???????What is the pathway that determines the diurnal lag time between soil respiration and soil temperature?

Soil respiration (Rs) is a critical component of the global carbon cycle in terrestrial ecosystems and is considered a function of soil temperature (Ts); however, Rs often shows a hysteresis effect with Ts. The mechanism causing this hysteresis pattern remains controversial, hindering our ability to accurately simulate and predict Rs in response to climate change. Here, a two-year field experiment was conducted to investigate the factors and pathways influencing the diurnal lag time between Rs and Ts (LTTs-Rs); we used an automatic chamber continuous measurement of Rs combined with 3 levels of precipitation manipulation: -30 % (P -30), +0% (CK), +30 % (P + 30) of ambient precipitation in rain-fed winter wheat (Triticum aestivum L.) and alfalfa (Medicago sativa L.) agroecosystems on the Loess Plateau, China. The results indicated that the monthly average diurnal dynamic peak time of Rs always occurred prior to that of Ts. The LTTs-Rs ranged from 1 h to 8 h in winter wheat under the three precipitation treatments over two years. The range was similar in alfalfa under the CK and P + 30 treatments; however, in alfalfa, the value ranged from 1 h to 10 h under the P _ 30 treatment. The LTTs-Rs value is determined by the direct pathway of photosynthetic product transport (the lag time between Rs and photosynthetically active radiation (PAR), LTRs-PAR) and the indirect pathway of heat transfer (the lag time between Ts and PAR, LTTs-PAR); that is, the LTTs-Rs is equal to the sum of the LTRs_PAR and LTTs-PAR. Moreover, the LTTs-PAR is equal to the sum of the lag time between air temperature (Ta) and PAR (LTTa_PAR) and the lag time between Ta and Ts (LTTs-Ta) in both winter wheat and alfalfa under the three precipitation treatments. Soil volume water content (VWC) affects LTTs-Rs by influencing the process (i.e., the transport process of heat and photosynthate) that comprises the LTTs-Rs. This is the first attempt to quantify the pathway processes that determine the LTTs-Rs and clarify the mechanism of the LTTs-Rs response to VWC. The results have important implications for our understanding of the LTTs-Rs production mechanisms and the accurate prediction of soil carbon emissions by future models.