(A)synchronous Availabilities of N and P Regulate the Activity and Structure of the Microbial Decomposer Community

Abstract

Nitrogen (N) and phosphorus (P) availability both control microbial decomposers and litter decomposition. However, these two key nutrients show distinct release patterns from decomposing litter and are unlikely available at the same time in most ecosystems. Little is known about how temporal differences in N and P availability affect decomposers and litter decomposition, which may be particularly critical for tropical rainforests growing on old and nutrient-impoverished soils. Here we used three chemically contrasted leaf litter substrates and cellulose paper as a widely accessible substrate containing no nutrients to test the effects of temporal differences in N and P availability in a microcosm experiment under fully controlled conditions. We measured substrate mass loss, microbial activity (by substrate induced respiration, SIR) as well as microbial community structure (using phospholipid fatty acids, PLFAs) in the litter and the underlying soil throughout the initial stages of decomposition. We generally found a stronger stimulation of substrate mass loss and microbial respiration, especially for cellulose, with simultaneous NP addition compared to a temporally separated N and P addition. However, litter types with a relatively high N to P availability responded more to initial P than N addition and vice versa. A third litter species showed no response to fertilization regardless of the sequence of addition, likely due to strong C limitation. Microbial community structure in the litter was strongly influenced by the fertilization sequence. In particular, the fungi to bacteria ratio increased following N addition alone, a shift that was reversed with complementary P addition. Opposite to the litter layer microorganisms, the soil microbial community structure was more strongly influenced by the identity of the decomposing substrate than by fertilization treatments, reinforcing the idea that C availability can strongly constrain decomposer communities. Collectively, our data support the idea that temporal differences in N and P availability are critical for the activity and the structure of microbial decomposer communities. The interplay of N, P, and substrate-specific C availability will strongly determine how nutrient pulses in the environment will affect microbial heterotrophs and the processes they drive.

Keywords: decomposition; microbial limitations; multiple element limitation; nutrient availability; nutritional constraints; successive fertilization; temporal variability.

FIGURE 1

Mass loss of the four different substrates (mean ± SD) as a function of sampling date. (A) cellulose, (B) Goupia glabra, (C) Simarouba amara, (D) Vochysia tomentosa. Fertilization treatments are shown with distinct symbols and curves (control: open triangle, dotted line; NP: black circles, solid line; Np: light gray square, solid line; Pn: dark gray diamond, solid line). Microcosms received the first dose of fertilizer (N, P, or NP) at d₀, and the second dose of fertilizers (i.e., P, N, or NP respectively) just after d₃₆ sampling. Stars indicate significant differences in the mass loss for each date considered separately, followed by a post hoc Tukey-HSD test (α = 0.05), with different letters indicating contrasted effects among the different fertilization treatments.

FIGURE 2

Litter SIR rates of the four different substrates (mean ± SD) as a function of sampling date. (A) cellulose, (B) Goupia glabra, (C) Simarouba amara, (D) Vochysia tomentosa. Fertilization treatments are shown with distinct symbols and curves (control: open triangle, dotted line; NP: black circles, solid line; Np: light gray square, solid line; Pn: dark gray diamond, solid line). Stars indicate significant differences in the mass loss for each date considered separately, followed by a post hoc Tukey-HSD test (α = 0.05), with different letters indicating contrasted effects among the different fertilization treatments.

FIGURE 3

Soil SIR rates of the four different substrates (mean ± SD) as a function of sampling date. (A) cellulose, (B) Goupia glabra, (C) Simarouba amara, (D) Vochysia tomentosa. Fertilization treatments are shown with distinct symbols and curves (control: open triangle, dotted line; NP: black circles, solid line; Np: light gray square, solid line; Pn: dark gray diamond, solid line). Stars indicate significant differences in the mass loss for each date considered separately, followed by a post hoc Tukey-HSD test (α = 0.05), with different letters indicating contrasted effects among the different fertilization treatments.

FIGURE 4

Microbial community structure in leaf litter and cellulose: (A) NMDS ordination of microbial community composition by substrate identity, (B) fungi:bacteria ratio across all substrate types (except cellulose) as a function of the different fertilization treatments at d₃₆ and d₇₄. NMDS was based on Bray–Curtis distances of 23 PLFA markers. With increasing distance between two points the community structure becomes more dissimilar. Vectors represent the proportion of the main microbial groups (bacteria, fungi) and their ratio (fungi:bacteria), pointing in the direction of higher abundances or a higher fungi:bacteria ratio.

FIGURE 5

Microbial community structure in the soil: (A) NMDS ordination of microbial community composition by substrate, (B) fungi:bacteria ratio across all substrate types (except cellulose) as a function of the different treatments at d₃₆ and d₇₄. See Figure 4 for further details.