Rapid response of moss-associated nitrogen fixation to nutrient additions in tropical montane cloud forests with different successional stages

Abstract

Nitrogen (N) fixation in association with mosses could be a key source of new N in tropical montane cloud forests since these forests maintain high humidity levels and stable temperatures, both of which are important to N fixation. Here, nutrient availability could be a prominent control of N fixation processes. However, the mechanisms and extent of these controls, particularly in forests at different successional stages, remains unknown to date. To address this knowledge gap, we investigated the impact of N, phosphorus (P) and molybdenum (Mo) additions on moss-associated N fixation in tropical montane cloud forests of two successional stages, an old-growth forest and an early-successional natural regrowth forest. We hypothesized that if N is available, N fixation rates would be rapidly reduced, while P and Mo would promote nitrogenase activity. Our results show that Mo additions did not affect N fixation rates, whereas N and P additions, in different doses and combinations, immediately reduced N fixation in both forests. Nonetheless, rates recovered within 1 year of nutrient additions. Nitrogen fixation rates associated with ground-covering mosses were similar in both forests. Interestingly, one year after the nutrient additions, N fixation rates across all the treatments were higher in the natural regrowth forests than the mature forests, suggesting more nutrient limitation in these regrowing forests, likely as a result of higher demand for growth. Our study highlights how moss-associated N fixation responds to changes in nutrient availability across distinct successional stages, deepening our understanding of processes that contributes to tropical montane cloud forests.

Supplementary information: The online version contains supplementary material available at 10.1007/s10533-024-01195-3.

Keywords: Forest succession; Molybdenum; Mosses; Nitrogen fixation; Phosphorus; Tropics.

Fig. 1

Mean ethylene production (nmol g dw⁻¹ h⁻¹, ± SE) associated with mosses from the control treatment across two different forest succession stages: natural regrowth (NR, n = 12) and old growth (OG, n = 12) forests. Different coloured bars represent the different sampling times for each forest type (dark green, 1D: 1 day after nutrient addition; light green, 1Y: 1 year after nutrient addition; and olive green, 1D₂: 1 day after second nutrient addition). Sampling time is shown individually. No statistical differences were seen across time or forests, tested individually or combined

Fig. 2

Mean ethylene production (nmol g dw⁻¹ h⁻¹, ± SE; n = 4 per treatment) associated with mosses in response to nutrient additions in two tropical montane cloud forests in Costa Rica in different successional stages (NR natural regrowth and OG old growth). Treatments are: control (CT), molybdenum (Mo), nitrogen (N), N and phosphorus (NP), and two levels of phosphorus (P_high and P_low) additions. Capital letters next to the forest type represent the different sampling time for each forest type (1D: 1 day after nutrient addition, 1Y: 1 year after nutrient addition; and 1D₂: 1 day after second nutrient addition). Different lowercase letters on top of error bars (SE) indicate statistically significant differences between treatments (Tukey HSD test)

Fig. 3

Response ratios (lnRR) relative to the control treatments of N fixation rates associated with mosses in response to nutrient additions. A positive lnRR (lnRR > 0) indicates promotion of N fixation rates and a negative lnRR (lnRR < 0) indicates a negative effect. Treatments are: control (CT), molybdenum (Mo), nitrogen (N), N and phosphorus (NP), and phosphorus in two levels (P_high and P_low). Two tropical montane cloud forests in Costa Rica under different succession stages are shown (NR: natural regrowth and OG: old growth). Capital letters represent the different sampling times for each forest type (A: 1 day after nutrients addition; B: 1 year after nutrients addition; and C: 1 day after second nutrients addition). Different small letters on top of error bars (SE) show statistically significant differences between treatments (Tukey HSD test). n = 4 per treatment