Comparison of Fertilizer Nitrogen Availability, Nitrogen Immobilization, Substrate Carbon Dioxide Efflux, and Nutrient Leaching in Peat-lite, Pine Bark, and Pine Tree Substrates

The objective of this study was to compare substrate solution nitrogen (N) availability, N immobilization, and nutrient leaching in a pine tree substrate (PTS), peatlite (PL), and aged pine bark (PB) over time under greenhouse conditions. Pine tree substrate was produced from loblolly pine logs (Pinus taeda L.) that were chipped and hammer-milled to a desired particle size. Substrates used in this study were PTS ground through a 2.38-mm hammer mill screen, PL, and aged PB. A short-term (28-d) N immobilization study was conducted on substrates fertilized with 150 or 300 mg L NO3N. Substrates were incubated for 4 days after fertilizing and NO3-N levels were determined initially and at the end of the incubation. A second medium-term study (10-week) was also conducted to evaluate the amount of N immobilized in each substrate when fertilized with 100, 200, 300, or 400 mg L N. In addition to determining the amounts of N immobilized, substrate carbon dioxide (CO2) efflux (mmol CO2/m –2 s) was also measured as an assessment of microbial activity, which can be an indication of N immobilization. A leaching study on all three substrates was also conducted to determine the amount of nitrate nitrogen (NO3-N), phosphorus, and potassium leached over 14 weeks under greenhouse conditions. Nitrogen immobilization was highest in PTS followed by PB and PL in both the shortand medium-term studies. Nitrogen immobilization increased as fertilizer rate increased from 100 mg L N to 200 mg L N in PL and from 100 mg L N to 300 mg L N for PB and PTS followed by a reduction or no further increase in immobilization when fertilizer rates increased beyond these levels. Nitrogen immobilization was generally highest in all substrates 2 weeks after potting, after which immobilization tended to decrease over the course of several weeks with less of a decrease for PTS compared with PL and PB. Substrate CO2 efflux levels were highest in PTS followed by PB and PL at each measurement in both the shortand medium-term studies. Patterns of substrate CO2 efflux levels (estimate of microbial populations/activity) at both fertilizer rates and over time were positively correlated to N immobilization occurrence during the studies. Nitrate leaching over 14 weeks was lower in PTS than in PB or PL through 14 weeks. This work provides evidence of increased microbial activity and N immobilization in PTS compared with PB and PL. Increased N immobilization in PTS explains the lower nutrient (primarily N) levels observed in PTS during crop production and justifies the additional fertilizer required for comparable plant growth to PL and PB. This work also provides evidence of less NO3-N leaching in PTS compared with PL or PB during greenhouse crop production despite the higher fertilizer rates required for optimal plant growth in PTS. 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 debrisbased 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 microorganisms, resulting in a draw on nutrient sources (primarily N) from the substrate solution (Gumy, 2001). The N extraction from the soil/substrate solution by microorganisms 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 microorganisms 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 HORTSCIENCE VOL. 44(3) JUNE 2009 781 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 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 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 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 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. Materials and Methods Short-term (4-week) N immobilization. Pine tree substrate used in this study was produced from loblolly pine trees ( 25-cm basal diameter) that were harvested at ground level and delimbed on 25 Apr. 2006 in Warsaw, VA. Trees were then chipped (including bark) with a Morbark Chipper (Winn, MI) operated by Wood Preservers Inc. (Warsaw, VA) on 26 Apr. 2006. Wood chips (2.5 cm · 2.5 cm · 0.5 cm) were further ground in a hammer mill (Meadows Mills, Inc., North Wilkesboro, NC) on 27 Apr. 2006 to pass through a 2.38-mm screen. Pine tree substrate ( 90% wood and 10% bark) was used fresh (uncomposted) and amended with 0.6 kg m calcium sulfate (CaSO4) because Saunders et al. (2005) reported improved growth of herbaceous species when CaSO4 was incorporated. Samples of PTS were tested for pH before potting and not amended with lime as a result of the relatively high pH ( 6.0) observed, which has been previously reported in freshly ground pine wood (Wright et al., 2008). Other substrates used in this study were an aged PB and PL [composed of 80% peat (Premier Tech, Quebec, Canada) and 20% perlite (v/v)]. Pine bark and PL were preplant-amended with dolomitic lime at a rate of 3.6 kg m and CaSO4 at the rate of 0.6 kg m. Substrates were prepared on 1 May 2006 and moistened to 50% moisture content (based on the substrate’s waterholding capacity) to facilitate lime reactions (in the peat and PB) and provide adequate moisture for Received for publication 2 Dec. 2008. Accepted for publication 23 Mar. 2009. The research was funded in part by the American Floral Endowment, Virginia Agricultural Council, the Virginia Nursery and Landscape Association, and the Virginia Tobacco Commission. The cost of publishing this paper was defrayed in part by payment of page charges. Technical assistance of Breanna Rau and Joyce Shelton is gratefully acknowledged. Use of trade names does not imply endorsement of the products named nor criticism of similar ones not mentioned. Graduate Research Assistant. Professor. To whom reprint requests should be addressed; e-mail [email protected]. 782 HORTSCIENCE VOL. 44(3) JUNE 2009 microbial activity in all substrates. The municipal water source (Blacksburg, VA) used to moisten the substrates had an alkalinity of 5.8 mEq L and pH of 6.8. Substrates were stored in closed containers for 8 d (after wetting) before the initiation of the experiment as suggested by Handreck (1992a). On 8 May 2006, 12 cm tall · 15 cm square (1.7-L) plastic containers were filled with the three substrates and placed on raised benches in the Virginia Tech (Blacksburg, VA) Greenhouse Facility (glass-covered) with average day and night temperatures of 24 and 19 C, respectively. Containers were irrigated with 500 mL of nutrient solution (beaker-applied) at the rates of 0, 150, or 300 mg L N as potassium nitrate (KNO3; Handreck, 1992a), which consequently supplied 420 mg L potassium (K) at the 150 mg L N rate and 840 mg L K at the 300 mg L N rate. Phosphorus was also supplied at 45 mg L P as phosphoric acid (H3PO4) as a result of published studies reporting the requirement (and immobilization) of P by microbial populations in soils and wood substrates (Gallardo and Schlesinger, 1994; Handreck, 1996). The 0 mg L N rate was added as a control to observe the effect of fertilizer on substrate CO2 efflux compared with a nonfertilized treatment. Fertilizer treatments were applied to substrates on Day 0 (day of potting) and every 7 d thereafter for 28 d for a total of five applications. After fertilizer applications (every 7 d), three replications of each treatment were removed from the greenhouse and analyzed for N immobilization using the NDI incubation and extraction procedure as described previously. Substrate solutions extracted before and after incubation at each sampling date were frozen and later analyzed for NO3-N with an Orion ion selective electrode (Thermo Electron, Beverly, MA) on 17 Aug. 2006. Substrate solution N levels were determined on Day 0 (initial) and on Day 4 (final) and the amount of N immobilized was calculated by determining the difference between Day 4 and Day 0. Total N loss (mg N per L of substrate) was then calculated for the total 4 d incubation. Nitrogen immobilization data from the 0 mg L N rate in all substrates were excluded from data analysis as a result of no N immobilization occurring at that rate. Between fertilizer/irrigation applications, containers were kept uncovered on open greenhouse benches and substrate moisture was determined by weighing representative containers of each substrate and applying tap water (same water source as previously described) to readjust substrate moisture to 70% of their waterholding capacities (determined before the initiation of this experiment). Substrate CO2 efflux levels were determined on all substrates and at all fertilizer rates as an estimate of microbial activity and potential N immobilization (Wang et al., 2003). Substrate CO2 efflux (mmol CO2/ m s) was determined each week on three container replications of each substrates at each fertilizer rate using a LI-COR 6250 (LICOR, Lincoln, NE) infrared gas analyzer (IRGA) equipped with a closed chamber constructed from a polyvinyl chloride pipe end cap designed to take nondestructive CO2 measurements from the substrate-filled containers. The chamber was placed on the substrate and pressed firmly to the surface before CO2 efflux measurements were taken. A gas sampling and return air port (constructed from 0.6-cm plastic tubing) from the chamber allowed air to be circulated from the chamber to the IRGA. Soil CO2 efflux rates were determined by measuring change in CO2 concentration (DC) over a 30-s period. The LI-COR 6250 was recalibrated before each sampling date and the system zeroed between treatment replications. Substrate CO2 efflux was measured at about the same