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Soil pH is one of the main drivers of soil microbial functions, including carbon use efficiency (CUE), the efficiency of microorganisms in converting substrate C into biomass, a key parameter for C sequestration. We evaluated liming effects after maize-litter addition on total CUE (including microbial residues), CUE of microbial biomass (CUE MB ), and fungal biomass on an acidic Acrisol with a low C. We established a 6-week incubation experiment to compare limed and unlimed Acrisol treatments and a reference soil, a neighboring Nitisol with optimal pH. Fungal biomass (ergosterol) increased ~ 10 times after litter addition compared with soils without litter, and the final amount was greater in the limed Acrisol than the Nitisol. Litter addition induced a positive priming effect that increased with increasing pH. The increases in soil pH also led to increases in litter-derived CO 2 C and decreases in particulate organic matter (POM)C. Thus, in spite of increasing microbial biomass C, CUE decreased with increasing pH and CUE MB was similar across the three soils. CUE MB was positively associated with saprotrophic fungi, implying that fungi are more efficient in incorporating litter-derived C into microbial, especially fungal biomass after 42 days. By including undecomposed maize litter and microbial residues, CUE provided a more comprehensive interpretation of pH and liming effects than CUE MB . Nevertheless, longer-term studies may provide further information on substrate-C turnover and the persistence of liming and pH effects.

Background and aims Studies verify that intercropping effects soil nutrient content, enzyme activity, aggregate stability, arbuscular mycorrhizal fungi (AMF) community and glomalin-related soil protein (GRSP) content in Red Soil (Ultisol in the USDA Taxonomy, Acrisol in the WRB Soil Taxonomy) on sloping farmland. However, the comprehensive contribution of soil nutrients, enzyme activity, AMF community and GRSP to the characteristics of water-stable aggregates under different planting patterns of maize and soybean are not fully understood. Methods A long-term field experiment commenced in 2018. Three treatments of maize ( Zea mays L.) monoculture, soybean ( Glycine max L.) monoculture and maize-soybean intercropping were established in an experimental field. The planting patterns, crop varieties and fertilizer rates of each plot were consistent for each of the four years of experiments (2018–2021). Results Results showed that intercropping can improve the concentrations of alkali-hydrolysable nitrogen, available phosphorus and total extractable glomalin-related soil protein, the activities of enzyme (urease, invertase, acid phosphatase and catalase) and the mean weight diameter (MWD) in the rhizosphere soil of maize and soybean. Moreover, results proved that intercropping can potentially increase AMF diversity and macro-aggregates (> 2.0 mm) in the maize rhizosphere and macro-aggregates (0.5-2.0 mm) in the soybean rhizosphere. Conclusion Intercropping of maize and soybean can increase soil aggregate stability in the rhizosphere of the two crops. The easily extractable glomalin-related soil protein was the main factor affecting soil aggregate stability and the formation of > 2.0 mm aggregates in the maize rhizosphere. Soil organic matter was the main factor affecting soil aggregate stability and the formation of 0.5–2.0 mm aggregates in the soybean rhizosphere.

Aims Plant roots can significantly alter soil pH and the chemical concentration and distribution of different elements in the rhizosphere environment. Here we ask whether cadmium (Cd) bioavailability in the rhizosphere of Cd-hyperaccumulator Sedum plumbizincicola can be influenced by root-induced effects on soil pH. Methods The Cd-hyperaccumulator S. plumbizincicola and the Cd non-hyperaccumulator ecotype Sedum alfredii were both grown in four different Cd-contaminated soils. We used the planar optode imaging technique to produce two-dimensional and high-resolution measurements of soil pH. Shoot excess cation concentration, root architecture and Cd concentrations ([Cd]) in soil pore water were also measured. Spatial analyses based on kernel density estimate of roots (KDE) and a Moran’s I correlogram were performed to assess spatial patterns and potential relationships among root distribution, soil pH and [Cd]. Results Both Sedum species showed root-induced increases in soil acidification (i.e. soil pH decreases of 0.1 to 0.62 units), which were clearly associated with greater root density of these plants. Remarkable excess cation uptakes by both Sedum species were detected and likely a driving factor for the root-induced acidification. The presence of the roots of S. plumbizincicola were then related to higher [Cd] in the rhizosphere than in bulk soil in Orthic Acrisol (+342%) and in Hydragric Antrosol soils (+296%). The hyperaccumulator S. plumbizincicola had larger root systems, higher acidification ability, and was associated with greater soil [Cd] than S. alfredii . Spatial patterns of root distribution and soil pH were similar between Sedum plants, however, spatial patterns of [Cd] differed across polluted soils. Conclusion Rhizosphere acidification induced by S. plumbizincicola plants can play an important role on soil Cd mobilization, but overall effects on soil Cd bioavailability will depend on intrinsic soil biogeochemical properties.

