Controls of primary productivity and nutrient cycling in a temperate grassland with year-round production

SEMMARTIN, M.; OYARZÁBAL, M.; LORETI, J.; OESTERHELD, M.

AUSTRAL ECOLOGY

Año: 2007 vol. 32 p. 416 - 428

1442-9985

Net primary production (NPP) and nutrient dynamics of grasslands are regulated by different bioticand abiotic factors, which may differentially affect functional plant groups. Most studies have dealt with grasslandsthat have extremely low or zero production over a significant period of the year. Here we explore the relativeimportance of a few environmental factors as controls of aerial and below-ground plant biomass production andnutrient dynamics in a grassland that is active throughout the year.We investigate their effect on the response ofthree main plant functional groups (warm- and cool-season graminoids and forbs). We conducted a factorialexperiment in a continuously grazed site in the Flooding Pampa grassland (Argentina). Factors were seasons(summer, autumn, winter and spring), and environmental agents (mowing, shade, addition of phosphorus [P] andnitrogen [N]). N addition had the largest and most extended impact: it tripled aerial NPP in spring and summerbut had no effect on below-ground biomass. This positive effect was accompanied by higher N acquisition andhigher soil N availability.Mowing increased aerial NPP in winter, increased root biomass in the first 10 cm duringautumn and winter and promoted N and P uptake by plants. Shading did not affect aerial NPP, but stimulated Nand P uptake by plants. P addition had no effect on aerial NPP, but increased shallow root biomass and its Ncontent in spring, and tripled P accumulation in plant biomass. The three plant functional groups differentiallyaccounted for these ecosystem-level responses. Graminoids explained the greater biomass production ofN-fertilized plots and mowing tended to promote forbs. These results suggest that the environmental controls ofaerial NPP in this grassland vary among seasons, differentially impact the major floristic groups, and affect theenergy and nutrient transfer to herbivores.