Greenhouse gas emissions from decomposition of biosolids as affected by stabilization method and soil moisture

Computational models are useful to estimate agricultural greenhouse gas emissions at regional scales. However, empirically based parameter values are required for the models to accurately represent carbon (C) and nitrogen (N) mineralization rates of different organic amendments in more and less humid regions or during wet and dry periods of the growing season. A controlled environment study was conducted to assess the rates of C and N mineralization in differently processed sewage sludge (biosolids) in wet and dry soil. Parameter values were estimated for use in modelling the degradation of three types of biosolids. A loam soil with either 49% water-filled pore space (WFPS) or 29% WFPS was amended with mesophilic anaerobically digested (digested), alkaline-stabilized, or composted biosolids. Headspace samples were collected and analysed for carbon dioxide (CO₂) and nitrous oxide (N₂O), and soil samples for nitrate (${\mathrm{NO}}_3^{-}$) and ammonium (${\mathrm{NH}}_4^{+}$). Four different first-order models were fitted to the cumulative CO₂–C and N₂O–N data (R² > 0.98), and soil ${\mathrm{NO}}_3^{-}$ (R² > 0.65) and ${\mathrm{NH}}_4^{+}$ (R² > 0.93) concentrations. CO₂–C data indicated that C mineralization was higher in soil with 49% WFPS than in soils with 29% WFPS. Seventy-nine percent of the C compounds in digested biosolids degraded in soil with 49% WFPS, compared with 52% for alkaline-stabilized biosolids and 8% for composted biosolids. The fitted coefficient values were similar for all of the four first-order models used in this study and provide useful information for parameterizing more sophisticated mechanistic models of the degradation of biosolids in soil.     

Effects of long-term phosphorus fertilizer inputs and seasonal conditions on organic soil phosphorus cycling under grazed pasture

Soil microbes and phosphatase enzymes play a critical role in organic soil phosphorus (P) cycling. However, how long-term P inputs influence microbial P transformations and phosphatase enzyme activity under grazed pastures remains unclear. We collected top-soil (0–75 mm) from a grazed pasture receiving contrasting P inputs (control, 188 kg ha⁻¹ year⁻¹ of single super phosphate [SSP], and 376 kg ha⁻¹ year⁻¹ of SSP) for more than 65 years. Olsen P, microbial biomass P, and acid and alkaline phosphatase enzyme activities were measured regularly over a 2-year period. Pasture dry matter and soil chemical properties were also investigated. Results showed that long-term P inputs significantly increased pasture dry matter, total N, and the concentrations of ${\mathrm{NO}}_3^{–}$–N but significantly decreased soil pH and the concentrations of ${\mathrm{NH}}_4^{+}$–N. Total C was not affected by P fertilization. Although Olsen P significantly increased with increasing long-term P inputs, microbial biomass P was similar under P fertilized treatments. Long-term P inputs decreased acid phosphatase activity but increased alkaline phosphatase activity. Microbial biomass P was similar across seasons in the control but decreased in spring and autumn while increased in summer and winter under P fertilized treatments. Acid and alkaline phosphatase activities were significantly affected by season and followed similar seasonal trends being maximum in summer and minimum in winter regardless of P treatment. Correlation and principal component analysis revealed that acid and alkaline phosphatase activities were significantly positively correlated with soil temperature and significantly negatively correlated with soil moisture. In contrast, Olsen P and microbial biomass P were weakly correlated with environmental conditions. The findings of this study highlight the intertwined relationship between organic P cycling and the availability of C and N in soil systems and the need to integrate both soil moisture and temperature in models predicting organic P mineralization, especially in the context of global climate change.     

