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Proper estimate of residue input as condition for understanding drivers of soil carbon dynamics

The close link between carbon (C) inputs to the soil and soil organic carbon (SOC) stocks is well documented for agroecosystems (Buya-novsky & Wagner, 1998). Hence, correctly calculated and unbiased C inputs are critical for the study and interpretation of soil C dynamics. In a recent paper, Luo, Feng, Luo, Baldock, and Wang (2017) demonstrate that SOC dynamics in agricultural systems are mainly driven by, in order of importance, the amount and quality of C inputs to the soil from plant residues, climate, and soil properties including the amount and composition of soil C. While we concur with Luo et al. (2017) that the average C input amount is the most influential variable on SOC change rate, their experimental set-up did not enable its weight to be accurately quantified. They apply a path analysis and make use of an extensive data set that comprises detailed soil properties from 90 Australian field trials. Meteorological variables were recorded at weather stations or were interpolated, and soil properties were measured. In contrast, plant C inputs from, for example, stubble were not measured, but were approximated by measured yields and harvest indices (HI ’ s). The method of estimation employed may have led to important biases and critical omissions. We are concerned about this approach for following reasons: