Effects Of Nitrogen Addition On Litter Basidiomycetes

Forests are increasingly affected by nitrogen inputs from air pollutants. Several studies have focused on the effects of nitrogen additions from fertilizer or actual or simulated air pollutants on litter decomposition in tropical (Hobbie and Vitousek, 2000) as well as temperate forests (Magill and Aber, 1988; Berg and Matzner, 1997; Carreiro et al, 2000; Schroter et al., 2003; Gallo et al, 2004; Waldrop et al., 2004; Chapter 10). High nitrogen concentrations can have paradoxical effects on litter decomposition rates, accelerating decay of labile components while inhibiting decay of recalcitrant highly lignified components in both temperate (Fog, 1988; Carreiro et al., 2000; Wardle, 2000) and tropical forests (Hobbie and Vitousek, 2000). Nitrogen additions often inhibit basidiomycete production or activity of ligninolytic enzymes during decomposition of refractory organic material (Kirk and Farrell, 1987; Cromack and Caldwell, 1992; Carreiro et al., 2000; Gallo et al., 2004; Waldrop et al., 2004). Shifts in the composition of a basidiomycete decomposer community in response to nitrogen additions may reflect reduced competitive advantage of fungi that produce hyphal cords and rhizo-morphs able to translocate nutrients into nutrient-depauperate food bases (Cromack and Caldwell, 1992; Boddy, 1993; Lodge, 1993). Cords and rhizo-morphs of basidiomycete decomposers disappeared from the litter layer of plots receiving complete fertilization (macro- and micronutrients) in Puerto Rico (Lodge, 1993). Such losses of cord and rhizomorph systems may accelerate losses of soil organic matter and nutrients on steep slopes (Lodge and Asbury, 1988; Lodge, 1993; Miller and Lodge, 1997).

The effects of nitrogen addition on basidiomycete litter decomposers were studied at two sites in the Luquillo Experimental Forest in Puerto Rico (D.J. Lodge et al., unpublished). The Bisley watershed (18°18'58"N, 65°44' 10"W) was a low-elevation (250-300 m asl) subtropical wet forest that received ^3,500 mm of rain per year. This was a late secondary forest in which the native forest was characterized by tabonuco trees (Dacryodes excelsa), but there was an abundance of nitrogen-fixing trees in the Fabaceae (Inga vera) that had been planted as overstorey for a coffee plantation. The Icacos site (18°16'32"N, 65°47'4"W) was a high-elevation (620 m asl) lower montane rainforest characterized by Palo Colorado trees (Cyrilla antillana) that received 4,000-4,500 mm of rain per year.

At each location, three pairs of plots (nitrogen addition and control) were matched for slope, dominant vegetation, elevation and watershed type. The nitrogen plots had received 50 kg N ha-1 year-1 since January 2002 (25 kgN ha-1 in two applications per year) as ammonium nitrate pellets (as in Hall and Matson, 1999; Magill et al., 2004). The control plots did not receive fertilizer. The location and maximum extent of three to five discrete basidiomycete litter mats was marked with flags in each of the plots at Icacos and Bisley before the first fertilization in January 2002, and again in June 2003. Mean fungal mat sizes increased over the 18-month study because they were originally marked during the dry season and remeasured during the rainy season. However, mat sizes increased more in control than in nitrogen treatment plots, though not quite significantly (one-sided test, P — 0.062), and were more marked for plots at higher elevation in Icacos than those at lower elevation in Bisley (Figure 3).

Changes in leaf attachment rates in response to nitrogen addition at Bisley and Icacos plots were determined by tethering numbered, freshly fallen leaves to marker flags with 1 m lengths of nylon line and placing them on basidiomycete mats (Table 1). The marker flags were placed downslope of the attached leaves, so the leaves were free to move. The leaves were checked at 8 days for the presence or absence of fungal attachments to the litter mat. At Icacos there was suggestion of correlation between nitrogen addition and decreased leaf attachment rate, after 8 days (P — 0.0625). Rate of leaf attachment decreased after fertilization but increased in control plots (Figure 4). In contrast, there was no relationship between nitrogen fertilization and leaf attachment at Bisley (Figure 4).

25000 20000 15000

" 10000 5000

Figure 3 Mean increases in decomposer basidiomycete litter mat size from the early dry season (January 2002, just prior to nitrogen additions to treatment plots) to the early wet season (July) 18 months later. Treatment plots received 25kgNha~1 each January and June in the form of ammonium nitrate pellets; paired control plots had no nitrogen additions. Forests were located at Bisley (250-300 m asl) and Icacos (620masl) in the Luquillo Mountains of Puerto Rico (three pairs of replicate plots at high and low elevations). Data on mean mycelial mat area for the five pairs of plots were pooled among sites. Pair-wise differences in mat area were analyzed using a one-sided Wilcoxon Signed Ranks Test (P = 0.0625; Hollander and Wolfe, 1973).

