| Crop management practices such as tillage, fertilization and crop rotations and associations with legumes have a crucial influence on soil properties and soil biological status. In order to investigate the short-term impact of tropical legumes in soil chemical and biochemical properties, a 3-year field study in Chiapas, Mexico, and a 1-year microplot study in Auburn, Alabama, were conducted. The first experiment evaluated the short-term effect of jackbean (Canavalia ensiformis [L.] DC.) intercropping with maize (Zea mays L.) under different tillage conditions and N fertilizer levels on soil properties and maize productivity.; Intercropped jackbean enhanced productivity, N concentration, yield and grain protein of corn, as well as soil total C and N, cation exchange capacity, exchangeable cations, and soil beta-glucosidase activity. Jackbean enhanced soil fertility status through biological N2 fixation, N transfer to maize, large biomass production and low C/N ratio of residues. The application of fertilizer at a rate of 150 kg N ha-1 significantly reduced soil pH and P availability, and generally had an inhibiting effect on soil enzyme activities. Results suggest that N fertilizer rates between 60 to 90 kg ha-1, and jackbean intercropping with maize are the best cultural practices for a balanced nutrient cycling in the system and a healthy level of enzyme activity, including potential antagonism against soilborne plant pathogens.; In general, highest levels of enzyme activity and chemical processes were observed in maize and jackbean rhizospheres and in the 0--5 cm layer of bulk soil. These results support previous findings on the rhizosphere effect enhancement on soil properties and emphasize the need to protect the top soil layer against erosion.; In the second experiment, conducted in Auburn, Alabama after one planting cycle, enhanced soil enzyme activities were observed in the rhizospheres of Crotalaria juncea L., Mucuna deeringiana (Bort) Merr., Indigofera hirsuta L., and Clitoria ternatea L., while poorest enzyme activity was consistently found in the cotton (Gossypium hirsutum L.) rhizosphere. Meloidogyne incognita J2 populations were significantly lower in legume rhizospheres, with the exception of these in jackbean and soybean ( Glycine max [L.] Merr.). Significantly correlated with enzyme activities, high counts of free-living nematode populations were found in Mucuna deeringiana, Crotalaria juncea, Indigofera hirsuta and soybean rhizospheres. These results suggest the potential benefits of using tropical legumes in rotation or in association with cotton or corn to enhance soil nutritional and biological status, suppressiveness to nematodes and, probably, to other soilborne plant pathogens. |
