In recent years, several peat and pine bark (PB) alternative substrates have been developed and researched in the United States and throughout the world. The interest in new substrates is in response to the increasing cost and environmental issues surrounding the use of peatmoss and the cost and availability of PB substrates. Many of the substrates investigated are wood-based or plant debris-based materials that have been processed for use as a container substrate from plants, including chinese tung tree (Aleurites fordi Hemsl.; Gruszynski and Kämpf, 2004), paper bark tree (Melaleuca quinquenervia Cav.; Poole and Conover, 1985), forest gorse (Ulex europaeus L.; Iglesias et al., 2008), tree fern (Dicksonia squarosa Swartz.; Prasad and Fietje, 1989), and miscanthus (Miscanthus sinensis Anderss.; Carthaigh et al., 1997) to name a few. Evaluation of these and other wood/plant-based substrates has proven successful in the production of vegetables (Pudelski and Pirog, 1984; Schnitzler et al., 2004), foliage plants (Roeber and Leinfelder, 1997), bedding plants (Boyer et al., 2008a; Wright and Browder, 2005; Wright et al., 2009), poinsettias and mums (Jackson et al., 2008b; Wright et al., 2008), and woody shrubs and trees (Boyer, 2008; Jackson et al., 2008a; Wright et al., 2006). Much attention is now focused on pine tree substrates (PTS) produced from loblolly pine trees that are ground (with or without bark, limbs, needles, and so on) in a hammer mill and clean chip residual (CCR), which is produced from byproducts of the pine tree harvesting process. These substrates can be hammer-milled to a size acceptable for use as a container substrate (Boyer, 2008; Fain et al., 2008a; Jackson et al., 2007; Wright and Browder, 2005).
In contrast to peat and PB, plant production in substrates composed of wood, or large portions of wood, have a tendency to become nitrogen (N) -deficient as a result of high rates of N immobilization (Handreck, 1991, 1993; McKenzie, 1958). Wood contains large amounts of useable/degradable carbon (C) compounds but only a small amount of nutrients available for micro-organisms, resulting in a draw on nutrient sources (primarily N) from the substrate solution (Gumy, 2001). The N extraction from the soil/substrate solution by micro-organisms lowers available nutrient supplies to plants, which in turn leads to plant nutrient deficiencies if additional N is not added to correct the problem (Bodman and Sharman, 1993; Handreck, 1993). Successfully producing crops in wood substrates will require new strategies in N management so that the collective amounts of N required by micro-organisms and by plants will be supplied in sufficient quantities to promote or maintain desired plant growth or to prevent nutrient deficiencies (Lunt and Clark, 1959; Worrall, 1985).
Several methods have been developed and used to reduce N immobilization in wood substrates and improve fertilizer management strategies during crop production: 1) composting wood materials has been shown to eliminate or significantly reduce the potential for N immobilization to occur during crop production by lowering the C:N ratio and allowing the initial breakdown, which requires high levels of N by micro-organisms (Gutser et al., 1983; Prasad, 1997); 2) a nutrient impregnation process used in the production of Toresa®, a commercial wood fiber substrate in Europe, mechanically grinds wood chips together with nutrient compounds in machines called retruders (Gumy, 2001; Schilling, 1999; Schmilewski, 2008; personal observation, Brian Jackson and Robert Wright at an Intertoresa AG Toresa® manufacturing facility in Hamburg, Germany, 13 Mar. 2007); 3) a technique called the Fersolin process impregnates wood material with sulfuric acid in the presence of hot gases (933 °C) resulting in a decrease in decomposable cellulose, which results in lower microbial activity and need for N (Bollen and Glennie, 1961); and 4) a process for treating wood materials by pyrolysis (a form of incineration that chemically decomposes organic materials by heat in the absence of oxygen) has been evaluated as a method to break down unstable and toxic wood components into more stable and nontoxic components that are resistant to microbial decay, which retards microbial N demand (Bollen and Glennie, 1961). The methods described are often expensive, time-consuming, and nonpractical for many substrate companies and growers. As a more practical approach, a common method for supplying nutrients to counteract microbial N immobilization from a substrate is by the application of additional fertilizer during crop production. This is the most commonly used and preferred method of countering the effects of N immobilization on plant growth (Gruda, 2005; Gruda et al., 2000; Wright et al., 2008).
