Depth dependence of temperature sensitivity of soil carbon dioxide, nitrous oxide and methane emissions

Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) are three important greenhouse gases (GHGs) associated with rising temperature. Their temperature sensitivities (Q(10)) play important roles in predicting soil-atmosphere gas exchange under global warming. Few studies have simultaneously evaluated the variation in Q(10) of CO2, N2O and CH4 emissions across different soil depths. Here, we sampled soils from 13 sites along a climatic transect in temperate grasslands of northern China, including meadow steppe, typical steppe and desert steppe. Soil samples were collected from five depths (0-10, 10-20, 20-40, 40-60 and 60-100 cm) and incubated at four temperatures (5, 15, 25 and 35 degrees C). We determined GHG emission rates, corresponding biotic (microbial biomass, microbial abundance and composition, and extracellular enzyme activities) and abiotic (soil carbon, nitrogen content, and soil pH) factors. The average Q(10) of CO2 and N2O emissions differed non-significantly among steppe types, while the effect of steppe type on Q(10) of CH4 emission was strongly affected by measured temperature ranges. Q(10) of CO2 and N2O emissions declined with soil depth, but showed no significant differences for CH4. The mechanisms driving the variations in Q(10) of GHG emissions among soil depths changed with steppe types. Q(10) of CO2 emission was significantly correlated with substrate quality and microbial biomass in meadow steppe and desert steppe, and with microbial abundance and composition in typical steppe. Q(10) of N2O emission was significantly influenced by microbial biomass in meadow steppe, soil pH in typical steppe, and substrate quality and microbial biomass in desert steppe, respectively. Q(10) of CH4 emission was significantly associated with microbial biomass in typical steppe, and with substrate quality in meadow steppe and desert steppe, respectively. Our findings emphasized the importance of considering steppe type when assessing vertical dynamics of Q(10) of GHG emissions.