Soil-Plant Phosphorus Dynamics and Microbial Activity as Influenced by Integrated Phosphorus Management in Wheat: Correlations with Grain and Biological Yields

Technical Summary: Soil-Plant Phosphorus Dynamics and Microbial Activity in Wheat under Integrated Management

Problem Statement
Phosphorus (P) deficiency in alkaline calcareous soils is a primary constraint on global wheat productivity, particularly in South Asia and Pakistan. In these soils, applied soluble P rapidly precipitates as stable calcium phosphates (e.g., hydroxyapatite), rendering 80–85% of applied fertilizer unavailable to crops. This inefficiency leads to economic losses for farmers, depletion of finite phosphate rock reserves, and environmental degradation through runoff. While integrated nutrient management (INM) combining organic amendments and biofertilizers is proposed as a solution, there is a lack of comprehensive data quantifying the simultaneous effects of diverse organic-inorganic P combinations and specific biofertilizers on soil physicochemical properties, microbial dynamics, and wheat productivity in these specific soil types.

Methodology
The study was conducted as a two-year field experiment (2020–2022) at the Agronomy Research Farm of The University of Agriculture, Peshawar, Pakistan, on silt loam calcareous soil (pH > 7.0, low organic matter). The experiment utilized a two-factor randomized complete block design (RCBD) with three replications.

• Factor A (P Sources): 21 treatments supplying a uniform rate of 90 kg P ha⁻¹. These included six sole sources: Single Super Phosphate (SSP), Rock Phosphate (RP), Sheep Manure (SM), Cattle Manure (CM), Legume Residues (LR), and Non-Legume Residues (NLR). The remaining 15 treatments were 50:50 mixtures of these sources (inorganic-inorganic, inorganic-organic, organic-organic).
• Factor B (Biofertilizers): Two treatments: Phosphate-Solubilizing Bacteria (PSB, a multi-strain culture) and Bioaab (Effective Microorganisms). An absolute control (no P, no biofertilizer) was included.
• Measurements: Post-harvest soil analysis included pH, organic matter (OM), and available P (AB-DTPA method). Plant analysis covered grain/straw P concentration, total P uptake, Phosphorus Use Efficiency (PUE), and Agronomic P Use Efficiency (APUE). Soil microbial populations were enumerated via serial dilution.
• Statistical Analysis: Data were analyzed using ANOVA, Pearson correlation, stepwise multiple regression, and Principal Component Analysis (PCA).

Key Contributions and Results

1. Soil Property Modifications:

   • pH Reduction: Integrated management significantly lowered soil pH. The control maintained a pH of 8.30, whereas the Cattle Manure (CM) + PSB treatment reduced it to 7.81. Plant residues (LR, NLR) also achieved low pH values (7.74–7.76). A strong negative correlation was found between soil pH and grain yield (r = -0.78).
   • Organic Matter (OM): OM ranged from 0.71% (sole SSP) to 1.41% (LR + NLR mixture). Sole animal manures increased OM to 1.20–1.26%, significantly higher than inorganic-only treatments. OM showed a strong positive correlation with grain yield (r = 0.86).
   • Available P: Available P increased from 2.49 mg kg⁻¹ (control) to a maximum of 4.79 mg kg⁻¹ (sole SM). Sole animal manures generally outperformed sole inorganic sources in maintaining available P, likely due to slower mineralization rates that bypass immediate fixation.

2. Microbial Dynamics:

   • PSB inoculation dramatically increased microbial populations (5.7 × 10⁸ cfu g⁻¹) compared to Bioaab (3.6 × 10⁸ cfu g⁻¹) and the control (2.5 × 10⁸ cfu g⁻¹).
   • Microbial population exhibited the strongest correlation with PUE (r = 0.91) and grain yield (r = 0.88), establishing it as a critical biological indicator of soil health in this system.

3. Plant Performance and Efficiency:

   • Yield and Uptake: The CM + PSB treatment achieved the highest grain P concentration (0.36%), total P uptake (21.8 kg ha⁻¹), and grain yield.
   • Efficiency Metrics: PUE ranged from 4.2% (NLR) to 14.4% (sole CM). APUE was maximized at 17.5 kg kg⁻¹ (SSP + SM). PSB inoculation increased PUE by 20% and APUE by 29% compared to Bioaab.
   • Synergy: The combination of animal manures (CM, SM) with PSB yielded superior results compared to plant residues or inorganic sources alone. Non-legume residues (NLR) showed minimal response to biofertilizers.

4. Predictive Modeling:

   • Stepwise regression identified soil available P and microbial population as the most significant predictors of grain yield, explaining 94% of the variation (R² = 0.94).
   • The model suggests that for every 1 mg kg⁻¹ increase in available P, grain yield increases by approximately 426 kg ha⁻¹, and for every 1 × 10⁸ cfu g⁻¹ increase in microbial population, yield increases by 185 kg ha⁻¹.

5. Temporal Effects:

   • Significant Year × Treatment interactions were observed for soil pH, OM, and APUE. The benefits of integrated management (specifically mixing organic and inorganic sources) became more pronounced in the second year, indicating cumulative improvements in soil health and P availability over time.

Significance and Claims
The paper claims that integrated P management, specifically the combination of Cattle Manure (CM) with Phosphate-Solubilizing Bacteria (PSB), represents the optimal strategy for wheat production in alkaline calcareous soils. This approach successfully:

• Mitigates soil alkalinity (reducing pH by ~0.5 units), thereby enhancing P solubility.
• Builds soil organic matter, which protects P from fixation and supports microbial activity.
• Maximizes P use efficiency, potentially reducing reliance on mined phosphate rock.
• Provides a predictive framework where biological indicators (microbial population) are as critical as chemical indicators (available P) for forecasting crop productivity.

The authors conclude that while the absolute available P levels (approx. 4.8 mg kg⁻¹) remain below traditional critical thresholds (12–15 mg kg⁻¹), the integrated approach creates a rhizosphere environment where P is effectively mobilized through acidification and chelation, allowing for high yields without excessive chemical fertilizer inputs. The study emphasizes that the efficacy of biofertilizers is highly dependent on the accompanying P source, with animal manures providing the necessary carbon substrates for PSB proliferation.