John E. Lloyd, Daniel A. Herms, Benjamin R. Stinner, and Harry A. J. Hoitink
We determined the effects of fertilization and mulching with composted yard waste and ground wood pallets on soil organic matter content, microbial activity, nitrogen cycling, and river birch growth in landscape microcosms. Dramatic effects of both mulches were apparent after only one season. Both mulches increased soil organic matter content and more than doubled microbial respiration. Effects on nitrogen availability, carbon:nitrogen (C:N) ratio, however, were highly dependent on the C:N ratio of the mulch. Composted yard waste (with a low C:N ratio of 17:1) dramatically increased total soil N and the rate at which inorganic N was released by decomposing organic matter. This increased plant available N and ultimately increased tree growth. Fertilization had no effect on the growth of trees mulched with yard waste, indicating that nutrients released by decomposition of the composted yard waste were able to fully meet both microbe and plant requirements. On the other hand, the high carbon and low nitrogen content of the ground wood mulch (C:N ratio of 51:1) stimulated microbial growth without adding much nitrogen to the soil. Microbes assimilated most of the existing soil nitrogen to support their own growth, 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 and increased the growth of river birch that had been mulched with ground wood.
Mulches have been shown to suppress weed growth, retain soil moisture, improve soil structure, and moderate soil temperatures (Robinson, 1988). The potential of organic mulch to establish nutrient regimes more similar to those of natural ecosystems has been recognized (Tukey and Schoff, 1963; Greenly and Rakow, 1995). However, the effects of mulch on nutrient cycling have not been investigated in ornamental landscapes.
Nutrients cycled as organic matter are decomposed by soil microorganisms (reviewed in Attiwill and Adams, 1993). The rate of decomposition is dependent on the total biomass of microbes in the soil, which in turn is dependent on organic matter content of the soil (Wardle 1992). In a process known as mineralization, nutrients are released from decomposing organic matter in inorganic forms which may be taken up by plants, or utilized by microbes to continue the decomposition process. The process by which nitrogen is assimilated by microbes and thus made unavailable to plants is referred to as nitrogen immobilization.
The amount of nitrogen released from decomposing litter that is available for plants is determined by the net balance between nitrogen mineralization and immobilization by soil microbes (Johnson, 1992). In environments where nitrogen is limiting, microbes generally out-compete plants for nitrogen, resulting in plant nutrient deficiencies and decreased plant growth (Kaye and Hart, 1997). In fertile soils, nitrogen levels may be high enough that neither the growth of microbes or plants is nitrogen limited.
The balance between nitrogen mineralization and immobilization is strongly influenced by the C:N ratio of the decaying organic matter (Facelli and Pickett, 1991). Populations of soil microbes are generally carbon-limited, and the addition of organic carbon to the soil stimulates the population growth of microbes until they eventually become limited by available nitrogen. Since soil microbes are stronger competitors for nitrogen than are plants, when the C:N ratio of litter is high (above 30:1), much of the available nitrogen pool will be immobilized by soil microbes, and therefore made unavailable to plants. Conversely, decomposition of organic matter high in nitrogen (C:N ratio lower than 30:1) increases the availability of nitrogen for plants by releasing nitrogen in excess of microbial demands. Fertilization can also relax competition between plants and microbes, thus stimulating plant growth (Aber 1992).
The availability of composted yard waste and ground wood pallets for use as landscape mulches has increased dramatically as a result of national policy to divert solid waste from landfills. These products vary widely in their C:N ratios and thus may have dramatically different effects on nitrogen availability and plant growth. Composted yard waste has the potential to release nutrients at optimal rates in slow-release form (Dick and McKoy, 1993), because its low C:N ratio (17:1) resembles high-quality forest litter. Conversely, the high C:N ratio (50:1) of woody materials is suspected to cause nutrient deficiencies by stimulating the growth of carbon-limited microbes (Robinson, 1988).
The objectives of our study were to determine how mulching with composted yard waste and ground wood pallets, with and without fertilization, impacts soil organic-matter content, microbial populations, nitrogen availability, and river birch growth.
Experimental Design
A field experiment was conducted in 1998 and 1999 in 48 landscape microcosms constructed on the Ohio Agricultural Research and Development Center’s Wooster campus. Microcosms were constructed by slicing a narrow trench 1 m deep around plots of ground 1 m2 in area. The trench was lined with heavy-duty PVC landfill liner, thereby isolating the soil environment of each plot and allowing for the establishment of replicated, randomized soil treatments. In the spring of 1998, one Heritage river birch (Betula nigra ‘Heritage’) was planted in each microcosm. The two-year-old trees averaged about 4.5 feet tall when they were planted. The experiment was designed as a randomized, complete block with six treatment combinations – three mulch treatments each applied with and without fertilization. Each of the six treatment combinations were replicated eight times.
