Effects of environmental conditions and leaf area index changes on seasonal variations in carbon fluxes over a wheat-maize cropland rotation.

Environmental conditions (EV) changes not only affect temporal variations in carbon fluxes directly, but affect them indirectly by impacting plant biotic traits. Investigating the extent of the effects of EV and biotic changes can help deepen our understanding of ecosystem carbon cycling. Therefore, we partitioned and quantified the contributions of EV and biotic changes' effects on seasonal variations in carbon fluxes (net ecosystem carbon exchange (NEE), and its components, i.e., gross ecosystem carbon exchange (GEE) and ecosystem respiration (RE)) in a (winter) wheat-(summer) maize rotation ecosystem from 2010 to 2012. A path analysis accompanied by Granger causality tests (GCTs), which filtered out several variables that were not causal for dependent variables, was used to calculate their respective contributions by integrating path coefficients. The seasonal variations in NEE, RE, and GEE were significantly and jointly affected by EV and the leaf area index (LAI) with R values ranging from 0.63 to 0.94 after the GCT. The path analysis indicated that the seasonal variations of carbon fluxes were dominated by the effects of EV changes (induced from varying EV for different fluxes, crops, and years), which contributed 60.7% (mean of two years), 64.5%, and 58.2% to wheat NEE, RE, and GEE, respectively, and 62.5%, 82.3%, and 58.1% to maize NEE, RE, and GEE, respectively. Overall, our study provided a new basis that future climatic changes may have important impacts on carbon exchanges in this rotation cropland.