Impact of grazing on soil biota in a Mediterranean grassland

Soil Biology and Biochemistry, 2006

This study focused on examining the impacts of cattle grazing on belowground communities and soil processes in humid grasslands. Multiple components in the soil communities were examined in heavily grazed and ungrazed areas of unimproved and improved bahiagrass (Paspalum notatum Flugge) pastures in south-central Florida. By using small (1-m  1-m) sampling plots, we were able to detect critical differences in nematode communities, microbial biomass, and mineralized C and N, resulting from the patchy grazing pattern of cattle. Soil samples were collected on three occasions between June 2002 and June 2003. Microbial C and N were greater ðPp0:01Þ in grazed than in ungrazed plots on all sampling dates. Effects of grazing varied among nematode genera. Most genera of colonizer bacterivores were decreased ðPp0:10Þ by grazing, but more persistent bacterivores such as Euteratocephalus and Prismatolaimus were increased, as were omnivores and predators. Higher numbers of persisters indicated that grazing resulted in a more structured nematode community. Some herbivores, particularly Criconematidae, were decreased by grazing. Abundance of omnivores, predators, and especially fungivores were strongly associated with C mineralization potential. Strong correlation of microbial C and N with nematode canonical variables composed of five trophic groups illustrates important links between nematode community structure and soil microbial resources. Including the analysis of nematode trophic groups with soil microbial responses reveals detection of grazing impact deeper into the hierarchy of the decomposition process in soil, and illustrates the complexity of responses to grazing in the soil foodweb. Although highly sensitive to grazing impacts, small-scale sampling could not be used to generalize the overall impact of cattle grazing in large-scale pastures, which might be determined by the intensity and grazing patterns of various stocking densities at the whole pasture level.

Naturwissenschaften, 2005

Soil acidity, nutrient availability and livestock density have a major influence on the belowground ecological community. As fast decomposition rates are due mostly to bacterial-based pathways and slower decomposition rates mostly to fungal-based pathways, it is helpful to condense empirical information in the so-called Nematode Channel Ratio (NCR). The NCR is shown to be a good indicator of efficiency in soil decomposition processes. We argue that in intensive agroecosystems, other fungivore members of the decomposer food web may outcompete the hyphal-feeding nematodes. We demonstrate how the NCR can be used to set ecological standards for sustainable use of the soil in agroecosystems. To summarize the interactions between the microbial resources and the decomposer nematofauna according to increasing land management, we propose the use of the fifth percentile as proxy for a sustainable environmental quality of grasslands on sandy soils, and the NCR mean as the upper threshold for low-stocked farms.

Belgian journal of …, 2005

This paper refers to the effects of large-scale seasonal fluctuations as well as experimentally induced small-scale variations of soil temperature and moisture on the structural and functional characteristics of a soil nematode community from a Greek Mediterranean grassland. Two levels of soil temperature, with a mean difference of 1.4° C, and two levels of moisture (2.3 % difference in water content) were created. The experiment lasted for a sixmonth period (from July to December). All nematode community parameters vary significantly with season, with the exception of the environmental constant (C env ), which reflects the intensity of competition. The Maturity Index (MI) is affected only by seasonally fluctuating soil conditions, while the Plant Parasite Index (PPI) is affected only by the small-scale differences in soil temperature and moisture. The high value (1.48) of bacterivorous to fungivorous ratio (B/F) indicates mainly bacterial mediated decomposition, resulting in good soil fertility. The B/F ratio is affected both by large and small-scale changes in soil conditions. Diversity and richness show dependence on seasonal variations as well as on small changes in temperature, while no effect of moisture is recorded. The correlations between nematode community parameters, microflora parameters and soil nitrogen pools show independence of nematode parameters from soil N-pools (N organic and N inorganic ) and parameters relating to microflora, as well as strong dependence on microbially-bounded nitrogen.

Soil sustainability implies a sufficient diversity and abundance of organisms to perform soil functions and to resist environmental stress. Previous studies have shown the importance of functional biodiversity for soil organisms. 2. Soil samples have been collected within the framework of a long-term monitoring programme in the Netherlands. Nematological and microbiological techniques were combined to facilitate a more comprehensive understanding of possible below-ground effects of land management. 3. A possible bias due to stochastic circumstances was investigated. The Mantel test showed that the diversity at species level is largely related to air temperature, but at genus level the effect of temperature disappears. No direct influence of rainfall on the soil biodiversity was found in our model. 4. To extrapolate our data to a national level, habitat-response relationships for soil organisms have been derived. Generalized linear models (GLMs) and Monte Carlo simulation allowed the estimation of the probability of occurrence at a given abundance for 95 nematode genera. 5. Our study describes the influence of abiotic conditions and land use intensity on the composition of nematode communities in grasslands on sand. The results obtained reveal a major influence of pH and livestock density on the diversity of the nematode community at both taxonomic levels as well as at different trophic levels (feeding habits). The presence and abundance of soil nematodes decrease with cattle pressure. 6. Functional diversity decreases with increasing management intensity. It is shown that the Shannon diversities of bacterial feeding nematodes and fungal feeding nematodes are strictly related to cattle pressure, whereas the bacterial biomass occurring under organic farming scores higher than in other farming systems.

