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The microbial metabolic quotient (MMQ), microbial respiration per unit of biomass, is a fundamental factor controlling heterotrophic respiration, the largest carbon flux in soils. The magnitude and controls of MMQ at regional scale remain uncertain. We compiled a comprehensive data set of MMQ to investigate the global patterns and controls of MMQ in top 30 cm soils. Published MMQ values, generally measured in laboratory microcosms, were adjusted on ambient soil temperature using long-term (30 yr) average site soil temperature and a Q₁₀ = 2. The area-weighted global average of MMQ_Soil is estimated as 1.8 (1.5–2.2) (95% confidence interval) μmol C·h⁻¹·mmol⁻¹ microbial biomass carbon (MBC) with substantial variations across biomes and between cropland and natural ecosystems. Variation was most closely associated with biological factors, followed by edaphic and meteorological parameters. MMQ_Soil was greatest in sandy clay and sandy clay loam and showed a pH maximum of 6.7 ± 0.1 (mean ± se). At large scale, MMQ_Soil varied with latitude and mean annual temperature (MAT), and was negatively correlated with microbial N:P ratio, supporting growth rate theory. These trends led to large differences in MMQ_Soil between natural ecosystems and cropland. When MMQ was adjusted to 11°C (MMQ_Ref), the global MAT in the top 30 cm of soils, the area-weighted global averages of MMQ_Ref was 1.5 (1.3–1.8) μmol C-mmol MBC⁻¹·h⁻¹. The values, trends, and controls of MMQ_Soil add to our understanding of soil microbial influences on soil carbon cycling and could be used to represent microbial activity in global carbon models.

Biochar, as a soil conditioner, has been widely used to promote the growth of maize, but most of the current research is short-term experiments, which limits the research on the long-term effects of biochar, especially the physiological mechanism of biochar on maize growth in aeolian sandy soil is still unclear. Here, we set up two groups of pot experiments, respectively after the new biochar application and one-time biochar application seven years ago (CK: 0 t ha -1 , C1: 15.75 t ha -1 , C2: 31.50 t ha -1 , C3: 63.00 t ha -1 , C4: 126.00 t ha -1 ), and planted with maize. Subsequently, samples were collected at different periods to explore the effect of biochar on maize growth physiology and its after-effect. Results showed that the plant height, biomass, and yield of maize showed the highest rates of increase at the application rate of 31.50 t ha -1 biochar, with 22.22% increase in biomass and 8.46% increase in yield compared with control under the new application treatment. Meanwhile, the plant height and biomass of maize increased gradually with the increase of biochar application under the one-time biochar application seven years ago treatment (increased by 4.13%-14.91% and 13.83%-58.39% compared with control). Interestingly, the changes in SPAD value (leaf greenness), soluble sugar and soluble protein contents in maize leaves corresponded with the trend of maize growth. Conversely, the changes of malondialdehyde (MDA), proline (PRO), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) manifested an opposite trend to the growth of maize. In conclusion, 31.50 t ha -1 biochar application can promote the growth of maize by inducing changes in its physiological and biochemical characteristics, but excessive biochar application rates ranging from 63.00-126.00 t ha -1 inhibited the growth of maize. After seven years of field aging, the inhibitory effect of 63.00-126.00 t ha -1 biochar amount on maize growth disappeared and changed to promoting effect.

