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

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

AUSTRAL ECOLOGY

The Ecological Society of Australia

Año: 2006 p. 1 - 38

1442-9985

Net primary production and nutrient dynamics of grasslands are regulated by different biotic and abiotic factors, which may differentially affect functional plant groups. Most studies have dealt with grasslands that have extremely low or zero production over a significant period of the year. Here we explore the relative importance of a few environmental factors as controls of aerial and belowground plant biomass production and nutrient dynamics in a grassland that is active throughout the year. We investigate their effect on the response of three main plant functional groups (warm and cool season graminoids and forbs). We conducted a factorial experiment 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 P, and N). N addition had the largest and most extended impact: it tripled aerial NPP in spring and summer and had no effect on belowground biomass. This positive effect was accompanied by higher N acquisition and higher soil N availability. Mowing increased aerial NPP in winter, increased root biomass in the first 10 cm during fall and winter and promoted N and P uptake by plants. Shading did not affect aerial NPP but also stimulated N and P uptake by plants. P addition had no effect on aerial NPP but increased shallow root biomass and its N content in spring, and increased P accumulation in plant biomass by three-fold. The three plant functional groups differentially accounted for these ecosystem-level responses. Graminoids explained the greater biomass production of N fertilised plots and mowing tended to promote forbs. These results suggest that the environmental agents of aerial NPP in this grassland vary among seasons and differentially impact the major floristic groups, which affects the energy and nutrients transfer to herbivores.Continuous biomass removal by grazing usually changes the resource allocation pattern of plants. These changes often increase resistance to tissue removal and derive  individuals with different morphometric traits, such as root to shoot or blade to sheath ratios. Shifts in morphometric traits, in turn, may alter nutrient cycling through changes in the average quality of litter that decomposes in soil. Previous work has showed that Paspalum dilatatum, a native grass from the Pampas grasslands, which inhabits a vast area and supports a wide range of grazing conditions, increases its blade to sheath ratio under continuous grazing with respect to ungrazed conditions. Here, we explored the consequences of these changes apparently associated with grazing regime on litter quality and nutrient dynamics during litter breakdown in soil. We separately analysed litter quality of blades and sheaths of P. dilatatum and determined, under controlled conditions, their decomposition and nutrient release kinetics over a maximum period of one year. We also studied the mineral nitrogen contents in soil amended with each litter type. Blade quality was significantly higher than sheath quality, nitrogen concentrations of blades and sheaths were approximately 1% and 0.6% respectively, and lignin to nitrogen ratios were ~5 and ~11 for blades and sheaths respectively. Phosphorus concentration, however, was similar in both litter types. Blades decomposed 10% faster than sheaths, released 20% more nitrogen,  and also released 15% more phosphorus than sheaths during the last half of the incubation period.  During the first three months, soil nitrogen content of litter amended incubations indicated immobilization with respect to non-amended control, but later blade incubations mineralised nitrogen whereas sheath incubations continued immobilising it. Results revealed that grazing potentially accelerates nutrient cycling during decomposition by increasing the blade to sheath ratio of P. dilatatum individuals, and suggest that this may be an important mechanism underlying grazing impact on nutrient cycling.