soil carbon

Greater Soil Carbon Sequestration under Nitrogen-Fixing Trees Compared with Eucalyptus Species

Resh, SC, D. Binkley, and JA Parrotta. 2002. Greater soil carbon sequestration under nitrogen-fixing trees compared with eucalyptus species RID A-2703-2010. Ecosystems 5 (3) (APR): 217-31.

Forests with nitrogen-fixing trees (N-fixers) typically accumulate more carbon (C) in soils than similar forests without N-fixing trees. This difference may develop from fundamentally different processes, with either greater accumulation of recently fixed C or reduced decomposition of older soil C. We compared the soil C pools under N-fixers with Eucalyptus (non-N-fixers) at four tropical sites: two sites on Andisol soils in Hawaii and two sites on Vertisol and Entisol soils in Puerto Rico. Using stable carbon isotope techniques, we tracked the loss of the old soil organic C from the previous C4 land use (SOC4) and the gain of new soil organic C from the C3, N-fixer, and non-N-fixer plantations (SOC3). Soils beneath N-fixing trees sequestered 0.11 + 0.07 kg m-2 y-' (mean ± one standard error) of total soil organic carbon (SOCT) compared with no change under Eucalyptus( 0.00 ± 0.07 kg m-2 y-1; P = 0.02). About 55% of the greater SOCT sequestration under the N-fixers resulted from greater retention of old SOC4, and 45% resulted from greater accretion of new SOC3. Soil N accretion under the N-fixers explained 62% of the variability of the greater retention of old SOC4 under the N-fixers. The greater retention of older soil C under N-fixing trees is a novel finding and may be important for strategies that use reforestation or afforestation to offset C emissions.

Atypical soil carbon distribution across a tropical steepland forest catena.

Johnson K.D., Scatena F.N., Silver W.L. Atypical soil carbon distribution across a tropical steepland forest catena. CATENA, In Press, Corrected Proof, Available online 4 August 2011, ISSN 0341-8162, DOI: 10.1016/j.catena.2011.07.008. (

Soil organic carbon (SOC) in a humid subtropical forest in Puerto Rico is higher at ridge locations compared to valleys, and therefore opposite to what is commonly observed in other forested hillslope catenas. To better understand the spatial distribution of SOC in this system, plots previously characterized by topographic position, vegetation type and stand age were related to soil depth and SOC. Additional factors were also investigated, including topographically-related differences in litter dynamics and soil chemistry. To investigate the influence of litter dynamics, the Century soil organic model was parameterized to simulate the effect of substituting valley species for ridge species. Soil chemical controls on C concentrations were investigated with multiple linear regression models using iron, aluminum and clay variables. Deeper soils were associated with indicators of higher landscape stability (older tabonuco stands established on ridges and slopes), while shallower soils persisted in more disturbed areas (younger non-tabonuco stands in valleys and on slopes). Soil depth alone accounted for 77% of the observed difference in the mean 0 to 60 cmSOC between ridge soils (deeper) and valley soils (shallower). The remaining differences in SOC were due to additional factors that lowered C concentrations at valley locations in the 0 to 10 cm pool. Model simulations showed a slight decrease in SOC when lower litter C:N was substituted for higher litter C:N, but the effects of different woody inputs on SOC were unclear. Multiple linear regression models with ammonium oxalate extractable iron and aluminum, dithionite–citrate-extractable iron and aluminum, and clay contents explained as much as 74% of the variation in C concentrations, and indicated that organo-mineral complexation may be more limited in poorly developed valley soils. Thus, topography both directly and indirectly affects SOC pools through a variety of inter-related processes that are often not quantified or captured in terrestrial carbon models.

Effects of nutrient additions on ecosystem carbon cycle in a Puerto Rican tropical wet forest

LI, YIQING; XU, MING; ZOU, XIAOMING 2006. Effects of nutrient additions on ecosystem carbon cycle in a Puerto Rican tropical wet forest. Global Change Biology 11, :1-10,.

