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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.

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.

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.

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.

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.

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.

The retention and mobility of arsenic (As) in soil depend on various physical and chemical factors. The knowledge of the sorption–desorption chemistry of As in soil is necessary for predicting the fate and behavior of As in soil environments. Therefore, this study assessed different organic (sugarcane bagasse and vermicompost) and inorganic amendments (steel slag and fly ash) for their impact on sorption–desorption of As in texturally different contaminated soils (of sandy clay (SC) and sandy clay loam (SCL) texture) to understand the effect of amendments on As retention and mobility. The results showed that the sorption data fitted well with both Langmuir and Freundlich isotherm equations. The As sorption capacity was significantly enhanced with the application of all amendments. At 30 °C, the adsorption maxima ( q max ) of SC soils enhanced to a greater extent following the order: steel slag (278 mg kg −1 ) > sugarcane bagasse (264 mg kg −1 ) > vermicompost (246 mg kg −1 ) > fly ash (242 mg kg −1 ). Whereas, in SCL, the order of q max was steel slag (145 mg kg −1 ) > sugarcane bagasse (132 mg kg −1 ) > fly ash (120 mg kg −1 ) > vermicompost (118 mg kg −1 ). Desorption index (DI) was invariably to > 1 at both temperatures with the application of amendments indicating hysteretic desorption of As. The free energy change (Δ G °) was negative in all treatments and soils (indicating a favorable sorption process) with positive entropy change (Δ S °) values. The study recommends steel slag as the most effective amendment for enhancing As (V) retention in contaminated soils, due to its higher sorption capacity compared to other amendments like sugarcane bagasse, vermicompost, and fly ash. The amendments generally improved As sorption in both soils, reducing As mobility and potentially limiting its environmental spread.

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.

Biochar is a carbon-rich solid material derived from the pyrolysis of agricultural and forest residual biomass. Previous studies have shown that biochar is suitable as an adsorbent for soil contaminants such as heavy metals and consequently reduces their bioavailability. However, the long-term effect of different biochars on metal extractability or soil health has not been assessed. Therefore, a 1-year incubation experiment was carried out to investigate the effect of biochar produced from bamboo and rice straw (at temperatures ≥500 °C) on the heavy metal (cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn)) extractability and enzyme activity (urease, catalase, and acid phosphatase) in a contaminated sandy loam paddy soil. Three rates (0, 1, and 5 %) and two mesh sizes (<0.25 and <1 mm) of biochar applications were investigated. After incubation, the physicochemical properties, extractable heavy metals, available phosphorus, and enzyme activity of soil samples were analyzed. The results demonstrated that rice straw biochar significantly (P < 0.05) increased the pH, electrical conductivity, and cation exchange capacity of the soil, especially at the 5 % application rate. Both bamboo and rice straw biochar significantly (P < 0.05) decreased the concentration of CaCl₂-extractable heavy metals as biochar application rate increased. The heavy metal extractability was significantly (P < 0.01) correlated with pH, water-soluble organic carbon, and available phosphorus in soil. The 5 % application rate of fine rice straw biochar resulted in the greatest reductions of extractable Cu and Zn, 97.3 and 62.2 %, respectively. Both bamboo and rice straw biochar were more effective at decreasing extractable Cu and Pb than removing extractable Cd and Zn from the soil. Urease activity increased by 143 and 107 % after the addition of 5 % coarse and fine rice straw biochars, respectively. Both bamboo and rice straw biochars significantly (P < 0.05) increased catalase activity but had no significant impact on acid phosphatase activity. In conclusion, the rice straw biochar had greater potential as an amendment for reducing the bioavailability of heavy metals in soil than that of the bamboo biochar. The impact of biochar treatment on heavy metal extractability and enzyme activity varied with the biochar type, application rate, and particle size.

We carried out mesocosm experiments using either the anecic earthworm Lumbricus terrestris or the endogeic earthworm Allolobophora chlorotica and loam, silt loam and sandy loam soils to investigate the differing impact of these earthworm of different ecotypes on aggregate formation (percentage water stable aggregates, %WSA) and soil water holding capacity (WHC), two soil properties that underpin many of the ecosystem services provided by soils. Earthworms significantly increased %WSA (by 16–56% and 19–63% relative to earthworm-free controls for L. terrestris and A. chlorotica, respectively). For L. terrestris, this increase was significantly greater in the upper 6.5 cm of the soil where their casts were more obviously present. Allobophora chlorotica treatments significantly increased WHC by 7–16%. L. terrestris only caused a significant increase in WHC (of 11%) in the upper 6.5 cm of the sandy loam soil. Linear regression indicated a consistent relationship between increases in %WSA and WHC for both earthworm species. However, for a given %WSA, WHC was higher for A. chlorotica than L. terrestris likely due to the known differences in their burrow structure. Overall, earthworms increased soil %WSA and WHC but the significant species/ecotype differences need to be considered in discussions of the beneficial impacts of earthworms to soil properties.