The role of interannual, seasonal, and synoptic climate on the carbon isotope ratio of ecosystem respiration at a semiarid woodland

摘要

The terrestrial carbon cycle is influenced by environmental variability at scales ranging from diurnal to interannual. Here, we present 5-years of growing season (day 131-275) observations of the carbon isotope ratio of ecosystem respiration (delta C-13(R)) from a semiarid woodland. This ecosystem has a large necromass component resulting from 97% Pinus edulis mortality in 2002, is dominated by drought-tolerant Juniperus monosperma trees, and experiences large variability in the timing and intensity of seasonal and synoptic water availability. Mean growing season delta C-13(R) was remarkably invariant (-23.57 +/- 0.4 parts per thousand), with the exception of particularly enriched delta C-13(R) in 2006 following a winter with anomalously low snowfall. delta C-13(R) was strongly coupled to climate during premonsoon periods (similar to May to June), including fast (<= 2 days) responses to changes in crown-level stomatal conductance (G(c)) and vapor pressure deficit (vpd) following rain pulses. In contrast, delta C-13(R) was relatively decoupled from G(c) and environmental drivers during monsoon and postmonsoon periods (July-August and September, respectively), exhibiting only infrequent couplings of delta C-13(R) to vpd and soil water content (SWC) with longer lags (similar to 8 days) and variable response slopes (both positive and negative). Notably, delta C-13(R) exhibited consistent dynamics after rainfall events, with depleted delta C-13(R) occurring within 1 h, progressive hourly delta C-13(R) enrichment over the remainder of the night, and net delta C-13(R) depletions over the multiple nights postevent in monsoon and postmonsoon periods. Overall this ecosystem demonstrated strong dependence of delta C-13(R) on precipitation, with an apparent dominance by the autotrophic delta C-13 signal in premonsoon periods when deep soil moisture is abundant and surface soil moisture is low, and weaker coupling during monsoonal periods consistent with increasing heterotrophic dominance when deep soil moisture has declined and surface moisture is variable.