Wet tropical forests play a critical role in global ecosystem carbon (C) cycle, but C allocation and the response of different C pools to nutrient addition in these forests remain poorly understood. We measured soil organic carbon (SOC), litterfall, root biomass, microbial biomass and soil physical and chemical properties in a wet tropical forest from May 1996 to July 1997 following a 7-year continuous fertilization. We found that although there was no significant difference in total SOC in the top 0–10cm of the soils between the fertilization plots (5.42  0.18 kgm2) and the control plots (5.27  0.22 kgm2), the proportion of the heavy-fraction organic C in the total SOC was significantly higher in the fertilized plots (59%) than in the control plots (46%) (Po0.05). The annual decomposition rate of fertilized leaf litter was 13% higher than that of the control leaf litter.We also found that fertilization significantly increased microbial biomass (fungi1bacteria) with 952  48mgkg1soil in the fertilized plots and 755  37mgkg1soil in the control plots. Our results suggest that fertilization in tropical forests may enhance long-term C sequestration in the soils of tropical wet forests.

The potential for carbon sequestration through reforestation of abandoned tropical agricultural and pasture lands

Silver, W.L. et al. (2000) The potential for carbon sequestration
through reforestation of abandoned tropical agricultural and pasture
lands. Rest. Ecol. 8, 394–407

Approximately half of the tropical biome is in some stage of recovery from past human disturbance, most of which is in secondary forests growing on abandoned agricultural lands and pastures. Reforestation of these abandoned lands, both natural and managed, has been proposed as a means to help offset increasing carbon emissions to the atmosphere. In this paper we discuss the potential of these forests to serve as sinks for atmospheric carbon dioxide in aboveground biomass and soils. A review of literature data shows that aboveground biomass increases at a rate of 6.2 Mg ha−1 yr−1 during the first 20 years of succession, and at a rate of 2.9 Mg ha−1 yr−1 over the first 80 years of regrowth. During the first 20 years of regrowth, forests in wet life zones have the fastest rate of aboveground carbon accumulation with reforestation, followed by dry and moist forests. Soil carbon accumulated at a rate of 0.41 Mg ha−1yr−1 over a 100-year period, and at faster rates during the first 20 years (1.30 Mg carbon ha−1 yr−1). Past land use affects the rate of both above- and belowground carbon sequestration. Forests growing on abandoned agricultural land accumulate biomass faster than other past land uses, while soil carbon accumulates faster on sites that were cleared but not developed, and on pasture sites. Our results indicate that tropical reforestation has the potential to serve as a carbon offset mechanism both above- and belowground for at least 40 to 80 years, and possibly much longer. More research is needed to determine the potential for longer-term carbon sequestration for mitigation of atmospheric CO2 emissions.

Estimating soil labile organic carbon and potential turnover rates using a sequential fumigation–incubation procedure

Zoua, X.M.; Ruanc,H.H.; Fua, Y.; Yanga, X.D.; Sha, L.Q. 2005. Estimating soil labile organic carbon and potential turnover rates using a sequential fumigation–incubation procedure.. Soil Biology & Biochemistry 37 :1923-1928.

Labile carbon is the fraction of soil organic carbon with most rapid turnover times and its oxidation drives the flux of CO2 between soils and atmosphere. Available chemical and physical fractionation methods for estimating soil labile organic carbon are indirect and lack a clear biological definition. We have modified the well-established Jenkinson and Powlson’s fumigation–incubation technique to estimate soil labile organic carbon using a sequential fumigation–incubation procedure. We define soil labile organic carbon as the fraction of soil organic carbon degradable during microbial growth, assuming that labile organic carbon oxidizes according to a simple negative exponential model. We used five mineral soils and a forest Oa horizon to represent a wide range of organic carbon levels. Soil labile organic carbon varied from 0.8 mg/g in an Entisol to 17.3 mg/g in the Oa materials. Potential turnover time ranged from 24 days in an Alfisol to 102 days in an Ultisol. Soil labile organic carbon contributed from 4.8% in the Alfisol to 11.1% in the Ultisol to the total organic carbon. This new procedure is a relatively easy and simple method for obtaining indices for both the pool sizes and potential turnover rates of soil labile organic carbon and provides a new approach to studying soil organic carbon.

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.
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