Soil microbiome biomass,activity, composition and CO2 emissions in a long-term organic and conventional farming systems

The implementation of environmentally friendly agricultural policies has increased the need to compare agricultural aspects of conventional (CON) and organic farming (ORG) systems. The objective of the present work was to compare the effects of an organic and conventional long-term experiment on bacterial and fungal biomass and activity, as well as soil CO₂ emission and readily available nitrogen forms in a soil cultivated with Helianthus annuus L. The microbial biomass was more active and abundant in ORG as well as soil CO₂ emission. Despite being less abundant, fungi were more active than bacteria in both ORG and CON experiments. 16S rRNA gene sequencing showed that the ORG treatment had a significantly greater bacterial richness than CON. Cyanobacteria, Actinobacteria and Proteobacteria were the most abundant phyla contributing more than others to the differences between the two systems. Moreover, the soil ${\mathit{NH}}_4^{+}$ and ${\mathit{NO}}_2^{-}$ content was not significantly different between ORG and CON, while ${\mathit{NO}}_3^{-}$ was less in ORG. ORG sunflower yield was significantly less compared with CON. While much remains to be discovered about the effects of these agricultural practices on soil chemical properties and microbial diversity, our findings may contribute to this type of investigation.     

Wetland-type conversion drives the C,N and P stoichiometry change of soil in river plain of lower of the Yellow River

Land use conversion on river plain has profound impacts on soil characteristics and elemental stoichiometry. Four wetland types (Riparian lower-beach wetland [RLW], Riparian higher-beach wetland [RHW], Cultivated wetland [CW] and Mesophytic wetland [MW]) were selected in the lower Yellow River area to investigate the consequence of wetland type conversion on soil carbon (C), nitrogen (N) and phosphorus (P) stoichiometry. The results demonstrated that wetland conversion induced significant spatio-temporal variations in soil C, N and P stoichiometry and physicochemical characteristics in soil. Frequent agricultural activities (fertilizer input) raised the nutrient content of natural wetland, particularly in surface soil (0–30 cm). Soil volumetric water content (VWC), soil bulk density (SBD), pH and soil enzyme activity varied significantly in different wetlands. Total carbon (TC) and total nitrogen (TN) contents in MW decreased with increasing soil depth (<40 cm layers), as did TN and total phosphorus (TP) contents in CW. On the other hand, TC, TN and TP contents in RLW and RHW did not change significantly with soil depth. However, the contents of TOC, ${\mathrm{NO}}_3^{-}–\mathrm{N}$ and Fe/Al-P, etc., varied among soil layers and among wetland types. Furthermore, the stoichiometric characteristics changed significantly in some soil layers, with mean values being less than the Chinese average. Statistically, significant positive correlations were determined between TC and TN (r = .56), TDC and TP (r = .62), N:P and pH (r = .57) (p < .05) and ${\mathrm{NO}}_3^{-}–\mathrm{N}$ and pH (r = .66, p < .01). VWC was negatively correlated with pH (r = −.56, p < .05), while C/P was negatively associated with soil temperature (ST) and SBD (r = −.55, r = −.64, p < .05). TDC, IP, TN, Fe/Al-P and ST were identified as the dominant factors, with the percentage of variance 41%, 20%, 12%, 9% and 6% respectively. These findings have a great scientific significance for the ecological conservation of wetlands in the lower Yellow River area.     

Investigating the spatio‐temporal variations of nitrate leaching on a tea garden hillslope by combining HYDRUS‐3D and DNDC models

