Submitted to: Scientia Agricola
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2009
Publication Date: 9/1/2009
Citation: La Scala, N., Lopes, A., Spokas, K.A., Archer, D.W., Reicosky, D.C. 2009. First-Order Decay Models to Describe Soil C-CO2 Loss After Rotary Tillage. Scientia Agricola. 66(5):650-657.
Interpretive Summary: Elevated atmospheric carbon dioxide, potential global warming concerns and prospective use of soil as a sink for carbon has attracted interest from farmers and land managers. Recent studies involving tillage methods indicate major gaseous loss of carbon (as carbon dioxide) immediately after tillage. There is a need to be capable of modeling this phenomenon, so better management strategies can be discovered. Tillage stimulates soil carbon losses by increasing aeration, changing temperature and moisture conditions, and breaking soil aggregates exposing light fraction organic matter, once protected, to decomposition. In this paper we evaluate a model to explain carbon dioxide emission after tillage by a rotary tiller as a function of the non-tilled emission plus a correction due to the tillage disturbance. Our hypothesis is that an additional amount of readily decomposable organic matter is made available to the soil organisms by tillage, exposing aggregate protected carbon, and thereby making it accessible to microorganisms. The model assumes that carbon in the readily decomposable organic matter follows a simple first-order reaction kinetics equation and that soil emissions are proportional to the carbon decay rate in soil, and the available labile soil carbon. Predicted and observed fluxes showed good agreement based on statistical measures. These results are significant to farmers and policy makers in that tillage results in substantial short-term gaseous losses of soil carbon. This information will assist scientists and engineers in developing improved tillage methods to minimize the gaseous loss and to improve soil carbon management.
Technical Abstract: In Brazil inventories have indicated that soil management and land use is the main cause of carbon dioxide (CO2) emission to atmosphere. Intense soil tillage is one of the processes that stimulates most of soil carbon (C) losses due to the increase in aeration, changes in temperature and moisture conditions, and breaking of soil aggregates exposing organic matter once protected to decomposition. To further understand the impact of tillage on CO2 emission we test the applicability of two conceptual models that describe the CO2 emission after tillage as a function of the non-tilled 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 air and microorganisms. The models assume that C in the readily decomposable organic matter follows a first-order reaction kinetics equation as: dC_soil(t)/dt = - k C_soil(t) and that soil C-CO2 emission is proportional to the C decay rate in soil, where C_soil(t) is the available labile soil C (g m-2) at any time (t) and k is the decay constant (time-1). Carbon emissions are addressed in terms soil C available for decomposition in the tilled and non-tilled plots in an experiment conducted on a bare soil after application of rotary tiller in several adjustments. Two possible relationships are derived between non-tilled (FNT) and tilled (FT) fluxes: F_t = F_nt + a_1 exp(-a_2 t) (model 1) and F_t=a_3 F_nt 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 k factor of labile C in the non-till plot. In model 1 the k factors are unequal and in model 2 the k factors are equal. Predicted and observed CO2 fluxes showed good agreement based on the coefficient of determination (R2) as high as 0.91. 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 non-tilled plots and also to the amount of labile C added to the readily decomposable soil organic matter due to tillage. The advantage of this approach is that temporal variability of tillage-induced emissions can be described by a simple analytical function that include the non-tilled emission plus an exponential term modulated by tillage and environmentally dependent parameters.