Agricultural intensification leads to higher nitrate levels in Lake Ontario tributaries.

Eutrophication remains the most widespread water quality impairment globally and is commonly associated with excess nitrogen (N) and phosphorus (P) inputs to surface waters from agricultural runoff. In southern Ontario, Canada, increases in nitrate (NO3-N) concentrations as well as declines in total phosphorus (TP) concentration have been observed over the past four decades at predominantly agricultural watersheds, where major expansions in row crop production at the expense of pasture and forage have occurred. This study used a space-for-time approach to test whether 'agricultural intensification', herein defined as increases in row crop area (primarily corn-soybean-winter wheat rotation) at the expense of mixed livestock and forage/pasture, could explain increases in NO3-N and declines in TP over time. We found a clear, positive relationship between the extent of row crop area within watersheds and NO3-N losses, such that tributary NO3-N concentrations and export were predicted to increase by ~0.4 mg/L and ~130 kg/km2 respectively, for every 10% expansion in row crop area. There was also a significant positive relationship between row crop area and total dissolved phosphorus (TDP) concentration, but not export, and TP was not correlated with any form of landcover. Instead, TP was strongly associated with storm events, and was more sensitive to hydrologic condition than to landcover. These results suggest that pervasive shifts toward tile-drained corn and soybean production could explain increases in tributary NO3-N levels in this region. The relationship between changes in agriculture and P is less clear, but the significant association between dissolved P and row crop area suggests that increased adoption of reduced tillage practices and tile drainage may enhance subsurface losses of P.

An assessment of the nutrient status of sugar maple in Ontario: indications of phosphorus limitation.

Soil acidification, caused by elevated anthropogenic deposition, has led to concerns over nutrient imbalances in Ontario's sugar maple (Acer saccharum Marsh.) forests. In this study, soil chemistry, foliar chemistry, crown condition, and tree growth were measured at 36 sugar maple stands that included acidic (pH < 4.4), moderately acidic (4.4 ≤ pH < 5.4), and non-acidic (pH ≥ 5.4) soil groups. Acidic sites had significantly lower foliar P, Ca, and Mg concentrations, and the Diagnosis and Recommendation Integrated System indicated that P, rather than Ca or Mg, was the most limiting nutrient. This is in spite of widespread reports of net Ca losses from acidified soils. Mass balance studies in the region indicate that in acidic forest soils, P input from deposition is greater than stream export. Low foliar P is therefore most likely due to low P availability to trees resulting from accumulation in organic matter/biomass and/or adsorption to Fe and Al hydroxides which are more prevalent in acidic soils. Despite differences in foliar nutrition, there were no significant differences in crown condition or tree growth across the study region, suggesting that low P availability is not yet having a widespread detrimental effect on tree health.

The adsorption and release of sulfur in mineral and organic soils of the Athabasca Oil Sands Region, Alberta, Canada.

Mineral soil and fibric peat from acid-sensitive western boreal catchments in the Athabasca Oil Sands Region of Alberta, Canada were evaluated for their ability to adsorb and release SO(4)(2-). Laboratory batch studies indicated that SO(4)(2-) adsorption in mineral soil from both the A and B horizons exhibits a limited response to elevated SO(4)(2-) concentrations, with the slope of initial mass isotherms <0.2 for all soils, likely due to low iron and aluminum oxide content. Although S retention is the dominant process in peat soils in the region, drought simulations in the lab using fibric peat collected from a poor fen exhibited as much as a five-fold increase in SO(4)(2-) concentration after drying and rewetting. Given the limited SO(4)(2-) adsorption capacity of mineral soils and the potential drought-induced S release from peatlands in this region where increased S deposition is expected, further investigation of acidification impacts is warranted.