The tropical forests of the Amazon Basin occur on a wide variety of different soil types reflecting a rich diversity of geologic origins and geomorphic processes. We here review the existing literature about the main soil groups of Amazonia, describing their genesis, geographical patterns and principal chemical, physical and morphologic characteristics. Original data is also presented, with profiles of exchangeable cations, carbon and particle size fraction illustrated for the principal soil types; also emphasizing the high diversity existing within the main soil groups when possible. Maps of geographic distribution of soils occurring under forest vegetation are also introduced, and to contextualize soils into an evolutionary framework, a scheme of soil development is presented having as its basis a chemical weathering index. We identify a continuum of soil evolution in Amazonia with soil properties varying predictably along this pedogenetic gradient.

Visible and near-infrared (Vis-NIR) diffuse reflectance spectroscopy with partial least squares (PLS) regression is a quick, cost-effective, and promising technology for predicting soil properties. The advantage of PLS regression is that all available wavebands can be incorporated in the model, while earlier studies indicate that PLS models include redundant wavelengths, and selecting specific wavebands can refine PLS analyses. This study evaluated the performance of PLS regression with waveband selection using Vis-NIR reflectance spectra to estimate the total carbon (TC) and total nitrogen (TN) in soils collected mainly from the surface of upland and lowland rice fields in Madagascar (n = 59; after outliers were removed). We used iterative stepwise elimination-based PLS (ISE-PLS) to estimate soil TC and TN and compared the predictive ability with standard full-spectrum PLS (FS-PLS). The predictive abilities were assessed using the coefficient of determination (R2), the root mean squared error of cross-validation (RMSECV), and the residual predictive deviation (RPD). Overall, ISE-PLS using first derivative reflectance (FDR) showed a better predictive accuracy than ISE-PLS for both TC (R2 = 0.972, RMSECV = 0.194, RPD = 5.995) and TN (R2 = 0.949, RMSECV = 0.019, RPD = 4.416) in the soil of Madagascar. The important wavebands for estimating TC (12.59% of all wavebands) and TN (3.55% of all wavebands) were selected from all 2001 wavebands over the 400–2400 nm range using ISE-PLS. These findings suggest that ISE-PLS based on Vis-NIR diffuse reflectance spectra can be used to estimate soil TC and TN contents in Madagascar with an improved predictive accuracy.

AimsThis study investigated the influence of climate and soil on the exudation rate and polysaccharide composition of aerial nodal root mucilage from drought-resistant and drought-susceptible maize varieties.MethodsTwo maize varieties were grown in two different soils (sandy-clay loam Acrisol and loam Luvisol) under simulated climatic conditions of their agroecological zones of origin in Kenya and Germany. The exudation rate of mucilage from the aerial nodal roots was quantified as dry weight per root tip per day and the mucilage was characterized for its polysaccharide composition.ResultsOn average, the mucilage exudation rate was 35.8% higher under the Kenyan semi-arid tropical than under the German humid temperate climatic conditions. However, cultivation in the loam Luvisol soil from Germany led to 73.7% higher mucilage exudation rate than cultivation in the sandy-clay loam Acrisol soil from Kenya, plausibly due to its higher microbial biomass and nutrient availability. The drought-resistant Kenyan maize variety exuded 58.2% more mucilage than the drought-susceptible German variety. On average, mucilage polysaccharides were composed of 40.6% galactose, 26.2% fucose, 13.1% mannose, 11% arabinose, 3.5% glucose, 3.2% xylose, 1.3% glucuronic acid, and 1% an unknown uronic acid. Overall, significantly higher proportions of the uronic acids were found in the mucilage of the plants grown in the Kenyan sandy-clay loam soil and under the Kenyan semi-arid tropical climatic conditions.ConclusionsMaize is able to enhance its mucilage exudation rate under warm climatic conditions and in soils of high microbial activity to mitigate water stress and support the rhizosphere microbiome, respectively.