time of day for each sampling date. The experimental design was completely randomized with three substrates, three fertilizer rates, and 15 replications per substrate for a total of 135 containers. Nitrogen immobilization and substrate CO2 efflux data were tested using the analysis of variance procedures and correlation analysis of SAS (Version 9.1; SAS Institute, Inc., Cary, NC) with treatment means separated by Duncan’s multiple range test (a = 0.05). Data were also subjected to regression analysis using SigmaPlot (Version 9.01; SPSS, Inc., Chicago, IL). Medium-term (10-week) immobilization. Pine tree substrate used in this study as well as the leaching experiment (described next) was produced from loblolly pine trees that were harvested at ground level and delimbed on 15 July 2007 in Blackstone, VA. Trees were then chipped (including bark) with a Bandit Chipper (Model 200; Bandit Industries, Inc., Remus, MI) on 17 July 2007. Wood chips were further ground in a hammer mill on 18 July 2007 to pass through a 2.38-mm screen. Other substrates used in this study included aged PB and PL. All substrates were amended similarly as described in the shortterm experiment. On 21 Aug. 2007, 12 cm tall · 15 cm square (1.7-L) plastic containers were filled with the three substrates and placed on raised benches in the Virginia Tech (Blacksburg, VA) Greenhouse Facility (glass-covered) with average day and night temperatures of 24 and 19 C, respectively. Fertilizer solutions were beaker-applied (500 mL) every 2 weeks (14 d) to the substrates at the rates of 100, 200, 300, or 400 mg L N as KNO3, which consequently supplied 280 mg L K for each 100 mg L N rate increase. Phosphorus was also supplied at 45 mg L as H3PO4. After fertilizer application and drainage for 1 h, three single container replications of each treatment were removed from the greenhouse and analyzed for N immobilization using the NDI incubation and extraction procedure (as described in the short-term experiment) on Week 0 (day of potting) and again every 14 d for 10 weeks for a total of six sampling dates. Substrate solutions extracted before and after incubation at each sampling date were frozen and later analyzed for NO3-N with an Orion ion selective electrode on 22 Jan. 2008. Nitrogen immobilization data were calculated in this study as described previously in the short-term experiment. Between fertilizer/irrigation applications, containers were kept uncovered on open greenhouse benches and substrate moisture was maintained at 70% in all substrates as described in the short-term experiment. Substrate CO2 efflux was determined every 2 weeks (alternate weeks of fertilizer applications) on three container replications of each substrates at each fertilizer rate using a LI-COR 6250 as previously described previously. The experimental design was completely randomized with three substrates, four fertilizer rates, and 24 replications per substrate for a total of 288 containers. Data were tested using the analysis of variance procedures and correlation analysis of SAS (Version 9.1; SAS Institute, Inc.). Data were also subjected to regression analysis using SigmaPlot (Version 9.01; SPSS, Inc.). Nutrient leaching. Pine tree substrate, PL, and PB used in this experiment were prepared as described previously in the medium-term experiment and amended similarly. On 21 Aug. 2007, 12 cm tall · 15 cm square (1.7-L) plastic containers were filled (by vol) with the three substrates. Fertilizer solutions were beaker-applied (500 mL) every 2 weeks (14 d) to the substrates at the rates of 100 or 300 mg L N prepared from KNO3, which consequently supplied 280 mg L K at the 100 mg L N rate and 840 mg L K at the 300 mg L N rate. Phosphorus was supplied at 45 mg L P as H3PO4 to both the rates (100 and 300 mg L N). Fertilizer applications were made to all substrates every other week through the conclusion of the study (14 weeks). Leaching of NO3-N, P, and K was monitored on six replicates of each treatment. Leachate collection devices were made by cutting 15-cm circular holes in the lids of 4.5L plastic buckets. Lids were then securely fitted on buckets and fallow substrate-filled containers were inserted halfway through the lids into the buckets. Buckets (with containers inserted) were then placed on raised benches in the Virginia Tech (Blacksburg, VA) Greenhouse Facility (glass-covered) with average day and night temperatures of 24 and 19 C, respectively. This system allowed only the leachate passing through the fallow containers to be collected after irrigations. After applying fertilizer solutions, containers were allowed to completely drain (1 h) before containers were removed from buckets for leachate collection. Leachate volume was determined and an aliquot was taken and subsequently frozen and