Temporal Variability of Soil Respiration in Experimental Tree Plantations in Lowland Costa Rica

The principal objective of this study was to determine if there is consistent temporal variability in soil respiration from different forest plantations in a lowland tropical rainforest environment. Soil respiration was measured regularly over 2004 to 2010 in replicated plantations of 15- to 20-year-old evergreen tropical trees in lowland Costa Rica. Statistically significant but small differences in soil respiration were observed among hours of the day; daytime measurements were suitable for determining mean fluxes in this study. Fluxes varied more substantially among months, with the highest average emissions (5.9 μmol·m−2·s−1) occurring in September and low emissions (3.7 μmol·m−2·s−1) occurring in January. Three of the six tree species had significantly increasing rates of soil respiration across 2004–2010, with fluxes increasing at an average of 0.09 μmol·m−2·s−1 per year: the three other species had no long-term trends. It was hypothesized that there would be a tradeoff between carbon allocation aboveground, to produce new leaves, and belowground, to sustain roots and mycorrhizae, but the relationship between canopy leaf fall—a surrogate for canopy leaf flushing—and soil respiration was significantly positive. The similarities observed among temporal trends across plantation types, and significant relationships between soil respiration, soil water content and soil temperature, suggest that the physical environment largely controlled the temporal variability of soil respiration, but differences in flux magnitude among tree species were substantial and consistent across years

Aboveground Tree Growth Varies with Belowground Carbon Allocation in a Tropical Rainforest Environment

Young secondary forests and plantations in the moist tropics often have rapid rates of biomass accumulation and thus sequester large amounts of carbon. Here, we compare results from mature forest and nearby 15–20 year old tree plantations in lowland Costa Rica to evaluate differences in allocation of carbon to aboveground production and root systems. We found that the tree plantations, which had fully developed, closed canopies, allocated more carbon belowground - to their root systems - than did mature forest. This increase in belowground carbon allocation correlated significantly with aboveground tree growth but not with canopy production (i.e., leaf fall or fine litter production). In contrast, there were no correlations between canopy production and either tree growth or belowground carbon allocation. Enhanced allocation of carbon to root systems can enhance plant nutrient uptake, providing nutrients beyond those required for the production of short-lived tissues such as leaves and fine roots, and thus enabling biomass accumulation. Our analyses support this deduction at our site, showing that enhanced allocation of carbon to root systems can be an important mechanism promoting biomass accumulation during forest growth in the moist tropics. Identifying factors that control when, where and for how long this occurs would help us to improve models of forest growth and nutrient cycling, and to ascertain the role that young forests play in mitigating increased atmospheric carbon dioxide

Changes in soil respiration across a chronosequence of tallgrass prairie reconstructions

Close relationships among climatic factors and soil respiration (Rs) are commonly reported. However, variation in Rs across the landscape is compounded by site-specific differences that impede the development of spatially explicit models. Among factors that influence Rs, the effect of ecosystem age is poorly documented. We hypothesized that Rs increases with grassland age and tested this hypothesis in a chronosequence of tallgrass prairie reconstructions in central Iowa, U.S.A. We also assessed changes in root biomass, root ingrowth, aboveground net primary productivity (ANPP), and the strength of soil temperature and moisture in predicting Rs. We found a significant increase in total growing season Rs with prairie age (R2 = 0.79), ranging from 714 g C m−2 in the youngest reconstruction (age 4) to 939 g C m−2 in the oldest prairie (age 12). Soil temperature was a strong predictor of intra-seasonal Rs among prairies (R2 = 0.78–0.87) but mean growing season soil temperature and moisture did not relate to total Rs. The increase in Rs with age was positively correlated with root biomass (r = 0.80) and ANPP (r = 0.87) but not with root ingrowth. Our findings suggest that growing season Rs increases with tallgrass prairie age, root biomass, and ANPP during young grassland development

Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils

Secondary forests and young forest plantations frequently have high rates of tree growth and NPP – higher even than mature forests in similar situations. The nutrients required to sustain this rapid growth are derived from the soil and by external inputs such as rainfall or, in the case of nitrogen, by biological N fixation. In our study of the effects of tree species within replicated experimental plantations in a moist, lowland tropical environment in Costa Rica, high rates of biomass accumulation and productivity were coupled with high rates of N accumulation and cycling. We applied extensive sampling through time within a mass-balance approach to address the question “Where does all the nitrogen come from?”
 Rates of N uptake by the vegetation in these plantations were extraordinarily high, even by tropical forest standards, reaching 412 kg N ha^-1^ yr^-1^. Rapid decay coupled to tight nitrogen cycling may have provided large amounts of available N for plant uptake, but do not explain the large quantities of N that accrued in the vegetation, up to 1075 kg N/ha over 16 years. Surface soil organic matter stocks in the plantations increased by as much as 320 kgC/ha in surface soils, but soil nitrogen varied differently. Soil N stocks to 1 m depth were depleted by an average of 2119 kg/ha relative to the mature forest. Thus, mineralization of soil organic nitrogen could have supplied the N that accrued in biomass over the 16-yr period, but this apparently occurred without concomitant net loss of soil C. The C:N ratios of soil organic matter (SOM)in the plantations indicated either replacement of SOM with more C-rich detritus or selective removal of N from existing SOM. Regardless, high productivity in these plantations apparently was supported in part by mining of soil nitrogen. Species varied, however. Depletion of soil N stocks was only 219 kg/ha under _Vochysia guatemalensis_, in which 1075 kgN/ha had accrued. Asymbiotic N fixation is the next most plausible mechanism for supplying plant-available N, and may be enhanced in stands with high availability of recently produced photosynthates