Soil amendment with biochar from oil palm biomass has been found to improve the quality of the infertile weathered soils and enhance crop productivity in the humid tropics. Meanwhile, the field information on microbial responses to oil palm-derived biochar application and its residual effect in acidic tropical soils is still limited. A field study was carried out over three cropping cycles of sweet corn on a Haplic Acrisol of Peninsular Malaysia. The soil was amended once with oil palm kernel shell (OPKS) biochar before the first cropping cycle, with or without inorganic fertiliser. Soil samples were taken at each harvesting stage and analysed for soil pH, cation exchange capacity (CEC), organic C, total N, available P, microbial biomass (C (MBC), N (MBN), and P (MBP)). Microbial biomass ratios (MBC : MBN, MBC : MBP) were calculated. The total bacterial and fungal populations were quantified from soil genomic DNA, employing qPCR amplification of the 16S rDNA and ITS gene. The sole application of biochar and its combined application with fertiliser, increased soil pH, CEC, organic C and N. Coapplication of OPKS biochar and NPK fertiliser hindered N loss in the second cycle. The bacterial and fungal abundance was stimulated following biochar treatment majorly due to the elevation of soil pH and CEC. The ratio of MBC : MBN had a significant negative correlation with N, signifying that this ratio could reflect soil N content and be used as a soil fertility indicator.

Background and aimsThere are controversies in the literature regarding the edaphic factors that may interfere with the occurrence of Rhizoctonia root rot in soybean, a disease caused by the soilborne fungus Rhizoctonia solani. The objective of this study was to determine which chemical and physical soil attributes are associated with the occurrence of Rhizoctonia root rot in soybean grown in a subtropical environment.MethodsFour fields with history of disease occurrence were selected, according to their soil class variability (Acrisol, Cambiol Ta, Cambisol Tb and Nitisol), in the municipality of Soledade, Rio Grande do Sul, Brazil. In each field, soil samples were collected in the 0–10 cm layer, inside and outside the bare patch with Rhizoctonia root rot, and in four repetitions for analysis of chemical and physical attributes.ResultsThe exchangeable contents and the saturation by aluminum and the extractable copper content were higher in the patch with the disease. On the other hand, the indicators of lower soil acidity (higher exchangeable calcium and magnesium content, pH value and base saturation) were the ones most associated with the area outside the patch. There was some association with soil texture regarding high clay content where the disease has been observed.ConclusionChemical attributes related to acidic soils are the ones most associated to the incidence of Rhizoctonia root rot in soybean grown in a subtropical environment in southern Brazil.

Addition of labile carbon (C) inputs to soil can accelerate or slow down the decomposition of soil organic matter (SOM), a phenomenon known as priming effect (PE). However, the magnitude and direction of PE is often difficult to predict, consequently making its relationship with labile C inputs and nutrient availability elusive. To assess this relationship, we added 13C labelled glucose (corresponding to 50% of initial soil microbial biomass C) to two soil types (Vertisol and Acrisol) with different concentrations of available N and from four land use systems (agricultural, pasture, grassland and shrubland). Parallel laboratory incubations i.e. short-term (6 days) and long-term (6 months), were set up to determine the effect of land use and soil type (N availability) on PE. Addition of labelled glucose in solution led to the retardation of SOM mineralization (negative PE) in both soil types and across all land use systems. This is attributed to preferential substrate utilization characterized by the higher mineralization of added glucose. Land use systems and soil types with higher N-availability displayed weaker negative PE, which is in line with the stoichiometric decomposition theory. In conclusion, our study demonstrate that N-availability plays a major role in determining mineralization of labile C inputs, magnitude and direction of PE in the studied dryland soils and land use systems. The fact that 15–27% of the added 13C remained in the soil at the end of the 6 months incubation and PE was negative, indicates that continuous labile C inputs could contribute to C immobilization and stabilization in these semiarid soils. Moreover, 13C glucose remaining in soils after 6 months in semi-natural pastures was comparable to those under natural grassland and shrubland systems especially in Acrisols. This demonstrates that incorporation and maintaining a perennial cover of native pastures has the potential to increase C sequestration in African semi-arid agricultural soils and landscapes.

According to the resource allocation model for extracellular enzyme synthesis, microorganisms should preferentially allocate their resources to phosphorus (P)-acquiring enzyme synthesis when P availability is low in soils. However, the validity of this model across different soil types and soils differing in their microbial community composition has not been well demonstrated. Here we investigated whether the resource allocation model for phosphatase synthesis is applicable across different soil types (Andosols, Acrisols, Cambisols, and Fluvisols) and land uses (arable and forest), and we examined which soil test P and/or P fraction microorganisms responded to when investing their resources in phosphatase synthesis in the soils. The ratio of alkaline phosphatase (ALP) to β-D-glucosidase (BG) activities in the arable soils and the ratio of acid phosphatase (ACP) to BG activities in the forest soils were significantly negatively related with the available inorganic P concentration. We also observed significant effects of available inorganic P, pH, soil types, and land uses on the (ACP + ALP)/BG ratio when the data for the arable and forest soils were combined and used in a stepwise multiple regression analysis. These results suggest that microbial resource allocation for phosphatase synthesis is primarily controlled by available inorganic P concentration and soil pH, but the effects of soil types and land uses are also significant.