Mulch and Fertilization Treatment
The three mulch treatments consisted of: (1) composted yard waste with a C:N ratio of 17:1; (2) ground wood pallets with a C:N ratio of 51:1; and (3) a bare soil control. Mulch was initially applied in fall of 1997. In spring of 1998 and 1999, old mulch was removed and replaced with fresh mulch. In all cases, mulch was applied to a depth of two inches. Beginning in October 1998, half of the microcosms from each mulch treatment were fertilized, and the other half were left untreated as controls. The fertilizer used was 18:5:4 NPK, with 56% of the N supplied in slow release form (methylene urea), and 44% of the N supplied in fast release form (17% ammonium nitrate and 27% water soluble urea). Fertilizer was applied at a rate of 3 lbs. N / 1,000 ft2 / yr in split applications, with half of the annual amount applied at budbreak in spring and half in early October.
Soil and Plant Measurements
In October 1998 and throughout 1999, soil from each microcosm was sampled to a depth of six inches. Soil samples were then analyzed to determine treatment effects on organic matter content, total nitrogen, nitrogen mineralization rate (the rate at which inorganic nitrogen is released from decomposing organic matter), the percent of nitrogen immobilized by microbes, and the amount of nitrogen existing in forms available to plants (NH4, NO3, NO2, and dissolved organic nitrogen). Only data from the October 1998 and April 1999 samples are reported here. Microbial respiration (as an index of microbial biomass) was measured in July 1999.
To determine treatment effects on nitrogen uptake by river birch, leaves were collected during June and September of 1998 and 1999 and analyzed for nitrogen content. The effect of the treatments on caliper growth of river birch was determined by measuring trunk diameter at 50 cm above ground at the beginning and the end of each growing season.
Soil Organic Matter, Microbial Activity, and Nitrogen Cycling
The mulch treatments had dramatic effects on soil organic matter, microbial activity, and nitrogen cycling that were readily apparent after only one season. Both mulches increased the organic-matter content of the soil, with the yard waste mulch having the most substantial effect, increasing organic matter content by nearly 40% (Figure 1). Both mulches also had dramatic effects on microbial biomass, more than doubling soil respiration (Figure 2).
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| Figure 1. Effects of composted yard mulch, groundwood mulch, and bare soil on soil organic matter in October 1998 and April 1999. Data are expressed as mean ± standard error. Means within a year with different letters are significantly different (p<0.05). | Figure 2. Effects of composted yard mulch, groundwood mulch, and bare soil on soil organic matter in October 1998 and April 1999. Data are expressed as mean ± standard error. Means within a year with different letters are significantly different (p<0.05). |
The composted yard waste and ground wood mulches had widely differing effects on nitrogen cycling. Total soil N was highest in the yard waste treatment, while the ground wood mulch had no effect on total N (Figure 3a). This is not surprising since the yard waste mulch has a relatively high concentration of N (1.91%) while the ground wood contains very little (0.65%). The rate at which inorganic N was released from decomposing organic matter (N mineralization rate) was also much higher in the composted yard waste than in the ground wood treatment (Figure 3b). By April 1999, the rate of N mineralization was also higher in the composted yard waste treatment than in the bare soil control (Figure 3b).
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| Figure 3. Effects of composted yard mulch, groundwood mulch, and bare soil on (a) total soil nitrogen, (b) rate of nitrogen mineralization, (c) percent of total soil nitrogen immobilized by soil microbes, and (d) plant available nitrogen (NH4, NO3, NO2, and dissolved organic nitrogen) in October 1998 and April 1999. Data are expressed as mean ± standard error. Means within a year with different letters are significantly different (p<0.05). |
The percentage of N that was immobilized in microbial biomass tended to be higher in the ground wood treatment than in the composted yard waste or bare soil control, although the differences were not great (Figure 3c). The net effect of all of these factors is that by April 1999, the amount of N available to plants was much higher in microcosms mulched with composted yard waste than those mulched with ground wood or in the bare soil treatment (Figure 3d).
Only limited effects of the October 1998 fertilizer application were evident in April 1999. Fertilization did not affect total soil N, but moderately increased the rate of N mineralization (Figure 4a) and slightly decreased the proportion of N that was immobilized by microbes (Figure 4b). The net effect was a limited increase in the amount of N available to plants (Figure 4c).