Aim To study the impact of long-term application of organic amendments and fertilizer nitrogen on C sequestration and its distribution among various physical pools of soil organic matter in rice-wheat system. Method We studied the distribution of organic C among physical pools of soil organic matter separated by size and density floatation techniques in a sandy loam soil after 11 years of rice-wheat cropping with continuous application of farmyard manure (FYM), rice straw (RS), and fertilizer nitrogen (N). Laboratory incubation experiments were conducted to estimate mineralizable C in soil and relate it to various organic C pools. Result Application of FYM and RS increased soil organic carbon (SOC) stocks in the surface soil by 33.7 % over sole application of fertilizer N. Conjoint use of FYM and RS along with fertilizer N caused the greatest (83.5 %) increase in SOC stocks. Particulate organic C (POC) constituted 23–34 % of SOC with 2.8 to 11.3 % as coarse POC (cPOC) and 17.5–22.6 % as fine POC (fPOC). The cPOC responded to management to a greater extent than fPOC and may thus be considered a more labile pool of SOC. The coarse particulate organic matter (cPOM) had wider C/N ratio (11.1 to 12.7) than the fine POM (fPOM; 8.2 to 9.9). Mineral associated organic C (MinOC) represented the greatest proportion (48–68 %) of SOC followed by heavy fraction (HFOC; 21–30 %) and light fraction organic C (LFOC; 5–15 %). Addition of FYM alone or in combination with RS enlarged the LFOC pool by 263 and 383 %, and HFOC pool by 62 and 127 %, respectively with insignificant effect on MinOC. Rice straw increased LFOC by 66 %, with no effect on HFOC. The C/N ratios generally decreased as the soil organic matter (SOM) fractions became finer and followed the order LFOM > iLFOM > HFOM > MinOM. Mineralizable C in the surface soil was significantly related to SOC (R2=0.90), LFOC (R2=0.72) and HFOC (R2=0.68). Conclusions Application of organic amendments in rice-wheat system has a major influence on SOC and the relative distribution among various C pools. The LFOC is most sensitive to management, followed by sand-sized HFOC and silt- and clay-sized MinOC pool suggesting thereby that these may be considered to represent active, slow and passive pools of SOC, respectively. The conjoint use of FYM, RS and fertilizer N could maintain SOC almost at the same level as for the uncultivated soil and this practice may help in maintaining the sustainability of rice-wheat cropping systems in the Indo-Gangetic plains.

AIMS: Biochar (BC) and humic acid product (HAP) soil amendments may improve plant performance under water-limited conditions. Our aim was to investigate if BC and HAP amendments, alone or in combination, will have positive and synergistic effects. METHODS: A three-factorial fully randomized study was carried out in the greenhouse for 66 days, including the factors ‘BC’, ‘HAP’ and ‘water regime’. Maize (Zea mays var. ‘Amadeo’ DKC-3399) was grown in pots (6 kg sandy soil pot⁻¹) amended with/without BC (0, 1.5 and 3 %; w/w) and with/without HAP (0 or an equivalent of 8 kg ha⁻¹). Two water regimes, limited and frequent (H₂O ₗᵢₘᵢₜ , H₂O fᵣₑqᵤ), were applied after day 28 following seedling establishment at 60 % water holding capacity (WHC). In the H₂O ₗᵢₘᵢₜ treatment, the soil water content was allowed to drop until wilting symptoms became visible (25–30 % WHC) while in H₂O fᵣₑqᵤ the WHC was brought to 60 % of the maximum on a daily basis RESULTS: BC but not HAP, added alone or in combination with BC, significantly increased the biomass yield and the water and N use efficiency of plants at both water regimes. The BC-mediated relative increase in the yield was equal with both watering regimes, refuting initial hypotheses. BC had generally a stimulating effect on water relations and photosynthesis, it increased the relative water content and the leaf osmotic potential, decreased the stomatal resistance and stimulated the leaf gas exchange (transpiration). Both, BC and pure HAP addition, stimulated photosynthesis by increasing the electron transport rate (ETR) of photosystem II (PSII) and of the ratio between effective photochemical quantum yield to non-photochemical quenching (Y(II)/Y(NPQ), revealing reduced heat dissipation. CONCLUSIONS: Biochar use in poor sandy soils can improve plant growth by improving soil-plant water relations and photosynthesis under both H₂O fᵣₑqᵤ and H₂O ₗᵢₘᵢₜ conditions. HAP loading, however, did not improve the effect of biochar or vice versa.

The combined application of organic resources (ORs) and mineral fertilizers is increasingly gaining recognition as a viable approach to address soil fertility decline in sub-Saharan Africa (SSA). We conducted a meta-analysis to provide a comprehensive and quantitative synthesis of conditions under which ORs, N fertilizers, and combined ORs with N fertilizers positively or negatively influence Zea mays (maize) yields, agronomic N use efficiency and soil organic C (SOC) in SSA. Four OR quality classes were assessed; classes I (high quality) and II (intermediate quality) had >2.5% N while classes III (intermediate quality) and IV (low quality) had <2.5% N and classes I and III had <4% polyphenol and <15% lignin. On the average, yield responses over the control were 60%, 84% and 114% following the addition of ORs, N fertilizers and ORs + N fertilizers, respectively. There was a general increase in yield responses with increasing OR quality and ORN quantity, both when ORs were added alone or with N fertilizers. Surprisingly, greater OR residual effects were observed with high quality ORs and declined with decreasing OR quality. The greater yield responses with ORs + N fertilizers than either resource alone were mostly due to extra N added and not improved N utilization efficiency because negative interactive effects were, most often, observed when combining ORs with N fertilizers. Additionally, their agronomic N use efficiency was not different from sole added ORs but lower than N fertilizers added alone. Nevertheless, positive interactive effects were observed in sandy soils with low quality ORs whereas agronomic use efficiency was greater when smaller quantities of N were added in all soils. Compared to sole added ORs, yield responses for the combined treatment increased with decreasing OR quality and greater yield increases were observed in sandy (68%) than clayey soils (25%). While ORs and ORs + N fertilizer additions increased SOC by at least 12% compared to the control, N fertilizer additions were not different from control suggesting that ORs are needed to increase SOC. Thus, the addition of ORs will likely improve nutrient storage while crop yields are increased and more so for high quality ORs. Furthermore, interactive effects are seldom occurring, but agronomic N use efficiency of ORs + N fertilizers were greater with low quantities of N added, offering potential for increasing crop productivity.

