A survey of Australian temperate pastures in summer and winter rainfall zones: soil nematodes, chemical, and biochemical properties

Data for soil nematode numbers, total microbial activity, microbial biomass carbon (C), and various soil chemical properties were collected from permanent and semi-permanent pastures at 108 locations in 2 contrasting environments: a summer rainfall zone in northern New South Wales (n = 60) and a winter rainfall zone in south-east South Australia and western Victoria (n = 48). Nematodes were also categorised according to their feeding habits and reproductive rates, and the abundance of various nematode groups was used to compute 3 indices that indicate the condition of the soil food web [enrichment index (EI), structure index (SI), and channel index (CI)]. At each location, pasture species herbage mass (kg DM/ha) was estimated and locations were grouped according to dominant species (lucerne, subterranean clover and phalaris in both rainfall zones; perennial ryegrass, winter rainfall zone only) as well as total soil microbial activity. The objective of the survey was to obtain a general indication of the biological status of soils used for pasture production in temperate regions of Australia, compare the soil biology in the 2 study areas, and determine whether it was influenced by pasture species composition. The most common plant-parasitic nematode was lesion nematode (Pratylenchus spp.), which was found at 67% of locations in the summer rainfall zone and 29% of those in the winter rainfall zone. Generally, there were more free-living nematodes in the soil than plant-parasitic nematodes. Numbers of free-living nematodes were highest in the winter rainfall zone, while in the summer rainfall zone there were more free-living nematodes in subterranean clover and phalaris pastures than lucerne pastures. Graphical representations of data for EI and SI indicated that the soil food webs at all sites in the summer rainfall zone were either structured or degraded. Food webs in the winter rainfall zone were more variable, with relatively high EI values indicating that more resources were available to support the soil biology. Climate had a major effect on CI values, with the mean in the winter rainfall zone being 28 (indicating a bacterial-dominant detritus food web) compared with 72 (indicating fungal dominance) in the summer rainfall zone. Mean total microbial activity was twice as high (2.6 v. 1.3 mu g fluorescein diacetate/g.min) in soils from the winter than the summer rainfall zone. Mean levels of total organic C, total nitrogen, and labile C were also higher for the winter rainfall zone than the summer zone. For both rainfall zones, mean microbial biomass C was 0.09 mg/g soil, labile C was 9.05% of total organic C, and microbial biomass C was 3.5% of labile C. In the summer rainfall zone, values for all measured biochemical properties were lower in lucerne pastures than in other pasture types. We concluded that the biological status of soils in the winter rainfall zone was better than soils in the summer rainfall zone, and that the biology under lucerne pastures in the summer rainfall zone differed from pastures dominated by phalaris and subterranean clover. Soils in both study areas were generally healthy, as they had relatively high levels of total organic C and labile C, high numbers of free-living nematodes, high microbial activities, and a general lack of problems associated with plant-parasitic nematodes. However, there were exceptions, as there was considerable within-site variability for some biological and biochemical parameters at some sites.