Changes In Soil pH Research Articles

Impact of grassland afforestation with contrasting tree species on soil phosphorus fractions and alkaline phosphatase gene communities

While grassland afforestation can enhance the net mineralization of soil organic phosphorus (P), the mechanisms involved and impacts of different tree species are not clear. Soil samples were taken from replicated adjacent field plots that had been maintained under unfertilized grazed grassland, radiata pine (Pinus radiata), and eucalyptus (Eucalyptus nitens) for 19 years. Soil phosphorus fractions were determined together with alkaline phosphatase activity and associated phoD and phoX gene analysis. Afforestation significantly decreased soil organic P, microbial biomass P, and soil alkaline phosphatase activity, and increased inorganic P bioavailability, although no differences were measured between radiata pine and eucalyptus. While distinct separation of phoD and phoX gene bacterial communities was associated with afforestation of grassland, separation between tree species was less pronounced. Shifts in phoD and phoX gene community structure were more related to soil moisture and pH than P status. Overall, we found that changes in soil moisture and pH associated with afforestation, rather than tree species per se, significantly affected the occurrence and diversity of bacterial alkaline phosphatase. This highlights the potential effects of changes in land-use and management on soil P dynamics and bioavailability.

Study on the new dynamics and driving factors of soil pH in the red soil, hilly region of South China.

Soil acidification has always been a substantial eco-environmental problem restricting agricultural development in the red soil region of southern China. It is necessary to determine the dynamic change in soil pH in this area to formulate regional agricultural and environmental management measures. Yujiang County, a typical county with red soil acidification in southern China, was selected as the study area. Based on soil data from 1982, 2007, and 2018, the spatiotemporal variation characteristics and the latest changes in soil pH in the county were analyzed. The results show that the soil pH in Yujiang County decreased from 5.66 to 4.74 and then increased to 4.96 from 1982 to 2018, showing a trend of first decreasing and then increasing. According to the spatial distribution characteristics of soil pH, the low soil pH values in the three periods were mainly distributed in the northern mountainous areas with more forestland and dry land area and some southern hilly areas, while the paddy soil pH values in the middle low hilly areas were relatively higher. The soil pH decreased rapidly from 1982 to 2007, showing a large area of acidification. In 2007, the proportions of acidic (4.5 < pH < 5.5) and strongly acidic (pH < 4.5) soils increased by 67.37% and 10.6%, respectively, compared with that in 1982. However, from 2007 to 2018, the soil pH of the whole county increased, and the acidification trend was alleviated, which is of great significance to the regional red soil ecological environment. Through the analysis of the main factors affecting the change in soil pH, it was found that the sharp decline in soil pH in Yujiang County during 1982-2007 was mainly caused by acid rain and excessive nitrogen application. From 2007 to 2018, no significant reduction in nitrogen fertilizer in this area occurred, and although the increase in soil organic matter contributed to alleviating soil acidification, the analysis showed that the decrease in acid rain was the main reason for the rise in soil pH in Yujiang County. At the same time, notably, there is a large area of soil in the area that is still acidic, and effective control of soil acidification is still an important ecological and environmental issue in this area. In order to further improve the pH value of soil in red soil region, it is suggested that on the basis of continuous improvement of acid rain, in addition to increasing soil organic matter by returning straw to field and other measures, appropriate amount of lime or alkaline biochar can be applied to better improve the soil ecological environment in red soil hilly region.

Effect of inorganic carbonate and organic matter in thermal treatment of mercury-contaminated soil

Thermal treatment of mercury (Hg)-contaminated soil was studied to investigate the desorption behavior of Hg at different temperatures. The soil samples were collected from two locations with different land uses around the mine and industrial site. The effect of soil properties such as inorganic carbonate minerals and organic matter content on Hg desorption was investigated to understand the thermal desorption process. The effect of soil composition on Hg desorption showed that behavior at 100 °C was similar, but a different behavior could be found at 300 °C. The thermal desorption efficiency at 300 °C is affected by the thermal properties of soils and the Hg desorption capacity of the soils. The Hg from both soil types was removed above 300 °C, and Hg was effectively removed from mine soil due to the partial decomposition of carbonate in the soil composition, while industrial soil showed that desorption would be restrained by Hg organic matter complexes due to organic matter content. Despite a relatively higher concentration of Hg in the mine soil, Hg removal efficiency was greater than that in the industrial soil. Sequential extraction results showed that only the Hg fractions (residual fractions, step 6) in mine soil changed, while the industrial soil was affected by changes in Hg fractions (step 3 to step 6) at 300 °C. Changes in soil pH during thermal desorption are also influenced by heating time and temperature. Therefore, the mechanisms of Hg desorption during thermal treatment were observed by soil properties. The volatilization of Hg in the soil is induced by organic carbon, while soil Hg release is controlled by organic matter complexes.

