Soil microbial biomass and nitrogen dynamics in a turfgrass chronosequence: A short-term response to turfgrass clipping addition
Introduction
Although turfgrasses, including golf courses, parks and home lawns, cover 14% of the cropland area in the USA and provide both recreational and environmental benefits (Beard and Green, 1994; Qian and Follett, 2002), there is a widespread concern that turfgrasses may not be ecologically sound due to N loss potentials associated with intensive management. Frequent mowing is a primary component of turf management. During the active growing season, turf is mowed as often as once a day. Mowing produces grass clippings that can filter into the canopy, to the soil surface and decompose. Understanding the dynamics of soil microbial biomass and N mineralization following clipping addition is central to formulating fertilizer best management practices that minimize N losses via leaching and denitrification.
Plant materials consist of a variety of organic compounds, and represent a C source for soil microbial metabolism. Incorporation of plant material into the soil generally stimulates microbial growth and activity, while the elemental composition of the material may have more specific effects on N mineralization and immobilization turnover (MIT). It is widely accepted that adding plant materials with C-to-N ratios <20 results in net N mineralization. However, several authors have reported substantial reductions in soil inorganic N (i.e., net N immobilization) following incorporation of plant materials having low C-to-N ratios (Jensen, 1994, Jensen, 1997). Nicolardot et al. (1986) found that adding organic materials with C-to-N ratios as low as 10 could result in short-term net N immobilization. This negative effect on soil inorganic N was thought to be caused by high C-to-N ratios of easily decomposable compounds, such as simple carbohydrates and lipids, that constitute the major C sources of early phase decomposition of plant materials in soil (Jensen, 1997; Andersen and Jensen, 2001).
Concomitant with its own decomposition, added plant material may trigger the decomposition of indigenous soil organic matter (priming effect), leading to potentially large fluctuations in soil inorganic N (Kuzyakov et al., 2000). The magnitude of priming seems to vary with the type of added organic matter (Kuzyakov et al., 2000). However, there is little information on soil microbial biomass and N mineralization dynamics following the addition of turfgrass clippings.
Inherent soil and microbial properties may, to some extent, influence the decomposition of certain plant materials. For example, soil N availability, an abiotic factor, regulated the decomposition of C-rich plant materials such as maize residue and wheat straw (Recous et al., 1995; Henriksen and Breland, 1999b). These authors observed that both C and N mineralization of C-rich plant materials decreased with lower soil N availability, as did N immobilized per unit of mineralized C. However, soil N availability had little effect on the mineralization of N-rich organic materials such as amino acids (Jones and Shannon, 1999). Several studies demonstrated that the size and activity of indigenous soil microbial populations affected the decomposition of soil incorporated plant materials (Allison and Killham, 1988; Franzluebbers et al., 1995; Henriksen and Breland, 1999a). Our previous work showed that turfgrass system age affected microbial biomass, activity, and N transformation processes as well as soil C and N content (Shi et al., 2006). In the present study, we examined short-term soil microbial biomass and N mineralization dynamics in a turfgrass chronosequence following the addition of turfgrass clippings. We hypothesized that soil microbial biomass and mineralization dynamics in response to clipping addition are dependent on turf age due to associated changes in soil and microbial attributes. Our objectives were (1) to evaluate the impacts of soil and microbial properties associated with turf age and land use on short-term C decomposition, N mineralization and nitrification following clipping addition, and (2) to estimate C decomposition and N mineralization dynamics of turfgrass clippings and subsequent influences on indigenous soil N mineralization.
Section snippets
Site description and soil sampling
Four golf courses were selected as study sites. Each was in or near the Pinehurst Resort and Country Club, located in the Sandhills region of North Carolina. The courses were established in 1907, 1979, 1996, and 2001 and were 97, 25, 8, and 3 yr old, respectively, when soil samples were taken in May 2004. They were in close proximity and had similar or identical soils (sand or loamy sand). Soil types were the Candor (Sandy, siliceous, thermic Arenic Paleudults) at the 97, 25, and 8 yr old courses
C dynamics following clipping addition
Soil microbial biomass remained stable during the 28-d incubation in the absence of added clippings (Fig. 1A). Biomass increased rapidly in all soils amended with clippings, being highest 7 d after their addition (P<0.05) (Fig. 1B). Thereafter, biomass declined to levels equivalent to those without clippings. The time course of soil microbial biomass in adjacent native pines was similar to that in turfgrass systems (Fig. 1).
Without clipping addition, cumulative CO2-C was a linear function of
Decomposition of turfgrass clippings
Decomposition of plant residues is initiated with the degradation of easily-decomposable compounds such as simple carbohydrates, nucleic acids, amino acids, proteins and lipids. The overall C-to-N ratio of easily decomposable compounds could be higher than the C-to-N ratio of whole plant material (Andersen and Jensen, 2001). Accordingly, plant residues which are characterized by low C-to-N ratios, and for which N mineralization would be predicted, often experience short-term net N
Acknowledgments
The study was financially supported by the Center for Turfgrass Research and Education, North Carolina, USA. We appreciate Dr. Bir Thapa's effort in providing bermudgrass and ryegrass tissues. We are grateful to Pinehurst Resort and Forest Creek County Club for their cooperation, to Mr. Howard Sanford for 15N analyses, to Ms Lisa Lentz for analyzing soil C and N, to Ms Susan Irvin for helping the experiment, and to Dr. Bruno Mary for kindly providing the FLUAZ program.
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