The understanding of the impact of nitrogen (N) fertilization on the field water cycle and corresponding water use efficiency (WUE) is very important for optimizing fertilization rates and conserving ...stressed water resources. We modeled soil moisture dynamics of maize (Zea mays L.), finger millet (Eleusine coracana Gaertn.), and lablab Lablab purpureus (L..) Sweet plots using calibrated HYDRUS‐1D model on two experimental sites (rain‐fed and irrigated) for three seasons under different N treatments. The results indicate that the effects of N depended on plant specific properties such as N‐fixation and drought tolerance, and on plant available water content governed by soil structure and rainfall seasonal variability. Maize WUE of plots which received 150 kg/ha of urea (46 %$\%$ N) were 10–30 kg/ha/mm higher than plots which received none; likewise, millet that received 50 kg/ha of urea had a 7–10 kg/ha/mm higher WUE than control plots in both experiments. However, differences in water cycle components were noticeable between N treatments only in the rain‐fed experiment, where higher N levels led to around 60 and 30 mm higher transpiration, 30 and 20 mm lower evaporation, and 30 and 15 mm lower percolation per season for maize and millet, respectively. In 2018, which was the driest year, the difference in maize WUE between the high and low N treatments was only 1 kg/ha/mm, which corresponded with low actual to potential transpiration ratios (<50%$<\!50\%$). This indicates higher sensitivity of maize to water stress compared to the other crops. The results of lablab indicate a positive impact of N fertilization on WUE only under water‐limited conditions.
Core Ideas
HYDRUS models calibrated and validated based on soil moisture of minimum three soil profiles per crop per treatment.
Transpiration reduction was calculated as a proxy for water stress and used to distinguish between nitrogen (N) and water stress effect.
Favorable soil conditions such as good pH and water availability appear to minimize N effect on field water cycle.
Precipitation amounts and plant‐specific properties interact to determine how N affects water balance and water use efficiency.
Core Ideas
A 3D soil–root model was used to investigate root–biopore interactions.
Known effects of biopores on root growth, i.e., increased root length and depth were reproduced.
Despite reducing ...root–soil contact, biopores led to increased water uptake in dry periods.
Biopores had a larger impact on water uptake for more compact and less conductive soils.
Roots are known to use biopores as preferential growth pathways to overcome hard soil layers and access subsoil water resources. This study evaluates root–biopore interactions at the root‐system scale under different soil physical and environmental conditions using a mechanistic simulation model and extensive experimental field data. In a field experiment, spring wheat (Triticum aestivum L.) was grown on silt loam with a large biopore density. X‐ray computed tomography scans of soil columns from the field site were used to provide a realistic biopore network as input for the three‐dimensional numerical R‐SWMS model, which was then applied to simulate root architecture as well as water flow in the root–biopore–soil continuum. The model was calibrated against observed root length densities in both the bulk soil and biopores by optimizing root growth model input parameters. By implementing known interactions between root growth and soil penetration resistance into our model, we could simulate root systems whose response to biopores in the soil corresponded well to experimental observations described in the literature, such as increased total root length and increased rooting depth. For all considered soil physical (soil texture and bulk density) and environmental conditions (years of varying dryness), we found biopores to substantially mitigate transpiration deficits in times of drought by allowing roots to take up water from wetter and deeper soil layers. This was even the case when assuming reduced root water uptake in biopores due to limited root–soil contact. The beneficial impact of biopores on root water uptake was larger for more compact and less conductive soils.
In this review, we introduce microbially-mediated soil processes, players, their functional traits, and their links to processes at biogeochemical interfaces e.g., rhizosphere, detritusphere, ...(bio)-pores, and aggregate surfaces. A conceptual view emphasizes the central role of the rhizosphere in interactions with other biogeochemical interfaces, considering biotic and abiotic dynamic drivers. We discuss the applicability of three groups of traits based on microbial physiology, activity state, and genomic functional traits to reflect microbial growth in soil. The sensitivity and credibility of modern molecular approaches to estimate microbial-specific growth rates require further development. A link between functional traits determined by physiological (e.g., respiration, biomarkers) and genomic (e.g., genome size, number of ribosomal gene copies per genome, expression of catabolic versus biosynthetic genes) approaches is strongly affected by environmental conditions such as carbon, nutrient availability, and ecosystem type. Therefore, we address the role of soil physico-chemical conditions and trophic interactions as drivers of microbially-mediated soil processes at relevant scales for process localization. The strengths and weaknesses of current approaches (destructive, non-destructive, and predictive) for assessing process localization and the corresponding estimates of process rates are linked to the challenges for modeling microbially-mediated processes in heterogeneous soil microhabitats. Finally, we introduce a conceptual self-regulatory mechanism based on the flexible structure of active microbial communities. Microbial taxa best suited to each successional stage of substrate decomposition become dominant and alter the community structure. The rates of decomposition of organic compounds, therefore, are dependent on the functional traits of dominant taxa and microbial strategies, which are selected and driven by the local environment.
