Variations in Belowground Carbon Storage and Soil CO2 Flux Rates along a Wet Tropical Climate Gradient

McGroddy, Megan; Silver, Whendee L. 2000. Variations in Belowground Carbon Storage and Soil CO2 Flux Rates along a Wet Tropical Climate Gradient. BIOTROPICA 32(4a): 614-624 .

We used a humid tropical elevation gradient to examine the relationships among climate, edaphic conditions, belowground carbon storage, and soil respiration rates. We also compared open and closed canopy sites to increase the range of microclimate conditions sampled along the gradient, and determine the effects of canopy openings on C and P storage, and C dynamics. Total soil C, the light C fraction, and all of the component fractions of the P pool were significantly related to soil moisture, and all but total soil C were also significantly related to temperature. Both labile and recalcitrant soil P fractions were negatively correlated with the light C fraction, while the dilute HCl-extractable P pool, generally thought of as intermediate in availability, was positively correlated with light C, suggesting that P may play an important role in C cycling within these systems. Total fine root biomass was greatest at 1000 m elevation and lowest at 150 m, and was strongly and positively correlated with soil moisture content. Soil respiration rates were significantly and negatively correlated with fine root biomass and the light C fraction. In forested sites, soil respiration rates were strongly and negatively correlated with total belowground C pools (soils 1 roots 1 forest floor). Belowground C pools did not follow the expected increasing trend with decreases in temperature along the gradient. Our results indicated that in humid tropical forests, the relationships among soil C and nutrient pools, soil respiration rates, and climate are complex. We suggest that frequent and prolonged anaerobic events could be important features of these environments that may explain the observed trends.


Exotic Earthworms Accelerate Plant Litter Decomposition in a Puerto Rican Pasture and a Wet Forest
Z. G. Liu and X. M. Zou
Ecological Applications
Vol. 12, No. 5 (Oct., 2002), pp. 1406-1417

Tropical land-use changes can have profound influence on earthworms that play important roles in regulating soil processes. Converting tropical forests to pastures often drastically increases the abundance of exotic earthworm populations such as Pontoscolex corethrurus. We initiated this study to examine the influence of exotic earthworms on the decomposition of plant leaves and roots in a tropical pasture and a wet forest of Puerto Rico. We employed two treatments: control with natural earthworm population, and earthworm reduction using an electroshocking technique. Decomposition rates of plant leaves on the ground surface and root materials within the surface mineral soil were estimated using a litterbag technique. To understand the role that exotic earthworms play in altering plant litter decomposition, we also compared soil CO2 evolution rates, soil microbial biomass, and physical and chemical soil properties between the controls and earthwormreduced plots during a one-year period. Earthworm populations in the electroshocked enclosures were reduced by 85% and 87% as compared with pasture and forest controls by the end of the experiment. Earthworm reduction significantly decreased the annual decay rates of plant leaves but had no effects on those of plant roots in both pasture and forest sites. Although the control plots had less mass remaining on every litterbag collecting date, significant treatment effects on leaf decomposition occurred only after 240 d in both sites. The decay rates were greater when organic materials had low carbon to nitrogen or phosphorus ratios. Soil respiration was also decreased in the earthworm-reduced plots. In contrast, soil microbial biomass C was not affected by earthworm reduction. Furthermore, there were no significant differences between the two treatments in soil bulk density, moisture content, pH, or temperature at either site. Our results suggest that exotic earthworms may accelerate leaf litter decomposition by elevating rates of litter consumption/digestion or microbial activity, rather than by improving soil physical/chemical conditions or altering microbial biomass.

Effects of carbon additions on iron reduction and phosphorus availability in a humid tropical forest soil

Liptzin, D., and Silver,W.L. (2009) Effects of carbon additions
on iron reduction and phosphorus availability in a humid
tropical forest soil. Soil Biol Biochem 41: 1696–1702.

chemical cycling through its interactions with carbon (C) and phosphorus (P).We used a laboratory study to explore the role of C quantity and quality in Fe reduction and associated P mobilization in tropical forest soils. Soils were incubated under an ambient atmosphere headspace (room air) with multiple levels of leaf litter leachate or acetate additions. Net Fe reduction occurred in all the treatments and at every time point. The more complex mixture of organic compounds in leaf litter leachate stimulated Fe reduction as much acetate, an easily fermentable C source. At the end of the experiment, Fe reduction was generally greater with higher C additions than in the low C additions and controls. The microbial biomass P had increased significantly suggesting rapid microbial uptake of P liberated from Fe. This occurred without increases in the available (NaHCO3) P pool. The immobilization of P by microbes during the incubation provides a P conservation mechanism in these soils with fluctuating redox potential, and may ultimately stimulate more C cycling in these highly productive ecosystems. Iron cycling appears to be an important source of P for the biota and can contribute significantly to C oxidation in upland tropical forest soils.

