A Method For Developing Soil Organic Matter Dynamics Monitoring Model For Cultivated Soil Systems

The decline in output of soil resources in recent years has aroused greater concern from several interested circles, including environmentalists, policy makers, soil scientists, and farmers, suggesting the need for continuous evaluation of the potential endurance of current agricultural practices in sustaining soil resources. Numerical methods are now increasingly being used to quantjfy such changes. Soil organic matter (SOM) change is the most dynamic indicator of soil quality due to cultivation. Alas, change in this parameter in the short run is hard to envisage. However, Soil microbial biomass, because of the easy with which it can be identjfied in the soils and due to its rapid response to perturbation in the short run is often used to account for SOM changes in soils. Using data from a short-term (60 days) greenhouse simulation study, a procedure of quantjfying effects of management on SOM and substrate decomposition is presented. Using microbial growth rate u(q), microbial efficiency in substrate utilization e(qj, spec jIic decomposition rates for added plant residues to two contrasting soils, red earth (Ferrasol) and black earth (Acrisol) were estimated The treatments included straw addition + buried, (Ti); straw addition + mineral N (T2); and straw addition + tillage, (T3). Sampling was done every 15 days. The study was conducted form October up to about early December. The objectives were: 1) to explore and assemble equations for characterizing SOM dynamics and residue decomposition in soils from literature, 2) to simulate decomposition processes under the influence of management factors in the presence of plant remains; 3) to show qualitative effects of the management factors on residue decomposition and soil microbial biomass carbon count; 4) to use data from the simulated experiment to quant~fr the effect of management perturbation on decomposition and SMBC dynamics; 5) to compute compound decomposition rates for the ecosystems studied and derive SMBC dynamics and residue decomposition models Straw decomposition rate was affected by external mineral N sources (Urea 46% N). Addition of an external N source sign jficantly increased decomposition rates. The study could not, however, fully account for the effect of tillage on residues because of the limited effect of the tillage method due to the art jficial barrier to mechanical interference supplied by the mesh bags. Based on the suggested methods of factor combination, the decomposition rates ranked in the order, Additive multiplicative 籑inimum multiplicative > Multiplicative. Straw placement, external N source and soil type sign ~icantly affected SMBC levels. xv? 7 In general, lower SMBC levels were predicted using additive multiplicative procedure compared with the observed values. In addition higher SMBC counts were registered in fertilized samples compared to unfertilized samples. Differences in SMBC absolute values for tilled samples compared to the control were visible at all sampling dates in both soil types under different factor combination procedure. Much lower values were predicted in red compared to black soil. SMBC values predicted with the mult,~olicative factor were higher than those predicted with the additive multiplicative and minimum multiplicative factor interaction procedures in both method...