Inversion of soil pH during the dry and wet seasons in the Yinbei region of Ningxia, China, based on multi-source remote sensing data

Dynamic monitoring of soil pH changes across different seasons is crucial to understanding soil alkalization and preventing land degradation in arid regions. Here, we explored the potential use of ground-measured hyperspectral data and Landsat 8 OLI remote-sensing images in the inversion of soil pH during the dry and wet seasons. The measured and image data of spectral reflectance and corresponding soil pH data obtained in the laboratory were used to analyze the spatiotemporal patterns of soil pH in the Yinbei region, Ningxia, China. First, the hyperspectral data were resampled to match the range of the image bands; then 11 spectral indices were calculated based on the two sets of spectral data. Principal component analysis (PCA), stepwise regression (SR), and gray relational analysis (GRA) were used to select feature variables (sensitive bands and spectral indices) from the spectral data. The back propagation neural network (BPNN), support vector machine (SVM), ridge regression (RR), and geographically weighted regression (GWR) were used to construct soil pH inversion models. A total of 24 models established using the different methods were compared in terms of the determination coefficients of the calibration set (Rc2) and prediction set (Rp2), root mean square error (RMSE), and relative percent deviation (RPD). The results revealed that the mean soil pH in the study area was higher during the dry season (9.28) than the wet season (9.11). The resampled hyperspectral data correlated well with the image data under different levels of soil alkalization (R2 > 0.9652). Of the different models examined, the global regression models (BPNN, SVM, and RR) were superior to the local regression model (GWR), with better inversion results obtained in the wet than the dry season. The BPNN and SVM models performed better than the RR model, and the PCA-SVM model based on measured data in the dry season achieved the best overall performance (Rp2 = 0.9724 and RPD = 5.76). These findings provide valuable information on the distribution of Solonetzs land in the study area, facilitating management of soil alkalization and degradation in the Yinbei region.

Effects of Landscape Positions and Landscape Types on Soil Properties and Chlorophyll Content of Citrus in a Sloping Orchard in the Three Gorges Reservoir Area, China

In recent years, soil degradation and decreasing orchard productivity in the sloping orchards of the Three Gorges Reservoir Area of China have received considerable attention both inside and outside the country. More studies pay attention to the effects of topography on soil property changes, but less research is conducted from the landscape. Therefore, understanding the effects of landscape positions and landscape types on soil properties and chlorophyll content of citrus in a sloping orchard is of great significance in this area. Our results showed that landscape positions and types had a significant effect on the soil properties and chlorophyll content of citrus. The lowest soil nutrient content was detected in the upper slope position and sloping land, while the highest exists at the footslope and terraces. The chlorophyll content of citrus in the middle and upper landscape position was significantly higher than the footslope. The redundancy analysis showed that the first two ordination axes together accounted for 81.32% of the total variation, which could be explained by the changes of soil total nitrogen, total phosphorus, total potassium, available nitrogen, available potassium, organic matter, pH, and chlorophyll content of the citrus. Overall, this study indicates the significant influence of landscape positions and types on soil properties and chlorophyll content of citrus. Further, this study provides a reference for the determination of targeted land management measures and orchard landscape design so that the soil quality and orchard yield can be improved, and finally, the sustainable development of agriculture and ecology can be realized.

Different regulation of soil structure and resource chemistry under animal- and plant-derived organic fertilizers changed soil bacterial communities

