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Purpose Increasing the pH of acid soils is a well-recognized means of improving their fertility; however, the effects that plants impose on rhizosphere soils in response to this change are not well understood. This research sought to investigate changes in phosphorus (P) availability in the rhizosphere of blue lupin ( Lupinus angustifolius ) in response to an increase in pH. Methods Blue lupin plants were grown in rhizoboxes using two contrasting acid soils at their native pH (5.3 and 4.7) and treated with lime to increase their pH to 6.3 in a replicated trial. Measurements of localized acid phosphomonoesterase activity and P flux were made next to lateral root segments using a combination of zymography and diffusive gradient in thin films (DGT). Rhizosphere and bulk soils were tested for pH, organic anions, exchangeable aluminium (Al), labile P and phosphomonoesterase activity. Root morphological traits, root and shoot yields and shoot nutrient concentrations were also recorded. Results Profiles of DGT-P fluxes across lateral roots showed mobilization of P in the soil with the higher organic P content, and depletion in the other. The extent of acid phosphomonoesterase activity in the rhizosphere decreased with soil pH increase. Shoot P uptake was strongly correlated with fine root length and total organic anions in the rhizosphere. The proportion of thin roots decreased at pH 6.3 compared to the native pH, whereas that of thick roots increased. Conclusion Soil pH increase to 6.3 using lime negatively affects the P acquisition by blue lupin due to the reduction of organic anion exudation, fine root length and the extent of acid phosphomonoesterase activity in the rhizosphere.

Legumes play critical dual roles in grazed grassland ecosystems; providing nitrogen inputs and high-quality feed for grazing livestock. However, many species fail to persist in acidic, low fertility soils. A glasshouse study was conducted to investigate the response of lucerne (Medicago sativa) to phosphogypsum (PG), lime and soluble P + S fertilizer (PS) application to two soils. Phosphorus and sulphur were applied through either PG (0, 1, 3 and 9 t ha−1) or P + S fertilizer at equivalent rates to PG. Both PG and PS were applied with or without lime, which was applied at 2 t ha−1. Yield and nutrient uptake of the lucerne was measured, while the soil was analyzed for pH, Olsen P and exchangeable aluminum. Yield responses were significantly different between the two soils. Maximum yields and P and S uptakes were obtained under PG 9 t ha−1 combined with lime. Exchangeable Al decreased in both soils under 1 ha−1 of PG compared with the control. At the highest rate, Olsen P increased by 8 and 6 mg kg−1 for PG and by 6 and 11 mg kg−1 for PS compared with the control for Glenmore and Molesworth soils respectively. Phosphogypsum showed positive effects on P and S bioavailability.

In this work, we modelled the response of soil water repellency (SWR) persistence to the decrease in moisture in drying soils, and we explored the implication of soil particle size distribution and specific surface area on the SWR severity and persistence. A new equation for the relationship between SWR persistence and soil moisture (θ) is described in this paper. The persistence of SWR was measured on ten different hydrophobic soils using water drop penetration time (WDPT) at decreasing levels of gravimetric water content. The actual repellency persistence showed a sigmoidal response to soil moisture decrease, where Ra(θ)=Rp/1+eδ(θ−θc). The suggested equation enables one to model the actual SWR persistence (Ra) using θ, the potential repellency (Rp) and two characteristic parameters related to the shape of the response curve. The two parameters are the critical soil moisture θc, where the Ra increase rate reaches its maximum, and the parameter δ affecting the steepness of the curve at the inflexion point of the sigmoidal curve. Data shows that both soil carbon and texture are controlling the potential SWR in New Zealand pastures.

BackgroundSoil salinity is a major factor in land degradation, affecting one billion hectares worldwide. Several strategies for overcoming salinity stress have been tested. Using inorganic amendments, such as phosphogypsum (PG) and natural gypsum (G), is one of the promising strategies for salinity mitigation.Materials and methodsA pot trial was conducted, under greenhouse conditions, using a saline soil (ECe soil = 11.7 dS m−1) and barley (Hordeum vulgare) as an indicator crop. We aimed to evaluate the effects of different sources of PG on soil salinity mitigation in comparison with G and consequently on barley yield components and metal uptake. Three treatments were tested: two phosphogypsum sources from two different phosphate industrial sites (PG1 and PG2), applied at four rates (0, 15, 30 and 45 t ha−1) and natural gypsum at one rate (15 t ha−1).ResultsResults showed that the weight and number of spikes and tillers, grain yield, and 1000-grain weight were all affected by the rate of PG, but not by PG source. The PG1 and PG2 enhanced the grain yield by 61% and 59% respectively, across rates compared to the control (0 t ha-1). Shoot and grains nutrients uptake were enhanced by PG addition with no effect on heavy metals contents of barley’s grains and shoots. Phosphogypsum appeared to be more efficient in promoting barley yield and nutrients uptake compared to G at 15 t ha-1. For instance, the grain yield, and the uptakes of P, S, K, and Mg were 11%, 100%, 64%, 70%, and 75% greater, respectively, across PG sources compared to G.Conclusion This study confirmed that PG is beneficial for saline soils remediation and fertilization and can replace natural gypsum.

