Application of a model of the decomposition of soil organic matter

Abstract A mathematical model of the decomposition of soil organic matter is tested against published experimental results and used to study important soil processes and to offer explanations of well known but incompletely understood experimental observations. The cycles of the three macronutrients N, P, and K through the plant-soil system are treated under both equilibrium and dynamic conditions. The equilibrium equations show that in many ecosystems, the annual-average soil solution concentrations of P and K are independent of the soil biota activity whereas the relative concentrations of the various N-forms in solution remain dependent on related microbe activity. The dynamic equations are compared with and shown to agree closely with a number of experimental observations, including (1) the overall pattern of decomposition and growth of heterotrophs and nitrifiers, (2) the immobilization and mineralization of N as a function of substrate C-N ratio, (3) wastage of substrate by various microbes, (4) the N priming effect, (5) the effect on microbes of oscillating low soil temperatures, and (6) the effect on microbes of soil moist-dry cycles. Among the conclusions drawn from the full model equations are the following: (i) the N priming effect is the consequence of a two-step substrate limitation involving first N and later C because of microbial waste metabolism, (ii) oscillating low soil temperatures result in lower population levels than does the mean temperature at least in part because nonlinearities give a net reduction in growth under oscillating conditions, (iii) moist-dry cycles, lethal to soil organisms, enhance CO 2 evolution in part because of the organisms' self-metabolism, (iv) plant and microbes compete for N to a greater or lesser extent depending on the kind of N available, the C:N ratio, the amount of leaching, and whether or not microbe use of NO 3 − is suppressed, and (v) in fertilizing some crops with NH 4 + , NO 3 − , or organic N, multiple-batch application is preferable with the mineral forms, whereas a single batch is best with the organic form because of differences in microbial immobilization.