Cattle grazing increases microbial biomass and alters soil nematode communities in subtropical pastures
Introduction
Grazing of grassland by large herbivores can have substantial indirect effects on soil organisms, and might have significant impact on plant productivity and community structure (Bardgett et al., 1998). Sustained productivity in grazing systems, including efficient recycling of nutrients and minimizing nutrient loss, depend upon soil biological processes and the interaction between grazing, nutrient mineralization, and soil biological communities (Yeates et al., 1997). Studies of the influence of domestic livestock on soil biological processes and communities in managed grasslands have produced contradictory results, indicating that more studies need to be done under a range of environmental and climatic conditions so that reasons for discrepancies among current studies may be resolved.
Results from research examining the impact of grazing on nematode communities have been mixed. Bardgett et al. (1997) observed declines in nematode abundance following cessation of grazing in upland pasture, which they attributed to declines in microbial abundance as a food source for many microbivorous nematodes. They also observed that grazing more than doubled nematode abundance at some locations but not others, which they attributed partly to differences in soil type and vegetation. Similarly, Freckman et al. (1979) observed a greater abundance of nematodes in a grazed than in an ungrazed grassland. However, Stanton et al. (1984) detected no differences in nematode communities between long-term grazed and ungrazed treatments, and heavy grazing by cattle had no effect on nematode biomass or seasonal densities (Smolik and Dodd, 1983). On the other hand, Wall-Freckman and Huang (1998) found that even though grazing treatments had no effect on nematode absolute abundances, they influenced the percent contribution of fungal-feeding nematodes, which were higher in long-term grazed than in an ungrazed treatment. In other studies, heavy grazing by sheep (30/ha) reduced nematode abundance (King and Hutchinson, 1976, King and Hutchinson, 1983), as did mowing to simulate grazing (Todd, 1996), but grazing by prairie dogs increased abundance of plant-parasitic nematodes (Ingham and Detling, 1984). Yeates and King (1997a) reported that percentage of bacterivorous nematodes was greater in an ungrazed grassland than in a grazed grassland. In a similar study (Yeates and King, 1997b), no differences were observed in nematode diversity between ungrazed native and improved grassland (changes in vegetation and fertilization), but a loss of plant-feeding species coupled with a marked increase in microbial-feeding species resulted in a lower total nematode maturity index (ΣMI) in the improved grassland. The variable results obtained in previous studies make it difficult to predict impacts of grazing on soil nematode communities.
Soil microbial biomass does not respond in a uniform way to herbivory by livestock or other large grazers in grazed ecosystems and has been observed to increase or decrease in response to grazing of the plant community (Bardgett and Wardle, 2003). Grazing livestock increase incorporation of surface litter into the soil, which can increase total soil organic matter. Furthermore, herbivory can increase root exudation of labile C compounds, which can stimulate growth of the rhizosphere microbial community (Bardgett et al., 1998; Holland et al., 1996; Yeates et al., 1997; Hamilton and Frank, 2001). Bardgett et al. (1997) showed that microbial biomass increased with increasing intensity of sheep grazing and declined following cessation of grazing. In other studies grazers either reduced microbial biomass or had no effects on microbial biomass (Harrison and Bardgett, 2004; Sakaran and Augustine, 2004; Tracy and Frank, 1998). Bardgett and Wardle (2003) proposed that positive, negative, or neutral effects from grazing on soil biota and ecosystem function are possible depending upon the balance of these effects. They suggested that positive effects are most common in ecosystems of high soil fertility, whereas negative effects are most common in unproductive ecosystems. However, mammalian grazing for two years in semiarid grassland in Kenya decreased soil microbial biomass in sites of both high and low fertility, despite the fact that grazing stimulated aboveground production in nutrient-rich sites and depressed it in nutrient-poor sites (Sakaran and Augustine, 2004). Therefore, there is a need to examine the response of the soil microbial biomass to grazing under different climatic and soil conditions.
