Document Type : Applicable
Authors
1
Department of Soil Science, Faculty of Agriculture, Shahid Bahonar University of Kerman
2
Environment Centre Wales, School of Environmental and Natural Sciences, Bangor University, Bangor, LL57 2UW, United Kingdom
3
Kerman Center for Agricultural and Natural Resources Research, Agricultural Research, Education and Extension Organization, Kerman, Iran
Abstract
Introduction: Pistachio is an important horticultural crop in Iran, extensively grown in arid and semi-arid regions, particularly in Kerman province. The cultivation of pistachios in calcareous soils presents significant challenges, including nutrient deficiencies, high salinity, elevated pH, and low organic matter content, all of which impair effective plant nutrition. Phosphorus stands as the second most important macronutrient after nitrogen in plant nutrition, playing a critical role in diverse metabolic functions essential for growth, development, and stress response. Understanding the dynamics of phosphorus (P) forms, especially non-labile P, in calcareous saline soils is crucial for optimizing P fertilizer use and minimizing adverse environmental impacts. Phosphorus fixation, a persistent issue associated with P fertilizer application, significantly reduces pistachio orchard productivity in eastern Iran. Excessive use of P fertilizers not only contributes to environmental issues such as nutrient imbalances, micronutrient deficiencies, and an increased risk of eutrophication, but also escalates production costs. Effective management of phosphorus fixation could reduce dependence on chemical fertilizers, limit phosphorus leaching into water sources, and alleviate eutrophication risks. This study investigates the vertical distribution of phosphorus forms, with a focus on non-labile P, in response to varying levels of phosphorus fertilizer application in pistachio-cultivated soils, aiming to inform sustainable nutrient management practices.
Materials and Methods: This study was conducted in a pistachio orchard in Kazem-Abad, located 39 km northwest of Kerman province (30°34'23"N, 56°49'44"E). The experiment followed a completely randomized design with three replicates to assess the effects of triple superphosphate (TSP) applied at rates of 200, 400, and 600 kg ha⁻¹, compared to a control treatment (no TSP). Soil samples were collected from four depths (0–10, 10–20, 20–30, and 30–50 cm), air-dried, and passed through a 2 mm sieve. Standard laboratory methods were used to determine the physicochemical properties of the soil. Inorganic phosphorus fractions (Pi fractions) were quantified using a sequential extraction method. Available P (Olsen P), total phosphorus (TP), total organic phosphorus (TPo), and total inorganic phosphorus (TPi) were also measured. Phosphorus concentrations in the extracts were determined using a colorimetric method. Data normality was assessed with the Kolmogorov-Smirnov test, and a Cox-Box transformation was applied when necessary to normalize the data. Analysis of variance (ANOVA) was performed using SAS 9.4 software to evaluate the effects of different phosphorus fertilizer levels on various phosphorus forms. Mean comparisons were conducted using the LSD at a 5% significance level (p < 0.05).
Results and Discussion: The results revealed a significant effect of fertilizer application rates on available phosphorus across all soil depth (p < 0.05). Available phosphorus concentrations increased progressively with higher TSP application rates, with the highest levels observed at 600 kg ha⁻¹. Compared to the control, this rate resulted in an average increase of 50% in total organic phosphorus, 40% in total inorganic phosphorus, and 43% in total phosphorus across the sampled soil layers. The proportions of Pi fractions also increased consistently with higher fertilizer rates across all depths. Non-labile phosphorus was the predominant inorganic P forms in all treatments and depths. Notably, application of 200 kg ha⁻¹ TSP enhanced both labile and moderately labile phosphorus, while higher rates (400 and 600 kg ha⁻¹) significantly increased non-labile phosphorus content. At 600 kg ha⁻¹, non-labile phosphorus concentrations were significantly higher than in the control. Statistical analysis showed significant correlations between available phosphorus, inorganic phosphorus fractions, and total phosphorus, with non-labile phosphorus exhibiting the strongest correlation with available phosphorus (r = 0.59). This suggests that non-labile phosphorus serves as a reservoir, releasing phosphorus to meet plant demands as labile P forms.
Conclusion: The study demonstrates that varying levels of phosphorus fertilizer application significantly influence the distribution of phosphorus forms, with higher application rates increasing the proportion of non-labile phosphorus. These findings highlight the critical role of non-labile phosphorus in sustaining long-term phosphorus availability for pistachio crops, thereby supporting soil fertility in calcareous, arid environments. To prevent excessive phosphorus accumulation and mitigate environmental risks such as eutrophication, careful management of fertilizer application is essential. The results advocate for the use of reduced chemical fertilizer rates and the exploration of stabilized phosphorus sources as sustainable alternatives. Such strategies can lower production costs, enhance soil health, and minimize environmental pollution, contributing to the long-term sustainability of pistachio cultivation in Iran.
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