Comparing soil organic carbon dynamics in plantation and secondary forest in wet tropics in Puerto Rico

YIQING, LI; XU, MING; ZOU XIAOMING; SHI§, PEIJUN; ZHANG, YAOQI 2005. Comparing soil organic carbon dynamics in plantation and secondary forest in wet tropics in Puerto Rico. Global Change Biology 11,: 239–248, doi: 10.1111/j.1365-2486.2005.00896.x.

We compared the soil carbon dynamics between a pine plantation and a secondary forest, both of which originated from the same farmland abandoned in 1976 with the same cropping history and soil conditions, in the wet tropics in Puerto Rico from July 1996 to June 1997. We found that the secondary forest accumulated the heavy-fraction organic carbon (HF-OC) measured by the density fractionation technique, more efficiently than the tree plantation did. Although there was no significant difference in total soil organic carbon (SOC) between the plantation (5.59  0.09 kgm2) and the secondary forest (5.68  0.16 kgm2), the proportion of HF-OC carbon to the total SOC was significantly higher in the secondary forest (61%) than in the plantation (45%) (Po0.05). Forest floor mass and aboveground litterfall in the plantation were 168% and 22.8% greater than those in the secondary forest, respectively, while the decomposition rate of leaf litter in the plantation was 23.3% lower than that in the secondary forest. The annual mean soil respirations in the plantation and the secondary forest were 2.32  0.15 and 2.65  0.18 gCm2 day1, respectively, with a consistently higher rate in the secondary forest than in the plantation throughout the year. Microbial biomass measured by fumigation–incubation method demonstrated a strong seasonal variation in the secondary forest with 804mgkg1 in the wet season and 460mgkg1 in the dry season. However, the seasonal change of microbial biomass in the plantation was less significant. Our results suggested that secondary forests could stock more long-term SOC than the plantations in the wet tropics because the naturally generated secondary forest accumulated more HF-OC than the managed plantation.

biogeochemistry of dissolved organic carbon entering streams

Kaplan, L. A., and J. D. Newbold, Biogeochemistry of dissolved organic
carbon entering streams, in Aquatic Microbiology: An Ecological Approach,
edited by T. E. Ford, pp. 139 – 165, Blackwell Sci., Malden,
Mass., 1993.

Patterns of Dissolved Organic Carbon in Transport

Patterns of Dissolved Organic Carbon in Transport
Louis A. Kaplan, Richard A. Larson and Thomas L. Bott
Limnology and Oceanography
Vol. 25, No. 6 (Nov., 1980), pp. 1034-1043

Two distinct patterns of dissolved organic carbon (DOC) in transport were observed in a southeastern Pennsylvania piedmont drainage basin under low flow conditions. In relatively undisturbed woodland spring seeps, DOC concentrations increased with distance from the groundwater sources as did the apparent percentage of high molecular weight DOC. Changes in pH, color, and phenolic-C paralleled those for total DOC in a seep, while the concentration of carbohydrate-C remained relatively constant. In headwater areas perturbed by humans, cattle, or waterfowl, elevated DOC concentrations decreased rapidly from point source inputs.

Diel Fluctuations of DOC Generated by Algae in a Piedmont Stream

Diel Fluctuations of DOC Generated by Algae in a Piedmont Stream
Louis A. Kaplan and Thomas L. Bott
Limnology and Oceanography
Vol. 27, No. 6 (Nov., 1982), pp. 1091-1100

Diel fluctuations in dissolved organic carbon (DOC) were measured in White Clay Creek, a piedmont stream in SE Pennsylvania. DOC concentrations, measured on 16 days from late March to early June and on 2 days in early November, 1978 and 1979, showed rapid increases from predawn minima to late afternoon maxima and gradual decreases after sunset. Concentrations increased during a single day by as much as 40% of the daily minimum. These measurements were made during periods of constant discharge and were not related to volume of flow or changes in groundwater DOC concentrations. Laboratory and microcosm experiments suggested that benthic algae excreted most of the DOM and that bacterial uptake modified its concentration and composition. When the diel pulse was strongest, net algal DOC excretion accounted for 20% of the total DOC exported from the watershed that day. Additional data show that the 14C method of measuring algal excretion is quantitatively and qualitatively inaccurate for periphyton incubated for 2-4 h.

