Nitrogen and Phosphorus Dynamics and Budgets in Annual Grasslands

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Abstract
N and P dynamics were studied in a grassland ecosystem dominated by exotic annual plants in central California. Biomass and N and P concentrations were observed over a 3—yr period for the dominant grasses, forbs, and legumes and plant residues (litter). The N and P contents were determined or estimated for various components of the ecosystem (e.g., live tops, live roots, soil organic matter, dead roots, litter, microflora, soil fauna, mineral N, solution P, and labile inorganic P). We estimated that >57, 125, and 89 kg/ha of N and 13.1, 14.7, and 14.1 kg/ha of P were mineralized during the 1972—1973, 1973—1974, and 1974—1975 growing seasons, respectively. However, N uptake by plants was 119, 87, and 68 kg/ha and P uptake was 134.1, 14.5, and 10.4 kg/ha, respectively. While the data showed that P was stable, indicating its cycle was relatively closed, the data of N showed that its cycle was open, with large inputs of N from symbiotic fixation the 1st yr of the study. Large losses of N from the system were estimated.