•The intensity of grazing affects soil air permeability.•Grazing exclusion allows recovery of soil structure, improving pore connectivity.•As grazing intensity increased, soil air permeability ...decreased.•The correlation of air-filled porosity versus air permeability may indicate pore connectivity.
The expansion of dairy farming in southern Brazil, mainly under a rotational grazing system, has generated soil physical changes with loss of soil quality, which in turn must be evaluated to determine how long they persist and how the soil recovers from these changes. The objective of this study was to evaluate the natural recovery of the soil physical attributes of a clayey Oxisol (Latossolo Vermelho distrófico típico, with clayey texture), which had been managed under different intensities of dairy cattle grazing: T0 = no grazing, T30 = consumption of 30 % of available forage, T50= consumption of 50 % of available forage and T80= consumption of 80 % of available forage. The rotational grazing system consisted of a grazing period (an average of 4 h) with a variable stoking rate to achieve forage consumption. For T0, zero animals, T30 three animals, T50 six animals and T80 nine animals per 100 m2 plot were used, followed by animal exclusion until the forage reached an average height of 35 cm. The grazing system was conducted for approximately 7 months, after which the animals were permanently removed from the area. The recovery of soil physical properties was monitored by soil sampling every three months for one year. The evaluated soil physical properties were porosity, bulk density and air permeability. The increase in grazing intensity negatively affected the soil physical quality. At all grazed areas, T30, T50 and T80, there was a reduction of the total porosity, the macroporosity and the soil air permeability, as well as an increase of the soil bulk density compared with the no grazed area, T0. However, 90 days after exclusion, increases in total porosity and macroporosity were observed. The angular coefficient between soil air-filled porosity and air permeability decreased as the grazing intensity increased and increased with time after grazing exclusion. Rotational grazing areas, when managed properly, can be an important tool for sustainable production. The use of grazing exclusion periods alleviates compaction, reducing the risk for forage production for the following grazing season.
In structured soils, earthworm burrows, root channels, shrinkage cracks, and interaggregate spaces form complex macropore networks relevant for preferential transport, turnover processes, and root ...growth. Macropore walls are often coated with organomineral material, which determine physicochemical properties such as wettability, sorption, and the cation exchange capacity (CEC). The objective here was to identify volume‐averaged mean macropore coating properties of larger intact soil cores (∼7,500 cm3) from Bt horizons of Luvisols developed from loess and glacial till. The quantification of organic C (OC) content and CEC of macropore surfaces was based on three‐dimensional images of X‐ray computed tomography (XRCT) of 231‐μm voxel resolution and a vesselness procedure to distinguish between biopores and cracks. Macropore surface areas were combined with millimeter‐scaled data of OC contents and CEC of macropore coating material. The surface of macropores that accounted for 5.6 % (loess‐Bt) and 4.6 % (till‐Bt) of the samples’ volumes represented approximately one‐third of the OC content and CEC of the bulk soil. Among the macropores, surfaces of larger biopores contributed most to OC content of the soil cores. The contribution of coated cracks and pinhole fillings to OC content was larger for the till‐Bt than for the loess‐Bt. Locally higher OC contents and CEC values emphasize the role of macropore surfaces in Bt horizons of Luvisols as geochemical hotspots and for mass exchange, especially during preferential flow and transport. Volume‐based coating properties may help improving macroscopic‐scale two‐domain flow and transport models.
► Root density of following wheat in the subsoil enhanced after 2 years of Lucerne. ► Lucerne as pre-crop improves water extraction from the subsoil by following summer wheat. ► Under humid temperate ...climate Lucerne as pre-crop reduces impact of dry spells in following spring crops.