Spatio‐temporal variations of nitrate‐nitrogen ( ${\mathrm{NO}}_3^{-}$‐N) leaching is driven by both soil hydrology and biogeochemistry. However, the widely used soil hydrology and biogeochemistry models have their weaknesses in simulating soil N cycling and soil water movement processes, respectively. In this study, we proposed an alternative approach by simply combining the HYDRUS‐3D and DNDC models to investigate the spatio‐temporal variations of ${\mathrm{NO}}_3^{-}$‐N leaching on a representative tea garden hillslope in Taihu Lake Basin, China. Results showed that the soil hydrology and N cycle were well simulated by HYDRUS‐3D and DNDC models, respectively. Based on the leaching equation, the soil water flux simulated by HYDRUS‐3D and soil ${\mathrm{NO}}_3^{-}$‐N content simulated by DNDC were combined to calculate the leachate ${\mathrm{NO}}_3^{-}$‐N concentrations with good accuracy. The accumulative ${\mathrm{NO}}_3^{-}$‐N leaching flux during the simulation year was 71.7 kg N ha^?1, with remarkable spatio‐temporal variations on this hillslope. Hot spots of ${\mathrm{NO}}_3^{-}$‐N leaching were observed in blocks 24, 27, 31, 34, 37, and 40 with accumulative leaching fluxes > 82.0 kg N ha^?1 y^?1. The spatial variation of ${\mathrm{NO}}_3^{-}$‐N leaching was mainly controlled by soil texture and soil hydraulic properties. Hot moments of ${\mathrm{NO}}_3^{-}$‐N leaching were observed after the applications of spring fertilizer (16 March) and basal fertilizer (30 October). The temporal variation of ${\mathrm{NO}}_3^{-}$‐N leaching was mainly controlled by precipitation and the spring fertilization. Methods and findings of this study will be benefit for the risk assessment of non‐point source N loss and the precise agricultural management.     

Effects of biochar addition on greenhouse gas emissions during freeze–thaw cycles in a black soil region,Northeast China

As global warming intensifies, the soil environment in middle to high latitudes will undergo more extensive and frequent freeze–thaw cycles (FTCs), which will significantly affect the carbon and nitrogen cycles of soil ecosystems and aggravate greenhouse gas (GHG) emissions. Biochar can increase soil organic carbon storage and mitigate climate change. To effectively control GHG emissions, soil supplemented with biochar at different application rates (0%, 2%, 4% and 6% [w/w]) under different numbers of FTCs (0, 3, 6, 9, and 12) was selected as the research object. The soil GHG emission characteristics in different experimental treatments and their relationships with soil physical and chemical properties were determined. Our results showed that N₂O and CO₂ emissions were promoted during FTCs, with values of 3.13–50.37 and 16.22–135.50 μg m⁻² h⁻¹, respectively. The order of N₂O and CO₂ emissions with respect to biochar application rate was as follows: 2% > 0% > 4% > 6%. CH₄ emissions were negative during FTCs, varying from −1.62 to −10.59 μg m⁻² h⁻¹, and negative CH₄ emissions were promoted by biochar. Correlation analysis showed that N₂O, CO₂ and CH₄ emissions were significantly correlated with pH, soil moisture and soil organic matter (SOM), total nitrogen (TN) and ${\mathrm{NH}}_4^{+}$–N contents (p < .01). The conceptual path model demonstrated that GHG emissions were significantly influenced by FTCs, moisture, SOM and biochar application rate. Our results indicate that the effects of FTCs on GHG emissions were greater than those of biochar application. Biochar application rates of 4% or 6% should be considered in the future to reduce soil GHG emissions in the black soil region of Northeast China. Our results can help provide a theoretical basis and effective strategy to reduce soil GHG emissions during FTCs in seasonally frozen regions.     

Evaluation of selected medicinal plant materials and dicyandiamide on nitrification of urea‐derived ammonium under laboratory conditions