The application of organic fertilizers affects soil aggregates, nutrient situations and bacterial communities. However, the direct and indirect effect of different resource-derived organic fertilizers on soil bacterial communities and their links to soil aggregate is poorly understood. Here, we carried out a field experiment for three years to investigate the linkages between soil aggregates and bacterial diversity in a dryland red soil. Six treatments were compared: (no fertilizer (CK), chemical fertilizer (CF), 60% chemical fertilizer + straw at 4500 kg ha−1 (RFS), 60% CF + biochar at 1500 kg ha−1 (RFB), 60% CF + pig manure at 15000 kg ha−1 (RFP), and 60% CF + vermicompost at 15000 kg ha−1 (RFV)). The application of pig manure or vermicompost (animal-derived organic fertilizer) enhanced the soil pH, the content of large macroaggregate (>2 mm), organic matter, available phosphorous and available nitrogen, while decreased the content of small macroaggregate (0.25–2 mm) and microaggregate (<0.25 mm) compared with other treatment. Relative abundances of the dominant bacterial phyla varied across chemical and organic fertilizer treatments, primarily by increasing Bacteroidetes, Gemmatimonadetes, and decreasing Chloroflexi, Acidobacteria and Planctomycetes. Organic fertilizers particularly animal-derived organic fertilizer enhanced the Richness, Shannon and Pielou indices of soil bacteria compared with chemical fertilizer treatment. The regulation of animal-derived organic fertilizers on bacterial community structure (accounting for 53.04% of the variance) was greater than that of plant-derived organic fertilizers (straw and biochar, accounting for 7.69% of the variance). The bacterial alpha diversity showed positive correlation with large macroaggregate, but a negative correlation with small macroaggregate. Further, structural equation models indicated that animal-derived organic fertilizers impacted bacterial diversity positively by regulating resource availability through soil aggregates. However, for the plant-derived organic fertilizers, bacterial diversity was negatively affected by soil aggregates alone, but this was not regulated by fertilization. In addition, animal-derived organic fertilizer also positively affected soil bacteria diversity indirectly via changing soil pH and resource availability. Overall, the different impacts of animal- and plant-derived organic fertilizers on soil bacterial community highlighting the pivotal role of soil habitat architecture and resource chemistry changed by organic fertilizers in the regulation of soil bacteria, which may influence soil quality and agricultural sustainable development.

Soil amendments and fumigation for the management of Fusarium wilt of bunching spinach in Ontario, Canada

Fusarium wilt, caused by Fusarium oxysporum f. sp. spinaciae, is a significant constraint to the summer production of bunching spinach in Ontario, Canada. Trials were conducted to assess soil amendments (limestone) and fumigants (metam sodium, chloropicrin and dazomet) for the suppression of Fusarium oxysporum f. sp. spinaciae. Dolomitic limestone applied at a rate of 10 and 20 t ha−1 reduced the incidence of fusarium wilt by 10 and 47%, respectively, even though there was little change in soil pH (7.4–7.5) or soil calcium concentration (1598 vs 1483 in 20 t lime vs control, respectively. Fusarium wilt severity was lower in the 2013 than in the 2014 growing season. In 2014, the fumigants metam sodium, chloropicrin and dazomet reduced the population of Fusarium spp. propagules in the soil by 84, 93 and 100%, respectively. Chloropicrin provided the best control of the spinach fusarium wilt pathogen compared to the other fumigants but was only assessed one year. Chloropicrin was the only treatment to truly reduced spinach Fusarium wilt by preventing vascular discolouration. Brassica juncea and calcium cyanamide were not effective at suppressing the disease. A high rate of nitrogen, consisting of polymer coated sulphur coated urea, was applied to provide the same rate of nitrogen as the Brassica juncea and calcium cyanamide treatments. Applying nitrogen in this formulation was as effective as metam sodium in reducing the incidence and severity of Fusarium wilt and should be evaluated further to determine the economics and environmental impact of this approach.

Comparing Biochar-Swine Manure Mixture to Conventional Manure Impact on Soil Nutrient Availability and Plant Uptake—A Greenhouse Study

