Effect of Biochar Type on Macronutrient Retention and Release from Soilless Substrate

A series of column studies were conducted to determine the influence of three different biochar types on nitrate, phosphate, and potassium retention and leaching in a typical greenhouse soilless substrate. A commercial substrate composed of 85 sphagnum peatmoss : 15 perlite (v:v) was amended with 10% by volume of three different biochar types including: gasified rice hull biochar (GRHB), sawdust biochar (SDB), and a bark and wood biochar (BWB). The non-amended control substrate, along with substrates amended with one of three biochar materials, were each packed into three columns. Columns were drenched with nutrient solution and leached to determine the impact of biochar on nutrient retention and leaching. Nitrate release curves were exponential and peaked lower, at later leaching events, and had higher residual nitrate release over time with each biochar amendment. The impact of biochar amendment on phosphate retention and release was more variable within and across the two experiments. In both experiments, the GRHB was a net source of phosphate, providing more phosphate to the system than the fertilizer application and hence obscuring any retention and release effect it might have. Potassium release varied by amendment type within each experiment, but within each amendment type was relatively consistent across the two experiments. All biochar types were a source of potassium, with GRHB providing more than SDB, but both providing far more potassium than the fertilizer event. The BWB amendment resulted in more leached potassium than the control substrate, but relatively little compared with GRHB and SDB amendments. Biochar is the charred organic matter that remains after pyrolysis of biomass or manure. The influence of biochar in mineral soil systems has been studied and reviewed extensively (Lehmann et al., 2011; Spokas et al., 2011; Verheijen et al., 2010). In contrast, the influence of biochars on soilless substrates used in greenhouse and nursery containers has not been studied adequately, and only a few citations, summarized previously (Altland and Locke, 2012), are tangentially relevant to current container production systems. Nonetheless, the collection of papers thus far seems to indicate the same potential benefits in soilless substrates including additions of some nutrients (Ruamrungsri et al., 2011), reduction in leaching of nitrates and phosphates (Beck et al., 2011), beneficial shifts in microbial populations (Graber et al., 2010), and improved physical properties (Dumroese et al., 2011). Recently, we reported that one form of mixed SDB retained and released nitrate and phosphate such that the concentration in the substrate would be moderated against fluctuations in nutrient concentration from intermittent fertilization events (Altland and Locke, 2012). Similarly, Beck et al. (2011) showed that amendment of an unspecified greenroof substrate with 7% biochar increased water retention and decreased total nitrogen (N) and phosphorus (P), nitrate, phosphate, and organic carbon in runoff. Biochars from different feedstocks yield different properties as a result of their differing particle sizes at the time of pyrolysis, inherent ash content of the feedstock (Demirbas, 2004), pyrolysis conditions (Singh et al., 2010), and storage conditions after processing (Spokas et al., 2011). The objective of this research was to determine the influence of three different biochar types on nitrate, phosphate, and P retention and leaching in a typical greenhouse soilless substrate. Materials and Methods A standard commercial soilless medium composed of 85 sphagnum peatmoss:15 perlite (v:v) (BM-6, Berger Peat Moss; SaintModeste, Quebec, Canada), which contained no incorporated macronutrient fertilizers, was selected as the base substrate for the study. We used three forms of biochar: GRHB (CharSil, Riceland Food, Inc., Stuttgart, AR), SDB obtained from a local bioenergy pyrolysis unit [Synterra Energy (formerly Red Lion Bio-Energy), Toledo, OH], and a BWB (Royal Oak Charcoal, Roswell, GA). The GRHB is a commercially available product, made by gasification of rice hulls at 815 C. The SDB uses hardwood sawdust from local lumber mills and is produced at 730 C. The BWB contains the bark and wood scraps in lumber production and is produced at 815 C. Each biochar type was characterized for its chemical properties (Table 1) and particle size distribution (Table 2). Percent carbon and N were determined with a PerkinElmer Series II CHNS/O Analyzer (PerkinElmer Instruments, Shelton, CT). Other macronutrients and micronutrients were determined with a Thermo Iris Intrepid inductively coupled plasma optical emission spectrometry (Thermo Electron Corp., Waltham, MA). Particle size distribution was determined by passing 30 g oven-dried (72 C) biochar through soil sieves ranging from 0.106 to 6.30 mm (Table 2). Particles 0.106 mm or less were collected in a pan. Sieves and pan were shaken for 3 min with a RX-29/30 Ro-Tap test sieve shaker (278 oscillations/min, 150 taps/min) (W.S. Tyler, Mentor, OH). The peatmoss substrate was amended volumetrically with 10% of each biochar type. Resulting substrates, including a non-amended control, were packed into glass columns 4.5 cm i.d. and 38 cm long, with a volume of 600 cm. Columns have a flat, false bottom above a stopcock to prevent compression and to control drainage of added solutions. Each biochar treatment was packed into three columns. Substrate was packed into a column by adding 150-cm increments of substrate, uniformly packing with a wand, and repeating until the column was full and contained 600 cm substrate. On 16 Aug. 2011, each column was saturated with 400 mL deionized (DI) water, enough to saturate the entire column of substrate, and let stand for 1 h. The solution was Table 1. Chemical composition of gasified rice hull biochar (GRHB), sawdust biochar (SDB), and bark and wood biochar (BWB) before amendment in a greenhouse substrate.