Within in a multidisciplinary project the effect of termination of cattle grazing on grassland and forest on soil microbial properties was investigated. The changes of microbial biomass (C m j c), basal respiration and the ecophysiological parameters (metabolic quotient and the C m j c :C org ratio) were small. Mainly in the dry summer of 1993 an increase of basal respiration, microbial biomass and the C m j C :C or g ratio was observed for the fenced in sites. The effects were not observed in the moist summer of 1995. For the pasture site, a substrate utilization assay employing 95 different C sources (Biolog) indicated some changes in the functional abilities of the bacterial communities after 9 years of protecting the site from grazing. The changes were attributed to a change of litter quality.

Journal of nematology, 2001

The effects of agricultural management on the soil nematode community were investigated in a field study at depths of 0 to 10 cm and 10 to 20 cm during a peanut (Arachis hypogaea) growing season in Israel. Nineteen nematode families and 23 genera were observed. Rhabditidae, Cephalobus, Eucephalobus, Aphelenchus, Aphelenchoides, Tetylenchus, Tylenchus, Dorylaimus, and Discolaimus were the dominant family and genera. Ecological measures of soil nematode community structure, diversity, and maturity indices were assessed and compared between the managed (by fertilization, irrigation, and pesticide application) and unmanaged fields. The total number of nematodes at a 10-cm depth during peanut-sowing, mid-season, and harvest periods was higher in the treated (managed) plot than in the control (unmanaged) plot. Bacterivores and fungivores were the most abundant trophic groups in both plots and both depths. The relative abundance of each group averaged 60.8 to 67.3% and 11.5 to 19.6% of the...

– Changes in grassland management induce disturbances that influence both soil functioning and soil fauna. This study aimed at determining the extent to which the composition of a grassland soil nematode community could be altered by a shift of grazing regime and the potential feedback that these alterations could provoke on grassland functioning. Therefore, we monitored the composition of the soil nematode community (i.e., plant-, bacterial-and fungal-feeders, omnivores and carnivores) of mesocosms that were sampled from two contrasted long-term field trials (high vs low grazing treatments) and subsequently subjected to high or no grazing for 2 years. The soil nematofauna responded faster and more strongly to the application of an intensive grazing regime on a previously extensively exploited system than the other way round. The application of an intensive grazing regime induced a significant increase in numbers of bacterial feeders and a decrease of the relative abundance of fungal-feeding nematodes. The nematofaunal community structure was determined by both the past and current grazing regimes throughout the 2 years of monitoring. Observed effects on soil microbivores seemed to reflect the 'immediate' above-ground primary production potential and to follow microorganism dynamics. On the other hand, observed effects on root-feeding nematodes seemed to reflect the integral effect of past and current grazing regimes on plant community root biomass and quality.

European Journal of Soil Biology, 2008

Journal of Arid Environments, 1999

Nitrogen amendments (0, 25, 50 and 100 kg NH4NO3/ha) were used to study the responses of primary production, microbial biomass and nematode population in desert soil. The study was conducted in the Israeli Negev Desert, a region characterized by low and randomly distributed rainfall. Over a 1-year study period, nitrogen amendments resulted in a significant (p<0·01) increase in soil microbial biomass. Soil microbial biomass also increased concomitantly with the increase in soil organic matter. The number of free-living nematodes in the soil increased with the increase in soil moisture, ranging from 43,000 individuals per square metre at the end of the summer to 351,000 individuals per square metre during the rainy season. No significant correlation was found between the nitrogen treatments and the nematode population, whereas a significant positive correlation was found between the nitrogen amendments and the above-ground biomass (r2=0·94, p<0·03). The nitrate proportion of the total soluble nitrogen in the soil also increased with the increase in soil moisture. This study provides baseline data for nitrogen amendments on soil microbial status, as well as insights into the importance of nitrogen in fertility in arid environments.

Biology and Fertility of Soils, 1993

Since the amount, intensity, and frequency of rainfall in desert regions vary strongly over space and time, the response by soil biota to this variability is of great importance. We conducted a study in the Negev desert in order to examine the immediate response by the soil nematode populations and the microbial biomass to varying amounts of water applied in a single pulse. Soil samples from the 0–10-cm depth were collected from areas undergoing four different wetting treatments, comprising 5, 10, 15, and 20 mm of water, and from a non-irrigated control soil. There was a correlation between diurnal variations in nematode populations and the diurnal fluctuations in soil moisture. The greatest abundance of nematodes was found in the soil treated with 20 mm water (970 individuals 100 g-1 dry soil) which was 2, 4, 5, and 14 times larger than that found in the soil treated with 15, 10, 5, and 0 mm of water, respectively. Bacterialfeeding and fungal-feeding nematodes accounted for approximately 95% of the total nematode population found in all treatments. The microbial biomass examined in the current study exhibited an immediate response to the wetting which was greater in soil treated with, 10, 15, and 20 mm of water compared with 0 and 5 mm. However, after 4 days (96 hours) the microbial biomass stabilized again at the basic level of the 0-mm control. However, our results indicated that the major trigger for changes in the nematode populations, and in the microbial biomass, was diurnal fluctuations in soil moisture, since peaks in nematode populations and in the microbial biomass were observed at various times of the day.