The use of swine manure as a source of plant nutrients is one alternative to synthetic fertilizers. However, conventional manure application with &gt;90% water and a low C:N ratio results in soil C loss to the atmosphere. Our hypothesis was to use biochar as a manure nutrient stabilizer that would slowly release nutrients to plants upon biochar-swine manure mixture application to soil. The objectives were to evaluate the impact of biochar-treated swine manure on soil total C, N, and plant-available macro- and micronutrients in greenhouse-cultivated corn (Zea mays L.) and soybean (Glycine max (L.) Merr.). Neutral pH red oak (RO), highly alkaline autothermal corn stover (HAP), and mild acidic Fe-treated autothermal corn stover (HAPE) biomass were pyrolyzed to prepare biochars. Each biochar was surface-applied to swine manure at a 1:4 (biochar wt/manure wt) ratio to generate mixtures of manure and respective biochars (MRO, MHAP, and MHAPE). Conventional manure (M) control and manure-biochar mixtures were then applied to the soil at a recommended rate. Corn and soybean were grown under these controls and treatments (S, M, MRO, MHAP, and MHAPE) to evaluate the manure-biochar impact on soil quality, plant biomass yield, and nutrient uptake. Soil organic matter significantly (&lt;0.05) increased in all manure-biochar treatments; however, no change in soil pH or total N was observed under any treatment. No difference in soil ammonium between treatments was identified. There was a significant decrease in soil Mehlich3 (M3) P and KCl extractable soil NO3− for all manure-biochar treatments compared to the conventional M. However, the plant biomass nutrient concentrations were not significantly different from control manure. Moreover, an increasing trend of plant total N and decreasing trend of P in the plant under all biochar-manure treatments than the controls were noted. This observation suggests that the presence of biochar is capable of influencing the soil N and P in such a way as not to lose those nutrients at the early growth stages of the plant. In general, no statistical difference in corn or soybean biomass yield and plant nutrient uptake for N, P, and K was observed. Interestingly, manure-biochar application to soil significantly diluted the M3 extractable soil Cu and Zn concentrations. The results attribute that manure-biochar has the potential to be a better soil amendment than conventional manure application to the soil.

Flood Pulse Irrigation of Meadows Shapes Soil Chemical and Microbial Parameters More Than Mineral Fertilization

While mineral fertilization increases agricultural yields, it also bears the risk of contaminating non-target ecosystems and negatively affecting soil chemical parameters and microbial communities. This calls for alternative and more sustainable agricultural practices that reduce the use of fertilizers. Flood pulse irrigation could be an alternative to mineral fertilization of hay meadows, since it increases the yield with little or no application of fertilizer. However, the positive and negative implications of flood pulse irrigation on soil chemical parameters and particularly soil microbial communities are still largely unknown. In this study, we assessed shifts in soil microbial communities (SMC) as a response to changes in soil chemical parameters after flood pulse irrigation and/or fertilization of meadows. We determined soil chemical (Corg, Ntot, water extractable N, P, K, pH) and microbial (phospholipid-derived fatty acids, PLFA) parameters of 12 meadows in a 2 × 2 factorial design, comprising flood pulse irrigation and fertilization. Corg, Ntot, and water content as well as microbial biomass were higher in flood-irrigated than in non-flooded soils. Soil microbial biomass positively correlated with Corg, Ntot, and water extractable N. Gram-negative bacteria significantly increased, whereas the fungi/bacteria ratio significantly decreased in flood-irrigated soils compared to non-flooded soils. Arbuscular mycorrhizal fungi were positively correlated with soil pH. Flood pulse irrigation seemed to promote the build-up of a larger soil carbon and nitrogen pool as well as higher water content and microbial biomass. By this, it potentially mitigated negative mineral fertilization effects such as changed soil pH and reduced carbon use efficiency. We conclude that flood pulse irrigation may represent a sustainable alternative to mineral fertilization.

The influence of lime application on the chemical and physical characteristics of acidic grassland soils with impeded drainage

AbstractSoil acidity and poor nutrient use efficiency are major limiting factors as regards output potential on heavy soils, soils which are dominated by high proportions of clay and organic matter, with impeded drainage, high buffering capacity and located in high rainfall areas. Lime is applied in order to counteract these limiting factors and in turn improve agricultural output and productivity. The current study investigates the effects of two commonly used lime products at three comparable treatment rates, ground lime (7.5, 5 and 2.5 tonne/ha) and granulated lime (7.5, 2.5 and 1.5 tonne/ha), applied across three distinct sites. The ability of each lime product and treatment rate to counteract soil acidity, increase nutrient availability and influence soil physical structure was assessed over time. On average across sites, 1 tonne/ha of each lime product increased soil pH by 0.15 and 0.21 pH units between ground and granulated lime, respectively. Site 3 experienced the greatest increase change in soil pH in comparison to the other two sites, largely due to lower clay content and cation exchange capacity. Granulated lime was 5.7 times more expensive than ground lime in its ability to reduce soil acidity. The high treatment rate showed the greatest reduction in soil acidity, aluminium and iron concentration as a mean across all sites. Morgan's soil test phosphorus concentration increased across all sites, with treatment rates having no effect on the rate of increase. There was evidence of reduced soil compaction and lime application showed no negative implication on soil physical structure.