Dry spells during the summer period affecting water uptake and plant growth in central Europe may occur more frequently in the future due to climate change. Improving the ability of crops to take up water from deeper soil layers is a potential strategy to secure water supply. The objective of this paper is to report on the effect of different preceding fodder crops on root growth and water uptake of spring wheat from the subsoil. Water extraction and root length density during grain filling of spring wheat were observed between anthesis and maturity in six different soil depths (0–15, 15–45, 45–60, 60–75, 75–90 and 90–105
cm) and with four different preceding crops: 1 year of fescue (Fes1Y), 2 years of chicory (Chi2Y), 2 years of lucerne (Luc2Y) and 3 years of chicory (Chi3Y). While there was no difference in total water extraction by wheat in the four crop sequences, water extraction from the deepest layer (90–105
cm) was significantly higher after 2 years of lucerne (Luc2Y). This was consistent with the root length densities measured in the 90–105 layer, which were 82, 89 and 112% higher in Luc2Y as compared to Fes1Y, Chi2Y and Chi3Y, respectively. Results suggest that lucerne as preceding crop supports deeper rooting and higher rooting density of following spring wheat enhancing access to water in deeper soil layers in response to prolonged dry spells. Effects facilitating root penetration like improved soil structure and higher nitrogen availability after lucerne are discussed. We conclude that suitable crop rotations with lucerne might be a cost-effective adaptation measure to overcome drought stress.
In conservation agriculture, conservation tillage potentially influences the physical, chemical, and biological quality of the soil. Although the effects of conservation agriculture on the soil’s ...physical properties have been studied in conventional management systems, studies on organic farming systems, especially concerning long-term changes, are scarce. This study summarizes the results of physical and mechanical soil parameters obtained over the initial 10 years of different conservation management treatments (plowing versus reduced tillage with and without compost application) in an organic field trial conducted in central Germany. Moreover, as a research objective, the effects of soil conservation measures on soil’s physical quality were evaluated. Differences in the soil’s physical quality during treatments were mainly detected in the topsoil. At a depth of 0.10–0.24 m, the total porosity and air capacity were lower, and the bulk density was higher in the reduced-tillage systems, compared to those of the plowed treatments. Additionally, the soil’s mechanical stability (precompression stress) was higher at a depth of 0.10 m for reduced-tillage systems combined with compost application. In addition, the soil’s aggregate stability was enhanced in the reduced-tillage systems (higher mean weight diameter, as determined via wet sieving). Overall, the reduced-tillage treatments did not exceed the critical physical values of the soil, nor affect the functionality of the soil (saturated hydraulic conductivity), thereby demonstrating its feasibility as a sustainable technique for organic farming. Future studies should include measures to ameliorate compaction zones in reduced-tillage treatments, e.g., by applying subsoiling techniques in combination with deep-rooting crops to prevent limited rooting space resulting from the high mechanical impedance, especially under dry soil conditions.
Soil structure controls soil hydraulic properties and is linked to soil aggregation processes. The aggregation processes of Oxisols are controlled mainly by clay mineralogy and biological activity. ...Computed microtomography (µCT) may be a tool for improving the knowledge of the hydraulic properties of these soils. Thus, this study brings an advance in the use of 3D image analysis to better comprehend the water behavior in tropical soils. In this work, three Oxisols were studied with the objective to (i) characterize the soil water retention curve (SWRC), the corresponding pore size frequency, and the saturated hydraulic conductivity (Ksat); (ii) use µCT to obtain, based on 3D images of soil structure and pore size distribution; and (iii) correlating parameters from SWRCs, Ksat, and µCT with other physical-hydric, chemical, and mineralogical attributes. Rhodic Haplustox—P1, Anionic Acrustox—P2, and Typic Hapludox—P3 were the three studied Oxisols. The differences among the SWRCs were related to the microgranular and block type’s structure morphology, which modified the soil pore space. The pore size frequency was calculated from SWRCs for pores with diameters of 87 ± 2 μm in P1, 134 ± 11μm in P2, and 175 ± 18 μm in P3. Pore size distribution from µCT was determined for the range of 20–100 µm, mainly with the highest percentages: 12 ± 1.09% for P1 and 12 ± 1.4% for P2. Pore connectivity was assessed from images by calculating Euler Numbers (EN), with the differences related to the biggest pore (ENbigpore): P1 (−44,223 ± 10,096) and P2 (−44,621 ± 12,573) showed more connected pores (ENbigpore) in comparison to P3 (−11,597 ± 6935). The parameter ENbigpore was decisive in understanding the water retention and conduction processes of the studied soils. The better-connected pore space increased Ksat in P1 (220 ± 0.05 mm h−1) and P2 (189 ± 0.1 mm h−1) in comparison to P3 (20 ± 0.3 mm h−1) and modified the shape of SWRCs.
