m and 165 m, respectively, and the death of microorganisms contributed 16% and 18%. The proposed method to quantify the source of increased soil labile P after DRW potentially contributes to reduce the release risk of P from the continent to the receiving aquatic systems.

Calibration methods for measuring the color of moist soils with digital cameras

The study of soil color is important for soil classification and as a diagnostic criterion reflecting a range of soil processes and soil properties. Field measurements of soil colors are affected by moisture content and other environmental and lighting condition. Here we evaluated calibration procedures to obtain colorimetrically corrected soil profile images on field moist soils using a gray card or histogram of reference soil images. In the laboratory, we further evaluated the colorimetric calibration methods on moist soil samples with varying moisture content and texture using an external scale (color scale with air-dry soil standards with known colors) or an internal scale (moist reference soil samples). The colors measured by a spectrometer were used as true colors of moist soil samples. The CIE L*a*b* color difference (ΔEab*) was used to evaluate the colorimetric accuracy between colors obtained from different devices. The results showed that the soil colors were greatly affected by soil moisture content. With the decrease in soil moisture, a significant increase in lightness (L*) and yellowness (b*) was observed. The field experiment showed that the colorimetrically accurate soil profile images at natural moist condition cannot be obtained by calibrating the digital image using a gray card or using the image of another camera. The laboratory experiments showed that colorimetrically accurate images can be obtained for sandy loam soil at a moisture content of less than 12% using the external standard. For soils with different texture, color characteristics, and moisture content (5–40%), colorimetrically accurate images of the samples can be obtained by using the internal scale. We conclude that the proposed calibration methods can be used to obtain colors of moist soils.

Soil carbon dynamics during drying vs. rewetting: Importance of antecedent moisture conditions

Soil moisture influences soil carbon dynamics, including microbial growth and respiration. The response of such ‘soil respiration’ to moisture changes is generally assumed to be linear and reversible, i.e. to depend only on the current moisture state. Current models thus do not account for antecedent soil moisture conditions when determining soil respiration or the available substrate pool. We conducted a laboratory incubation to determine how the antecedent conditions of drought and flood influenced soil organic matter (SOM) chemistry, bioavailability, and respiration. We sampled soils from an upland coastal forest, Beaver Creek, WA USA, and subjected them to drying and rewetting treatments. For the drying treatment, field moist soils were saturated and then dried to 75, 50, 35, and 5% saturation. In the rewetting treatment, field moist soils were air-dried and then rewet to 35, 50, 75, and 100% saturation. We measured respiration and water extractable organic carbon (WEOC) concentrations and used 1H-NMR and FT-ICR-MS to characterize the WEOC pool across the treatments. The drying vs. wetting treatment strongly influenced SOM bioavailability, as rewet soils (with antecedent drought) had greater WEOC concentrations and respiration fluxes compared to the drying soils (with antecedent flood). In addition, air-dry soils had the highest WEOC concentrations, and the NMR-resolved peaks showed a strong contribution of protein groups in these soils. Both NMR and FT-ICR-MS analyses indicated increased contribution of complex aromatic groups/molecules in the rewet soils, compared to the drying soils. We suggest that drying introduced organic matter into the WEOC pool via desorption of aromatic molecules and/or by microbial cell lysis, and this stimulated microbial mineralization rates. Our work indicates that even short-term shifts in antecedent moisture conditions can strongly influence soil C dynamics at the core scale. The predictive uncertainties in current soil models may be reduced by a more accurate representation of soil water and C persistence that includes a mechanistic and quantitative understanding of the impact of antecedent moisture conditions.

