Tree species controls over nitrogen and phosphorus cycling in a wet tropical forest
Data files
Sep 11, 2024 version files 435.34 KB
Abstract
Wet tropical forests play an important role in the global carbon (C) cycle, but given current rates of land-use change, nitrogen (N) and phosphorus (P) limitation could reduce productivity in regenerating forests in this biome. Whereas the strong controls of climate and parent material over forest recovery are well known, the influence of vegetation can be difficult to determine. We addressed species-specific differences in plant traits and their relationships to ecosystem properties and processes, relevant to N and P supply to regenerating vegetation in experimental plantations in a single site in lowland wet forest in Costa Rica. Single tree species were planted in a randomized block design, such that climate, soil (Oxisol), and land-use history were similar for all species. In Years 15-25 of the experiment, we measured traits regarding N and P acquisition and use in four native, broad-leaved, evergreen tree species, including differential effects on soil pH, in conjunction with biomass and soil stocks and fluxes of N and P. Carbon biomass stocks increased significantly with increasing soil pH (P = 0.0184, previously reported) as did biomass P stocks (P = 0.0011). Despite large soil N pools, biomass P stocks were weakly dependent on traits associated with N acquisition and use (N2 fixation and leaf C:N, P < 0.09). Mass-balance budgets indicated that soil organic matter (SOM) could supply the N and P accumulated in biomass via the process of SOM mineralization. Secondary soil P pools were weakly correlated with biomass C and P stocks (r = 0.47, P = 0.08), and were large enough to have supplied sufficient P in these rapidly growing plantations, suggesting that alteration of soil pH provided a mechanism for liberation of soil P occluded in organo-mineral soil complexes, and thus supply P for plant uptake. These results highlight the importance of considering species’ effect on soil pH for restoration projects in highly weathered soils. This study demonstrates mechanisms by which individual species can alter P availability, and thus productivity and C cycling in regenerating humid tropical forests, and the importance of including traits into global models of element cycling.
README: Tree species controls over nitrogen and phosphorus cycling in a wet tropical forest
https://doi.org/10.5061/dryad.cjsxksndg
Give a brief summary of dataset contents, contextualized in experimental procedures and results.
Description of the data and file structure
This data set is organized as follows. The first sheet in this Excel file is labeled ‘Metadata’ and this sheet contains information about the data. The Metadata sheet is organized by these columns: Study name; General information; Components; Abbreviations; Description; Frequency/Dates of measurements; Sheet name within this file; and Notes. Each remaining sheet in this Excel file is labeled as per the Metadata column ‘Sheet name within this file’. The sheets include:
- Biomass C,N,P conc
- Biomass CN, CP, NP
- C Budget
- N Budget
- P Budget
- Fine-root biomass
- Surface litter mass
- Soil bulk density
- Soil C
- Soil N
- Soil P
- Litterfall OM
- Litterfall C
- Litterfall N
- Litterfall P
- Branchfall
- Fine-root growth
- Soil respiration
- N fixation
- Trait Ordination Data
- Change in soil CNP
The ‘C budget’, ‘N budget’ and ‘P budget’ sheets incorporate many of the other data sheets. Abbreviations are defined in the ‘Metadata’ sheet, but generally repeated within the other sheets. Across all sheets, missing data are denoted by a period (.).
Sharing/Access information
Data on soil pH from this study are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.07k4p
The rDNA sequence file is available from the NCBI Sequence Read Archive (accession number SRX766152).
Code/Software
Data were centered and scaled data prior to principle components analysis (vegan::rda; v.2.6-4, Oksanen et al. 2022) in R (v.4.2.2; R Core Team 2022). SAS Programs used included: the MANOVA, MIXED, and REG procedures. For comparisons including the Mature Forest, we used the Satterthwaite adjustment for degrees of freedom. Pairwise comparisons for significant overall F-tests were conducted using P-values adjusted by Tukey’s Honestly Significant Difference method.
Methods
How was the data set collected?
