No abstract

Cumulative daily load time series show that the early 2000s marked a step-change increase in riverine soluble reactive phosphorus (SRP) loads entering the Western Lake Erie Basin from three major tributaries: the Maumee, Sandusky, and Raisin Rivers. These elevated SRP loads have been sustained over the last 12 yr. Empirical regression models were used to estimate the contributions from (i) increased runoff from changing weather and precipitation patterns and (ii) increased SRP delivery (the combined effects of increased source availability and/or increased transport efficiency of labile phosphorus [P] fractions). Approximately 65% of the SRP load increase after 2002 was attributable to increased SRP delivery, with higher runoff volumes accounting for the remaining 35%. Increased SRP delivery occurred concomitantly with declining watershed P budgets. However, within these watersheds, there have been long-term, largescale changes in land management: reduced tillage to minimize erosion and particulate P loss, and increased tile drainage to improve field operations and profitability. These practices can inadvertently increase labile P fractions at the soil surface and transmission of soluble P via subsurface drainage. Our findings suggest that changes in agricultural practices, including some conservation practices designed to reduce erosion and particulate P transport, may have had unintended, cumulative, and converging impacts contributing to the increased SRP loads, reaching a critical threshold around 2002.

Global food production crucially depends on phosphorus (P). In agricultural and urban landscapes much P is anthropogenic, entering through trade. Here we present a long-term, largescale analysis of the dynamics of P entering and leaving soils and aquatic systems via a combination of trade, fluvial transport, and waste transport. We then report net annual P inputs,

The use of leguminous crops as plowdown has stimulated interest in their contribution to soil fertility. The contribution of N from single‐year plowdown to a succeeding corn (Zea mays L.) crop was evaluated in two field experiments located at Elora, Ontario, on a Guelph‐London loam series Grey‐Brown Luvisolic (Typic Hapludalf) soil. In Experiment 1, 20 cultivars of alfalfa (Medicago sativa L.) and 10 cultivars of red clover (Trifolium pratense L.) were compared. Experiment 2 included two cultivars each of alfalfa and red clover under three management treatments. Nitrogen yield of plowdown legumes was considered in relation to the yield and wholeplant N concentration of the succeeding corn crop. In both experiments, whole‐plant N concentration of corn was significantly higher following alfalfa than following red clover. This suggested that alfalfa N was more readily mineralized than that of red clover, as predicted by a decomposability index based on the C/N ratio and lignin concentration of the plowdown material. However, corn yield following alfalfa plowdown was not always greater than that following red clover. Factors other than N‐supply may have influenced corn yield response to legume plowdown. Although significant cultivar variation in plowdown N yield was observed, there appeared to be no association with succeeding corn yield and N concentration. In general, legume plowdown supported corn yields equivalent to those achieved with 90 to 125 kg ha−1 of fertilizer N.

Soybean isoflavone concentrations vary widely, but the contribution of soil fertility and nutrient management to this variability is unknown. Field experiments from 1998 to 2000 on soils with low to high exchangeable potassium (K) concentrations evaluated K application and placement effects on isoflavone concentrations and composition of soybean in various tillage and row-width systems. Soybean seed yield and concentrations of daidzein, genistein, glycitein, leaf K, and seed K were measured. Significant increases in daidzein, genistein, and total isoflavone were observed with direct deep-banded K or residual surface-applied K on low-K soils. Positive effects of K fertilization on isoflavones were less frequent on medium- to high-testing K soils. Both individual and total isoflavones were often positively correlated with seed yield, leaf K, and seed K on low-K soils. Appropriate K management could be an effective approach to increase isoflavone concentrations for soybeans produced on low- to medium-K soils.