Impact of phosphorus drip fertigation practices on chickpea physiology, yield, and nutrient dynamics in soil-plant continuum under Mediterranean climate

The agricultural sector in the Mediterranean basin faces many challenges related to the use of water and mineral resources for crop production and food security for an exponentially growing population. Most countries in arid and semi-arid areas suffer from water scarcity and soil nutrient depletion which severely impact the productivity of different cropping systems currently challenged by climate change and the high pressure on natural resources. Given its role in several plant biophysiological processes, such as plant photosynthesis, carbon fixation, energy transfer and allocation, and plant growth and development, phosphorus (P) nutrition take increasing attention in the last decades. The limited resources of rock phosphate as well as the low use efficiency of phosphorus fertilizers push the scientific community to urgently rethink the strategies of P fertilizer management at different scales.
In this dissertation, we focus on the study of the impact of P agricultural management practices on P availability and mobility under drip fertigation systems and the consequent effects on plant photosynthesis, nutrient uptake, plant growth and productivity, and nutrient use efficiency. A series of agronomic pot experiments were conducted under controlled conditions to investigate the impact of various P drip fertigation practices on phosphorus availability in Moroccan alkaline soils, on chickpea physiology, productivity, and nutrient uptake. The objective of the first trials was to study the potential use of inorganic soluble polyphosphates (Poly-P) as slow-release fertilizer under drip fertigation. The impact of the chemical properties of polyphosphate and orthophosphate (Ortho-P) fertilizers (polymerization degree, solubility, and interactions with other nutrients), as well as P application frequency on the availability of phosphorus in soil (Olsen-P) and its relocation into deeper soil layers, were investigated. The second pot trials were conducted in controlled conditions to evaluate the interactive effect of P fertilizer form (Ortho-P vs Poly-P) and soil moisture conditions on chickpea photosynthetic activity, water and nutrient uptake, and their consequent effects on biomass accumulation and nutrient use efficiency. The mechanisms underlying phosphorus nutrition impacts on chickpea photosynthesis efficiency, grain yield, and nutrient uptake and use efficiency were explored using different plant sensors and analytical methods like chlorophyll fluorescence, plant phenotyping, and plant nutrient analysis. Based on the results of pot trials, two-years field experiments were conducted in a 2.5 ha plot. The objective of these field experiments was to evaluate the potential use of electromagnetic induction as a fast and reliable technique to characterize soil properties. Soil electrical conductivity (ECa), crop yield data, and soil chemical properties were used as input data to delineate chickpea fertigation management zones using unsupervised clustering analysis. The variable rate P application strategy was evaluated as an efficient agricultural practice to increase the chickpea grain yield and quality and to improve phosphorus use efficiency at the field scale in Moroccan semi-arid conditions.
The results from pot experiments showed that soil phosphorus availability varied significantly between fertilizer forms and fertigation frequencies. As compared to the Ortho-P fertilizer, P becomes less mobile in soils fertigated with Poly-P forms (Poly-100 and Poly-53) and its availability is maintained until harvest. The analysis of Olsen-P at the harvest stage showed that the higher P availability in soil was obtained with Poly-P forms with higher values in the 0-5 and 5-10 cm soil layers than in the 10-20 cm. Moreover, weekly fertigation (Fweek) revealed the best results in terms of P availability compared to fertigation at sowing (Fsow) or 3-day frequency (F3days) and all P fertilizers significantly improved chickpea grain yield, seed quality, and water productivity, compared to the unfertilized control. In addition, our findings showed that the photosynthesis efficiency was significantly improved in fertigated chickpea plants compared to the control treatment as revealed by the electron transport chain between photosystem II and I, which was significantly enhanced. The polyphosphate fertilizer (Poly-A: Poly-53) increased the number of electron acceptors of the photosynthetic linear electron transport chain compared to the other P fertilizer forms. Furthermore, the interactive effect between P fertilizer forms and soil moisture conditions resulted in several significant changes in chickpea phenotypic and physiological traits. The stomata density and conductance, chlorophyll content, photosynthesis efficiency, biomass accumulation, and plant nutrient uptake were significantly improved under P drip fertigation with varied values depending on P fertilizer form and irrigation regime. Our results suggested that the P fertilizer form and irrigation regime providing chickpea plants with enough P and water, at the early growth stage, increased the stomatal density and conductance, which significantly improved the photosynthetic performance index (PIABS) and P use efficiency (PUE), and consequently biomass accumulation and nutrient uptake. The significant correlations established between leaf stomatal density, PIABS, and PUE supported the above hypothesis. Based on these results from the pots experiments we can conclude that Poly-P fertilizers can be recommended as an effective source of phosphorus for plants, due to their slow-release properties and the frequency of P application through the drip fertigation system which can be reduced while ensuring high crop yields. However, the effectiveness of Poly-P fertilizers was greatly reduced under water stress conditions, unlike the Ortho-P form which kept stable positive effects on the chickpea's physiological traits. On other hand, the field experiment showed that the soil spatial variability can be properly assessed by the electromagnetic induction sensor (CMD MiniExplorer), which represents basic information to conduct an oriented soil sampling design. Moreover, the statistical correlation analysis as well as the multiple regression model used for the prediction of chickpea yield greatly contributed to identifying the main soil properties influencing chickpea grain yield in the studied site. The combination of soil and plant sensing data (ECa, Ca, P, and GY) and their processing by geostatistical tools (semi-variograms, interpolation by ordinary kriging and IDW methods, and fuzzy c-means clustering algorithm) were very useful to delineate chickpea drip fertigation management zones (MZs). The application of variable rate P fertilizers through a drip fertigation system in the identified MZs increased the chickpea grain yield, seed size, and phosphorus use efficiency by 12%, 9%, and 18% respectively.
Based on these results, we can conclude that the optimization of P fertigation practices, especially the choice of adequate P fertilizer form and application frequency, considering the availability of irrigation water and soil properties, can greatly contribute to improving crop yield and phosphorus use efficiency at field scale. Moreover, the integration of precision agricultural practices like variable rate application could be of great interest to farmers in semi-arid regions like Morocco to enhance water and mineral resource use efficiency which in return contributes to improving farmers' income, preserving the ecosystem, and ensuring sustainable agricultural production systems.