DGT technique to assess P mobilization from greenhouse vegetable soils in China: A novel approach

Use of diffusive gradients in thin-film technique to predict the mobility and transfer of nutrients and toxic elements from agricultural soil to crops-an overview of recent studies

The purpose of this review was to survey the recent applications of the diffusive gradients in thin films (DGT) technique in the assessment of mobility and bioavailability of nutrients and potentially toxic elements (PTEs) in agricultural soil. Many studies compared the capabilities of the DGT technique with those of classical soil chemical extractants used in single or sequential procedures to predict nutrients and PTE bioavailability to crops. In most of the published works, the DGT technique was reported to be superior to the conventional chemical extraction and fractionation methods in obtaining significant correlations with the metals and metalloids accumulated in crops. In the domain of nutrient bioavailability assessment, DGT-based studies focused mainly on phosphorous and selenium labile fraction measurement, but potassium, manganese, and nitrogen were also studied using the DGT tool. Different DGT configurations are reported, using binding and diffusive layers specific for certain analytes (Hg, P, and Se) or gels with wider applicability, such as Chelex-based binding gels for metal cations and ferrihydrite-based hydrogels for oxyanions. Overall, the literature demonstrates that the DGT technique is relevant for the evaluation of metal and nutrient bioavailability to crops, due to its capacity to mimic the plant root uptake process, which justifies future improvement efforts.

Leaching and fractionation of phosphorus in intensive greenhouse vegetable production soils

Greenhouse vegetable production systems use excessive phosphorus (P) fertilizer. This study is set out to look into the P fractionation, mobility, and risk of P leaching in ten greenhouse soils. The mean P concentrations in leachates varied from 0.4 to 1.6 mg l-1 (mean of 30 days of soil leaching). Between 5.7 and 31.0 mg kg-1 of P was leached from soils during 30 days of column leaching. Organic matter (OM) and Olsen-extractable P (Olsen P) correlated strongly with cumulative P leached after 5, 10, 15, 20, 25, and 30 days of leaching. The high correlation between OM and Olsen P with cumulative P leached at 5 days of leaching suggests that in future leaching experiments, the leaching period should be extended to 5 days of leaching. The first two P fractions correlated significantly with the total P leached in the primary days of leaching. The pH had little effect on P leaching but had a significant impact on soluble and exchangeable P fraction, suggesting that P mobility would increase in these calcareous greenhouse vegetable soils as pH rose. The calculated change point (194 mg kg-1) was high, indicating that a high percentage (40%) of the studied greenhouse soils had exceeded the change point. In conclusion, due to the high degree of P saturation and change point in greenhouse vegetable soils, P mobilization is a significant risk, and the findings can be used to provide future direction for fertilizing greenhouse vegetable soils.

Available and total phosphorus background levels in soils: a calcareous and semi-arid region

It is critical to understand the risk of element pollution in soils by evaluating their background levels. Phosphorus (P) content in agricultural soils needs to be assessed from agronomic and environmental standpoints. The current study intended to calculate the background levels of available and total P in soils. To achieve this goal, 50 sites without human activities were selected. Soils were sampled from the surface and subsurface of each site (100 soil samples). The available P forms in soils were extracted using the water-extractable P (WEP), calcium chloride-extractable P (CCEP), and Olsen-extractable P (OEP) methods. The first two extractants are being used to evaluate P leaching from soils, while the last one is being used as an agronomic indicator. The methods used to calculate background levels were the iterative 2-δ technique (2-δ) and the calculated distribution function (CDF). Results showed that the upper limits of background levels using 2-δ method were 1.45, 0.92, 8.12, and 424.4 mg kg^-1 for WEP, CCEP, OEP, and total P, respectively. Also, the upper limits of background levels using CDF method were 1.42, 1.15, 12.09, and 447.6 mg kg^-1, for WEP, CCEP, OEP, and total P, respectively. It can be concluded that using these background levels, which for the first time were calculated for P, would enable us to have an accurate examination of P excess as a result of human activities.

