Phosphorus dynamics in boreal podzols converted from forest to agriculture

Kedir, Amana Jemal (2020) Phosphorus dynamics in boreal podzols converted from forest to agriculture. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Phosphorus (P) is a non-renewable and vital element for all life and the second most yield-limiting nutrient next to nitrogen in agriculture. The global and regional soil P pools, fluxes and the governing biogeochemistry vary with soil, vegetation, and management types. Limited understanding of the dynamics of P fluxes and pools in the Newfoundland (Nfld) natural and agricultural Podzols hampers decisions on best management options. To fill this knowledge gap, I: (a) assessed the levels and status of P in Nfld farm soils and their relationship with the recommended fertilizer rates across locations and managements; (b) evaluated the utility of ten P-extraction and analysis tests (P-test) for P extractability in natural grassland, as well as forested and agricultural soils; (c) determined P adsorption isotherms for forested and agricultural soil horizons; (d) investigated the P adsorption capacity of long-term managed soils; and (e) carried out a greenhouse trial to evaluate the plant uptake of P from soil layers relevant to soils converted from forested to agricultural use. The provincial Mehlich-3 P-test confirmed the relationship between measured P and the recommended P rates, but also suggests possible overfertilization. Soil P-test measurements varied with management status and were affected by the soil organic matter (SOM), Al, Fe, and soil depth or horizon. Citric acid extracted significantly more P for all tested soil conditions except for newly converted soils. Phosphorus adsorption capacity varied by soil horizon, depth, and management. Phosphorus availability and uptake in recently converted soils from forested to agriculture varied with soil depth and P sources. Given the variability in results and the possibility for a mismatch between P application and P uptake for soils, differentially across a range of conversion ages and managements, it is critical that further agronomic work focuses on the detailed understanding of P species and the effect of management on the P biogeochemistry, with a focus on the role of SOM and mineral contents. It is recommended that targeted calibration of P-test and recommendations be carried out to cover all sources of variability in soils that have undergone conversion to confirm the relationship between P sources, extractability, and uptake for different conversion and management states.

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