La Scala, N  FCAVUNESP  
Lopes, A  FCAVUNESP  
Spokas, Kurt  
Archer, David  
Reicosky, Donald 
Submitted to: European Journal of Soil Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: October 24, 2008 Publication Date: February 16, 2009 Repository URL: http://hdl.handle.net/10113/29599 Citation: La Scala, N., Lopes, A., Spokas, K.A., Archer, D.W., Reicosky, D.C. 2009. ShortTerm Temporal Changes of Bare Soil CO2 Fluxes Described by FirstOrder Decay Models. European Journal of Soil Science. 60(2):258264. Interpretive Summary: To gain further insight into the mechanisms of tillage induced carbon dioxide (CO2) losses, we investigated the application of two different mathematical models to simulate the emission of CO2 following tillage. The models were based on the assumption that CO2 emission after tillage is a function of the nontilled emission plus a correction due to the tillage disturbance. Our hypothesis is that an additional amount of labile carbon (C) is made available to the soil organisms by tillage, exposing aggregate protected C, and thereby making it accessible to microorganisms. The two models were both firstorder decay models, but differed from each other in the first model assumed different rates of organic matter decay following tillage and the second model assumed equal rates of decay following tillage. Both models performed well. However, the model based on the assumption that the decay rates before and after tillage were equal, fit the observed field data better than the model with unequal decay rates. The advantage to this modeling is that the amount of CO2 lost can be predicted by utilizing the notill flux as a surrogate for the tillage emissions. With further experiments it is anticipated that the effect of various tillage implements will be able to be quantified thereby improving the ability to predict CO2 tillage emission losses as a consequence of tillage. This information will assist scientists and engineers in developing improved tillage methods to minimize the gaseous loss and to improve soil carbon management and assist farmers to develop new management techniques for enhancing soil carbon. This research will be of direct benefit to the farmers to enable them to maintain crop production with minimal impact to the environment. Technical Abstract: To further understand the impact of tillage on carbon dioxide (CO2) emissions, we compare the performance of two conceptual models that describes the CO2 emission after tillage as a function of the nontilled emission plus a correction due to the tillage disturbance. Our hypothesis is that an additional amount of labile carbon (C) is made available to the soil organisms by tillage, exposing aggregate protected C, and thereby making it accessible to microorganisms. The models assumes that C in the readily decomposable organic matter follows a firstorder reaction kinetics equation as: dCsoil/dt = kCsoil(t) and that soil CCO2 emission is proportional to the C decay rate in soil, where Csoil(t) is the available labile soil C (g m2) at any time (t). Emissions are addressed in terms soil C available to decomposition in the tilled and nontilled plots. Two possible relationships are derived between nontilled (FNT) and tilled (FT) fluxes which are: Ft =Fnt + a1 * exp(a2t) (model 1) and Ft=a3 * Fnt * exp(a4t) (model 2), where t is time after tillage. The difference between these two models comes from an assumption related to the k factor of labile C in the tilled plot and its similarity to the k factor of labile C in the nontill plot. In model 1 the k factors are unequal and model 2 the k factors are equal. Predicted and observed CO2 fluxes showed good agreement based on determination coefficient (R2), index of agreement and model efficiency, with R2 as high as 0.97. Comparisons also reveal that model 2, the model where all C pools are assigned the same k factor produces a better statistical fit over the other model. The four parameters included in the models are related to the decay constant (k factor) of tilled and nontilled plots and also to the amount of labile carbon added to the readily decomposable soil organic matter due to tillage. The advantage to this modeling approach is that temporal variability of tillageinduced emissions can be described by analytical functions that include the nontilled emission plus an exponential term modulated by tillage and environmentally dependent parameters.
