Long-term effects of base cation fertilization on nutrient cycling and species composition of a sugar maple stand in southern Québec: application of the Rb/K reverse tracer

To improve the understanding of the long-term effect of forest potassium (K)-fertilization, I revived a 23-year-old base cation fertilization experiment in a sugar maple stand in southern Quebec (45°59.6'N 73°59.9'W). My main objectives were to determine (i) the long-term percent of K derived from fertilizer in the soil-plant system using the rubidium (Rb)/K reverse tracer method, (ii) if fertilization had altered the diversity of the understory plant community, and (iii) if there was still an effect on the growth and nutrient status of the sugar maple trees. In 1988, six 40 m × 40 m plots were delineated in a sugar maple stand. In June 1989, three of these plots received 500, 250, and 250 kg•ha-1 of K2SO4, CaMg(CO3)2, and CaCO3, respectively as fine particulate matter, and the other three were left as control. Mid-canopy leaves of sugar maple trees were sampled annually in August from 1988 to 1991 and 2011 to 2012. In 2012, understory species were surveyed using the Braun-Blanquet method and ten of the most abundant and ubiquitous species were sampled for leaf chemistry. Leaves of both understory and canopy sugar maple trees were digested for nutrient concentrations using trace metal grade HNO3. In 2012, two soil pits were dug in each plot and individual horizons were sampled. Soil was extracted with 1 M NH4Cl. K, Ca, Mg, and Rb concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) for both digested and extracted samples. Growth of sugar maple trees was assessed using tree cores taken in 2012. Fertilization increased sugar maple leaf K two years following fertilization, but the effect was no longer detectable in 2011/2012. Only Ca and Mg showed increased concentrations in the leaves the fall of the year of fertilization. The percent of leaf K derived from fertilizer in 2011/12 is nearly what it was in the fall of 1989, two months after fertilization. There was no significant difference between control and fertilized plots in the concentration of K in the sapwood of the sugar maple trees, but there was 27 ± 2 % of K derived from fertilizer in the wood in 2012. There was no difference in relative growth of sugar maple, or understory vegetation diversity or species richness in fertilized plots. Nutrient analysis of understory vegetation showed no differences in concentrations of leaf K between treatments, but percent K from fertilizers amounted to over 25 % in several species. There was no effect of treatment on soil chemistry but there was a large proportion of the original fertilizer recovered in the soil profile. The total amount of K from fertilizer held in the soil-plant system was 54 kg•ha-1 or 24 % of the original 225 kg•ha-1 of fertilizer K applied. Overall, these results suggest: (i) a short-term improvement of K status following fertilization, (ii) efficient recycling of K fertilizer in the soil-plant system of maple forests, and (iii) no obvious long-term change in understory species composition following forest fertilization.

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