Changes in soil bacterial communities with increasing distance from maize roots affected by ammonium and nitrate additions

It is well known that the interactive effect of nitrogen (N) fertilizer and plant root would cause distinct microbial communities and associations between root surface and bulk soil. However, little is known about the continuous changes of these microbial parameters along an increasing distance from roots under different N forms. Ammonium (NH4+) and nitrate (NO3−) are two major inorganic nitrogen (N) forms available for plant growth, and may have different effects on both plant growth and soil microbiome. Here, a rhizobox containing a root zone (root growing area) and various soil compartments (0–0.5 cm, 0.5–1 cm, 1–2 cm, 2–4 cm, and 4–9 cm from the root zone) was designed to investigate the spatial distribution of soil chemical properties, bacterial diversity, and community structure along a distance from maize (Zea mays L.) roots with the addition of 15N-labeled NH4+ or NO3− in an acidic red soil. Addition of NH4+ or NO3− reduced soil bacterial diversity only in the maize root zone. The structures of soil bacterial communities differed more significantly with addition of NH4+ or NO3− in the root zone and 0.5 cm away from the root zone compared with other soil compartments. Soil pH was the major driver of bacterial community assembly during plant uptake of N. Maize roots recruited acidophilic bacteria (e.g. Acidibacter, Burkholderia, and Catenulispora) under NH4+ treatment, and recruited bacteria that prefer higher pH (e.g. Sphingomonas, Sphingobium, Azospirillum, and Novosphingobium) under NO3− treatment. We concluded that the spatial patterns of soil bacterial communities at different distances from the maize root zone were influenced by the combined effects of the N form and rhizosphere, mainly as a result of changes in soil pH during the uptake of NH4+ and NO3− by maize roots.

Comparative study on different organic acids for promoting Solanum nigrum L. hyperaccumulation of Cd and Pb from the contaminated soil

Organic acids-assisted phytoremediation is a promising strategy to remove metal pollutants from the soil. However, few reports have focused on the mechanisms of organic acids promoting the uptake of heavy metals by hyperaccumulators. In this study, 5 types of organic acids, namely polybasic carboxylic acids, acidic amino acids, acidic plant growth regulators, phosphoric and gluconic acids, were comprehensively investigated the effects on the solubility of Cd and Pb in the soil along with their uptake by Cd hyperaccumulator Solanum nigrum L. The results indicated that the addition of Hydroxyethylidene-1,1-diphosphonic acid (HEDP) and d-Gluconic acid (D-GA) effectively extracted the most of acid-extractable and some of reducible and oxidizable fractions of Cd and Pb in the soil, with the extraction rates of 64.8% and 34.4% for total Cd and 53.6% and 30.0% for total Pb, respectively. HEDP and D-GA significantly increased the accumulations of Cd (57.1% and 35.0%) and Pb (43.4% and 31.9%) by S. nigrum without the inhibition of its biomass, making the great removal efficiencies of Cd (1.35% and 1.16%) and Pb (0.039% and 0.036%) from the soil. The enhanced phytoremediation efficiency of S. nigrum was due to the increase of the extractable Cd and Pb in the rhizosphere but little changes of soil pH and enzyme activities (catalase and urease). Among all of organic acids, HEDP may be an alternative to EDTA because of its characteristics of environmental friendliness and high efficiency.

Diagnosis and management of green algae in low land paddy fields of Cauvery delta zone, Tamil Nadu

Rice is a prime food crop for Asian countries. Wet land rice cultivation contributes maximum grain yield than dry land rice. Cauvery delta is a predominant area for rice cultivation in Tamil Nadu. Green algae growth during Kuruvai (June -August) season is a serious problem in wet land rice. These algae growth create anaerobic condition and prevent rice root respiration. The entire rice root was uprooted and floated on the stagnated water during initial stage. There is no preliminary study for green algae control in rice field. Soil and water samples were collected and analyzed for the nature of occurrence. Laboratory and field experiments were conducted to find out the remedial measures. The results of soil and water sample analysis showed that use of bore well water and dumping of phosphatic fertilizers leads to salt accumulation which favours the green algal growth. The results of the laboratory experiment revealed that the CuSO4 londox power, propiconazole and hexaconazole showed moderate inhibition on 5th day after treatment. The findings from field experiment indicated that use of conoweeder, alternate wetting and drying and CuSo4 drenching @ of 2.5 kg/ha when green algae appearance has just noticed or 5.0 kg/ha when severe growth occurred was effective in managing the green algae. Among all measures, alternate wetting and drying is the best management practices. CuSO4 drenching reduces around 70% of the growth. Even though CuSO4 react negatively with algae growth, soil pH changes and salt concentration play a major role on the CuSO4 action towards green algae. In order to maintain soil health condition, biofertilizer application, crop rotation, green manure trampling to be practised to recover the soil from alkaline pH, removal of accumulated salt and to control the algae growth using CuSO4.