Background : Poor utilization of urea fertilizer and N losses from agriculture lands demands alternate fertilization practices to reduce N losses and improve utilization, i.e., application of nitrification inhibitors. Aims : This study was aimed to evaluate and compare the influence of dicyandiamide (DCD) and selected medicinal plant materials and on N transformations, nitrification inhibition and recovery of applied N. Methods : Treatments included: urea nitrogen (UN), UN + DCD, UN + Gingiber officinale, UN + Viola odorata, UN + Sewertia chirata, UN + Azadirachta indica, UN + Sphaenathus indicus, UN + Allium sativus, UN + Artemisia absenthium, UN + Fumaria indicus, UN + Caesalpinea bondusella, UN + Barberis lyceum, and an un‐amended control. Urea was applied at 200 mg N kg^?1 soil, while DCD and medicinal plant materials were applied at of 1% and 20% of applied urea, respectively. Results : Medicinal plant materials inhibited nitrification of urea‐derived ${\mathrm{NH}}_4^{+}\text{-}N$. On an average of medicinal plant materials treatments, 51% of ${\mathrm{NH}}_4^{+}\text{-}N$ was still present in soil compared to 17% ${\mathrm{NH}}_4^{+}\text{-}N$ in UN treatment without medicinal plant materials after 28 days. Similarly, ${\mathrm{NO}}_3^{-}\text{-}N$ was 76.54 mg kg^?1 in UN treatment compared to 34.40 mg kg^?1 in UN + medicinal plant materials treatments, indicating 55% reduction in nitrification. Apparent nitrogen recovery (ANR) in UN treatment was 65% compared to 74% in UN + DCD treatment. ANR in treatments, where UN was amended with medicinal plant materials, varied between 58 to 70%. Conclusions : The use of DCD and medicinal plant materials with UN significantly reduced ${\mathrm{NH}}_4^{+}\text{-}N$ oxidation and nitrification ( ${\mathrm{NO}}_3^{-}\text{-}N$). In general, medicinal plant materials were more effective in regulating N transformations and, thus, offer a suitable alternate fertilization practice to reduce N losses and improve fertilizer utilization.     

Ammonium fertilization increases pearl millet yield by promoting early root growth,higher tillering,and water use during grain filling in a low P Sahelian soil

Background : Adaptation of pearl millet [Pennisetum glaucum (L.) R. Br.] to low soil phosphorus (P) at early seedling stages and efficient P fertilizer application are crucial for its survival in the West African Sahel. While addition of ${\mathrm{NH}}_4^{+}\text{-}N$ to P in the microdose technique has been reported to stimulate early growth of pearl millet, there is little information regarding root length (RL) at different timings. Aims : Our study aimed at assessing the effects of added ${\mathrm{NH}}_4^{+}\text{-}N$ and ${\mathrm{NO}}_3^{-}\text{-}N$ to P on (1) pearl millet agronomic traits such as root and shoot growth and (2) the water use dynamics during the cropping cycle. Methods : Twenty‐four “RhizoTubes” filled with P‐deficient soil were used to grow pearl millet under three treatments: (T1) placed P addition alone at 0.4 g per seed hole, P_placed, (T2) P_placed + ${\mathrm{NH}}_4^{+}\text{-}N$, and (T3) P_placed + ${\mathrm{NO}}_3^{-}\text{-}N$. At 2, 3, 4, and 5 weeks after sowing (WAS) we took non‐destructive measurements of RL. Water use (transpiration) was measured from 3.5 to 9 WAS. Results : At early growth, roots in the topsoil of T2 were longer than T3 and T1 roots (at 4 WAS: 129.3 cm for T2, 87.5 cm for T3, and 93.3 cm for T1, p < 0.05). Total RL at 2 WAS correlated positively with seedling height and final grain yield. Fertilization with ${\mathrm{NH}}_4^{+}\text{-}N$ reduced the time to flowering and increased the number of tillers and grain yield, whereas addition of ${\mathrm{NO}}_3^{-}\text{-}N$ increased vegetative dry matter at harvest and water use efficiency. Conclusions : Our findings suggest that fertilization with ${\mathrm{NH}}_4^{+}\text{-}N$ plays a critical root stimulating role at early growth stages, seemingly by increasing lateral root initiation, which carries through to a larger water use during grain filling and higher grain yield.     

Co‐occurrence of organic and inorganic N influences asparagine uptake and amino acid profiles in white clover

