Effects of Organic Mulches and Fertilization on Soil Microbial Activity, Nutrient Availability, and Growth of River Birch Daniel A. Herms1, John E. Lloyd1,2, and Benjamin R. Stinner1
[email protected]
1. Department of Entomology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, Ohio 2. Current affiliation: Department of Plant, Soil, and Entomological Sciences, University of Idaho, Moscow, Idaho Abstract We determined the effects of mulching with composted yard waste and recycled ground wood pallets, with and without fertilization, on soil organic matter, microbial activity, nitrogen cycling, and growth of river birch, Betula nigra. Both mulches increased soil organic matter content and more than doubled microbial respiration, which is consistent with the hypothesis that soil microbes are generally carbon-limited. Effects on nitrogen availability, however, were highly dependent on the C:N ratio of the mulch. Mulching with composted yard waste, with a low C:N ratio of 17:1, dramatically increased total soil N as well as N mineralization rate. This increased plant available N, as well as foliar nitrogen concentration and growth of river birch. Fertilization had no effect on the growth of trees mulched with composted yard waste, which indicates that composted yard waste serves as a high quality organic fertilizer that can meet fully the nutrient requirements of both microbes and plants as it decomposes. On the other hand, mulching with high C:N (125:1) wood pallets stimulated microbial growth while adding little nitrogen to the soil. Microbes immobilized a high proportion of the limited nitrogen pool, thereby slowing the growth of river birch. These results are consistent with the hypothesis that soil microbes out-compete plants for nitrogen, and that addition of organic matter with high C:N ratios can induce nutrient deficiencies in plants by stimulating microbial growth. Fertilization relaxed this competition, increasing the growth of river birch that had been mulched with ground wood. This study demonstrates conclusively that organic mulches can have major effects on soil fertility and plant growth that are dependent on their C:N ratio. Understanding the dominant influence of soil microbes on nitrogen availability is key to understanding the dynamics of mulch effects on soil fertility.
Introduction Nutrient cycles have been studied thoroughly in forested and agricultural ecosystems (Facelli and Pickett, 1991; Wardle, 1992; Attiwill and Adams, 1993). In ornamental landscapes, however, nutrient cycling has received little attention. Mulches are used widely to suppress weeds, conserve soil moisture, and enhance aesthetics (Robinson, 1988). The potential of mulch to increase organic matter and establish patterns of nutrient cycling more similar to natural ecosystems has also been recognized (Tukey and Schoff, 1963; Greenly and Rakow, 1995). However, the effects of mulch on nutrient cycling and soil fertility have not been investigated in ornamental landscapes (Borland, 1988; Gleason and Iles, 1998). Nutrients are cycled in forested ecosystems as organic matter is decomposed by soil microorganisms, resulting in mineralization of nitrogen and other nutrients (conversion to inorganic forms), followed by plant and microbial uptake (reviewed in Attiwill and Adams, 1993). Recent evidence suggests that plants can also utilize dissolved organic nitrogen released from decomposing organic matter (Chapin et al., 1993; Kaye and Hart, 1997; Nasholm et al.,
Daniel A. Herms, John E. Lloyd, and Benjamin R. Stinner
1998, 2000). Rate of litter decomposition is correlated with soil microbial biomass, which in turn is directly linked to soil organic matter content (Wardle, 1992). In most agricultural and forest soils, microbial growth is resource limited and increases rapidly in response to additions of available carbon (Johnson, 1992; Wardle, 1992; Barrett and Burke, 2000). Litter quality is a key factor affecting microbial growth and rate of decomposition (Facelli and Pickett, 1991). Microbial biomass responds faster to more readily biodegradable litter than to more recalcitrant carbon sources, such as those with high lignin content (Melillo et al., 1982; Entry and Backman, 1995). Soil microbes act both as a source and sink of available N through opposing processes of mineralization and immobilization (sequestration of inorganic N in microbial biomass), and subsequent remineralization of nitrogen as soil microbes die and are decomposed (Keeney, 1980; Johnson, 1992; Jonasson et al., 1996). Plants and microbes compete for the same pool of available nutrients (Kaye and Hart, 1997; Hodge et al., 2000). Hence, the availability of nitrogen for plants is determined by the net balance between the rate at which nitrogen is released from decomposing organic matter, and the rate at which mineralized N is immobilized by soil microbes (Keeney, 1980; Johnson, 1992; Attiwill and Adams, 1993; Mary et al., 1996). The balance between nitrogen mineralization and immobilization is strongly influenced by the C:N ratio of the decomposing organic matter (Facelli and Pickett, 1991; Kaye and Hart, 1997). Organic matter with a C:N ratio greater than 30:1 does not contain enough nitrogen to support microbial growth (Aber, 1992; Kaye and Hart, 1997), and microbes must scavenge additional nitrogen from the soil (e.g. Frey et al., 2000). Since soil microbes are considered stronger competitors for nutrients than are plants (Kaye and Hart, 1997; Hodge et al., 2000), much of the available nitrogen pool will be immobilized by soil microbes and be unavailable to plants (Aber and Melillo, 1980; Downs et al., 1996; Wang and Bakken, 1997). Conversely, decomposition of organic matter with a C:N ratio less than 30:1 increases nitrogen availability for plants because nitrogen is mineralized in excess of microbial requirements (Aber, 1992; Johnson, 1992; Kaye and Hart, 1997). When soil microbes are limited by nitrogen, fertilization can decrease competition between plants and microbes, thus stimulating the growth of both (Aber, 1992; Johnson, 1992; Vanotti, et al. 1995; Magill and Aber, 1998). Materials available for use as mulch in the landscape industry have changed considerably in recent years as a result of efforts to divert solid wastes from landfills (Glenn, 1999). 'Yard wastes' such as leaves and grass clippings are often collected and removed from ornamental landscapes (Bormann et al., 1993), which disrupts nutrient cycles and increases reliance on inorganic fertilizers (Craul, 1994; Osmond and Platt, 2000). However, much of the more than 30 million tons of yard waste generated annually in the United States (McKeever, 1999) is composted for use in landscapes (Glenn, 1999). Because the low C:N ratio (