Previously, studies of the impact of cattle grazing densities on nematode communities in subtropical rangelands in south-central Florida showed that nematode communities were influenced more by rainfall (McSorley, 1996), season, and pasture types (McSorley and Frederick, 2000) than by cattle stocking densities. The lack of effect from stocking densities was surprising, since it is expected that any disturbance that influences the food source or environment of nematodes will be reflected in the abundance and composition of the nematode communities (Wasilewska, 1989). It is anticipated that cattle grazing should increase the input of organic matter into the soil, increase microbial activities, and thus enhance microbial feeding nematodes. However, cattle often graze discontinuously and create small-scale patchiness in vegetation that may influence the spatial distribution of belowground communities and processes. Under these conditions, it is possible that the common soil sampling practice of collecting and compositing systematic soil cores from a given area may not be adequate for assessing the impact of grazing because soil from intensively grazed and less intensively grazed grass patches are mixed together. Similar concern about the heterogeneity within ecosystems has been raised by Augustine and Frank (2001) and by Bardgett and Wardle (2003). Therefore, in the current project, the impact of cattle grazing on nematode communities and labile C and N pools was investigated in small-scale plots (1×1 m2) in heavily grazed and completely ungrazed pasture areas to pinpoint grazing effects in a heterogeneous environment. Soil samples were taken following three grazing seasons to examine the consistency of grazing impact. The specific objective of this study was to determine the effect of cattle grazing on nematode communities, microbial biomass, and readily mineralizable pools of C and N. In addition, we examined whether grazing had predictable effects on different groups of nematodes and whether these effects could be linked to changes in soil organic matter and, in particular, to microbial biomass and rapidly cycling pools of C and N.
Section snippets
Site description
The study was conducted at the MacArthur Agro-Ecology Research Center at Buck Island Ranch located in Highlands County, Florida (27°09′N, 81°11′W). Soil samples were collected from heavily grazed and ungrazed sites on three sampling dates at termination of three successive grazing seasons. Two sets of samples (6 June 2002, 3 June 2003) were collected from the semi-native pasture that has never been fertilized, was dominated by a mixture of bahiagrass (Paspalum notatum Flugge) and native
Impact of grazing on soil C and N
Grass was much taller in ungrazed plots than in grazed plots on all sampling dates, providing a good indication of grazing intensity in the sampling plots (Table 1). For the semi-native pastures, the lower grass heights in 2002 than in 2003 were likely due to the fact that the pastures were burned in February 2002 and thus the grass height measurement represents only 4 months of growth during the late winter/spring dry season. Grass productivity was not determined at the sampling
Impacts of grazing on soil C and N pools
Microbial biomass C and N were strongly enhanced by grazing at this subtropical grassland in south-central Florida. Since grazing habits of cattle involve mostly grazing in patches rather than browsing over wide areas, heterogeneity of resources entering the soil is likely to create variability in grazing effects. In this experiment, the grazing impact was restricted to a small-scale sampling plot with a defined sample representing grazing at a high cattle-stocking density without any visible
Conclusions
The current study demonstrated the importance of examining grazing effects on soil biological communities under a variety of climatic conditions, and revealed the importance of small-scale patchiness in grazing effects on soil biological communities. Grazing resulted in a clear increase in soil microbial biomass C and N forms, and also significantly influenced soil nematode communities. However, the impacts of grazing on nematodes depended upon nematode life strategy (as indicated by c–p
Acknowledgements
The authors thank Mr. J. J. Frederick and R. P. Menne for their technical assistance, and Dr. K. B. Nguyen and A. Peterson for reviewing the paper. This project is supported in part by a grant from the US Department of Agriculture, and in part by the Florida Agricultural Experiment Station and the MacArthur Agro-ecology Research Center. This manuscript is approved as Journal Series No. R-10907 and contribution No. 82 from the MacArthur Agro-ecology Research Center.
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