Background : Direct plant uptake of organic nitrogen (N) may be important for plant N nutrition, but we lack knowledge of how the concentration and form of external N influence organic N uptake and plant N status. Aims : We investigated the uptake of the amino acid asparagine (Asn) in white clover in the presence of different inorganic, organic and total N concentrations. Methods : Actively N₂‐fixing white clover seedlings were for one week exposed to combinations of ${\mathrm{NO}}_3^{-}$ (3–30 µmol N kg^?1 sand DW) and Asn (3–30 µmol N kg^?1 sand DW), whereafter the Asn uptake rate was determined by addition of ¹³C₄‐Asn. Shoot and root amino acid profiles were also analyzed. Results : Increasing ${\mathrm{NO}}_3^{-}$ and total N concentrations decreased ¹³C₄‐Asn uptake rates and internal clover Asn content. In addition, total N and ${\mathrm{NO}}_3^{-}$ also affected amino acid profiles, with Asn, Asp, Glu, Gln, Cys, Gly, Pro, Ser, and Ala being more related to the low N doses, and Thr, Val, Ile, Leu, Phe, Tyr, Trp, and Met being more abundant at increasing N doses. Conclusions : Asn uptake rate in white clover is reduced by increasing inorganic N. Plant amino acid profiles are likely to be a more sensitive indicator of N supply.     

Effective stress and shear strength parameters of unsaturated soils as affected by compaction and subsequent shearing

Wheeling induced compression and shearing forces are main stresses accounting for soil deformation and changes of hydraulic, gaseous and thermal properties. There are reports about the combined effects of compaction and subsequent shearing on soil hydraulic properties, but their consequences on soil strength properties (i.e., effective stress and shear strength) need to be further analysed. This study investigated the dynamics of soil mechanical properties as affected by pore water pressure ($u_{\mathrm{w}}$) during compaction and shearing. Soil samples from an A-horizon of Gleysols derived from glacial sediment and a Stagnic Luvisol from loess were analysed. The repacked and structured samples were compressed under static and cyclic loading and then sheared at two speeds (0.3 and 2.0 mm min⁻¹) with three loading levels (50, 100, and 200 kPa). During each stress application, the $u_{\mathrm{w}}$, chi factor (χ) and effective stress (${\sigma }^{\prime }$) were measured and calculated. The shear strength ($\tau$), angle of internal friction ($\phi$) and cohesion ($c$), were determined and fitted by the Mohr–Coulomb failure criterion. The results showed that compaction and shearing increased $u_{\mathrm{w}}$ and χ in all homogenized soils while on structured soils this phenomenon only occurred when the applied loading stress exceeded the soil precompression stress. The increased $u_{\mathrm{w}}$ resulted in soil hydraulic and mechanical stresses, which ultimately reduced the ${\sigma }^{\prime }$, especially at −6 kPa initial matric potential. Soils with finer texture, higher loading stresses and faster shear speed normally exhibited more reduced ${\sigma }^{\prime }$ values. The structured soil had higher $\tau$ values with higher $\phi$ and $c$ compared to the homogenized soils. The changes of $u_{\mathrm{w}}$ at high loading stress (i.e., 200 kPa) may overlap the normal pattern of the Mohr–Coulomb failure line that results from the theoretical Mohr envelope. Thus, to minimize the destruction of soil structure and stability induced by wheeling, it is important to consider field water content, traffic loading and wheeling speed.     

Ammonium‐driven nitrification plays a key role in increasing Mn availability in calcareous soils

Background : Manganese deficiency often becomes a yield limiting factor, particularly on calcareous soils, even though the total soil manganese content is usually sufficient. Although it is known that acidifying N fertilizers can improve Mn availability, the reason of this effect is still unknown. Aim : Our aim was to investigate the effect of stabilized ammonium fertilizers as a tool to distinguish between physiological‐ and nitrification‐induced acidification. Method : Two pot experiments with Triticum aestivum L. and one soil incubation experiment using different nitrogen forms (CN = calcium nitrate, AN = ammonium nitrate, AS = ammonium sulfate, ATS = ammonium thiosulfate) with and without addition of nitrification inhibitors (DCD, Nitrapyrin, Piadin, DMPP) were conducted to examine the effect on Mn availability in the soil and Mn uptake by the plants at different development stages (EC 31 und 39). Results : With increasing fertilizer ${\mathrm{NH}}_4^{+}$ content a higher Mn concentration was detected: CN: 32 µg Mn g^?1 DM, AN: 39 µg Mn g^?1 DW, AS: 55 µg Mn g^?1 DM, ATS: 109 µg Mn g^?1 DM. The addition of a nitrification inhibitor resulted in a significantly lower rhizosphere pH compared to the non‐stabilized fertilizer. Surprisingly, the use of different nitrification inhibitors led to unchanged (CN, AN) or lower Mn concentrations of wheat. Especially in the ${\mathrm{NH}}_4^{+}$ treatments (AS and ATS), this negative effect was very evident (AS+DCD: 42 µg Mn g^?1 DM; ATS+DCD: 55 µg Mn g^?1 DM). Conclusions : Mn availability was enhanced by ongoing nitrification process rather than physiological acidification. Compared to other N forms, ammonium thiosulfate led to the highest Mn availability in bulk soil.     