Core Ideas
For preferential flow modeling, different macropore types must be considered.
A method was developed for separating biopores and cracks in 3D images from XRCT.
The method enabled a more ...objective determination of structuring element sizes.
The voxel‐based approach was found useful for quantification of macropore types.
In structured soils, earthworm burrows, root channels, shrinkage cracks, and interaggregate spaces form complex macropore networks. Depending on the type and morphological properties, each macropore surface type is coated with specific organo‐mineral compounds, differently affecting sorption and mass exchange during preferential flow and turnover processes. For a quantitative, macropore type–specific analysis using X‐ray computed tomography (XRCT) with subsequent three‐dimensional (3D) image analysis, a discrimination of biopores from cracks and interaggregate spaces is necessary. We developed a method that allows separating biopores from other larger macropores in 3D images from XRCT of intact soil cores. An image‐processing workflow using the MAVI (Modular Algorithms for Volume Images) software framework ToolIP (Tool for Image Processing) was created to handle XRCT 3D images. Masking steps enabled to retain the surface roughness in the resulting two images of separated biopores and cracks. As a key point, the sizes of the structuring elements used in the spherical opening and dilation were objectively determined. For this purpose, maximum differences in the pore shapes between the 3D images of cylindrical biopores vs. more flat cracks and unregularly interaggregate spaces were focused. At the given resolution of 231‐μm voxel edge length, an optimum size of 2.5 voxels was found for both processing steps. The voxel‐based approach is applicable to XRCT 3D images of different spatial resolution and appears useful for the quantification of physicochemical surface properties of different macropore types for soil volumes, enabling a more precise description of preferential flow and transport.
Mucilage released by plant roots affects hydrological and mechanical properties of the rhizosphere. The aim of this study was to disentangle the effects of the factors mucilage and soil moisture on a ...range of soil mechanical parameters in a sand and a loam. Both substrates were homogenised and filled into cylinders at bulk densities (ρb) of 1.26 and 1.47 g cm−3 for loam and sand, respectively. Chia seed (Salvia hispanica L.) mucilage concentrations of 0, 0.02, 0.2 and 2 g dry mucilage kg−1 dry soil were tested at four different gravimetric water contents in loam (θg = 0.34, 0.19, 0.14 and 0.09 g g−1) and three in sand (θg = 0.20, 0.06 and 0.04 g g−1). To quantify the influence of water content on the effect of mucilage on mechanical soil properties, two sets of samples were prepared, one for a micro penetrometer test, the other to measure bulk soil properties. Penetration tests were performed at 120 mm h−1 using a universal testing machine with a high‐precision sensor equipped with a penetrometer conus resembling a root. Mechanical energies were determined by calculating the area of the time–force curves. The energy required for a root to grow in a loam at permanent wilting point was decreased from 0.31 J in the control to 0.26 J in the 2 g kg−1 mucilage treatment, whereas it increased from 0.05 J in the control to 0.08 J at the highest water content. Pre‐compression stress (σpc), compression index (Cc), swelling index (Cs) and elasticity index were determined with a confined uniaxial compression test. σpc was increased by addition of mucilage in both substrates whereas the response on compressibility and elasticity was specific to substrate and water content. Here mucilage had a stronger impact on sand—the substrate with lower initial compressibility and elasticity. We conclude that the effect of mucilage on soil mechanical properties and subsequently on plant growth depends on the combined response of substrate and water content.
Highlights
The influence of soil moisture on the mechanical effects of mucilage concentration was examined.
Mucilage decreased the energy required for root growth in a loam at the permanent wilting point.
Higher mucilage concentrations increased compressibility and elasticity in sand.
Effect of mucilage on mechanical parameters depends on interaction between substrate and moisture.
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