Efficacy of secondary biosolids fertilization in a managed Douglas-fir forest in the coastal western hemlock zone of British Columbia

Secondary biosolids were applied to 16- and 26-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations (SI50 = 27 m and 36 m, respectively) in the Southern Moist Submaritime Coastal Western Hemlock biogeoclimatic subzone (CWHmsl), British Columbia, in order to investigate the effects on soil chemical properties, tree nutrient status, stand growth and yield, soil moisture availability and tree water status, tree photosynthesis, leaf area index and water use efficiency. Comparisons were made between liquid biosolids (3% dw) and dewatered biosolids (30% dw) rewatered with liquid biosolids (1:3, w/v), and between biosolids applications in spring, summer and fall, between 0 (control), 750, 1000 and 1500 kg-N/ha biosolids treatments, and between 225 kg-N/ha urea/arrurionium nitrate treatment and the biosolids treatments in order to determine the appropriate biosolids type, application season and rate(s) for the study area. Soil and foliar samples were taken, soil and tree water potential, photosynthesis and leaf area index, and tree growth were measured over a three year period (1993-95) following treatment. Secondary biosolids treatments significantly increased forest floor total N, NH₄⁺-N, NO₃⁻-N, total available N (NH₄⁺-N + NO₃⁻-N), and extractable P, but significantly reduced forest floor C/N ratio in the first and second year after application. The N and P variables were also increased in the mineral soil, but to a lesser extent. pH, electric conductivity, and exchangeable K, Ca and Mg were generally unaffected by the biosolids application. Current year foliar N and S concentrations were significantly increased, while foliar S0₄-S was significantly decreased by the biosolids treatments, and the increase or reduction was correlated to the dose of applied biosolids. The biosolids effects on foliar P, K, Ca and Mg were not significant. The secondary biosolids treatments also greatly increased foliar nutrient ratios between N and other nutrients, and foliar Cu, Zn and Fe concentrations. Application of secondary biosolids significantly increased current year foliage, shoot and bud growth, current annual increments of DBH, height, and stand basal area, irrespective of biosolids application season, especially in the second year following treatment. There were no significant differences in the responses of soil chemistry, tree nutrition and growth between the 3% solids biosolids treatment and the dewatered/rewatered (with 3% biosolids) biosolids treatment. The spring biosolids application resulted in largest increases in soil N availability, foliar N concentration and growth in comparison with the summer and fall applications. Both 1000 and 1500 kg-N/ha biosolids treatments significantly increased soil N and P availability and foliar N concentration, but the former resulted in greater increases in tree growth than the latter; the 750 kg-N/ha treatment also increased tree growth, but to a lesser extent. The 225 kg-N/ha fertilizer treatment had a similar effect on soil N availability, tree N nutrition and growth, compared to the 750 kg-N/ha biosolids treatment, but it had little effect on total N, extractable P, and C/N ratio in the forest floor and mineral soil. The biosolids treatment increased soil water potential in the pot trial without seedlings, but there was no significant difference between biosolids treatments and control in the field trial with trees. Tree twig xylem water potential was significantly increased by the 1000 and 1500 kg-N/ha biosolids treatments during dry summertime, irrespective of biosolids application season. The secondary biosolids treatments significantly increased tree net photosynthetic rate, irrespective of stand age. Instantaneous water use efficiency and a measure of leaf area index were also increased in the biosolids-amended stands. The results in this study demonstrate that secondary biosolids fertilization significantly improved soil N and P availability, tree nutrition and growth, increased soil water storage, leaf area and photosynthetic efficiency, and reduced tree water stress, and that dewatered biosolids rewatered with 3% biosolids (1:3, v/v), spring, and 750-1000 kg-N/ha are considered to be the most appropriate biosolids type, application season and rate(s), respectively, in the study area.Forestry, Faculty ofGraduat