The Role of Rapid Flow Paths for Nitrogen Transformation in a Forest Soil: A Field Study with Micro Suction Cups

Hagedorn F, Mohn J, Schleppi P, Flu¨ hler H (1999) The
role of rapid flow paths for nitrogen transformation in
a forest soil: A field study with micro suction cups.
Soil Sci Soc Am J 63:1915–1923

Preferential flow is a common phenomenon in soils. This study was conducted to investigate the significance of rapid flow paths for N transformation in a forested Humaquept in central Switzerland. Fifty micro suction cups, each with a surface area of 12 mm2, were installed in a regular grid in the uppermost 5 cm. First, the location of each "microcup" relative to main flow paths was estimated based on the response to applications of a dye, SO2-4, and Cl-. Then, a N-addition experiment was carried out to study the N transformation at locations along flow paths and within the soil matrix. Only 23 of 50 microcups responded to the application of the dye within the first 24 h, which indicates that a large portion of the soil volume is not in contact with the infiltrating rainwater. Those microcups which responded to the added dye were regarded to be located along flow paths. At depths below 2 cm, under temporarily reducing conditions, sampling locations in or near flow paths had higher NO-3 concentrations (20-25 microM) than those of the soil matrix (below 12 microM). Within 24 h after a simulated rainfall, the IMGf1.gif" BORDER="0"> ratio decreased more in the flow paths (between -2.4 and -4.9 mol mol-1) than in the soil matrix (-0.7 to -0.8 mol mol-1), which indicates an enhanced denitrification at these locations. In the subsequent dry period, nitrification started 2 d earlier and was more pronounced along flow paths. The results of this study suggest that flow paths are microhabitats with an increased N transformation compared with the soil matrix.

Variability of DOC and nitrate responses to storms in a small Mediterranean forested catchment

Bernal, S., A. Butturini, and F. Sabater (2002), Variability of DOC and
nitrate responses to storms in a small Mediterranean forested catchment,
Hydrol. Earth Syst. Sci., 6, 1031– 1041.

Severe drought periods followed by intense rainfall often leads to major floods in Mediterranean catchments. The resulting hydrology is complex and the response of solutes in the streams is often unpredictable. This study aimed to identify the most relevant factors controlling the hydrological responses to storms of an intermittent Mediterranean stream and to link those factors with dissolved organic carbon (DOC) and nitrate during storm events. Measurements of climate, hydrology, DOC and nitrate concentrations during 26 storm events over three hydrological years were analysed. The contribution of the storm events to the total DOC and nitrate annual export was also calculated. Nitrate was mainly mobilised during high flow, while most of the DOC export occurred during baseflow. Solute concentrations peaked after drought periods and the solute export was maximal during the largest rainfalls (i.e.>100 L m-2). One single large storm contributed some 22% of the total annual export of DOC, and about 80% of that of nitrate. Discharge was a good predictor of neither DOC nor nitrate responses, so variables other than discharge were considered. Factor Analysis was used to identify the main factors controlling the biogeochemical responses. Antecedent moisture conditions and the magnitude of the storm event were the most relevant factors and accounted for 63% of the total variance. Solute responses during high flow were highly variable. However, solute concentration changes showed a significant and moderate relationship with the factors controlling the hydrological responses (i.e. Δ DOC v. the antecedent moisture conditions and Δ NO3-N v. the magnitude of the storm event).

Hydrologic flowpaths influence inorganic and organic nutrient leaching in a forest soil

Asano, Y., Compton, J. E. & Church, R. M. Hydrologic flowpaths influence
inorganic and organic nutrient leaching in a forest soil. Biogeochem. 81,
191-204 (2006).

Hydrologic pathways through soil affect element leaching by determining the relative importance of biogeochemical processes such as sorption and decomposition. We used stable hydrogen isotopes of water (dD) to examine the influence of flowpaths on soil solution chemistry in a mature spruce–hemlock forest in coastal Oregon, USA. Soil solutions (50 cm depth, n = 13) were collected monthly for 1 year and analyzed for dD, major ions and dissolved organic carbon (DOC) and nitrogen (DON). We propose that the variability of dD can be used as an index of flowpath length and contact time. Throughfall variability in dD was much greater than soil solution variability, illustrating that soil solution integrates the variation in inputs. Lysimeters with greater variation in dD presumably have a greater proportion of flow through rapid flowpaths such as macropores. The variation in soil solution dD for individual lysimeters explained up to 53% of the variation in soil solution chemistry, and suggests that flowpaths influence leaching of some constituents. Soil solutions from lysimeters with greater dD variation had higher DOC and DON (r2 = 0.51 and 0.37, respectively), perhaps because transport via macropores reduces interaction of DOM with the soil matrix. In contrast, nitrate concentrations were highest in lysimeters with a small variation in dD, where long contact time and low DOC concentrations may yield higher net nitrification. Our results demonstrate the utility of stable isotopes to link flowpaths and soil solution chemistry, and illustrate how the spatial complexity of soils can influence ecosystem- level nutrient losses.
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