Understanding Microbes Helps Explain Nutrient Cycling

As urban areas continue to expand into the rural landscape, agricultural land is converted to turfgrass cover in the form of golf courses, parks, athletic fields and lawns. With urbanization expected to increase 79 percent in the United States over the next 25 years (Alig et al, 2004), turfgrass ecosystems inherit an even more prominent role in urban nutrient cycling, water management and carbon interactions.

Previous research has examined nutrient use and carbon sequestration by turfgrass and other natural and agricultural ecosystems. However, little is known about the tiny creatures that orchestrate these processes: soil microorganisms. The study of soil microorganisms is difficult on a number of levels. First, we can't see them, so they are out of sight and out of mind. Second, most methods for studying these critters are rife with difficulties. Third, while we can identify thousands of different species, we only really know what a small percentage of them are doing in the soil. In short, we have a lot left to learn. By understanding what microbes are doing and under what conditions they dominate, we can learn more about how to improve nutrient and organic matter cycling in turfgrass areas.

QUICK TIP

Soil microorganisms have an enormous influence on plant-soil interactions crucial for maintaining healthy turfgrass. Perhaps the most important of these interactions is the cycling of nutrients, especially nitrogen. As much as half of applied nitrogen fertilizer can be consumed by the microbial community within three days. This immobilized nitrogen is only temporarily unavailable to the plant and is re-released, or mineralized, over time depending on climate and soil properties along with the demographic and activity of the microbial pool. Substantial amounts of both soluble and slow-release nitrogen fertilizer sources undergo several transformations between ammonium, nitrate and organic nitrogen before finally being absorbed by the plant. Soil microorganisms are the behind-the-scenes facilitators of nitrogen immobilization, mineralization and transformations in the soil.

Organic matter accumulation is another turf-management issue of particular concern to golf course managers because of its significance on playing conditions. Once again, the rate of organic matter degradation and accumulation is dependent on the population and activity of the soil microbial community. With such important roles in these fundamental processes in turf management, an increased understanding of this active and complex population could enable turf managers to use these soil microbes to their advantage. Understanding and potentially manipulating this community could have significant implications on turf-management strategies.

Studying soil microorganisms is notoriously difficult, and there are several different methods that can be used, each with their own inadequacies. The method we used to study the multitude of microorganisms in the soil was to identify them by their unique fatty acids (PLFA-FAME analysis) and then lump the individual species into broadly defined groups. We can then study how the groups dominate in different environments. To obtain a representative sample, we collected soil samples from 42 turfgrass sites in southern Wisconsin. We then analyzed the soil to see the relative distribution of the various functional groups and then compared the functional groups to the soil properties. The groupings, or functional groups, of microorganisms are listed here:

• Gram positive bacteria (GPB) have a rigid cell wall and are tolerant of stressful conditions. These bacteria are usually dominant in extreme environments, and can form spores and remain in a dormant condition until favorable growing conditions return.
• Gram negative bacteria (GNB) have weak cell walls and require a "biofilm" to survive. These bacteria are important for all aspects of the nitrogen cycle (immobilizing, mineralizing, nitrifying and denitrifying). These bacteria are usually dominant in agricultural ecosystems and prairies.
• Saprophytic fungi (SF) are important for breaking down plant material and other organic matter. These fungi typically dominate forest and other ecosystems that produce large amounts of relatively recalcitrant materials.
• Arbuscular mycorrhizal fungi (AMF) form symbiotic relationships with plants and trade water and nutrients (especially phosphorus) for carbon-containing compounds made by the host plant.
• Total biomass is the sum of all these functional groups plus all the additional microorganisms that did not fit into one of the above functional group categories. Very little is known about the function of these uncategorized species.

1 2 

• Podcasts
• Online Exclusives
• Industry News
• The Links
• Golfdom Insider
• Turfgrass Trends
• Special Reports
• The Golfdom Report
• Archives