The effects of prescribed fire on soil nematodes in an arid juniper savanna

Prescribed fire produced a landscape with two types of severely burned patches: charred shrub patches and charred patches with tree trunks at the center. Soil nematodes were more abundant in burned and unburned juniper (Juniperus monosperma) tree patches than in yucca-shrub patches. There were no differences in nematode abundance between burned and unburned patches during the late spring and summer samples. Nematode abundance was significantly (p < 0.05) higher in unburned patches than in burned patches in the early spring samples, reflecting large differences in soil moisture between unburned and burned patches. There were no differences in soil nematode abundance between burned and unburned patches at oneyear post-burn and three-year post-burn sites. When all samples were pooled, taxonomic diversity, ecological indices, and abundance of trophic groups (bacteria-feeders, fungi-feeders, and omnivore-predators) were higher in unburned than burned patches. These results suggest that the long-term (up to three years post-burn) effects of fire on soil nematodes are indirect, i.e., by loss of tree canopies, litter accumulation, and shrub foliage, which affects soil temperatures and water redistribution.

Responses of soil nematode community to monoculture or mixed culture of a grass and a legume forage species in China

Mixed cultivation of fast-growing grasses and nitrogen(N)-fixing legumes for forage production is widely considered effective for obtaining sustained high forage yields without depleting soil N levels.However,the effects of monoculture and mixed culture of these species on soil food webs are poorly understood.In this study,soil nematode communities were examined as indicators of the soil food web structure of monoculture and mixed culture of grass and legume at three N levels,i.e.,338(low),450(moderate),and 675(high)kg N ha-1 year-1,across 2 years in wet and dry seasons,using the grass Paspalum wetsfeteini and the legume Medicago sativa(alfalfa),both commonly cultivated worldwide.Repeated-measures analysis of covariance showed that compared with grass monoculture,legume monoculture and grass-legume mixture increased abundances of herbivorous,bacterivorous,and fungivorous nematodes in the soil food web under the low and moderate N fertilization levels.Principal response curve results showed that the abundance of Helicotylenchus,a plant parasite,was significantly higher under legume monoculture than other planting systems at the low N fertilization level.Structural equation model analysis indicated that the legume increased bacterivore abundance,while increasing N fertilization decreased omnivore abundance.The legume might increase the quantity and quality of food resources for soil biota,resulting in the bottom-up control of soil nematode communities.Our results indicate that targeted control of a soilborne pathogen,Helicotylenchus,is required in alfalfa-based planting systems.In addition,high inorganic N application,which is detrimental to legume-rhizobia symbiosis,nullified the otherwise positive effects of legumes on soil nematodes.