NITROGEN AND PHOSPHORUS CYCLING IN GRAZED AND UNGRAZED PLOTS IN A TEMPERATE SUBHUMID GRASSLAND IN ARGENTINA

1. The distribution and dynamics of N and P were studied for two adjac ent grazed and ungrazed areas of native grassland in the Flooding Pamp a, Argentina, over the winter-spring season of maximum primary product ion. We evaluated how grazing effects on vegetation structure altered patterns of nutrient cycling at the ecosystem level. Nutrient transfer s were estimated from element concentrations measured in plant and soi l pools at minimum and peak standing crop and from previous work in th e study area. 2. Total nutrient amounts in vegetation (9.4-12.6 g m(-2 ) N and 0.50-0.76 g m(-2) P) did not differ consistently between graze d and ungrazed grassland, but nutrient location reflected the effect o f cattle on phytomass structure and species composition. Plant pools o f N and P were mainly located in below-ground organs, to the extent of 80-90% in grazed and 63-75% in ungrazed vegetation. Nutrients in abov e-ground green material from grazed prairie were 50% of the content fo r ungrazed prairie. At peak standing crop the grazed grassland contain ed more than 50% N and P in forb biomass, whereas protected vegetation had most nutrients in graminoids. 3. Nitrogen concentration in live s hoots declined through the season while it was increasing in roots, in dicating preferential below-ground allocation. Seasonal patterns of pl ant N were amplified by continuous grazing. Phosphorus allocation appe ared to be closely regulated in this grassland. Although changes in sh oot P concentrations were subtle, P allocation to shoots seemed higher in ungrazed grassland. Shoot N:P ratios suggested that vegetation was relatively more P-limited in early winter, but more N-limited in late spring. 4. In general, N and P contents tended to increase in living vegetation and to decrease in dead plant compartments over the season studied. Nutrient uptake by roots was 30-50% higher in the grazed plot , in correspondence with enhanced mineralization rates. Concentrations of N and P in graminoid live shoots were also higher under grazing, a lthough grazing decreased the overall translocation of N to shoots. Li vestock consumption represented more than 40% of above-ground nutrient translocation, and thus grazing increased nutrient how through the be low-ground plant pathway. Strong demand from underground sinks in graz ed vegetation probably contributed to increased root uptake. Grazing d id not affect soil-available nutrients, although it did accelerate nut rient cycling rates, mostly of P. 5. Ammonia volatilization and N expo rts through cattle removal were of similar magnitude (0.22g m(-2) year (-1) N), and were 60% of wet deposition. Therefore, N tended to accumu late in this grassland. Annual P outputs in animal biomass produced a small net loss of P from the managed ecosystem.