In-situ multiple parameter calibration and mapping using a mobile soil sensor

ABSTRACT The purpose of this study was to estimate local calibration models for 30 chemical and 4 physical soil properties, based on visible and near-infrared subsurface soil reflectance spectra (VNIR-SRS). Further, we discussed a case of decision-making for precision farming by a grower, using high-resolution soil maps. The experimental sites were located in rotational upland fields of taro in Saitama Prefecture, Japan. The parent materials in the experimental sites were Light Clay, Clay Loam, and Sandy Clay Loam. A mobile soil sensor (SAS3000) attached to a tractor was used to measure the VNIR-SRS (350 to 1700 nm). A total of 231 field-moist soil samples and corresponding VNIR-SRS data were collected from 23 fields (6 growers), from 2017 to 2018 period, to estimate local calibration models for the 34 properties. A total of 34 local calibration models were estimated using the second derivative pretreatment and partial least squares regression analysis. Excluding several outliers, the coefficient of determination (R 2) for the local calibration models was 0.81 or more. Immediately after soil sensing in the field, we generated high-resolution soil property maps for the growers’ fields, using the VNIR-SRS data and the 34 local calibration models stored in the SAS3000. The grower involved in the map-based decision-making recalled past crop management, growth, yield, etc., in the target field, and selected a cost-effective soil management technique. In addition, the six growers in the study stressed the importance of understanding the high-resolution relative variability among multiple soil properties, between fields and within a given field.

Effects of cadmium perturbation on the microbial community structure and heavy metal resistome of a tropical agricultural soil

The effects of cadmium (Cd) contamination on the microbial community structure, soil physicochemistry and heavy metal resistome of a tropical agricultural soil were evaluated in field-moist soil microcosms. A Cd-contaminated agricultural soil (SL5) and an untreated control (SL4) were compared over a period of 5 weeks. Analysis of the physicochemical properties and heavy metals content of the two microcosms revealed a statistically significant decrease in value of the soil physicochemical parameters (P < 0.05) and concentration of heavy metals (Cd, Pb, Cr, Zn, Fe, Cu, Se) content of the agricultural soil in SL5 microcosm. Illumina shotgun sequencing of the DNA extracted from the two microcosms showed the predominance of the phyla, classes, genera and species of Proteobacteria (37.38%), Actinobacteria (35.02%), Prevotella (6.93%), and Conexibacter woesei (8.93%) in SL4, and Proteobacteria (50.50%), Alphaproteobacteria (22.28%), Methylobacterium (9.14%), and Methylobacterium radiotolerans (12,80%) in SL5, respectively. Statistically significant (P < 0.05) difference between the metagenomes was observed at genus and species delineations. Functional annotation of the two metagenomes revealed diverse heavy metal resistome for the uptake, transport, efflux and detoxification of various heavy metals. It also revealed the exclusive detection in SL5 metagenome of members of RND (resistance nodulation division) protein czcCBA efflux system (czcA, czrA, czrB), CDF (cation diffusion facilitator) transporters (czcD), and genes for enzymes that protect the microbial cells against cadmium stress (sodA, sodB, ahpC). The results obtained in this study showed that Cd contamination significantly affects the soil microbial community structure and function, modifies the heavy metal resistome, alters the soil physicochemistry and results in massive loss of some autochthonous members of the community not adapted to the Cd stress.

Anaerobic Respiration in the Unsaturated Zone of Agricultural Soil Mobilizes Phosphorus and Manganese.

Anaerobic conditions mobilize phosphorus (P) in soils and sediments. The role of anaerobic microsites in well-drained soil on P migration is unknown. This study aimed to identify mechanisms that control field-scale vertical P mobility as affected by organic fertilizers that may trigger variable redox conditions. Soils were sampled at different depths in a well-drained Luvisol after 19 years of application of organic fertilizers. The concentrations of P and manganese (Mn) in 0.45-μm-filtered extracts (10-3 M CaCl2) of field-moist soil samples were strongly correlated (r = + 0.95), and both peaked in and below the compacted plough pan, suggesting that reductive processes mobilize P. Waterlogged soil incubations confirmed that anaerobic respiration comobilizes Mn and P and that this leads to the release of colloidal P and iron (Fe). The long-term applications of farmyard manure and immature compost enhanced the concentrations of Mn, Fe, and aluminum (Al) in the soil solution of subsurface samples, whereas less such effect was found under the application of more stable organic fertilizers. Farmyard manure application significantly enhanced soil P stocks below the plough layer despite a small P input. Overall, multiple lines of evidence confirm that anaerobic respiration, sparked by labile organic matter, mobilizes P in this seemingly well-drained soil.

