Abstract
Forest fertilisation with nitrogen (N) typically increases N leaching for 1–2 years. Some studies have reported effects also after clearcutting. This study presents an analysis of soil-water chemistry data from the 3rd to the 15th year after clearcutting of fertilised experimental plots on a low-fertility site in Sweden. Before clearcutting in 1987, study plots had been fertilised with NH4NO3 in 1967, 1974, and 1981, resulting in total applications ranging from 0 to 1800 kg N·ha−1. In 1989, disc trenching was simulated by manual digging on small subplots within the fertilised main plots. Soil-water samples were collected at a depth of 50 cm. Previous N fertilisation and site preparation, respectively, affected (p < 0.05) the total N and NO3–-N concentrations and pH of soil water, but no statistical interaction between fertilisation and site preparation was found. The NO3–-N concentration was elevated for total N applications above 720 kg·ha−1 (mean NO3–-N concentration of 0.93 mg·L−1 for 1080 kg N·ha−1, 1.6 mg·L−1 for 1440 kg N·ha−1, and 2.4 mg·L−1 for 1800 kg N·ha−1 compared with 0.20 mg·L−1 for the control) and lower after simulated disc trenching (0.63 mg·L−1) than in nontrenched soil (1.3 mg·L−1). The elevations in the soil-water NO3–-N concentration for the fertiliser treatments seemed to be related to changes in the soil N store created by previous fertilisation.
Highlights
Forest fertilisation with N generally increases stem volume growth in boreal forests on mineral soil (Binkley and Högberg 2016)
The NO3--N concentration tended to increase with increasing fertiliser application rate from 0.2 mg l-1 for the control to 2.4 mg l-1 for the 1800N regime while pH decreased from 6.1 to 5.0 (Figs. 2-3)
This is based on the results from the soil study reported by Högbom et al (2001) and the fact that elevations in the NO3--N concentration were accompanied by reductions in pH (Figs. 2-3)
Summary
Forest fertilisation with N generally increases stem volume growth in boreal forests on mineral soil (Binkley and Högberg 2016). Lower C/N ratios in the organic layer have been detected after fertilisation (Hyvönen et al 2008). Fertiliser-induced changes in the soil N store could influence the environmental response to subsequent forestry operations like clearcutting and site preparation, as proposed by Tamm et al (1974). Clearcutting generally increases the soilwater NO3- concentration and export and previous fertilisation may intensify the increase (Tamm et al 1974; Gundersen et al 2006). After harvesting previously fertilised experimental sites, increases, decreases, and no detectable effects on the soil-water NO3-
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