AGRICULTURAL WATER QUALITY IN COLD ENVIRONMENTS SPECIAL SECTION Core Ideas• Floodwater DRP concentration increased with time of flooding in amended and unamended soils. • Increase in floodwater DRP concentration was less under simulated snowmelt than summer flooding. • Rate of P diffusion from pore water to floodwater was less under simulated snowmelt flooding. • Gypsum reduced floodwater DRP in one soil with DRP concentrations >1 mg L −1 , but not in the other. • Woodchip biochar was ineffective in reducing P release from soils to overlying floodwater.

The maximum benefits that may be obtained from site‐specific N management will not be realized until we improve our understanding of the spatial variability in plant available nitrogen (PAN) under different soil and crop management practices across the landscape. The objectives of this study were to quantify the influence of soil factors on the spatial and temporal variability of N availability to corn (Zea mays L.) grown under different tillage and legume cover crop conditions. Three management treatments were established on a variable landscape in southern Ontario in 1999, 2000, and 2001: barley (Hordeum vulgare L.) followed by corn under no‐tillage, barley followed by corn under spring plowing and secondary tillage, and barley underseeded with red clover (Trifolium pratense L.) that was plowed down the next spring and followed by corn. The PAN contents were measured through the growing seasons and related to soil properties, management, and their interactions using regression analysis. The spatial patterns of PAN were temporally stable, suggesting a temporal consistency in the spatial patterns of factors influencing PAN. The variation in soil temperature among landscape positions was very small, often only 1 to 2°C, and its contribution to the spatial variation in PAN was considered negligible. Soil water made only a small contribution to the variability in PAN because the seasonal average water‐filled pore space (SAWFPS) exhibited little variation across the landscape and often fell within the nonlimiting water range (NLWR) for N mineralization. Most of the variation in PAN within a season in this landscape was accounted for by variation in organic carbon (OC) content.

An understanding of the spatial and temporal variability of N uptake at a landscape scale is required to implement site-specific N management. We determined the spatial variation of in-season N uptake and N nutritional status of corn (Zea mays L.) in a variable Abbreviations: CHU, crop heat unit; CT, conventional tillage; DM, aerial dry matter; Mgt, management treatment effect; N, fertilizer N; N c , critical N concentration in the aerial dry matter; N i , N concentration in the aerial dry matter at a given time; NT, no-tillage; 1N, fertilizer N added; 0N, zero fertilizer N; OC, organic carbon; Pos, landscape position effect; RC, red clover; WC, gravimetric water content. R.S. Dharmakeerthi, Dep. of Soils and Plant Nutrition, Rubber Research Inst. of

Despite the large number of studies on biochar and soil properties, few studies have investigated the effects of biochar in contrasting soils. A study was conducted including four rice-husk biochar rates (0%, 0.1%, 0.5% and 1%) to understand the effects on selected soil properties of two Alfisols (sand and sandy loam) in Sri Lanka. Significant changes in soil properties including increases in pH, cation exchange capacity (CEC), organic carbon, water retention at field capacity and saturated hydraulic conductivity, and reduction in bulk density, were observed at higher rates of biochar (0.5% and 1%). Mean-weight-diameter increased only at 1% biochar application rate in sandy soil, whereas it significantly increased across all the rates in sandy loam soil over the control. Electrical conductivity showed no significant increase in either soil, indicating no threat of salinity. Biochar showed a potential for ameliorating acidity, especially in slightly acidic sandy soil. Soil aggregation and water flow improved markedly in sandy loam soil over sandy soil. Further, CEC and water retention of sandy soil had pronounced effects compared with sandy loam soil. Our study highlights the importance of soil type in determining the value of rice-husk biochar as a soil amendment to improve soil aggregation, water retention and flow and CEC.

Although there are economic and environmental reasons to manage fertilizer‐nitrogen (N) more effectively in variable landscapes, the impact of weather and its interaction with soil properties/landscape attributes or management practices has received little attention. The objectives of this study were to assess the magnitude of temporal and spatial variability in soil and plant N in a variable landscape under different management practices and to assess the dependence of their temporal variability on readily available weather variables such as air temperature and rainfall. The experiment was conducted from 1997 to 2003 on a simple slope under three maize (Zea mays L.) based cropping systems. Soil and shoot N were measured through the growing season and the sum used as a measure of plant available N (PAN). Values of PAN varied with year, treatment, landscape position, and year × treatment and year × treatment × position interaction terms. The effects were quantified for each management treatment using multiple regression analyses to relate PAN to soil organic carbon (OC), cumulative degree days (CDD), and cumulative rainfall (CRF) in different periods within the growing season. Plant Available Nitrogen was most strongly influenced by rainfall early in the growing season and exhibited a nonlinear response to OC and CRF. The regression model predicted spatial patterns that were generally stable when applied to historical weather data; PAN increased with OC in 12 of the 15 yr. The analyses illustrate the feasibility of combining soils and weather data to predict N dynamics in variable landscapes.