Nitrogen and phosphorus cycling in grazed and ungrazed plots in a temperate subhumid grassland in Argentina.
The distribution and dynamics of N and P were studied for two adjacent grazed and ungrazed areas of native grassland in the Flooding Pampa, Argentina, over the winter-spring season of maximum primary production. Effects of grazing on vegetation structure and patterns of nutrient cycling at the ecosystem level were evaluated. Nutrient transfers were estimated from element concentrations measured in plant and soil pools at minimum and peak standing crop and from previous work in the study area. 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 grazed and ungrazed grassland, but nutrient location reflected the effect of cattle on phytomass structure and species composition. Plant pools of 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 above-ground green material from grazed prairie were 50% of the content for ungrazed prairie. At peak standing crop the grazed grassland contained more than 50% N and P in forb biomass, whereas protected vegetation had most nutrients in graminoids. Nitrogen concentration in live shoots declined through the season while it was increasing in roots, indicating preferential below-ground allocation. Seasonal patterns of plant N were amplified by continuous grazing. Phosphorus allocation appeared to be closely regulated in this grassland. Although changes in shoot 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. 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, although grazing decreased the overall translocation of N to shoots. Livestock consumption represented more than 40% of above-ground nutrient translocation, and thus grazing increased nutrient flow through the below-ground plant pathway. Strong demand from underground sinks in grazed vegetation probably contributed to increased root uptake. Grazing did not affect soil-available nutrients, although it did accelerate nutrient cycling rates, mostly of P. Ammonia volatilization and N exports through cattle removal were of similar magnitude (0.22 g m-2 year-1 N), and were 60% of wet deposition. Therefore, N tended to accumulate in this grassland. Annual P outputs in animal biomass produced a small net loss of P from the managed ecosystem.