The status of phosphorus levels in Iranian agricultural soils - a systematic review and meta-analysis.

Phosphorus (P) inputs are essential for maximizing agronomic potential, yet high P inputs and subsequent P losses can cause eutrophication of water bodies. There is a need to evaluate P contents in agricultural soils globally both from an agronomic and environmental perspective. This systematic review and meta-analysis estimated the pooled mean levels of P contents of Iran. In this study, data on available and total P contents of Iran's calcareous soils was compiled (main focus on Olsen P) and compared to (i) estimated Iranian background and global agricultural soil P contents, and (ii) agronomic and (iii) environmentally critical Olsen P values. The pooled mean estimate from the meta-analysis indicates that the levels of Olsen P across 425 soil samples (27 studies) were 21.3 mg kg-1 and total P across 190 soil samples (12 studies) 805.5 mg kg-1. Using 26 mg kg-1 as the agronomic critical Olsen P value above which no increase in crop yield occurs, crops grown on 61% of the soil samples in the investigated region would respond to P fertilizer and 20% of soils are currently in the optimum category (26-45 mg kg-1 Olsen P). The environmentally critical Olsen P value (~ 63 mg kg-1), defined as the amount above which P leaches from soil rapidly, was exceeded by 11% of soils with a further 4% of soils with elevated eutrophication risk. To maximize crop yields while maintaining a minimal risk of P leaching in Iran's calcareous soils, we suggest an ideal Olsen P of 26 mg kg-1. The outcomes from this study inform about the P status of Iranian soils and could help update recommendations for P fertilizer applications in calcareous soils globally. The framework presented here could further be adopted to evaluate the P status in other soil types.

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.

Minimizing phosphorus leaching from a sandy clay loam caused by phosphorus fertilizers.

At moderate to high fertilization rates, sandy-textured soils can leach much phosphorus (P) threatening surface water quality. High rates are used to compensate for P leaching, but there is also potential to reduce P leaching by using different P fertilizers. We examined the effect of poultry manure (PM), sheep manure (SM), triple superphosphate (TSP), sewage sludge of Sanandaj (SSS), sewage sludge of Toyserkan (SST), and biochars of Sanandaj and Toyserkan sewage sludges (BSSS and BSST, respectively) applied at a rate of 100 mg P kg-1 (equivalent to 220 kg P ha-1 yr-1, the current regional practice for capital applications designed to raise and maintain soil P in the region) on P leaching over 10 pore volumes (equivalent to 589 mm rainfall) through a sandy clay loam soil widespread in Iran (and the Middle East). Phosphorus leaching losses decreased in the following order: TSP > SM > PM > SST > BSSS > control > SSS > BSST. The leachability of fertilized soil was best estimated by measurement of the mobile KCl-P fraction. At the capital application rate used, SSs or their biochars represented the least risk of P leaching and could be used in place of highly soluble manures or TSP to either protect water quality or maintain more P in the soil. However, this should only occur after confirming that this substitution does not impair agronomic performance.

Simulating phosphorus leaching from two agricultural soils as affected by different rates of phosphorus application based on the geochemical model PHREEQC.

Phosphorus (P) leaching from agricultural soils, in consequence of long-term utilization of P fertilizers, decreases the water quality and leads to eutrophication. The effect of monopotassium phosphate (MKP) at the rates of 0, 50, 200, 400, and 800 mg P kg-1 on P and certain cations leaching from two agricultural soils (loam and sandy loam soils) was investigated in a laboratory study. Soil treatments were packed in columns with 5 cm in diameter, up to 10 cm. Soil columns were leached using distilled water solution for 20 pore volumes, and the leachates were analyzed for pH, electrical conductivity (EC), calcium (Ca), sodium (Na), potassium (K), and P. To simulate the concentrations of K and P in leachates, the PHREEQC model was utilized. In addition, the P vertical distribution in different depths of the soil columns after the leaching experiment was investigated using Olsen-extractable P (Olsen-P). Generally, as the MKP rates increased, the mean (mean of 20 pore volumes) value of pH and Ca concentration in leachates decreased, but the mean value of EC, Na, and K concentrations in leachates increased. In early pore volumes, the P concentration in all treatments begins to rise, then begins to fall. The application of different rates of MKP fertilizer increased the cumulative amount of P leached in both studied soils. Significant relations were obtained for the rates of MKP application and the cumulative amount of P leached. Overall, the model did a good job of simulating K and P concentrations in leachates, as well as the trend of K and P leaching. In both treated soils with increasing of fertilizer rates, the Olsen-P status in all depths increased, and the P content increased with depth. The Olsen-P contents before the leaching experiment for each treatment were predicted, and power equations significantly described its relation with mean P concentration in leachates. Higher application rates of MKP (400 and 800 mg P kg-1) resulted in much higher P concentrations in leachates than the threshold value (0.1 mg l-1), and these rates should not be used in agricultural soils, whereas applying 50 mg P kg-1 to agricultural soil could be a reasonable rate for preventing P losses.

