This research assesses the fate of newly added carbon (C) substrates in soils over a two year period, comparing microbial versus mineralogical control over C loss versus stabilization in the field. The sites included a long substrate age gradient (LSAG) across wet Hawaiian forests, and a climate and weathering gradient across Hawaiian grasslands. A long-term field fertilization experiment was also used. This design allowedus to assess the effects of soil mineralogy, soil C and nutrient status, and cover type on the fate of new C. Replicate soil columns were inserted into soils to 25 cm at each site (n = 3), including 13C-labeled glucose or glycine addition, charcoal addition, and control columns.
Results/Conclusions
Among the forest sites, the youngest soil had the smallest standing stock of soil C and microbial biomass, with the largest stocks at the intermediate-aged site. In grassland sites, the wetter site had greater stocks of soil C and microbial biomass compared with the drier site. After two years, a substantial amount of added 13C was recovered in soil columns in both forest and grassland soils, indicating annual-scale stability of the added compounds. Comparing the two low-molecular weight compounds, 2.8x and 3.1x greater retention occurred for glucose versus glycine after one and two years, respectively, suggesting that glucose may be a better precursor for relatively stable soil C. 13C NMR spectroscopy also revealed substantial retention of charcoal across sites, with greater charcoal retention in the dry grassland site versus all wetter grassland and forest sites. Improving our understanding of the chemical and environmental conditions that promote soil C stabilization will ultimately help us predict ecosystem-climate feedbacks.