Soil, surface water and ground water phosphorus relationships in a partially harvested Boreal Plain aspen catchment

Soil phosphorus (water-extractable) measured in harvested and forested areas of a headwater aspen forested catchment in north-central Alberta was related to surface and ground water total dissolved phosphorus (TDP). No differences in water-extractable soil phosphorus concentrations ([ext-P]) were observed between harvested and forested areas. Topographic position explained most of the variance in the [ext-P] of surface soils. Soil [ext-P] in surface horizons was large in upslope areas compared to low-lying areas, ephemeral draws, and wetlands. Forest floor and surface organic soils (0-10 cm) had greater concentrations of ext-P (>70 μg g−1) and total P (tot-P) (>1000 μg g−1) than mineral soils ([ext-P] <2 μg g−1 and [tot-P] <300 μg g−1). Phosphorus buffering capacity was small in organic surface soils (EPC0 > 5000 μg L−1) and large in mineral soils (EPC0 A horizon = 100-400 μg L−1; EPC0 B horizon < 100 μg L−1). This was reflected in greater levels of TDP in surface water (range = 2-2350 μg L−1, median = 85 μg L−1) and soil water (range = 22-802 μg L−1, median = 202 μg L−1) which flowed through organic soils, compared with small concentrations of TDP in ground water which flowed through mineral soils (range = 0-1705 μg L−1, median = 23 μg L−1). Our results indicate that increases in ground water TDP following harvest are unlikely due to the large adsorption affinity of mineral soils. Phosphorus-rich surface soils have a large potential for phosphorus release to surface water but this does not differ between harvested and forested areas. Sub-humid climatic conditions and rapid aspen regeneration lead to soil moisture deficits and limited surface runoff which may reduce harvesting effects on P mobilization on the Boreal Plain.