Shim Jee H, Pendall Elise, Morgan Jack A, Ojima Dennis S
Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523, USA.
Oecologia. 2009 May;160(2):321-33. doi: 10.1007/s00442-009-1302-4. Epub 2009 Mar 4.
In semi-arid regions, where plants using both C(3) and C(4) photosynthetic pathways are common, the stable C isotope ratio (delta(13)C) of ecosystem respiration (delta(13)C(R)) is strongly variable seasonally and inter-annually. Improved understanding of physiological and environmental controls over these variations will improve C cycle models that rely on the isotopic composition of atmospheric CO(2). We hypothesized that timing of precipitation events and antecedent moisture interact with activity of C(3) and C(4) grasses to determine net ecosystem CO(2) exchange (NEE) and delta(13)C(R). Field measurements included CO(2) and delta(13)C fluxes from the whole ecosystem and from patches of different plant communities, biomass and delta(13)C of plants and soils over the 2000 and 2001 growing seasons. NEE shifted from C source to sink in response to rainfall events, but this shift occurred after a time lag of up to 2 weeks if a dry period preceded the rainfall. The seasonal average of delta(13)C(R) was higher in 2000 (-16 per thousand) than 2001 (20 per thousand), probably due to drier conditions during the 2000 growing season (79.7 mm of precipitation from April up to and including July) than in 2001 (189 mm). During moist conditions, delta(13)C averaged -22 per thousand from C(3) patches, -16 per thousand from C(4) patches, and -19 per thousand from mixed C(3) and C(4) patches. However, during dry conditions the apparent spatial differences were not obvious, suggesting reduced autotrophic activity in C(4) grasses with shallow rooting depth, soon after the onset of dry conditions. Air and soil temperatures were negatively correlated with delta(13)C(R); vapor pressure deficit was a poor predictor of delta(13)C(R), in contrast to more mesic ecosystems. Responses of respiration components to precipitation pulses were explained by differences in soil moisture thresholds between C(3) and C(4) species. Stable isotopic composition of respiration in semi-arid ecosystems is more temporally and spatially variable than in mesic ecosystems owing to dynamic aspects of pulse precipitation episodes and biological drivers.
在半干旱地区，同时存在采用C(3)和C(4)光合途径的植物，生态系统呼吸作用的稳定碳同位素比率（δ(13)C(R)）在季节和年际间变化很大。更好地理解这些变化的生理和环境控制因素，将改进依赖大气CO(2)同位素组成的碳循环模型。我们假设降水事件的时间和前期湿度与C(3)和C(4)禾本科植物的活动相互作用，以确定净生态系统CO(2)交换（NEE）和δ(13)C(R)。田间测量包括2000年和2001年生长季节整个生态系统以及不同植物群落斑块的CO(2)和δ(13)C通量、植物和土壤的生物量及δ(13)C。NEE随着降雨事件从碳源转变为碳汇，但如果降雨前有干旱期，这种转变会在长达2周的时间滞后后发生。2000年δ(13)C(R)的季节平均值（-16‰）高于2001年（-20‰），这可能是因为2000年生长季节（4月至7月包括7月的降水量为79.7毫米）比2001年（189毫米）更干燥。在湿润条件下，C(3)斑块的δ(13)C平均为-22‰，C(4)斑块为-16‰，C(3)和C(4)混合斑块为-19‰。然而，在干旱条件下，明显的空间差异并不明显，这表明干旱条件开始后不久，具有浅生根深度的C(4)禾本科植物的自养活性降低。气温和土壤温度与δ(13)C(R)呈负相关；与更多湿润生态系统相比，水汽压亏缺对δ(13)C(R)的预测能力较差。呼吸成分对降水脉冲的响应可以通过C(3)和C(4)物种之间土壤湿度阈值的差异来解释。由于脉冲降水事件和生物驱动因素的动态变化，半干旱生态系统中呼吸作用的稳定同位素组成在时间和空间上比湿润生态系统更具变异性。