Table 1 Numbers of tethered, freshly fallen leaves of Dacryodes excelsa and Clusia krugiana placed on litter mats in paired nitrogen loading and control plots before and after nitrogen fertilization in June 2003

Plots

Bisley

Icacos

3 pairs, +N and control

3 pairs, +N and control

2 weeks before

10 D. excelsa leaves per

6 C. krugiana leaves per

fertilization

plot (1 per mat)

plot (1 per mat)

1 week after

20 D. excelsa leaves per

20 C. krugiana and 20 D.

fertilization

plot (1 per mat)

excelsa (2 each per mat)

Note: Rate of leaf attachment to the mats by basidiomycetes was determined at 8 days. Paired comparisons between treatments within forest type were made using Wilcoxon Signed Ranks tests (Hollander and Wolfe, 1973).

Note: Rate of leaf attachment to the mats by basidiomycetes was determined at 8 days. Paired comparisons between treatments within forest type were made using Wilcoxon Signed Ranks tests (Hollander and Wolfe, 1973).

The decreased growth and inhibition of new resource capture by white-rot basidiomycetes following nitrogen addition at Icacos fits with previous studies that showed nitrogen inhibition of ligninolytic enzyme production by white-rot fungi (Kirk and Farrell, 1987; Cromack and Caldwell, 1992; Carreiro et al, 2000; Gallo et al., 2004; Waldrop et al., 2004), which probably affects their competitive abilities (Carreiro et al., 2000). The lack of response to nitrogen addition in the lower elevation forest at Bisley may have been related to the natural abundance of nitrogen there. Lowland tropical forests are often saturated with nitrogen (Silver et al., 2005). The low-elevation forest at Bisley had more nitrogen cycling via fine litter than forests at higher elevation in the Luquillo Mountains (Lodge et al., 1991).

Figure 3 Mean increases in decomposer basidiomycete litter mat size from the early dry season (January 2002, just prior to nitrogen additions to treatment plots) to the early wet season (July) 18 months later. Treatment plots received 25kgNha~1 each January and June in the form of ammonium nitrate pellets; paired control plots had no nitrogen additions. Forests were located at Bisley (250-300 m asl) and Icacos (620masl) in the Luquillo Mountains of Puerto Rico (three pairs of replicate plots at high and low elevations). Data on mean mycelial mat area for the five pairs of plots were pooled among sites. Pair-wise differences in mat area were analyzed using a one-sided Wilcoxon Signed Ranks Test (P = 0.0625; Hollander and Wolfe, 1973).

Bisley Icacos Bisley Icacos

Before fertilization After fertilization

Figure 4 Percent of marked leaves attached to litter mats by decomposer basidiomycete fungi after 8 days in subtropical forest at low and high elevations (Bisley and Icacos, respectively) in the Luquillo Mountains of Puerto Rico, immediately before and after a nitrogen addition. Suppression of basidiomycete leaf attachment rates was suggestive in the high-elevation forest at Icacos (P = 0.0625) but not at low elevation in bisley.

Bisley Icacos Bisley Icacos

Figure 4 Percent of marked leaves attached to litter mats by decomposer basidiomycete fungi after 8 days in subtropical forest at low and high elevations (Bisley and Icacos, respectively) in the Luquillo Mountains of Puerto Rico, immediately before and after a nitrogen addition. Suppression of basidiomycete leaf attachment rates was suggestive in the high-elevation forest at Icacos (P = 0.0625) but not at low elevation in bisley.

Slope in Degrees

Figure 5 Mean frequency of decomposer basidiomycete litter mats moving up- or downslope as a function of degree of slope. Movements of mats over 18 months were recorded as +1 for upslope, —1 for downslope and 0 for no movement relative to slope axis. The slopes were determined using a level and plumb line. Data were pooled from two forests at low and high elevations in the Luquillo Mountains of Puerto Rico (Bisley and Icacos, respectively) and divided among five slope classes at 5° intervals: 0-5, >5-10, >10-15, >15-20 and >20-25°. The effect of slope on the mean frequency of mat movement was analyzed using regression analysis in excel (Version 9.0.3821, Microsoft Corp.).

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  • annett
    Is basidiomycetes a nitrogen fixer?
    7 years ago

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