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| Figure 4. Effects of fertilization on (a)rate of nitrogen mineralization, (b) percent of total soil nitrogen immobilized by soil microbes, and (c) plant available nitrogen (NH4, NO3, NO2, and dissolved organic nitrogen) in April 1999. Data are expressed as mean ± standard error. Means within a year with different letters are significantly different (p<0.05). |
Nitrogen Uptake and Growth of River Birch
Mulch and fertilization effects on the foliar nitrogen content of river birch did not become apparent until 1999, when foliar N content was higher in the composted yard waste treatment than in the ground wood pallet or bare-soil control (Figure 5). Fertilization also increased foliar N levels (Figure 5).
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| Figure 5. Effects of composted yard mulch, groundwood mulch, and bare soil, with or without fertilization, on foliar nitrogen content of river birch in June 1999. Data are expressed as mean ± standard error. Means within a year with different letters are significantly different (p<0.05). |
The effects of the mulch and fertilizer treatments on tree growth tended to correspond with their effects on plant available nitrogen in the soil. In 1998, the radial trunk growth of river birch was greater in the composted yard waste mulch treatment (6.9 ± 0.4 mm) than the ground wood (4.7 ± 0.4 mm) and bare soil treatments (4.5 ± 0.4). In 1999, river birch trunk growth was again greatest in plots mulched with composted yard waste, and was slowest in the wood pallet treatment (Figure 6). Fertilization increased the growth of trees mulched with ground wood, but had no effect on trees mulched with yard waste or grown in bare soil (Figure 6).
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| Figure 6. Effects of composted yard mulch, groundwood mulch, and bare soil, with or without fertilization, on foliar nitrogen content of river birch in June 1999. Data are expressed as mean ± standard error. Means within a year with different letters are significantly different (p<0.05). |
Both mulches tested in this study increased soil organic matter and microbial respiration after only one season. These results are consistent with the hypothesis that soil microbes are carbon-limited and that the addition of organic carbon can dramatically increase microbial biomass in the soil.
The effects that the increased organic matter and microbial activity had on nitrogen availability, however, were highly dependent on the C:N ratio of the mulch applied. Composted yard waste (with a low C:N ratio of 17:1) dramatically increased total soil N. Although the percentage of N immobilized by microbes was nearly as high in the yard-waste treatment as in the ground-wood treatment, the rate at which inorganic N was released by decomposing organic matter was much greater in the yard-waste treatment.
The higher level of total soil N and rate of N mineralization in the yard-waste treatment, coupled with a lower percentage of nitrogen immobilized by microbes, yielded much higher levels of plant available N (NH4, NO3, NO2, and dissolved organic N). This led to increased foliar N content of river birch and ultimately increased tree growth, which was greater in the yard-waste than in the ground-wood or bare-soil treatments. Fertilization had no additional effect on the growth of trees mulched with yard waste, indicating that nutrients released by decomposition of the composted yard waste were able to fully meet the nutritional requirements of both microbes and the plant.
On the other hand, mulching with ground wood (with a high C:N ratio of 51:1) decreased the foliar nitrogen content and growth of river birch. The high carbon and low nitrogen content of the ground wood mulch stimulated microbial growth without adding significant quantities of nitrogen to the soil. Microbes assimilated most of the existing soil nitrogen (immobilizing more than 85%) to support their own population growth, thereby limiting the amount of N available to support plant growth. Fertilization increased the growth of trees mulched with ground wood by increasing the level of available nitrogen.
These results are consistent with the hypothesis that soil microbes out-compete plants for available nitrogen, and that addition of organic matter with high C:N ratios can induce nutrient deficiencies in plants by stimulating microbial growth. Fertilization relaxed the competition between plants and microbes in the ground wood treatment by increasing nutrient availability, thereby increasing plant growth.
Previous studies have concluded that organic mulch can increase tree growth by conserving soil moisture and enhancing soil structure (Greenly and Rakow, 1995; Iles and Dosmann, 1999). Our study indicates that organic mulches can also impact tree growth through effects on nutrient availability. Mulches with a low C:N ratio such as composted yard waste can increase tree growth by increasing soil organic matter content, microbial biomass activity, and plant available nitrogen. On the other hand, mulches with a high C:N ratio such as those derived from ground pallets can stimulate microbial growth without increasing the soil nitrogen pool, thereby inducing nutrient deficiencies that slow plant growth.
We are grateful to the many people who helped make this study possible. The OARDC Grounds Department and Physical Plant provided invaluable assistance constructing the microcosms. Kurtz Brothers, Inc., provided the mulch used in this study. Bruce Birr (USDA Forest Service, North Central Forest Experiment Station) conducted the foliar nitrogen analysis. Matthew Ayres (Department of Biology, Dartmouth College) made the soil respiration measurements. Technical assistance was provided by Don Beam, Eric Bodle, Marie Egawa, Carolyn Glynn, Catie Lloyd, Ben Marthey, Dave McCartney, and Bill Styer.
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