Variation in abundance, diversity, and composition of nirK and nirS containing denitrifying bacterial communities in a red paddy soil as affected by combined organic-chemical fertilization

The long-term effects of different combined organic–chemical fertilizer regimes on denitrifying communities remain unclear. In this study, we performed quantitative PCR and sequencing analyses to examine the abundance, diversity, and structure of two nir (nirK and nirS) denitrifying communities in response to five fertilization treatments, namely, no nitrogen (N) fertilizer (NF), chemical fertilizer (CF), pig manure + chemical fertilizer (PM), biogas slurry + chemical fertilizer (BF), and milk vetch + chemical fertilizer (GM). All N fertilizer treatments received equal amounts of total N, phosphorus (P), and potassium (K). Compared with the NF treatment, combined organic–chemical fertilization significantly increased abundances of the nirK and nirS genes (particularly in the GM treatment) (P < 0.05). Chemical fertilizer application, with or without organic materials, shifted the nirK-type community relative to that under the NF treatment. In comparison, only a distinguished effect of combined organic-chemical fertilizations on nirS-type denitrifying communities was observed. Fertilizer-induced changes in soil pH and organic matter were identified as the factors contributing most to observed variations in the abundance and composition of the two nir denitrifying communities. Nitrite reductase (NIR) activity was mirrored by the changes in nirS gene abundance. Network analysis revealed that nirK- and nirS-type denitrifying communities formed ecological clusters that were closely associated with carbon addition, and indicated that nirS operational taxonomic units (OTUs) had more opportunities to interact with each other or with nirK OTUs. Overall, a high sensibility of nirK-type denitrifying communities to N fertilization was observed, whereas the nirS-type denitrifiers were more responsive to organic–chemical fertilization than chemical fertilizer application alone, and the addition of carbon was established to be the principal factor implicated in shaping the co-occurrence network patterns of nir denitrifying communities.

Changes in physicochemical properties, enzymatic activities, and the microbial community of soil significantly influence the continuous cropping of Panax quinquefolius L. (American ginseng)

In the production of the natural medicinal plant American ginseng, replantation typically fails due to continuous cropping obstacles. However, the cause is still not clear and needs more research. Soil samples were collected from (a) maize fields where American ginseng had never been planted, (b) fields where American ginseng had just been harvested, and (c) fields where maize had been planted for 2, 4 and 6 years respectively after American ginseng. We investigated the physicochemical properties, the enzymatic activities, and the soil microbial community structure and composition of the samples. We found that the content of soil salt, NH4+-N, and NO3−-N increased significantly in samples associated with the production of American ginseng, whereas the soil pH, carbon-to-nitrogen ratio, alkaline phosphatase, and cellulase activity all significantly decreased and gradually recovered to the pre-planting level. Moreover, the bacterial diversity decreased, while fungal diversity and richness increased; fungal richness continued to increase in farmlands replanted maize. The relative abundance of some microbial communities was changed significantly and was gradually restored with a longer time to replant maize. Pearson’s correlation analysis shown that significantly changed microbial communities were significantly associated with changes in soil pH, soil salt and nitrogen content, alkaline phosphatase, and cellulase activity. Changes in soil pH, soil salt and nitrogen content caused changes in microbial community structure and composition, as well as cellulase and alkaline phosphatase activity. These changes may cause the continuous cropping obstacles of American ginseng and may be improved by planting maize.

Immobilization of Cd and Pb in a contaminated acidic soil amended with hydroxyapatite, bentonite, and biochar