later analyzed for NO3-N with an Orion ion selective electrode on 17 Jan. 2008, and P and K concentrations were analyzed on 31 Jan. 2008 with a Spectro Ciros Vision ICP (Spectro Analytical Instrument, Mahwah, NJ). Between fertilizer/irrigation applications, containers were kept uncovered on open benches in a greenhouse and substrate moisture was maintained at 70% as described in the medium-term experiment. HORTSCIENCE VOL. 44(3) JUNE 2009 783 The experimental design was completely randomized with three substrates, two fertilizer rates, and six replications per substrate for a total of 36 containers. Data were tested using the analysis of variance procedures of SAS (Version 9.1; SAS Institute, Inc.). Results and Discussion Short-term (4-week) N immobilization. At each measuring date and at both fertilizer rates, the amount of N immobilization was highest in PTS compared with PB and PL, and the amount of immobilization in PB was higher at all dates and fertilizer rates compared with PL (Table 1). At both fertilizer rates (150 mg L and 300 mg L N), N immobilization occurred in the first days after potting (Day 0) and increased with each measurement date through the end of the study (28 d) in all substrates except PL at the high fertilizer rate (Table 1). The initial immobilization in all substrates explains the need for starter charge fertilizers that are typically added to commercial substrate mixes. Quick-release starter charge fertilizers supply N to the substrate on planting, which can offset the N immobilized by microbes. Immobilized N was equal at both fertilizer rates for PL and PB at all five measuring dates, whereas PTS had a higher amount of N immobilization at the 300 mg L N rate through Day 21 compared with the 150 mg L N rate (Table 1). On Day 28, N immobilization was the same (15 and 16 mg per L substrate) at both fertilizer rates in PTS. Immobilization amounts were at least as high at Day 28 (end of the experiment) at both fertilizer rates and in all substrates compared with Day 0 indicating the continuous occurrence of N immobilization over the course of 28 d during crop production (Table 1). However, at Day 28, more than twice the amount of N was immobilized in PTS compared with PB and more than five times the amount immobilized in PL (Table 1). Substrate solution NO3-N levels, reflective of N immobilization, were lower in PB and PTS compared with PL after the 4-d incubation at both fertilizer rates at all measurement dates with levels in PTS being the lowest of all the substrates (Table 2). The levels of NO3-N recovered in PTS at the low fertilizer rate were very low at the first two measurements (less than 1.0 mg L N) but increased by the last measurement date (9.3 mg L N; Table 2). At all measuring dates (0, 7, 14, 21, and 28), the NO3-N levels after incubation in PTS remain lower at both fertilizer rates than PB and PL (Table 2). The lower substrate solution NO3-N levels in PTS (resulting from more N immobilization) compared with PB have also been reported even when plant growth was similar (Jackson et al., 2008a) and in PL (Jackson et al., 2008b; Wright et al., 2008). The reason for higher initial (Day 0) substrate solution N levels at all measuring dates (Table 2) in PB and PL compared with PTS at the 300 mg L N rate can be explained by the already existing N in the substrates from previous fertilizer applications that remained in the substrate as a result of the lower amount of N immobilization occurring in those substrates each week (Table 1). Short-term fast-growing crops (e.g., bedding plants) grown in PL or PB are commonly fertilized at rates between 100 and 200 mg L N, which is unacceptable for plants growing in PTS (Jackson and Wright, 2009). Based on the increased N immobilization in PTS compared with PB and PL at both fertilizer rates (Table 1) and as a result the higher NO3-N levels remaining in substrate solution in PB and PL after 4 d compared with PTS (Table 2), it is clear why decreased plant growth is usually observed in PTS at relatively low (100 to 200 mg L N) fertilizer rates. Similar to these results, Sharman and Whitehouse (1993) observed less available N in substrate solution at low fertilizer rates (75 and 150 mg L N) in wood-based substrates, but at higher fertilizer rates (300 mg L N), N levels in substrate solution were higher and closer to levels in peat. Work by Table 1. Nitrate nitrogen (NO3-N) immobilized (milligrams) in peat-lite, pine bark, and pine tree substrate (PTS) at five sampling dates over 28 d (short-term experiment) in containers fertilized with two rates of N from potassium nitrate (KNO3). Substrates NO3-N immobilized z N rate (mg L) Day 0 Day 7 Day 14 Day 21 Day 28 Significance Peat-lite 150 2.4 d 3.0 d 3.2 d 3.0 d 2.9 c L** Q* 300 2.2 d 2.5 d 2.8 d 2.8 d 2.4 c NS