Grazing in a porous environment. 2. Nematode community structure

The influence of soil matric potential on nematode community composition and grazing associations were examined. Undisturbed cores (5 cm diameter, 10 cm depth) were collected in an old field dominated by perennial grasses on a Hinckley sandy loam at Peckham Farm near Kingston, Rhode Island. Ten pairs of cores were incubated at −3, −10, −20 and −50 kPa matric potential after saturation for 21–28 or 42–58 days. Nematodes were extracted using Cobb's decanting and sieving method followed by sucrose centrifugal-flotation and identified to family or genus. Collembola and enchytraeids present were also enumerated because they are grazers that reside in air-filled spaces. Direct counts of bacteria and fungi were made to estimate biovolume using fluorescein isothiocyanate and fluorescein diacetate stains, respectively. Trophic diversity and maturity indices were calculated for nematode communities. Three patterns of matric potential effect were observed for nematode taxa. One, there was a consistent effect of matric potential for all seasons for Alaimus, Monhysteridae, Prismatolaimus, Paraxonchium and Dorylaimoides. Two, some effects of matric potential were consistent among seasons and other effects were inconsistent for Aphelenchoides, Aphelenchus, Cephalobidae, Coomansus, Eudorylaimus, Huntaphelenchoides, Panagrolaimidae, Paraphelenchus, Sectonema, and Tripyla. Third, effects of matric potential were always inconsistent among seasons for Aphanolaimus, Aporcelaimellus, Bunonema, Rhabditidae, and Tylencholaimus. As predicted, fungal and bacterial biomass responded oppositely to matric potential. Total bacterial biomass was greater at −3 kPa than −10, −20 and −50 kPa (P=0.0095). Total fungal biomass was greater at −50, −20 and −10 kPa than −3 kPa (P=0.0095). Neither bacterial-feeding, fungal-feeding nor predacious nematodes correlated significantly with bacterial or fungal biomass. Omnivorous and predacious nematodes correlated positively with number of bacterial-feeding nematodes; predacious nematodes also correlated positively with fungal-feeding nematodes. Numbers of Collembola and enchytraeids were more often correlated positively with microbial-grazing nematode numbers in drier than moist soils. From this study, we propose two mechanisms that may explain nematode community structure changes with matric potential: differential anhydrobiosis and/or enclosure hypotheses. The later suggests that drying of soil generates pockets of moisture in aggregates that become isolated from one another enclosing nematodes and their food in relatively high concentrations creating patches of activity separated by larger areas of inactivity.