Grazing in a porous environment. 2. Nematode community structure

The influence of soil matric potential on nematode community composition an d 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 pair s of cores were incubated at -3, -10, -20 and -50 kPa matric potential afte r 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 al so enumerated because they are grazers that reside in air-filled spaces. Di rect counts of bacteria and fungi were made to estimate biovolume using flu orescein isothiocyanate and fluorescein diacetate stains, respectively. Tro phic diversity and maturity indices were calculated for nematode communitie s. Three patterns of matric potential effect were observed for nematode tax a. One, there was a consistent effect of matric potential for all seasons f or Alaimus, Monhysteridae, Prismatolaimus, Paraxonchium and Dorylaimoides. Two, some effects of matric potential were consistent among seasons and oth er effects were inconsistent for Aphelenchoides, Aphelenchus, Cephalobidae, Coomansus, Eudorylaimus, Huntaphelenchoides, Panagrolaimidae, Paraphelench us, Sectonema, and Tripyla. Third, effects of matric potential were always inconsistent among seasons for Aphanolaimus, Aporcelaimellus, Bunonema, Rha bditidae, and Tylencholaimus. As predicted, fungal and bacterial biomass re sponded 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 g reater at -50, -20 and -10 kPa than -3 kPa (P=0.0095). Neither bacterial-fe eding, fungal-feeding nor predacious nematodes correlated significantly wit h bacterial or fungal biomass. Omnivorous and predacious nematodes correlat ed positively with number of bacterial-feeding nematodes; predacious nemato des also correlated positively with fungal-feeding nematodes. Numbers of Co llembola and enchytraeids were more often correlated positively with microb ial-grazing nematode numbers in drier than moist soils. From this study, we propose two mechanisms that may explain nematode community structure chang es with matric potential: differential anhydrobiosis and/or enclosure hypot heses. The later suggests that drying of soil generates pockets of moisture in aggregates that become isolated from one another enclosing nematodes an d their food in relatively high concentrations creating patches of activity separated by larger areas of inactivity.