According to the FAO, 828 million people were facing acute food insecurity in 2021. Fertilization is a critical input factor in crop production and food security achievement. Therefore, fertilization is a critical input factor in crop production and food security achievement. However, there is room for improvement in the application of fertilizers in certain regions. Thus, new fertilizers with a relatively low cost could enhance farmers’ access to these essential inputs. Phosphogypsum (PG) is used as fertilizer because it contains many nutrients essential for plant growth, including calcium, sulfur, and phosphorus. A two-year field experiment was conducted using two Moroccan PG products (PG1 and PG2, obtained from two different industrial sites), applied at four rates (0, 1.5, 3, and 4.5 t/ha). The aim was to assess the impact of PG source and rate on barley crops, including yield component, nutrients uptake, and heavy metals content. The study’s findings revealed that as the rate of PG application increased, there were significant enhancements in the number of spikes, tillers, grains, total biomass, grain yield, and thousand-grain weight. In fact, when compared to the control, the application of 1.5, 3, and 4.5 t/ha of PG led to a remarkable increase in grain yield by 21%, 34%, and 39%, respectively. Furthermore, the uptake of nutrients (N, P, K, Ca, Mg, and S) by the shoots and grains was significantly influenced by the PG application rates, with higher rates resulting in greater nutrient uptake. Notably, the application of PG had no discernible impact on the heavy metal content in shoots, grains, or soil.

The direct application of phosphate rock (PR) has been found suitable for acidic soils. Still, efforts are needed to improve its reactivity to match grassland P demand. This research aimed to investigate changes in the dissolution of two Moroccan sedimentary PRs (Ben Guerir and Khouribga) in response to four rates of phosphogypsum (PG)—a by-product of the phosphate fertilizer industry. We conducted a 60-day incubation study using two acid soils from New Zealand. The soils were treated with PRs at 100 mgP kg −1 of soil either alone or combined with PG, which was applied at 0, 1, 3 and 9 t ha −1 (approximately the equivalent of 0, 0.9, 2.7, and 8.1 g of PG kg −1 of soil, respectively). The dissolution rates were determined from the differences in residual calcium (Ca) extracted with 1 M HCl. Soil pH, Olsen P, exchangeable aluminium (Al) and Ca and Ca saturation were analyzed at the end of the experiment. Phosphate rocks and PG’s physicochemical properties were characterized. Phosphogypsum addition increased Olsen P by 34% and 59% at 9 t ha −1 compared to 0 t ha −1 in Molesworth and Lindis Peaks soils, respectively. However, PG did not affect the dissolution of P Rs in the different of soil types. Khouribga PR was more reactive than Ben Guerir PR, especially in the Molesworth soil where soil pH and base saturation were lower and P retention was higher compared to Lindis Peaks soil. Particle size distribution was the key factor that contributed to the observed greater reactivity of the Khouribga PR. Both P Rs showed dissolution rates >50%, suggesting their suitability for direct application on acid soils. Being an important source of sulphur and some P, PG if combined with PR, can promote and complement PR’s direct use as fertilizer on acid soils. Moreover, the development of new fertilizer products by combining these two materials should be encouraged.

Liming effects on soil phosphorus (P) availability via biological P cycling are not clear. We conducted an 18-month field experiment on a long-term (60 years +) permanent fertilized grassland in a relatively dry environment. The aim was to examine the impact of liming on P biochemical processes and dynamics. Lime was applied at the beginning of the experiment to produce a soil pH range of 5.4–7.0, with no fertilizer P treatments. Soil sampling was conducted throughout the experimentation period at 0–75 mm. All soils were analysed for moisture content, pH, Olsen P, resin P, exchangeable aluminium (Al), microbial biomass P (MBP) and enzyme activities. At the final sampling, the soil samples were analysed for total C, total N and anaerobic mineralizable N (AMN). A sequential P fractionation was conducted for 0–30 mm depth samples. Liming effects on soil pH and P processes were limited to the surface 30 mm only, where labile inorganic P (P i ) fraction increased by 42% at pH 7.0 compared to pH 5.4, while labile and moderately labile organic P (P o ) decreased by 33% and 25%, respectively. Strong positive relationships were found between microbial P and: soil pH, labile P i , total C and AMN. Absolute activities of acid and alkaline phosphomonoesterases were not affected by liming. However, their specific activity decreased by 47% and 28%, respectively at pH 7.0 compared to pH 5.4. Absolute enzyme activity of phosphodiesterase correlated strongly and positively with labile P i . Our findings demonstrate that liming enhances plant P availability under field conditions in long-term fertilized extensive grassland. However, the effects are limited to near-surface depths in the soil.