This study aims to investigate the dynamic changes over time in the immobilization efficiency of hydroxyapatite, bentonite, and biochar for Cd and Pb co-contaminated soil given the potential remobilization of immobilized heavy metals for long-term stabilized soils. Hydroxyapatite, bentonite, and biochar were applied only once at 2.5% (w/w) of soil during a 5-year pot experiment with pepper-cabbage (Capsicum annuum L.-Brassica pekinensis) rotation. The dynamic changes of soil pH, soil organic carbon (SOC), TCLP (toxicity characteristic leaching procedure)-, DTPA-, and PBET (simulated human gastric solution extraction)-Cd and Pb in soil, fractionation of Cd and Pb in soil, as well as the dynamic variations of bioaccessible Cd and Pb in vegetables were determined. The results indicated the application of hydroxyapatite, bentonite, and biochar significantly increased the proportion of residual fraction of Cd and Pb, and reduced the mobility, bioavailability, and bioaccessibility of Cd and Pb analyzed by TCLP, DTPA, and PBET, respectively. Moreover, the concentrations of TCLP-, DTPA-, and PBET-Cd and Pb gradually increased over time. The proportions of acid-extractable Cd and Pb in amended treatments in 2019 were higher than those in 2016. The increment for acid-extractable Cd fraction was 4.69%, 5.53%, and 4.19% in hydroxyapatite, bentonite, and biochar, respectively. While the increment for acid-extractable Pb fraction was 1.48%, 2.08%, and 1.91%, respectively. The bioaccessible Cd and Pb in pepper and cabbage significantly decreased after remediation while no remarkable changes were observed for Pb from 2017 to 2019. The three amendments were effective ameliorants for Cd and Pb co-contaminated acidic soil, but the immobilization efficiency decreased over time. Moreover, hydroxyapatite showed the highest efficiency on metal stabilization and vegetable safety.

Representing methane emissions from wet tropical forest soils using microbial functional groups constrained by soil diffusivity

Abstract. Tropical ecosystems contribute significantly to global emissions of methane (CH4), and landscape topography influences the rate of CH4 emissions from wet tropical forest soils. However, extreme events such as drought can alter normal topographic patterns of emissions. Here we explain the dynamics of CH4 emissions during normal and drought conditions across a catena in the Luquillo Experimental Forest, Puerto Rico. Valley soils served as the major source of CH4 emissions in a normal precipitation year (2016), but drought recovery in 2015 resulted in dramatic pulses in CH4 emissions from all topographic positions. Geochemical parameters including (i) dissolved organic carbon (C), acetate, and soil pH and (ii) hydrological parameters like soil moisture and oxygen (O2) concentrations varied across the catena. During the drought, soil moisture decreased in the slope and ridge, and O2 concentrations increased in the valley. We simulated the dynamics of CH4 emissions with the Microbial Model for Methane Dynamics-Dual Arrhenius and Michaelis–Menten (M3D-DAMM), which couples a microbial functional group CH4 model with a diffusivity module for solute and gas transport within soil microsites. Contrasting patterns of soil moisture, O2, acetate, and associated changes in soil pH with topography regulated simulated CH4 emissions, but emissions were also altered by rate-limited diffusion in soil microsites. Changes in simulated available substrate for CH4 production (acetate, CO2, and H2) and oxidation (O2 and CH4) increased the predicted biomass of methanotrophs during the drought event and methanogens during drought recovery, which in turn affected net emissions of CH4. A variance-based sensitivity analysis suggested that parameters related to aceticlastic methanogenesis and methanotrophy were most critical to simulate net CH4 emissions. This study enhanced the predictive capability for CH4 emissions associated with complex topography and drought in wet tropical forest soils.

Elevated CO2 shifts soil microbial communities from K‐ to r‐strategists

AbstractAimsSoil microbes are key to myriad processes in terrestrial ecosystems. Elevated CO2 represents a dominant driver of global climate change; however, it remains unclear to what extent elevated CO2 impacts soil microbial communities at ecosystem and global scales. Here, we sought to address the following questions: (a) Do the compositions of microbial communities shift from K‐ to r‐strategists under elevated CO2? (b) What is the extent of the compositional shifts of microbial communities affected by elevated CO2 concentrations, experimental duration, ecosystem types and/or background climates? (c) Are the responses of microbial communities to elevated CO2 associated with changes in soil pH and carbon and nitrogen availabilities?LocationGlobal.Time period1998–2020.Major taxa studiedSoil microbes.MethodsWe performed a global meta‐analysis of 965 observations from 122 studies, which tested the effects of elevated CO2 on microbial communities. The data covered broad variations in ecosystems, climate, CO2 concentrations, experimental duration, and soil factors.ResultsWe revealed that elevated CO2 decreased the K‐ to r‐strategist ratios with decreasing fungi : bacteria, Gram+ : Gram– bacteria, and Acidobacteria : Proteobacteria ratios, and increased bacterial biomass, microbial biomass carbon, Gram– bacteria, and Acidobacteria abundance. Moreover, the shifts from K‐ to r‐strategists were more pronounced under higher CO2 concentrations and longer experimental durations. The responses of microbial attributes to elevated CO2 did not differ significantly among croplands, forests and grasslands. Furthermore, the response of microbial biomass to elevated CO2 was negatively correlated with the response of soil pH, while those of bacterial biomass and fungi : bacteria ratios were positively correlated with those of soil organic carbon and soil carbon : nitrogen ratios, respectively.Main conclusionsOur results suggest that elevated CO2 shifts soil microbial communities from K‐ to r‐strategists, and provide supportive evidence for understanding responses of soil microbial processes to elevated CO2.

