The litter-bag method was used to assess the disappearance rates of organic C, N, and P from the plant detritus of a sawgrass (Cladium jamaicense Crantz) tidal marsh in North Florida. A steady-state kinetic model was used to simulate the effects of C, N, and P cycling dynamics on the quantity and quality of soil organic matter in the marsh. Four basic parameters were required for the model, namely, (i) net annual primary production of C, N, and P, (ii) soil organic C, N, and P in the biologically active layer, (iii) disappearance rate of dead standing crop, and (iv) disappearance rates of sawgrass C, N, and P from the litter-bag. The decomposition of sawgrass C, N, and P can be described by a two-component model through which a labile fraction and a refractory fraction of the primary production are defined. The rate constants of the labile fraction are one to two orders of magnitude greater than those of the refractory fraction. Simulation study indicated that the time required for soil C, N, and P to reach the steady state were 30, 60, and 70 yr, respectively. The simulation also indicated that the C/N ratio of the soil organic matter changed most significantly in the first 10 yr of the marsh development, whereas the N/P ratio remained relatively the same during the same period of time. The C/N and N/P ratios and the mean fraction remaining of soil organic matter all approach their respective steady-state values, which are 23, 13, and 0.47, respectively. The observed C/N and N/P ratios of the soil organic matter were 21 and 13, respectively. The calculated labile fractions of the sawgrass were 0.34, 0.205, and 0.488 for organic C, N, and P, respectively. The mean retention time of C, N, and P in the labile fraction was 8.5 months. The MRTs of the refractory fractions were 10, 29, and 33 yr for C, N, and P, respectively. The bulk of the soil organic matter consists almost entirely of the refractory fraction of the primary production. According to the simulation, decomposition rate constants of plant detritus were the most significant and sensitive parameters that determine the quantity and quality of the soil organic matter. The annual N and P cycling rates between the aboveground sawgrass and the soil were estimated to be 18.8 g/m2 and 1.9 g/m2, respectively.
This research was supported partially by a research grant from SEA/CR, USDA. Grant no. FLAX-79006.