Aim: In the current Anthropocene, many ecosystems are being simultaneously invaded by multiple alien species. Some of these invasive species become more dominant and have greater environmental impacts than others. If two potentially dominant species invade the same area, the combined impact has been reported to be either (a) domination by one species, that is, the competitive dominance of one invader, or (b) invasion meltdown, where the combined impact is much greater, that is, a synergistic effect. We studied the effects of the invasion of two alien plant species that are known to strongly decrease native plant species diversity: the Persian walnut Juglans regia and goldenrod Solidago canadensis. Location: We examined native vegetation diversity in abandoned fields (in Poland)where neither species had invaded, only one species had invaded, and both species had invaded.Methods: Field survey data were analysed using generalized linear mixed models and ordination techniques.Results: When goldenrod invaded alone, it caused a larger decrease in species richness and cover (74%) than when walnut invaded alone (58%). When walnut and goldenrod co-occurred in abandoned fields, walnut was dominant and strongly decreased goldenrod density by 87%. However, the combined impact on native species diversity was much lower (15% decrease in native plant diversity) than when either goldenrod or walnut invaded alone. Main conclusions:In contrast to many other studies, our study does not support the occurrence of an invasion meltdown. Instead, our results show that even when one invader dominates, its negative effect on plant diversity can be strongly modified by the presence of another invasive species. K E Y W O R D Sbiodiversity, competition, competitive hierarchy, dominant invader, facilitation, invasion ecology, invasion meltdown, nitrogen, theory of coexistence

The aim of this study was to evaluate the effects of cover type (control, agro-textile or perforated plastic film) and harvest date (60 or 75 days after planting and at full physiological maturity) on the yield, quality and cost-effectiveness of early harvest potato cultivation. The study was conducted in the years 2008-2011, at the Gorzyń Experimental-Education Laboratory for Soil and Plant Cultivation, Gorzyń Branch, part of the University of Natural Sciences in Poznań. The experiment was set up in a split-plot randomized block design with four replications. Covers increased the total and marketable tuber yields at early harvest dates, in particular on the 60th day after planting, compared to the reference. Tubers cultivated under covers were also found to contain higher amounts of dry matter and starch than those which were not covered. The proportion of tuber fractions with a diameter between 4.6 and 5.5 cm and above in the total yield was found to be strongly dependent on cover type. The proportion of these fractions was significantly lower under plastic film than under Agro-textile. Over the 3 years cycle, high gross margins were achieved on the 60th and 75th days after planting with perforated film and agro-textile.Resumen El propósito de este estudio fue evaluar los efectos del tipo de cobertura (testigo, agrotextil, o plástico perforado) y fecha de cosecha (60 o 75 días después de la siembra y a madurez fisiológica total) sobre el rendimiento, calidad y costo-efectividad en el cultivo de papa de cosecha temprana. El estudio se desarrolló en los años 2008-2011, en el Laboratorio Experimental de Educación para Suelo y Cultivo de Plantas Gorzyń, rama de Gorzyń, perteneciente a la Universidad de Ciencias de la Vida en Poznań. El experimento se estableció en un diseño de bloques divididos al azar con cuatro repeticiones. El propósito del trabajo fue evaluar los efectos de tipo de cobertura (testigo, agro-textil, o capa de plástico perforada) y fechas de cosecha (60 y 75 días después de la siembra, a madurez fisiológica completa de los tubérculos) sobre rendimientos y calidad de tubérculos, así como en los resultados económicos del cultivo de papa para cosecha temprana. Las coberturas aumentaron los rendimientos de tubérculo total y comercial en las fechas de cosecha temprana, en particular a los 60 días después de la siembra, al compararse con la referencia. De la misma manera, se encontró que los tubérculos bajo cobertura contenían mayores cantidades de materia seca y almidón que los no cubiertos. El margen alto de retribución neta se alcanzó a los 60 y 75 días después de la siembra, en los tratamientos con plástico perforado y agrotextil, que fueron usados en un ciclo de tres años.

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Efficient nutrient use is critical to ensure economical crop production while minimizing the impact of excessive nutrient applications on the environment. Nitrogen (N) is a key component of agricultural production, both as an input to support crop production and as a waste product of livestock production. Increasing concern for future sustainability of agricultural production and preservation of the natural resource base has led to the development of nutrient budgets as indicators and policy instruments for nutrient management. Nutrient budgets for N have been developed by the Organization for Economic Co-operation and Development (OECD) as agri-environmental indicators to compare the evolving conditions in member states, and are also used by the US Department of Agriculture Natural Resource Conservation Service (USDA-NRCS) to develop nutrient management plans. Here, we examine the crop and animal production systems, drivers impacting management choices, and the outcome of those choices to assess the utility of gross annual N balances in tracking the progress of management decisions in minimizing the environmental impact of agricultural production systems. We use as case studies two very different agronomic production systems: Mississippi, USA and Poland. State and country level data from the US Department of Agriculture and OECD databases are used to develop data for the years 1998-2008, and gross annual N balances are computed. Examination of agricultural production practices reveals that the gross annual N balance is a useful tool in identifying differences in the magnitude and trends in N within agricultural systems over large areas. Significant differences in the magnitude of the N budget were observed between the highly diversified, small-scale agriculture common to Poland, and the large-scale, intensive agriculture of Mississippi. It is noted that use of N balance indices can be problematic if the primary intent is to reveal the impact of economic drivers, such as crop prices, or management choices, such as tillage or crop rotation. Changes in cropping systems in response to commodity prices that improve N balance can be masked by detrimental growing conditions, including edaphic, biotic and weather conditions, that are outside of the producers' control. Moreover, use of large area-scale indices such as country or state-wide balances may mask the severity of localized nutrient imbalances that result from regionalized production systems that overwhelm the nutrient balance, such as confinement livestock production. Development of a policy to address environmental impact and establish sustainable production systems must consider the year-to-year variability of drivers impacting agricultural production, and the spatial heterogeneity of nutrient imbalance.

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