Soil incubation study showed biogas digestate to cause higher and more variable short‐term N2O and N2 fluxes than mineral‐N

Today, a large share of mineral fertilizer is substituted by biogas digestates. Biogas digestates are known to promote N₂O production, compared to mineral fertilizer. In particular, the initial phase following fertilizer application is crucial for the N gas release as N₂O and also N₂. However, this period impact has been rarely investigated, especially not across various field sites. Thus, undisturbed soil cores from two fertilizer types (biogas digestate vs. mineral fertilizer) at five sites with different site characteristics were investigated in a short‐term laboratory experiment under N₂‐free helium–oxygen incubation atmosphere. Across sites, biogas digestate soil cores showed significantly higher absolute N₂O fluxes compared to mineral fertilizer soil cores, even though this effect was dominated by samples from one site (Dornburg with the highest biogas digestate fertilization rate). Also relative N₂O fluxes showed a similar tendency. On average, absolute and relative N₂ fluxes differed between the two fertilizer types, while N₂ fluxes were highest at the Dornburg site. A N₂O/(N₂O+N₂) ratio of denitrification below or equal to 0.5 clearly highlighted the importance of N₂O reduction to N₂ for three of five the biogas digestate soil cores. Soil characteristics like bulk density and water‐filled pore space as proxies for gas diffusivity in soil, as well as N availability ( ${\mathrm{NO}}_3^{-}$, ${\mathrm{NH}}_4^{+}$), significantly affected the N₂O and N₂ fluxes from the biogas digestate soil cores. While this study presents data on short‐term N₂O and N₂ fluxes, there is a need for further studies in order to investigate the dynamics, the duration of the observed effects and their significance at the field scale.     

High concentrations of sodium and chloride ions have opposing effects on the growth of the xerophyte Pugionium cornutum under saline conditions

Background : The research on plant salt tolerance has mainly focused on Na⁺, but Cl^? has been relatively neglected. Previous studies have found that the xerophyte Pugionium cornutum, an important forage grass in the arid and semi‐arid regions of northwestern China, could synergistically accumulate high quintiles of Na⁺ and Cl^? in its shoots under NaCl treatments. However, the separate effects of these ions on the adaptation of P. cornutum to saline conditions have not been investigated. Aims : In this study, the response of P. cornutum to Na⁺ and Cl^? was analyzed. Methods : Four‐week‐old seedlings were treated with additional 50 mM NaCl, Na⁺‐specific solution containing 50 mM Na⁺ with a mix of ${\mathrm{NO}}_3^{-}$, H₂ ${\mathrm{PO}}_4^{-}$, and ${\mathrm{SO}}_4^{2-}$ as counter anions, and Cl^?‐specific solution containing 50 mM Cl^? with a mix of K⁺, Ca²⁺, and Mg²⁺ as counter cations. Results : Compared with the normal growth condition irrigated with Hoagland solution, the Na⁺‐specific solution severely impaired the growth and photosynthesis of P. cornutum due to the high accumulation of Na⁺ in shoots and the deterioration of tissue K⁺ homeostasis; while the Cl^?‐specific solution significantly increased shoot fresh and dry biomass. The Cl^?‐specific solution could also increase the turgor pressure in leaves for enhancing osmotic adjustment, which should be mainly attributed to the large accumulation of Cl^?, since the concentrations of other ions, including K⁺, Mg²⁺, Ca²⁺, H₂ ${\mathrm{PO}}_4^{-}$, and ${\mathrm{SO}}_4^{2-}$, in tissues under Cl^?‐specific treatment were maintained at the same levels as those observed under the normal condition. Conclusions : P. cornutum displays an excellent tolerance to moderate Cl^? but not to Na⁺, and the large accumulation of Cl^? should play a positive role in stimulating the growth of P. cornutum under salt stress.     

