Document: AMY-3-59-70

Disruption of nitrification affects carbon cycling in the Patagonian shrub-grass steppe.

AUSTIN, A.T.* ¹, O.E.SALA ¹, R.B.JACKSON ², N.LOPEZ ¹ and B.MENDEZ ¹

Universidad de Buenos Aires, Buenos Aires ARGENTINA ¹
Duke University, Durham, NC 27708 USA ²

Abstract:
We conducted a manipulative ecosystem experiment in Patagonia, Argentina, where we inhibited nitrification (transformation of ammonium to nitrate) in the soil with the use of nitrapyrin. Nitrapyrin is known to target the activity of bacteria in the genus Nitrosomonas, which is responsible for a large fraction of autotrophic nitrification in terrestrial ecosystems. The experimental site was a semi-arid shrub steppe in temperate South America (170 mm mean annual precipitation), with vegetative cover a near equal proportion of shrubs and grasses in a bare soil matrix. We measured numerous times during the two-year period several ecosystem response variables including inorganic soil nitrogen concentrations, rates of net mineralization and nitrification, ¹⁵N natural abundance of shrubs and grasses, indirect measures of primary productivity of shrubs and grasses, and decomposition and nutrient release. Additionally, chromosomal DNA extraction was performed on soil samples using standard phenol methods and PCR amplification to assess changes in Nitrosomonas populations. Addition of nitrification inhibitor resulted in substantially lower Nitrosomonas populations in soils with the addition of nitrapyrin, particularly in bare soil areas. Soil nitrate concentrations and rates of net nitrification were signficantly lower in inhibited soils (P<0.01), but had significantly higher ammonium and total inorganic nitrogen concentrations (P<0.001 and P<0.0001 respectively). ¹⁵N values of grass foliar samples were significantly more depleted (P<0.01) in the control than in treated plots, suggesting nitrogen losses were smaller where nitrification was inhibited. However, there was a significant negative effect of nitrification inhibition on total plant aerial cover (P<0.01) and rates of decomposition. These results suggest that disruption in the functioning of the nitrogen cycle may have detrimental effects on plant carbon gain in this semi-arid ecosystem in spite of higher overall nitrogen availability.

Keywords: nitrification inhibition, nitrogen cycling, carbon cycling, global change, carbon-nitrogen interactions, 15-N, Patagonian steppe, nitrapyrin, Argentina

This abstract is being presented at: 12:00 PM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.