Nitrogen cycling in livestock systems of the Pampas and Campos of Argentina and Uruguay. Has N openness changed?

PIÑEIRO, G.; PARUELO, J.M.; OESTERHELD, M.; AUSTIN, AMY T.

Valdivia, Chile

Workshop; II Internacional Workshop: Risk assessment of Agricultural Intensification on N deposition on pristine forest and plantations in Southern Chile; 2005

Universidad Austral de Chile

Nitrogen cycling is affected by domestic herbivores as a large fraction of N stored in the vegetation is consumed by livestock, changing the magnitude and rate of soil N turnover. We explored the effects of grazing on N cycling by two methodological approaches. First, using a biogeochemical simulation model (CENTURY), we explored the alterations caused by European livestock introduction in 11 Rio de la Plata grassland sites in Argentina and Uruguay. Second, we evaluated the hypothesis raised from the simulations with data collected at grazed/non-grazed paired sites. CENTURY simulations were performed considering a constant annual N input through time of 7.18 kg/ha.year of N (average of published values for all sites, actual range 3.5 to 10.2 kg/ha.year of N), and N return to soil from faeces and urine as 80% of N consumed. We simulated 400 years of grazing, from year 1600 to the present, progressively increasing stocking rates to present values. In the field, we measured soil N contents at 4 selected field exclosures (were herbivores were removed and N accumulated) located throughout the region. The data provided an estimate of the minimum amount of N that is entering these grazed ecosystems. We also measured 15N contents, in aboveground plant organs, roots and soil (0-30 cm) in 3 exclosures and on its grazed adjacent sites to assess N cycling. CENTURY simulations showed increased  N loss in grazed sites immediately after introduction of domestic herbivores, and ecosystems were still losing N at the end of the simulation (3.55 kg/ha.year of N). Increased N emissions came from urine and faeces patches while emissions from soils and vegetation were reduced (50%). Greater emissions decreased grasslands N pools after 400 years of grazing from 4980 to 3960 kg/ha on average, with 95% of ecosystem N stored in the soil. Alteration of the N cycle due to herbivores introduction resulted in an increase of N openness and turnover under grazing. N openness (N input/N pools) varied from 0.14% before and 0.18% after 400 years of livestock introduction and  from 1.74% to 2.85%, considering gross mineralization as an indicator of ecosystem N availability (N input/N gross mineralization). Rates of N turnover, defined as the mean time that a molecule of N completes an entire ecosystem cycle, increased after grazing from 21.8% to 22.1% under constant N inputs. N turnover was evaluated relative to the amount of N stored in active cycling pools (N gross min+N returns+Natm inputs/N in active cycling pools). N accumulation in soils inside exclosures was significantly higher than N inputs previously measured or estimated. Soils and aboveground vegetation of exclosures had increases of 65 kg/ha.year of N, and less than <1 kg/ha.year of N, respectively, while roots lost N at 3.6 kg/ha.year, relative to the nearby grazed pairs. Legumes were not present in exclosures nor in the grazed pairs. We are not aware of papers that estimated or measured rates of non-symbiotic N fixation in the region, but for other regions, it has been estimated as equivalent to dry and wet deposition. Assuming these proportions, our estimates of N deposition in this region could be much higher then previously thought (31 kg/ha.year of N). ä15N increased  in soils and decreased in aboveground plant parts and roots in grazed grasslands. This pattern can be interpreted as a result of both higher N emissions and faster turnover. Thus, changes in natural abundance of 15N were in agreement with CENTURY findings. Our work, based on modeling and field experiments, suggests that domestic herbivores changed N cycling in the Pampas and Campos, increasing N emissions, N openness and N turnover.