Effects of organic carbon and clay contents on structure-related properties of arable soils with high clay content

Understanding of factors governing soil structural features is necessary for managing key processes affecting crop productivity and environmental impacts of agriculture, for example, soil water balance, aeration, and root penetration. Organic matter is known to act as a major binding agent in soil aggregation and thus constitutes a central pillar in soil structure formation. However, knowledge of the structural role of organic matter or carbon (OC) in soils highly rich in clay-sized particles (<0.002 mm) is limited. In this study, the effects of clay and OC contents on aggregate stability, water holding capacity, near-saturated hydraulic conductivity, total porosity, and pore size distribution were assessed in cultivated fields with high clay content located in private crop production farms in southern Finland. Significant positive correlations were found between OC content and proportion of water stable aggregates and specific pore sizes from the range of 30 μm up to 1 mm diameter determined by image analysis. Porosities on a smaller size range derived from water retention measurements likewise showed a positive correlation with OC in <0.2 μm sizes. On the range of 0.2–1 μm, a negative relationship was observed, which induced a negative effect of OC on soil plant available water reserves. In line with the positive correlation between OC and larger soil pores, free water, representing the amount of water that can be drained by gravity, exhibited a positive relationship with OC suggesting that OC content can enhance aeration of soils with high clay content. Compared to OC, clay content tended to have an adverse effect on soil structural properties. Clay correlated negatively with pores larger than 30 μm, free water content, and extrapolated field saturated hydraulic conductivity. Further, our imaging results showed how saturated hydraulic conductivity was controlled by pore morphology, and there was a power law relationship between the conductivity and critical pore diameter. $K\propto d_c^2$ in agreement with the percolation theory. Overall, the structural impacts and hydrological implications of OC and clay in heavy clay soils vary by pore size ranges and their emergent practical impacts are thus not straightforward.     

Biochar effects on soil chemical properties and mobilization of cadmium (Cd) and lead (Pb) in paddy soil

This study focused on the effects of biochar (BC) application on soil chemical properties and mobilization of cadmium (Cd) and lead (Pb) in the paddy soil. BC was applied at the rate of 0, 10, 20 and 40 t ha⁻¹, respectively. BC application caused a significant increase in soil organic carbon contents (SOC), pH, nitrate–nitrogen (-N),and available phosphorus contents (AP) in the top and subsurface soil, while SOC contents in the subsurface soil decreased with increasing rate of BC. BC40 effectively reduced the mobility of Cd and Pb from the top layer to the subsurface soil, while concentrations of Cd and Pb in the topsoil remained unchanged. Path analysis showed that the direct path coefficient AP was highest; SOC, -N and AP had a negative direct effect on the Cd and Pb in subsurface soil. Soil pH and -N had a high negative indirect effect through AP. The decision coefficient decreased in the following order: pH, AP, SOC, -N and -N. Regression analysis showed that soil Cd and Pb had a significant linear correlation with soil AP, whereas soil Pb also had a significant linear correlation with soil pH. In conclusion, BC40 can alter soil chemical properties and reduce the mobility of Cd and Pb from the top layer to the lower subsurface of the paddy soil.     

