Assessment of pasture management practices on microbial biomass, composition, and functional diversity

View the project final report

• 2008 annual report

Commodities

• Agronomic: general hay and forage crops, grass (misc. perennial), hay

Practices

• Animal Production: feed/forage, grazing management, grazing - continuous, grazing - rotational
• Education and Training: on-farm/ranch research, participatory research
• Natural Resources/Environment: biodiversity
• Production Systems: agroecosystems
• Soil Management: soil analysis, soil microbiology

Soil resilience is a concept deemed important to our understanding of soil quality and sustainability and is thought to be facilitated by functional diversity of soil biota. To better understand the potential of ecosystems to respond to disturbance, more information on microbial community and functional group composition, abundance, and their distribution within soils is necessary. While the diversity of microorganisms in soils is thought to be high, some management practices, such as conventional tillage, can deteriorate soil physical properties, and bring about a decline in total microbial biomass. The objective of this research was to compare soil microbial communities under different pasture management strategies on farms across southern Wisconsin. We used a hybrid phospholipid fatty acid (PLFA) and fatty acid methyl ester (FAME) analysis to measure microbial biomass, fungal to bacterial ratios (f:b), and characterize by lipid biomarkers the following functional guilds; gram-positive and negative bacteria (Gm+ and Gm-), arbuscular mycorrhizal fungi (AMF), and actinomycetes. For multivariate analysis of microbial community structure, principal components analysis (PCA) was performed.

Introduction:

Human activities over the last several centuries have altered earth’s abiotic and biotic processes (Vitousek and Mooney 1997). Alteration of terrestrial surfaces may force changes in ecosystem processes (Randerson et al., 2002). For example, the conversion from prairie to agriculture has reduced soil carbon and nitrogen stocks, which maintain soil fertility (Mann 1986). Grazing herbivores have been shown to influence plant species composition, aboveground plant structure, and quantity and quality of organic inputs (Bardgett et al.,1998), soil N cycling (Gueswell et al., 2005, LeRoux et al., 2003), root biomass (Holland and Detling 1990), carbon (C) enhancement of the rhizosphere by root exudation (Mawdsley and Bardgett 1997), microbial biomass (Bardgett et al., 1997), and f:b ratios (Bardgett et al., 1996).

Soil microorganisms play an important role in soil quality, and understanding the impacts of human activities, including grazing, is important to our understanding of the role microorganisms play in soil processes and maintenance of soil quality. The impact of land use changes on microbial- mediated processes may be influenced by whether the process is performed across a wide range of microbes or whether it is performed by a narrower group (Balser et al., 2006). To better understand the potential of ecosystems to respond to disturbance, more information on microbial community composition and functional groups, and how they are affected is necessary (Balser et al., 2006, Patra et al., 2005). While the impact of livestock production practices on aboveground biomass is well studied, their impacts on belowground heterotrophic communities are not.

Livestock grazing of pastures is a growing phenomenon in the upper Midwest. Jackson-Smith et al. (1996) estimated that about 15% of all dairy operations in Wisconsin maintained some form of grazing as a management strategy. An update to this study (Ostrom and Jackson-Smith 2000) indicated this estimate had grown to 22% by 2000. Grazing dairy livestock in pasture is contrasted with confinement systems where feed is mechanically harvested and transported to a centralized location where livestock spend most if not all of their time. Social and economic benefits of a particular form of grazing management, Management Intensive Rotational Grazing (MIRG), have been demonstrated in many settings (Parker 1992, Fales et al., 1995, Frank et al., 1995, Paine et al., 1999). More specifically, MIRG, which entails livestock grazing in relatively small paddocks at high densities (150 to 250 animal units•ha-1), but for short durations (1 to 3 days), has been touted as beneficial to both graziers and grazers (Undersander et al., 1993, Paine et al., 2000). But while MIRG’s benefits for production economics are increasingly well established, its ecological consequences remain mostly anecdotal and the scientific basis for how these ecosystems function is lacking.

The objectives of this research were to:

1) quantify total microbial biomass, f:b ratios, characterize microbial community composition by functional guild, and
2) compare variability of response variables of pasture management treatments across farms.