We addressed the relationships between tree species and accrual of C, N, and P, using a long-term, replicated field experiment containing four native tree species grown in plantations under similar climate, parent material and land-use history in a tropical wet forest in Costa Rica. Our approach was to identify mechanisms that had two main criteria: (1) the potential to influence availability of N and P for plants, and thus productivity (as indicated by their biomass accumulation) and (2) a direct connection to traits that could differ among species in their attributes, and thus drive species-specific differences in ecosystem properties. We used various approaches to evaluate several mechanisms in relation to specific traits, while also considering that plant species differ in a plethora of traits that could interact to result in a non-significant correlation between a particular mechanism and trait.
Field measurements
The field experiment was established on a 12-ha site that had been mature wet tropical forest until deforestation in 1955, planted to pasture, and grazed until abandonment in 1987. In 1988, a stratified randomized complete block experiment was established with individual species planted within each plot. The design contained twelve 0.25-ha (50 m × 50 m) plots in each of four blocks. Each plot was divided into four quadrants to facilitate random sampling within plots. Eleven of the 12 plots within a block were planted with a single tree species, with a tree spacing of 3 × 3 m. The 12th plot was left unplanted as a Control, with those plots receiving the same management, except that no trees were planted. We established a fifth block (150 x 200 m in size) in Mature Forest, <150 m from the experimental site, such that soils, climate, and historical vegetation were similar to the adjacent experiment. By the time of this study, only four of the 11 tree species planted had survived with complete canopy coverage across at least 3 of the 4 blocks (four-letter codes in parentheses): Hieronyma alchorneoides Allemao (Hial); Pentaclethra macroloba (Willd.) Ktze, (Pema); Virola koschnyi Warb., (Viko); and Vochysia guatemalensis Donn. Sm. (Vogu).
To evaluate differences among species related to N and P acquisition, use, and storage, we measured C, N, and P concentrations and stocks of these compartments: plant biomass (above- and belowground), surface litter, and mineral soil (five depth intervals, 0-100 cm). Components of the planted (overstory) biomass included: canopy (leaves, flowers and fruits); twigs (diameters <1 cm); branches (diameters 1-10 cm); large branches and boles (diameters > 10 cm); stumps; stump-roots; woody roots (>2 mm diameter); and fine roots (≤ 2 mm diameter). Components for the regenerating (unplanted, understory) vegetation included the same components. See ‘Metadata’ sheet for full details.
We measured above- and belowground fluxes of organic matter, C, N, and P of the planted species and the vegetation regenerating on its own, referred to as the ‘overstory’ and ‘understory’ respectively. Components measured included: fine litterfall; branchfall; aboveground biomass increments (Δ biomass) of the overstory and understory, coarse-root and stump root biomass increments and fine-root growth (overstory + understory). We assayed asymbiotic N2 fixation in surface soil (0-5 cm) and surface litter. Turnover time was calculated as the quotient of biomass and NPP within each component, expressed in years. Soil respiration was measured in-situ approximately monthly for two years (Nov 2011-Oct 2013) in 3-4 quadrats per plot. Sampling
Laboratory methods
Organic C and total N for both plants and soil were analyzed by dry combustion (DC) using a Thermo-Finnigan EA Flash (Series 1112, EA Elantech, Lakewood, NJ). Duplicates were run on all samples. Plant tissue P was measured by microwave-assisted acid digestion and analyzed using inductively coupled plasma optical emission spectroscopy (Kingston and Haswell 1997). In total, 2704 plant samples were analyzed for C, N, and P.
We used the acetylene reduction assay to estimate nitrogenase activity and thus evaluate asymbiotic N2 fixation. Gas analysis was conducted on a Tracor 540 gas chromatograph (GC) fitted with a carbosphere column (Alltech, 5' × 1/8" SS COL P/W ALUMINA F-1 60/80 1 EA, 5664PC). The oven and FID temperatures were 100 °C and 175 °C respectively. Regarding the incubation period, analysis of ethylene results gave no evidence of substrate depletion within 24 h, nor inhibition of activity over 7 d. Nitrogenase activity was low enough that a 7-d period was needed to detect activity; thus, data presented are for the 7-d incubation period. The C2H2 reduced by the sample over the incubation period was calculated as: ΔC2H4 (μmol C2H4 L-1 g-1 hr-1) × headspace (ml/1000) / incubation period (hrs) / sample mass (g dry wt). Potential N2 fixation was calculated using a molar ratio of 3:1 ethylene produced per N fixed.
