Growth chamber and laboratory experiments evaluated the effects of seed-soil microsite characteristics on seed germination. When corn (Zea mays L. ‘Pioneer 3541’), soybean [Glycine max (L.) Merr. ‘Corsoy 79’], velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and giant foxtail (Setaria faberi Herrm. ♯ SETFA) were seeded among soil aggregates 2.50 to 1.25 cm in diameter in the growth chamber, there was decreased seedling emergence with decreasing frequency of irrigation. These same species seeded inside artificial soil aggregates showed increased seedling emergence with decreasing frequency of irrigation. The germination of corn and soybean seed inside 0.4-g, fully moist soil aggregates incubated in the laboratory under high relative humidity conditions was significantly decreased relative to seed incubated in the absence of soil. Velvetleaf and giant foxtail germination was significantly reduced by 0.1-g soil aggregates. Rice (Oryza sativa L.) had 68% emergence from 1.0-g, fully moist soil aggregates, whereas larger seeded corn and soybean had only 20 and 10% emergence, respectively. Germination inhibition of giant foxtail seed by fully moist soil was partially reversed by incubating seed-containing aggregates in an atmosphere of 75% oxygen.

Our study examines the effect of seasonal rains on soil organic C dynamics in a tropical deciduous forest ecosystem in Western Mexico. At the end of the wet season, an accumulation of labile nutrient forms developed and was maintained during the dry season. This accumulation enhances microbial activity in the first rains of the wet season. For example, the litter samples of the dry season had a higher C and N mineralization than those of the wet season. Similarly, the January soil samples had higher C mineralization than October soil samples (55 and 34 μg C g-1 d-1, respectively). These results suggest that the quality of C is strongly affected by the seasonality of rains, which in turn influences microbial activity. This seasonality also influences nutrient redistribution between soil aggregate fractions. Chemical changes across seasons suggest that soil organic matter associated with macro-aggregates represents the main source of energy for microbial activity at the beginning of the wet season, while micro-aggregates protect the labile nutrient forms during the growing season.