E.J. Velthorst

More new carbon in the mineral soil of a poplar plantation under Free Air Carbon Enrichment (POPFACE): Cause of increased priming effect?

[1] In order to establish suitability of forest ecosystems for long-term storage of C, it is necessary to characterize the effects of predicted increased atmospheric CO2 levels on the pools and fluxes of C within these systems. Since most C held in terrestrial ecosystems is in the soil, we assessed the influence of Free Air Carbon Enrichment (FACE) treatment on the total soil C content (C-total) and incorporation of litter derived C (C-new) into soil organic matter (SOM) in a fast growing poplar plantation. C-new was estimated by the C3/C4 stable isotope method. C-total contents increased under control and FACE respectively by 12 and 3%, i.e., 484 and 107 gC/m(2), while 704 and 926 gC/m(2) of new carbon was sequestered under control and FACE during the experiment. We conclude that FACE suppressed the increase of C-total and simultaneously increased C-new. We hypothesize that these opposite effects may be caused by a priming effect of the newly incorporated litter, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates.

Soil organic matter dynamics in density and particle size fractions as revealed by the 13C/12C isotopic ratio in a Cerrado's oxisol

Abstract In order to better understand the dynamics of soil organic matter (SOM) in Oxisols and the impact of converting native cerrado (savannah) into pasture, we studied the dynamics of different physically separated SOM pools at different depths in a cerrado oxisol (Typic Haplustox), under natural conditions and after 23 years of cultivated pasture ( Brachiaria spp . ) via the replacement of the native C (C 3 -derived) by pasture C (C 4 -derived). Organic C stocks of the original cerrado (15±3 kg m −2 ) and pasture (17±3 kg m −2 ) were not significantly different, which was attributed to the high biomass production of the tropical grasses and the protective effect of the high clay content (>800 g kg −2 ). We observed that 89–91% of the total organic C accumulated in the clay+silt fraction. The replacement of cerrado-derived C by pasture-derived C was in average 36%, 34%, and 19% for A p , AB 1 , and B w2 horizons, respectively, suggesting a fast turnover rate of organic C regardless of the high clay content. The replacement decreased in the order: free low-density organic matter (LDOM)>heavy fractions (sand, silt, clay)>occluded-LDOM. The lower replacement of the occluded-LDOM compared to the heavy fractions was attributed to protection inside aggregates and to a possible accumulation of C 3 -derived charcoal (black carbon). After 23 years of pasture, about 50% of the total organic C in the free-LDOM in the topsoil was still from cerrado, indicating that a significant part of this fraction was relatively recalcitrant. Charcoal fragments observed in the fraction suggested that the recalcitrance was probably due to charred material.

Effects of fire on soil organic matter in a "cerrado sensu-stricto" from Southeast Brazil as revealed by changes in δ13C.

We studied the effect of increasing fire incidence on litter and soil organic matter in a Dark Red Latosol (Typic Haplustox) under a “cerrado sensu-stricto” vegetation in Southeast Brazil. After 21 years, C4-grass population significantly increased in the plot under high fire incidence (“cerrado 3”) compared to the plots of low fire incidence (“cerrado 1” and “cerrado 2”). In general, the variability of organic carbon (OC) content, organic nitrogen (ON) content and δ13C, both in litter and in the topsoil, was higher in “cerrado 3” than in the other plots. The higher fire incidence in “cerrado 3” decreased the litter stock of carbon and nitrogen by 3703±930 and 104±26 kg ha−1, respectively, in relation to the other plots. However, no difference in carbon (195.0±8.2 Mg ha−1) and nitrogen (13.4±1.0 Mg ha−1) stock in the first meter of soil was observed between the plots. The values of δ13C increased between 2.06‰ and 3.39‰ under C3 plants, and decreased 8.31‰ under C4 plants from litter to topsoil. For all plots, the δ13C profile showed an increase in δ13C with depth. In “cerrado 1” and “cerrado 2”, the δ13C values ranged from −25.80‰ to −23.19‰, typical for a C3-dominated vegetation profile. In “cerrado 3”, the replacement of C3- by C4-derived C was in average 34.6% throughout the profile. We conclude that: (i) the studied undisturbed “cerrado sensu-stricto” is a C3-dominated vegetation; (ii) biannual fire incidence for 21 years reduced the C3 dominance and decreased the stock of C and N in litter, but not in the soil; (iii) in the same period, at least about one-third of the C stock in the soil, or 67.5 Mg ha−1 (to 1-m depth), was replaced by C4-derived below ground litter; and (iv) in choosing a δ13C reference profile for soils under cerrado, it is necessary to take fire incidence into account.