The impact of nitrogen source and crop rotation on nitrogen mass balances in the Mississippi River Basin.

Nitrogen (N) leaching to surface waters from grain farms in the Mississippi River Basin (MRB), USA, is the primary cause of hypoxia in the Gulf of Mexico. Regional-scale N mass balances indicate that a small, intensively cropped area of the upper MRB contributes disproportionately to nitrate loading. These aggregate balances miss small-scale variability, especially that caused by differences in farm management. We constructed N mass balances for a gradient of farm types, from corn-soybean monocultures to diversified grain farms that rely on biological N fixation (BNF) as a primary N source, to compare the relative efficiency of diverse farming systems in the MRB. Five-year N balances were calculated for a most and least productive field on each farm using data collected from interviews with 95 grain farmers in Iowa, Ohio, Minnesota, and Wisconsin; from legume biomass and corn grain samples collected from a subset of farms; and published values from the literature. Nitrogen balances ranged from high average annual surpluses (149 kg N x ha(-1) x yr(-1)) to large deficits (80 kg N x ha(-1) x yr(-1)), and differed based on N source and crop rotation. Fields with > 50% of total N additions from legume N sources and fields with complex crop rotations that included both annual and perennial species were approximately in balance (3.7 kg N x ha(-1) x yr(-1) and 5.7 kg N x ha(-1) x yr(-1), respectively) compared to fertilizer-based practices in corn-soybean rotations with average annual surpluses near 35 kg N x ha(-1) x yr(-1). Surplus N was also inversely related to the proportion of total N inputs from BNF for medium (80-160 kg N x ha(-1) x yr(-1)) to high (> 160 kg N x ha(-1) x yr(-1)) N rates. Diversified farmers were more likely to adjust their management practices in response to environmental variability compared to fertilizer-based farmers. Taken together, results from this study suggest that significantly reducing surplus N in agroecosystems will require reducing N inputs and increasing C availability to support the internal biological mechanisms for storing N in farm fields.

Hydration State, Metabolism, and Hatching of Mono Lake Artemia Cysts.

Artemia monica, the only macrozooplankton in Mono Lake, California, is unique among brine shrimp in that it produces encysted diapause embryos that sink to the lake bottom where they overwinter. Currently, the lake's salinity is about twice as high as it was 40 years ago and, at equilibrium, it is projected to fluctuate between 169-248 g/l. Here we describe the effects of salinity on the termination of diapause, hatching, carbohydrate metabolism, and hydration of the cysts. As expected, hatching is much more sensitive to salinity than is termination of diapause. Carbohydrate metabolism, which involves the conversion of trehalose to glycerol and is required for hatching, responds to increasing salinity as reported in other Artemia species: increasing amounts of glycerol must be synthesized as salinity is raised. The unfreezable water in these embryos is 0.29 g H2O/gram dry weight (gdw) cysts, similar to values reported for other biological systems. This result and previous studies suggest that water probably becomes limiting at hydration levels of about 0.60 g H2O/gdw cysts. In Mono Lake water, the cysts reach this critical hydration at a salinity between 140-160 g/l, equivalent to approximately 3780-4330 mOsm/ kg. We conclude that Artemia monica will cease to exist within this salinity range and doubt that it can hatch beyond this limit, which is imposed by the requirement of metabolic processes for minimal amounts of cellular water.