Nitrogen and Phosphorus Dynamics and Budgets in Annual Grasslands

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.

Plant phenology and life span influence soil pool dynamics: Bromus tectorum invasion of perennial C3–C4 grass communities

“…These processes further impact soil C and N by affecting microbial processes (e.g., mineralization and immobilization) via changes in microbial substrate availability (Woodmansee and Duncan 1980, Rickard 1985a, Jackson et al 1988, Wedin and Tilman 1990, Hart et al 1993. It follows that, in conjunction with water availability, the phenology and life span of the dominant species in a plant community should exert substantial control on available soil C and N pools.…”

Biotic Translocation of Phosphorus: The Role of Deer in Protected Areas

“…Woodmansee and Duncan [15] showed that the P cycle in grassland systems is nearly closed based on mass balance without harvests, and Schütz et al [13] explicitly adopted the assumption that there are no other existing P sinks or P sources. They found no transfer of P between the short-grass and tall-grass portions in the 10.7 ha study site.…”

“…First, P is a rather immobile element and often a limiting factor as evidenced by terrestrial plants taking up much more P than exists in soils as organic and inorganic P [8,15,45]. Under minimal natural input and output rates, ecosystems often accumulate P until a steady state is reached and biological recycling is maximal both at the organismal and ecosystem levels [3,4,6,39].…”

“…Central to the biogeochemical cycle of P is its virtual immobility which, in absence of harvesting, results in nearly closed local cycles and often makes it a limiting factor [8,15,45]. The concentration factor for soil-plant systems is possibly the highest known [46]: terrestrial plants and zoomass contain 500-550 and 30-50 million tons of P, respectively, in contrast to only 50 million tons of inorganic and organic P in soils [8].…”

Changes of the concentrations of dissolved chemical compounds migrating into ground water through biogeochemical barriers in an agricultural landscape

“…Contrary to phosphorus, nitrogen is included in an open cycle. N-NH 4 + , N 2 O, NO, NO 2 , and N 2 spread very easily, and are observed in high amounts in precipitation as well as in run-off (Woodmansee, Duncan 1980). The object of this study was to estimate how small ponds, located in agricultural fields and in afforestation, fulfill the function of a biogeochemical barrier in relation to nitrogen and phosphorus compounds migrating in ground water.…”