Effect of organic and inorganic phosphorus fertilizers on phosphorus availability and its leaching over incubation time.

The use of organic and inorganic phosphorus (P) fertilizers in agricultural soils is very common, and few studies have been conducted to study the effect of different P sources on relative P extractability (RPE) and leaching using different P extractants and degree of P saturation (DPS), over a long period of time. Thus, this study was conducted to investigate the effect of incubation time and different P sources on RPE, DPS, and to predict the concentration of P leached from soil using different P extractants. In order to achieve these goals, nine sewage sludges (SSs), two biochars, animal manure (AM), poultry manure (PM), wheat residue (WR), diammonium phosphate (DAP), and triple superphosphate (TSP) were added to the soil as much as 100 mg P kg-1 in a 163 days incubation experiment. On average across all amendments and incubation periods, Mehlich-3 extractable P (M3EP) gave the highest mean RPE (42.9%, SE = 7.1%), with water-extractable P (WEP) the lowest (4.6%, SE = 0.93%), and Olsen-extractable P (OEP) (38.3%, SE = 6.3%) in between. Among SSs and based on average across of all incubation periods, soils treated with Shiraz and Takestan SSs were the least soluble source of P, while the highest soluble source of P were soils treated with Kermanshah and Tehran SSs. The results indicated that soil samples taken 16 days following the addition of amendments should reflect agronomic and environmental purposes aiming to assess available and the potential P loss from agricultural soils. The split line model perfectly fitted to the relation between OEP and M3EP (r = 0.93). The DPSs were calculated and the P leaching rate was estimated. Based on OEP, the soils treated with TSP and DAP were at high risk, the medium risk was for soils treated with Kermanshah, Saveh, Tehran, Rasht, Sanandaj, and Isfahan SSs, and PM. Control soil, and soils treated with WR were at no risk, and the soils treated with Arak, Shiraz, and Takestan SSs, ABC, WBC, and AM were classified as low risk.

Assessment risk of phosphorus leaching from calcareous soils using soil test phosphorus.

Accurate estimation of phosphorus (P) leaching is important because excess P may reduce surface and ground water quality. Little attention has been paid to estimate P leaching from soil tests in calcareous soils. The relation between different soil tests P (STP), P sorption index (PSI) and degree of P saturation (DPS) and leaching of P were examined for assessing the risk of P loss from calcareous soils. Columns leaching repacked with native soils were leached with either distilled water or 10 mM CaCl2 solutions, separately. Four leaching events were performed at four days, and 28.7 mm of distilled water or 10 mM CaCl2 solutions was applied at each leaching events. Compared with distilled water, CaCl2 had a small ability to solubilize P from soils. Concentration of P in leachate in both leaching solutions was exceeding 0.1 mg l-1 associated with eutrophication. Cumulative P leached P was ranged from 0.17 to 18.59 mg P kg-1 and 0.21-8.16 mg P kg-1, when distilled water and 10 mM CaCl2 solutions were applied, respectively and it was higher in sandy clay loam soils compared with clay soils. Among evaluated environmental soil P tests, PCaCl2-3h (P extracted by 10 mM CaCl2 for 3 h), PCaCl2-1h (P extracted by 10 mM CaCl2 for 1 h) were more accurate than other soil P tests for predicting P concentration in the leachates in both leaching solutions and accounting for 83% and 72% of variation of P concentration, respectively. The water extractable P (WEP) (r = 0.771) and Olsen-P (POls)(r = 0.739) were significantly related to the leached P concentration using distilled water solution in a split line model, with a change point of 27.4 mg P kg-1 and 61.5 mg P kg-1, respectively. Various DPS were calculated and related to the leached P concentration. Based on P extracted by Mehlich-3 (PM3) and HCl (PHCl) and PSI, the change point of the relationship between leached P concentration and DPSM3-3 (PM3(PM3+PSI)×100) and DPSHCl-2 (PHCl(PHCl+PSI)×100) for both leaching solutions was approximately the same, thus a mean value of 49% for DPSM3-3 and 73% for DPSHCl-2 was obtained. Soils were grouped into four categories of increasing P leaching potential based on WEP, POls, and DPSM3-3. The results indicated that 8.00%-25.50% of the soil grouped in no risk category whereas 8.00%-13.70% of the soils fell into the high risk category.