For soil, total P was extracted in HCl:HNO3 (4:1 by volume) (Crosland et al. 1995) and P in the digests was measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). For measurement of total organic P, we used the ignition method (Kuo 1996), in which organic P in the soil was converted to inorganic P by oxidation at 550° C. Phosphorus in ignited and unignited subsamples was then extracted with 0.5N H2SO4 for 16 h. ICP-AES was used to measure P in the extracts. Organic P in the soil was calculated by subtracting P concentration in the unignited sample from P concentration in the ignited sample. Modified Olsen-extractable P was determined by extraction with NaHCO3 and ethylenediaminetetraacetic acid (Kuo, 1996). Sequential P extractions were conducted to target secondary minerals following Chadwick et al. (1994) to quantify the P associated with these phases, and to determine whether they were altered by the experimental tree species. Following an initial extraction of labile nutrients with unbuffered 1 M ammonium acetate, we extracted soils with acid ammonium oxalate, which dissolves SRO Fe and Al phases, and then with citrate-dithionite, which dissolves most crystalline Fe oxyhydroxides (as well as substituted or co-precipitated Al). These extractions have long been used for assessing secondary mineralogy of highly weathered soils (Chadwick et al. 1994, Torn et al. 1997). The sum of Poxalate and Pdithionite quantified in this procedure is hereafter referred to as ‘secondary soil P.’ It is important to note that the ‘organic’ and ‘secondary’ pools partially overlap because a portion of the organic P that is liberated by combustion will also likely be solubilized by the oxalate or dithionite extraction; that is, secondary mineral-bound P includes organic P that is bound with these
How has the dataset been processed?
Field and laboratory data were either immediately entered into Excel spreadsheets from hand-written field data sheets, or from data-capturing apparati (e.g., LICOR 8100). Datasets were then formatted for analyses in SAS and R. All data sets were regularly backed up on storage devices.
To confirm species variation in traits and better understand the relationships among traits, we summarized and visualized these multivariate traits with PCA. We used 40 traits for which we had data for every plot in the experiment. Data were centered and scaled data prior to principle components analysis. We also ran permanova as a mixed model with species, block, and their interaction using Type III sums of squares with residual randomization permutation procedures.
For factorial analyses, the experimental unit was the plot. For variables with multiple measurements per plot, the mean was used in the analysis. The ‘treatment’ consisted of the vegetation type; for most variables, this consisted of the four planted tree species, but for some it included the Mature Forest and/or the Control. For variables in which measurements included only the four planted species, the design was a randomized complete block (n = 15, with 4 species × 4 blocks, minus one Vochysia plot). For analyses including the Mature Forest, the design was an incomplete block design (n = 23 if it also included the Control). Blocks were considered a random effect. We tested for homogeneity of variances and normality of distributions.
ANOVA was used to evaluate differences among vegetation types in element (C, N, and P) concentrations, stoichiometries, and stocks in vegetation (by component), surface litter (by component), soil (5 depth intervals, including a vegetation type × depth interaction term), N2 fixation (by substrate, including a vegetation × substrate interaction. We first conducted a MANOVA to determine whether all elements could be evaluated in one test for a given response variable. In tests for all variables, the interactions among elements were significant, thus individual ANOVAs were done for each element. To assess redistribution of N and P within the soil profile, we compared changes in N and P stocks within the rooting zone (0-30 cm) and below (30-100 cm) over the 8-y period of the project. Procedures were conducted in SAS (Littell et al. 1996).
Multiple regression models were used to evaluate the relative effects of explanatory variables, i.e., species traits that could influence N and P acquisition and use, on the response variables of stocks of biomass C, N, and P.