Nitrogen (N) and phosphorus (P) are critical to pasture productivity; however, limited information is available on how the single and combined additions of N and P affect soil P fractions and seasonal changes in microbial and biochemical processes linked to P cycling under pasture systems. A two-year field trial was conducted where N (0 or 250 kg ha −1  yr −1 ) and P (0 or 50 kg ha −1  yr −1 ) were applied in a full factorial design to an intensively managed grass-pasture system. Changes in plant growth and nutrient uptake, soil microbial biomass P, soil phosphatase activities, and soil inorganic and organic P fractions were assessed by regular sampling. Phosphorus addition increased Olsen P and shoot P uptake but not shoot biomass compared to the control. In contrast, N addition decreased Olsen P by 23% but increased both shoot biomass and P uptake by 1.6-fold, compared to the control. Microbial biomass P was irresponsive to N and P additions. Phosphatase enzyme activity significantly increased in summer under N addition, which was linked to labile organic P mineralization. After two growing seasons, N addition alone significantly decreased readily-available inorganic P, labile inorganic P, moderately labile inorganic P, and labile organic P by 75, 19, 7, and 28%, respectively, compared to the control. On the other hand, combined N and P addition significantly decreased readily-available inorganic P, labile inorganic P, and labile organic P by 39, 26, and 28%, respectively, but had no impact on moderately labile inorganic P compared to P addition alone. The findings of this study revealed that short-term N fertilization to N-limited grass-pastures can accelerate P cycling by mobilizing labile inorganic and organic P as well as moderately labile inorganic P pools. However, N fertilization combined with P applications exceeding plant requirements cannot mobilize moderately labile inorganic P, which accumulates under high P sorbing soils.

The biochemical drivers of phosphorus (P) availability and cycling are sensitive to changes in soil pH. However, reports of lime-induced pH modification effects on P availability are (1) inconsistent, (2) focused mainly on chemical changes, and (3) often limited to the bulk soil. Using lupin as an indicator species, we aimed to evaluate the effects of lime-induced soil pH change, from 5.3 to 6.0, on biochemical processes responsible for P mobilization and dynamics in the rhizosphere of two different lupin species. Indicator species, blue lupin ( Lupinus angustifolius ) and Russell lupin ( Lupinus polyphyllus ), were grown in a P-deficient acid grassland soil without P inputs for 11 weeks in a glasshouse. The rhizosphere soils were analyzed for enzyme activity, microbial P, and carboxylates. Both bulk and rhizosphere soils were analyzed for P fractions, exchangeable aluminum (Al), and pH. Plant yields and shoot P uptake were measured. Labile inorganic P (P i ) increased in the rhizospheres of both lupin species, likely due to P i desorption combined with labile organic P (P o ) mineralization, induced by rhizosphere pH elevation after liming. Soil pH increase promoted microbial P immobilization and reduced phosphomonoesterase activity in the rhizosphere, leading to an accumulation of P o mainly as moderately labile and stable P o forms. Total carboxylate concentration (TCC) increased with soil pH increase. Variation in shoot P uptake was mostly explained by TCC. These results indicate that (1) lime application strongly affects P biochemical processes and dynamics in the rhizosphere of lupins. (2) Russell lupin utilizes less P than blue lupin and is unresponsive to liming.

PurposeLegume establishment and persistence in New Zealand hill and high-country soils are largely limited by high soil acidity and associated aluminum (Al) toxicity. The present study aimed to evaluate the effect of four rates of phosphogypsum (0, 1, 3, and 9 t ha−1) on Al speciation in the soil solution and to examine which species are mostly impacting total dry matter (TDM) yield of lucerne.MethodsGlasshouse and incubation experiments were conducted using three acid soils with different exchangeable Al concentrations: Molesworth, Glenmore, and Lindis Peaks. The distribution of Al species was modeled using visual Minteq. Partial least square (PLS) regression was used to evaluate the relationships between Al3+ and other variables in the soil-soil solution system.ResultsIn the planted and incubated Molesworth soils, Al3+ and hydroxylated Al (Al–OH) fractions decreased significantly at 1 and 3 t of phosphogypsum ha−1 compared to 0 t ha−1. However, in the planted Glenmore and incubated Lindis Peaks soils, these two fractions remained unchanged. The contribution of variables in Al3+ concentration depended on the soil type. However, the loading plot of the whole soil data set (n = 62) showed that Al–OH, base saturation, soil/soil solution pHw, and exchangeable Al were the main explanatory variables for the variation in Al3+ concentration. The TDM yield of lucerne was better explained by Al3+, Al-F, and Al-DOM than exchangeable Al.ConclusionsReasonable amounts of phosphogypsum (1 to 3 t ha−1) can help to alleviate Al toxicity in acid soils (pH ≤ 5.3), but higher application rates should be avoided.