The most frequently used and accepted method for determining N immobilization in soilless substrates is the nitrogen drawdown index (NDI) procedure developed by Handreck (1992a, 1992b). The NDI procedure involves saturating or “charging” a substrate with a KNO3 fertilizer solution containing 75 mg·L−1 N and then incubating the substrate at 22 °C for 4 d. Substrate solution nitrate nitrogen (NO3-N) levels are determined immediately after saturation on Day 0 and then again after Day 4 (the incubation period). The NDI is then calculated by the following formula (NO3-N measured on Day 4/NO3-N measured on Day 0 × 100). The resulting index is a value between 1.0 and 0.0 with a value of 1.0 representing no N loss during the 4-d incubation and an index value of 0.0 indicating complete N loss after 4 d. Substrates composed of large amounts of wood materials (high C:N ratio) will immobilize all, or nearly all, of the N during the 4-d incubation when using 75 mg·L−1 N, making it impossible to determine the maximum amount used by micro-organisms. Handreck (1992b) has recommended that the N concentration in the saturating solution be 150 mg·L−1 N when substrates with a high demand for N are being tested or that the incubation time be decreased to obtain measurable amounts of N remaining in the substrate after incubation. Similarly, Sharman and Whitehouse (1993) suggest that saturating solutions with concentrations of 150, 200, or 300 mg·L−1 N be used in N immobilization tests on materials with high C:N ratios.
Nitrogen immobilization in soils and organic materials results from microbial assimilation of ammonium nitrogen (NH4-N) and NO3-N into proteins, nucleic acids, and other organic complexes contained within microbial cells (Davet, 2004). Carbon dioxide (CO2) release represents the final stage of oxidation of organic substrates (Davet, 2004). Because root respiration is also a source of CO2 in the soil/substrate, it is important to take into account that the CO2 measured is not solely a result of microbial respiration. Soil CO2 efflux is influenced by a number of factors, including soil/substrate quality and organic matter content, temperature, soil moisture, root biomass, nutrient availability, and microbial activity and biomass (Casadesus et al., 2007; Fog, 1988; Wang et al., 2003).
The estimation of microbial populations (e.g., bacteria, fungi, protozoa) in soils or soilless substrates may be accomplished by several methods, for example by counting the population (by either microscopy or plating on agar), chloroform fumigation procedure, quantifying carbon mineralization, or by assaying some unique component of biomass such as ATP, extracellular dehydrogenase, or by measuring the metabolic activity of the population (Blagodatsky et al., 2000; Boyer et al., 2008b; Carlile and Dickinson, 2004; Henriksen and Breland, 1999; Needelman et al., 2001; Turner and Carlile, 1983; Vance et al., 1987). Measuring the metabolic activity of a microbial population (respiratory activity) involves monitoring CO2 evolution or O2 consumption. Techniques for monitoring CO2 evolution from soil were pioneered by Waksman (1932) and are still widely used in studies of microbial activity in soils and soilless substrates (Gough and Seiler, 2004; Jackson et al., 2008a; Pronk, 1997; Söderstrom et al., 1983; Turner and Carlile, 1983). Microbial activity (estimated by CO2 efflux from soils) increases in response to N fertilization in N limiting soils (Zhang and Zak, 1998) and to phosphorus (P) fertilization in P-limiting soils (Gallardo and Schlesinger, 1994). Microbial activity has also been reported to decrease in response to high rates of N fertilization of forest soils (Smolander et al., 1994; Thirukkumaran and Parkinson, 2000). Less work has been completed on soilless substrates compared with field or forest soils using CO2 efflux to monitor/estimate microbial activity.
In addition to N immobilization, nutrient leaching in PTS has been proposed as a possible reason for the lower electrical conductivity and nutrient levels observed in PTS compared with peat-lite (PL) or PB during plant production (Jackson, 2008; Wright and Browder, 2005; Wright et al., 2008). Nutrients such as NO3-N and orthophosphate anions (P) have been shown to leach from horticulture crop production areas and are a major concern for growers and environmental agencies. Although P is considered rather immobile in many soils, it is more readily leached from soilless container media (Broschat, 1995; Yeager and Wright, 1982). Limited information is available on nutrient leaching from wood substrates, and no information is available on nutrient leaching in PTS during crop production. This is an important issue in light of the higher fertilizer requirements reported for PTS (Jackson et al., 2008a; Jackson and Wright, 2009; Wright et al., 2008), which increases the potential for nutrient leaching.
Most nursery and greenhouse producers base their fertility management on previous growing experiences with PL and PB substrates. These fertility practices may not be applicable when growing crops in PTS in light of the higher fertilizer requirements, limited understanding of N immobilization timing and rate, and its unknown leaching potential. Determining the extent and timing of N immobilization and nutrient leaching in PTS therefore needs to be determined for more accurate nutrient management (application timing and rates) strategies when producing plants in this substrate. The objective of these studies was to compare N immobilization, substrate CO2 efflux, and nutrient leaching rates in PL, PB, and PTS over time under greenhouse conditions.
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