The effects of topsoil thickness on winter annual weed growth and nutrient concentration were assessed for three consecutive years in soybean plots. The topsoil treatments had high fertility levels, uniform textures, and no herbicides were used in the study. Common chickweed composed 75% of the winter annual weed species. Weed biomass production decreased as topsoil thickness decreased from 22.5 cm to 0. Topsoil thickness of 22.5 cm produced 800 kg ha–1 more weed growth than 0 cm topsoil. The weed biomass grown in thicker topsoil had higher total amounts of N, K, Mg, and Ca.

Compounds of the 20 elements needed by living organisms are relatively soluble in water and therefore vulnerable to being leached and lost from terrestrial ecosystems during mineralization. Intact systems have thus acquired retention mechanisms that sequester nutrients and minimize losses. Such mechanisms should be particularly important where rainfall is very high but direct evidence of retention mechanisms in tropical forests is scarce (Snaddon et al. 2012, Turner et al. 2007).

Difficulties to simply transfer trophic cascade theory from temperate to tropical lakes are now well recognized. Many mechanisms trying to explain top-down divergences between these systems have been proposed, such as lack of key species of herbivorous zooplankton, absence of seasonality in fish reproduction, cyanobacteria development, or differences in fish foraging behaviour. Very few studies have considered bottom-up mechanisms, in particular differences in nutrient recycling and nutrient limitation between the two types of ecosystems. According to the ecological stoichiometry theory, fish-induced alterations of food-web structure could modify the efficiency of consumer-driven nutrient recycling by changing the relative biomass contribution of species in food webs. Consequently, by mostly considering top-down processes, one could underestimate consequences on nutrient availability for phytoplankton growth. In this paper, we compared the results of two mesocosm experiments carried out in temperate and tropical areas, each manipulating food-web structure via the presence or absence of fish. We found trophic cascades in both experiments, but differences between fishless and fish treatments were greater in temperate than in tropical systems. In the tropical experiment, the observed effects could not be supported by classical zooplankton community alteration or by cyanobacteria prevalence. Our results suggest a key contribution of fish nitrogen-excretion to phytoplankton growth in mostly nitrogen-limited tropical systems. Differences in stoichiometric response to food-web structure alteration between temperate and tropical lakes could thus represent a major difference between the two systems. Our study stresses the need for further studies that would allow robust generalization on the functioning of freshwater temperate and tropical ecosystems.

Swine production is a major component of Corn Belt agriculture; thus development of a sustainable Corn Belt agriculture depends on sustainable swine production systems. Swine are versatile enough to adapt to sustainable concepts, and swine production raises several opportunities to enhance sustainability. These include: 1) feeding with increased use of forages and by-product feeds; 2) nutrient cycling through improved handling of manure; 3) low-capital housing systems that offer an improved environment for the operator and reduced financial risk; 4) management systems suited to the swine's behavior; and 5) preventive approaches to swine health and a broader genetic base. The challenge beyond identifying the opportunities is research and technology transfer and incorporation of sustainable concepts into ecologically based swine production systems.