Effect of coffee husk and cocoa pods biochar on phosphorus fixation and release processes in acid soils from West Cameroon

Acid soil in West Cameroon has limited phosphorus (P) availability which limits plant growth. This is mainly because of low pH, high levels of exchangeable aluminium (Al) and iron (Fe) and fixation of P. In this study, acid soils, sampled in Bafang, were amended with biochar produced from coffee husks (CH) and cocoa pod husks (CP) at two different temperatures (350 and 550 °C) in other to evaluate the effect on the physicochemical properties of the acid soil and the effect on P sorption and desorption. The soil was amended with biochar at a rate of 0, 20, 40 and 80 g/kg and incubated for 7 and 60 days. Physicochemical properties of all soil–biochar samples were determined followed by sorption experiments and data fitted in the Langmuir and Freundlich isotherm models in other to evaluate soil P sorption capacity and its affinity to soil amended with biochar. Moreover, desorption studies were done to evaluate the availability of P in soil amended with biochar after sorption. The outcomes of this study reveal an increase in soil pH, electrical conductivity (EC), available P, soil organic carbon and a drastic decrease in exchangeable Al and Fe. The point of zero charge of biochar-amended soil was higher than the control and increased with amendment rate. The experimental data of the sorption of P on soils and soil–biochar samples fits into Langmuir and Freundlich models (R² > 0.9) suggesting that the P adsorption is controlled by both model mechanisms. Soil–biochar mixture results in a decrease in the sorption capacity as compared with the control and the decrease was predominant with increasing amendment rate. At amendment rates of 20, 40 and 80 g/kg after 7 days of incubation, $Q_{\max }$ for SCH350 were 2267, 2048 and 1823 mg/kg which increased to 2407, 2112 and 1990 mg/kg after 60 days of incubation. This tendency was observed for all biochar inputs with respect to the increase in incubation days. Furthermore, desorption of P from soil–biochar mixtures was enhanced with biochar added at greater rate and produced at higher temperature. The desorption percentage was increased by more than around 10% for all biochar types from 20 mg/kg to 80 mg/kg amendment. Thus, biochar addition to acid soils reduces P fixation to acid soil and improves P desorption to soil solution, thereby providing more available P in the soil solution and better conditions for plant growth.     

Potassium is a potential toxicant for Arabidopsis thaliana under saline conditions

Background and aims : Most physiological and biochemical studies on salt stress are NaCl‐based. However, other ions (e.g., K⁺, Ca²⁺, Mg²⁺, and ${\mathrm{SO}}_4^{2-}$) also contribute to salt stress in special circumstances. In this study, salt stress induced by various salts was investigated for a better understanding of salinity. Methods : Arabidopsis thaliana plants were stepwise acclimated to five iso‐osmotic salts as follows: NaCl, KCl, Na₂SO₄, K₂SO₄, and CaCl₂. Results and Conclusions : Exposure to KCl and K₂SO₄ led to more severe toxicity symptoms, smaller biomass, and lower level of chlorophyll than exposure to NaCl and Na₂SO₄, indicating that Arabidopsis plants are more sensitive to potassium salts. The strongly reduced growth was negatively correlated with the accumulation of soluble sugars observed in KCl‐ and K₂SO₄‐treated plants, suggesting a blockage in the utilization of sugars for growth. We found that exposure to KCl and K₂SO₄ suppressed or even blocked sucrose degradation, thus leading to strong accumulation of sucrose in shoots, which then probably inhibited photosynthesis via feedback inhibition. Moreover, K⁺ was more accumulated in shoots than Na⁺ after corresponding potassium or sodium salt treatments, thus resulting in decreased Ca²⁺ and Mg²⁺ concentrations in response to KCl and K₂SO₄. However, K₂SO₄ caused more severe toxicity symptoms than iso‐osmotic KCl, even when the K⁺ level was lower in K₂SO₄‐treated plants. We found that Na₂SO₄ and K₂SO₄ induced strong accumulation of tricarboxylic acid intermediates, especially fumarate and succinate which might induce oxidative stress. Thus, the severe toxicity symptoms found in K₂SO₄‐treated plants were also attributed to ${\mathrm{SO}}_4^{2-}$ in addition to the massive accumulation of K⁺.