Dynamic Changes in Soil Phosphorus Accumulation and Bioavailability in Phosphorus-Contaminated Protected Fields

Soil phosphorus accumulation resulting in a high risk of phosphorus pollution is due to high multiple vegetable cropping indexes and excessive fertilizer input in protected fields. Therefore, this study explored the bioavailability of soil-accumulated phosphorus to improve fertilization and reduce the risk of soil phosphorus contamination in protected fields. A field trial was performed in Yanbian Prefecture, China to study the phosphorus bioavailability after continuous spinach planting without phosphate fertilizer applications. Results indicated that with increasing numbers of planting stubbles, soil inorganic phosphorus and occluded phosphorus changed little, while water-soluble and loose phosphorus, aluminum-phosphate, iron-phosphate, and calcium-phosphorus decreased first and then increased. Soil available phosphorus declined linearly. For planting spinach in protected fields, the threshold of soil phosphorus deficiency is 200 mg kg^-1. A soil phosphorus supply potential model was established between x (the soil available phosphorus) and y (the numbers of planting stubbles): y = 6.759 + 0.027x, R = 0.99, which can be used to predict how planting stubbles are needed to raise the soil available phosphorus above the critical value of phosphorus deficiency for spinach. These results will provide the theoretical guidance for rational phosphorus fertilizer applications and control agricultural, non-point pollution sources in protected fields.

Excessive phosphorus inputs dominate soil legacy phosphorus accumulation and its potential loss under intensive greenhouse vegetable production system

Phosphorus (P) is an essential element for crop growth and it plays a critical role in agricultural production. Excessive P applications has become a serious concern in Chinese greenhouse vegetable production (GVP) systems. Nevertheless, P accumulation (legacy P) in GVP profile soils and its potential loss remain poorly documented. Hence, this study aimed to response this issue via paired collection of 136 soil samples (0-30, 30-60 and 60-90 cm depth) and 41 vegetable samples from both plastic greenhouses (PG) and solar greenhouses (SG) in Shouguang, Shandong province. Results showed that the annual input of P ranged from 772 to 2458 kg ha^-1 for different vegetables through the whole growing season versus little vegetable P uptake (ranging from 47.8 to 155 kg ha^-1). Results also revealed significant P accumulation in both SG and PG profile soils. Compared to arable soils (background soils), legacy P to the depth of 90 cm in PG and SG soils were 3.28 and 11.16 Mg P ha^-1, respectively. The content of total P in PG and SG soils significantly increased with cultivation duration. The maximum environmental capacity of P in SG soils was 187 Mg ha^-1, and the maximum number of years for safe planting was 38 yrs. After four years of cultivation, P loss would occur in these soils and the loss rate of P increased with cultivation duration. Opposite to PG soils, a potentially higher risk of P losses took place in SG soils. Our results also demonstrated that excessive P inputs driven by intensive agricultural practices dominated legacy P accumulation within the profile soils and its losses in GVP systems. Site-specific P managements, including improving P use efficiency, reducing further P surplus and reusing legacy P in soils, are urgently needed to minimize P loss. At the same time, the potential loss of subsoil P could not be neglected.

Alum split applications strengthened phosphorus fixation and phosphate sorption in high legacy phosphorus calcareous soil