In recent years, plants in sandy soils have been impacted by increased climate variability due to weak water holding and temperature buffering capacities of the parent material. The projected impact spreads all over the world, including New England, USA. Many regions of the world may experience an increase in frequency and severity of drought, which can be attributed to an increased variability in precipitation and enhanced water loss due to warming. The overall benefits of biochar in environmental management have been extensively investigated. This review aims to discuss the water holding capacity of biochar from the points of view of fluid mechanics and propose several prioritized future research topics. To understand the impacts of biochar on sandy soils in-depth, sandy soil properties (surface area, pore size, water properties, and characteristics) and how biochar could improve the soil quality as well as plant growth, development, and yield are reviewed. Incorporating biochar into sandy soils could result in a net increase in the surface area, a stronger hydrophobicity at a lower temperature, and an increase in the micropores to maximize gap spaces. The capability of biochar in reducing fertilizer drainage through increasing water retention can improve crop productivity and reduce the nutrient leaching rate in agricultural practices. To advance research in biochar products and address the impacts of increasing climate variability, future research may focus on the role of biochar in enhancing soil water retention, plant water use efficiency, crop resistance to drought, and crop productivity.

Saturated hydraulic conductivity (Ks) is a crucial parameter that influences water flow in saturated soils, with applications in various fields such as surface water runoff, soil erosion, drainage, and solute transport. However, accurate determination of Ks is challenging due to temporal and spatial uncertainties. This study addresses the knowledge gap regarding the long‐term behavior of Ks in sandy soils with less than 10% fine particles. The research investigates the changes in Ks over a long period of constant head tests and examines the factors influencing its variation. Two sandy samples were tested using a hydraulic conductivity cell, and the hydraulic head and discharge were recorded for over 50 days. The results show a general decline in Ks throughout the test, except for brief periods of increase. At the end of both tests, there are noticeable reductions in the saturated hydraulic conductivities of the samples, with one sample being 96% and the other sample 91% less than the maximum recorded saturated hydraulic conductivity during the tests. Furthermore, the relationship between flow rate and hydraulic head gradient does not follow the expected linear correlation from Darcy's law, highlighting the complex nature of sandy soil saturated hydraulic conductivity. The investigation of soil properties in three different sections of the samples before and after the tests revealed a decrease in the percentage of fine particles and a shift in specific gravity from the bottom to the top of the sample, suggesting particle migration along the flow direction. Factors such as clogging by fine particles and pore pressure variation contribute to the changes in Ks. The findings of this research show the importance of considering changes of saturated hydraulic conductivity during constant‐head laboratory tests. Therefore, this study provides evidence for the requirement to further assess the laboratory methods for measurement of the saturated hydraulic conductivity in sandy soil mixtures. Key Points The estimation of saturated hydraulic conductivity through constant head tests demonstrates sensitivity to the duration of the tests Observations indicate an alteration in particle size distribution before and after testing within different sections of the samples Flow rate and hydraulic gradient vary over the test duration, emphasizing the complex hydraulic system within the samples

