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

Abstract

The objective of this study was to compare substrate solution nitrogen (N) availability, N immobilization, and nutrient leaching in a pine tree substrate (PTS), peat-lite (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−1 NO3-N. 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−1 N. In addition to determining the amounts of N immobilized, substrate carbon dioxide (CO2) efflux (μmol CO2/m−2·s−1) 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 short- and medium-term studies. Nitrogen immobilization increased as fertilizer rate increased from 100 mg·L−1 N to 200 mg·L−1 N in PL and from 100 mg·L−1 N to 300 mg·L−1 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 short- and 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.