Legume–microbiome interactions unlock mineral nutrients in regrowing tropical forests

Legume trees form an abundant and functionally important component of tropical forests worldwide with N2-fixing symbioses linked to enhanced growth and recruitment in early secondary succession. However, it remains unclear how N2-fixers meet the high demands for inorganic nutrients imposed by rapid biomass accumulation on nutrient-poor tropical soils. Here, we show that N2-fixing trees in secondary Neotropical forests triggered twofold higher in situ weathering of fresh primary silicates compared to non-N2-fixing trees and induced locally enhanced nutrient cycling by the soil microbiome community. Shotgun metagenomic data from weathered minerals support the role of enhanced nitrogen and carbon cycling in increasing acidity and weathering. Metagenomic and marker gene analyses further revealed increased microbial potential beneath N2-fixers for anaerobic iron reduction, a process regulating the pool of phosphorus bound to iron-bearing soil minerals. We find that the Fe(III)-reducing gene pool in soil is dominated by acidophilic Acidobacteria, including a highly abundant genus of previously undescribed bacteria, Candidatus Acidoferrum, genus novus. The resulting dependence of the Fe-cycling gene pool to pH determines the high iron-reducing potential encoded in the metagenome of the more acidic soils of N2-fixers and their nonfixing neighbors. We infer that by promoting the activities of a specialized local microbiome through changes in soil pH and C:N ratios, N2-fixing trees can influence the wider biogeochemical functioning of tropical forest ecosystems in a manner that enhances their ability to assimilate and store atmospheric carbon.

Rapid soil and vegetation changes at regional scale in continental Antarctica

Antarctica is the last pristine environment on Earth, its biota being adapted to the harsh and extreme polar climate. Until now, soil formation and vegetation development in continental Antarctica were considered very slow due to the extreme conditions of this polar desert. Since the austral summer 2002/2003, a long-term monitoring network of the terrestrial ecosystems (soils, vegetation, active layer thickness) has been established at Victoria Land (VL) across a > 500 km latitudinal gradient of coastal sites (73°–77°S). In only one decade large ecosystem changes were detected. Climate was characterized by a significant increase of thawing degree days in northern VL and of autumn air temperature. No extreme climatic events (such as hot spells) where detected in the study period. Soil chemistry suffered large quantitative changes, clearly indicating rapid pedogenetic processes. In most soils the upper layers exhibited a strong alkalinization (pH increases up to 3 units) and increases in conductivity, anions and cations (in particular of SO4 and Na). The largest changes were observed in soils with low vegetation cover. Statistically significant differences in soil chemistry were detected between soils with high and low vegetation cover, the former showing lower pH, conductivity, Na and Cl. Most plots exhibited changes of total cover, species richness and floristic composition, with vegetation expansion in soils with low vegetation cover and the largest increase recorded at Apostrophe Island (northern VL). Principal Component Analysis (PCA) identified the main trend of vegetation change, with a shift from lower to higher cover and a secondary trend of change associated with a gradient of water availability, consistent with an increase in water instead of snow. Redundancy analysis (RDA) identified the trend of change in soil chemistry with increases in pH, conductivity, anions and cations associated with the concomitant decrease in C, N, NO3, PO4. The RDA confirmed that soil changes were associated with a gradient of vegetation change (from low to high cover) as well as of water availability, as already indirectly outlined by the PCA. Field manipulation experiments carried out at five locations of the network between 73°S and 77°S, simulating increases of precipitation from snow or water additions didn’t induce changes in soil pH, indicating that pulse events of snow accumulation or melting could not trigger persistent soil pH changes. These data allow hypothesize the occurrence of a main ecosystem change occurring at regional scale at Victoria Land. The slight air warming and its consequences on soil chemistry and vegetation, further highlight the sensitivity of the fragile Antarctic ecosystems to the consequences of even small changes in climate.