Saline water evaporation and salt precipitation in porous media are prevalent in many arid and coastal areas, posing significant environmental and engineering challenges while offering the possibility of enhancing the thermal conductivity of granular materials. Evaluating the impact of salt precipitation and developing effective strategies to manage and utilize saline soils in affected regions requires a comprehensive understanding of precipitation dynamics. This study examined the salt precipitation distribution induced by brine drying and evaluated its influence on soil thermal conductivity. The salt content profile along the depth was determined after drying within sandy soils under various brine concentrations, saturation levels, and drying temperatures. X‐ray computed tomography was used to characterize precipitation patterns at the pore scale. Each sample's thermal conductivity was measured after drying to assess the impact of salt precipitation. Results show that thermal conductivity enhancement depends on both salt content and pore habit. Salt cementation occurs when precipitation initiates at low brine content with disconnected small brine clusters, enhancing the contacts between particles and linearly increasing thermal conductivity. Conversely, if salt precipitation starts at higher brine content, large brine clusters remain interconnected, and capillary‐driven flow brings water and salt to the drying front, leading to pore‐filling salt within the sample, which has a limited impact on thermal conductivity. Plain Language Summary Evaporation of saline water in porous media leads to salt precipitation, posing environmental and engineering challenges in many arid and coastal regions. This phenomenon affects thermal conductivity, important for geothermal energy, and hydraulic conductivity, relevant to hydrogen storage and carbon sequestration in brine reservoirs. Understanding how salt precipitation patterns vary at both the sample and pore scales is crucial for predicting changes in porous media properties. In this study, we investigate the distribution of salt precipitation in sandy soils due to saline water evaporation. Using X‐ray tomography, we identify two distinct pore habits: contact cementation and pore filling. We also assess how salt precipitation influences soil thermal conductivity. By testing samples with different salt concentrations, moisture levels, and drying temperatures, we demonstrate that salt precipitation significantly impacts thermal conductivity depending on the amount and pore habits of salt. These findings enhance our understanding of how brine evaporation and salt precipitation affect soil properties, providing valuable insights for predicting and managing these effects in engineering practices. Key Points Salt precipitation increases thermal conductivity in sandy soils Insights gained into salt distribution at both sample and pore scales Two internal salt deposition habits: grain cementation and pore filling

Phosphorus (P) fertilizer is critical to maintain a high yield and quality of alfalfa (Medicago sativa L.). There are several fertilizer types and soil types in China, and the application of a single type of P fertilizer may not be suitable for present-day alfalfa production. In order to select the optimal combination of alfalfa and soil type and fertilizer type for improving P utilization efficiency. We conducted a greenhouse pot experiment, calcium superphosphate (SSP), diammonium phosphate (DAP), ammonium polyphosphate (APP), potassium dihydrogen phosphate (KP), and no-fertilizer control treatments were applied to alfalfa in sandy and saline-alkali soils. The response of alfalfa root morphology and rhizosphere processes to different P fertilizers was investigated. The results showed that shoot biomass of alfalfa was slightly higher in sandy soil than in saline-alkali soil. Shoot biomass of alfalfa increased by 223%-354% in sandy soil under P treatments compared with the control, and total root length increased significantly by 74% and 53% in DAP and SSP treatments, respectively. In saline-alkali soil, alfalfa shoot biomass was significantly increased by 229% and 275% in KP and DAP treatments, and total root length was increased by 109% only in DAP treatment. Net P uptake of alfalfa in DAP treatment was the highest in both soils, which were 0.73 and 0.54 mg plant , respectively. Alfalfa shoot P concentration was significantly positively correlated with shoot and root biomass ( < 0.05, 0.01 or 0.001) whereas negatively correlated with acid phosphatase concentration ( < 0.05). Improvement of plant growth and P uptake induced by P fertilizer application was greater in sandy soil than in saline-alkali soil. DAP and KP was the most efficient P fertilizers in both sandy soil and saline-alkali soil.

In this paper, we provide direct evidence of the importance of root hairs on pore structure development at the root–soil interface during the early stage of crop establishment. This was achieved by use of high-resolution (c. 5 μm) synchrotron radiation computed tomography (SRCT) to visualise both the structure of root hairs and the soil pore structure in plant–soil microcosms. Two contrasting genotypes of barley (Hordeum vulgare), with and without root hairs, were grown for 8d in microcosms packed with sandy loam soil at 1.2 g cm−3 dry bulk density. Root hairs were visualised within air-filled pore spaces, but not in the fine-textured soil regions. We found that the genotype with root hairs significantly altered the porosity and connectivity of the detectable pore space (> 5 μm) in the rhizosphere, as compared with the no-hair mutants. Both genotypes showed decreasing pore space between 0.8 and 0.1mm from the root surface. Interestingly the root-hair-bearing genotype had a significantly greater soil pore volume-fraction at the root–soil interface. Effects of pore structure on diffusion and permeability were estimated to be functionally insignificant under saturated conditions when simulated using image-based modelling.