The within-field spatial variation in rice grain Cd concentration is determined by soil redox status and pH during grain filling

Rice is a major dietary source of the toxic trace metal Cd. Large variation in Cd concentration in rice grain was documented by global and regional surveys, with this variation difficult to predict from soil tests. Even within individual fields, a large spatial variation is often observed but the factors controlling this within-field spatial variation are still poorly understood. In the present study, we used field- and laboratory-based experiments to investigate the effects of a gentle slope gradient within paddy fields (a common farmers’ practice to facilitate water flow from the inlet to the outlet) on Cd availability and grain Cd concentrations in unlimed and limed soils. In our field experiments, a gentle slope resulted in large spatial variations in soil redox potential (Eh) and pH upon soil drainage during rice grain filling. As a result of these variations in Eh and pH, there was a 6- to 7-fold spatial within-field variation in grain Cd concentrations, which were the highest in the irrigation inlet area associated with higher Eh values and the lowest in the outlet area with lower Eh values. Our results highlight that soil Eh, and more importantly, field-moist soil pH during grain filling determine grain Cd concentration and accordingly, incorporating measurements of soil redox status (or water content) and pH of field moist soils (rather than air-dried soils) during grain filling may improve the prediction of grain Cd concentrations. Delaying drainage during grain filling or increasing soil pH by liming is effective in reducing grain Cd accumulation.

Consequences of crude oil contamination on the structure and function of autochthonous microbial community of a tropical agricultural soil

Crude oil contamination of soil matrices is a persistent problem with deleterious consequences due to the recalcitrant, toxic and mutagenic properties of its constituents. To decipher the effects of crude oil contamination on the microbial community structure and function of an agricultural soil, field moist soil microcosms 2S (agricultural soil) and AB6 (agricultural soil polluted with crude oil) were set up. Taxonomic profiling of the two microcosms using next generation shotgun sequencing revealed massive decline in the number of recovered sequences from 3,267,616 (2S) to 250,241 (AB6). It also revealed the dominance of the phyla Actinobacteria (46.86%), and Firmicutes (51.20%) in 2S and AB6 with preponderance of Conexibacter (11.40%), and Singulisphaera (4.43%) in 2S, and Bacillus (38.52%), Sphingobium (10.51%), and Clostridium (7.06%) in AB6, respectively. Gas chromatographic fingerprints of residual crude oil in AB6 revealed complete disappearance of 50% of the hydrocarbon fractions at the end of 42 days while the others were degraded to < 6% of their initial concentrations. Functional annotation of the predicted ORFs in the two metagenomes revealed diverse metabolic features of the autochthonous microbial community. It also revealed the exclusive detection of diverse genes in AB6 metagenome responsible for degradation of various classes of hydrocarbons and the detoxification, transport and resistance to heavy metals. This study has established the deleterious effects of crude oil contamination on the microbial community structure of a tropical agricultural soil and revealed the adaptive features of the microbial community to various environmental stressors.

Temporal variability in extractable manganese concentrations in air-dry and field-moist soil.

When soils are removed from the field and processed for laboratory studies, changes in chemical properties may occur. Field-moist soil (approximately 40 percent water on an air-dry basis) was collected and passed through a 2 mm screen. After thorough mixing, half the soil was air dried (approximately 3 percent moisture). Samples were further divided with half of the field-moist and air-dried soil stored at room temperature and the other half stored in a refrigerator at 5 °C. Quadruplicate samples of each soil were analyzed at one-week intervals by extracting the equivalent of 10 grams of oven-dry soil with 20 mL of 0.01 M CaCl2 for 48 h followed by analysis using atomic absorption spectrophotometry. Preliminary results showed no significant difference between refrigerated and room-temperature soil manganese concentrations, but differences between air-dry and field-moist soil manganese concentrations were observed. Air-dried soil exhibited higher extractable manganese, even when accounting for the dilution effect of the soil moisture. This indicates the difference is due to changes in the soil solution thermodynamics rather than to dilution of a highly soluble manganese salt by the higher soil moisture. Soil analysis will continue for a period of twelve weeks to examine the changes in extractable manganese for both moisture contents.