High phosphorus (P) saturation arising from historic P inputs to protected vegetable fields (PVFs) drives high P mobilisation to waterbodies. Amendment of soils with alum has shown potential in terms of fixing labile P and protecting water quality. The present 15 month pot experiment investigated P stabilisation across single alum application (Alum-1 treatment, 20 g alum/kg soil incorporated into soil before the maize was sown), alum split applications (Alum-4 treatment, 5 g alum/kg soil incorporated into soil before each crop was sown i.e. 4 × 5 g/kg) and soil only treatment (Control). Results showed that the Alum-1 treatment caused the strongest stabilisation of soil labile P after maize plant removal, whereas the P stabilisation effect was gradually weakened due to the transformation of soil non-labile P to labile P and the reduced active Al³⁺ in soil solution. For the Alum-4 treatment, soil labile P decreased gradually with each crop planting and was lower than the Alum-1 treatment at the end of the final crop removal, without any impairment on plant growth. The better P stabilisation at the end of Alum-4 treatment was closely correlated with a progressive supply of Al³⁺ and a gradual decrease of pH, which resulted in higher contents of poorly-crystalline Al, Fe and exchangeable Ca. These aspects were conducive to increasing the soil P stabilisation and phosphate sorption. In terms of management, growers in continuous cropping systems could utilise split alum applications as a strategy to alleviate P losses in high-P enriched calcareous soil.

Increased risk of phosphorus and metal leaching from paddy soils after excessive manure application: Insights from a mesocosm study

Livestock manure has gradually become an alternative fertilizer for maintaining soil fertility, whereas excessive application of manure leads to the release of phosphorus (P) and toxic metals that may cause complex environmental risks. To investigate the accumulation and migration of P within soil profiles, a mesocosm experiment was conducted to analyze the content and leaching of soil P, metals, and dissolved organic carbon after different fertilization treatments, including control (no fertilizer, CK), chemical fertilizer (CF), chemical fertilizer combined low (CF + LPM) and high (CF + HPM) rate of manure application. Results showed that a high rate of manure application significantly enhanced the accumulation of total soil P (by ~14%) and P availability (easily-available P, by ~24%; Olsen-P, by ~20%) in topsoil, and also increased the content of easily-available organic P (EA-Po) in both topsoil and subsoil compared to the CK treatment. The migration of dissolved inorganic and organic P (DIP and DOP) in leachate within soil profiles was strengthened by manure application. Moreover, significant positive correlations between P, metals, and dissolved organic carbon (DOC) in leachate indicated that downward co-migration occurred within the soil profiles, and also suggested that excessive manure application can intensify the risk of P loss by increasing the migration of manure-derived DOC. Overall, our findings provide insights into P accumulation and migration within soil profiles after excessive manure application, which is useful for predicting the potential risk of P and metal leaching from paddy soils.

Sensitivity of Three Phosphate Extraction Methods to the Application of Phosphate Species Differing in Immediate Plant Availability

Extractive tests for determining the plant-availability of soil phosphorus (P) give varying results due to the inherently different characteristics of the extraction solution. Generally, classical soil P tests such as the Olsen or calcium acetate/lactate (CAL) method do not give an indication on the total amount of plant available P, but merely give an indication of the equilibrium between soil and extraction solution. It is also not entirely clear which fractions of P are directly determined through the various methods of extraction, i.e., determined P must not be immediately plant available, as is the case for rock phosphate. It is therefore possible that extraction methods either over or under estimate the amount of P available for plant consumption. In this research, we compared three methods of soil P determination (CAL, Olsen and diffusive gradients in thin films (DGT)) with regards to their ability to determine P species (Ca(H2PO4)2, CaHPO4, Ca3(PO4)2 and Inositol-6-hexakisphosphate) added to soils of high sorption capacity, immediately after as well as two weeks after application. For each of the methods, it could be shown that sorption processes in the soil immediately (0 days incubation) fix P to a point where it is not extractable through any of the described methods. These sorption processes continue over time, leading to a further decrease of determined P. The acidic CAL extraction method gives higher results of extractable P compared to the Olsen method. Due to the extraction of Ca3(PO4)2, the CAL method may overestimate immediately plant-available P. The most suitable methods for the determination of immediately plant available P may therefore be the Olsen and DGT methods. Organic IP6 is not determined by any of the extraction methods. At low concentrations of soil P, the DGT method may fail to give results.