Substrate and grazing controls on nitrogen mineralization in Fuegian soils

ANCHORENA J, MENDOZA R.E. AND A. CINGOLANI.

Rosario

Congreso; IX International Rangeland Congress; 2011

Society for Range Mangement

Ungulate grazing can control soil fertility by changing species composition and the type of litter that enters the soil. This effect is, however, dependent on habitat productivity: rich soils enhance N mineralization while poor soils reduce it (Bardgett and Wardle 2003). We studied soil potential N mineralization comparing sheep grazing regimes along a gradient of habitat productivity in the Magellanic steppe. Using Landsat images we selected 14 sites in paddocks showing fence-contrasts. Sites were located in three substrates: rich and poor moraines, and meltwater plains (Collantes et al. 1999). We made vegetation censuses, soil profile descriptions and collected soil samples (5 replicates). Each site was assigned to different grazing regimes, based on informants’ reports. Analyses included C, N, P, C/N, pH, CEC and cations. Potential mineralization was measured incubating the samples at 28ºC during 70 days. The leachate was analyzed periodically for NO3 and NH4. Relationships between soil variables were summarized with a PCA, and mineralized N correlated with the ordination axes. One-way ANOVAs were performed with sites grouped by substrate to determine differences in N mineralized (Nmin). PCA axis 1 (80% of total variance), a gradient of soil quality (Fig. 1), was positively correlated with Nmin and N mineralized as NO3 (p < 0.01), and negatively with N mineralized as NH4 (p < 0.05), indicating the substrate control and supporting the idea that in poor soils NH4 is the main source of N for plants. On poor substrates, high selective grazing changed plant composition to communities dominated by Empetrum, known by its recalcitrant litter. This change resulted in a shift from NO3-N to NH4-N (p < 0.01). Under non-selective grazing soils increased Nmin (c. 20 times, p < 0.001) and NO3/N (c. 8 times, p < 0.001). Grazing intensity did not affect N mineralization rates in nutrient-rich soils. As postulated, plant communities induced by different grazing regimes differed in soil chemical quality and in the nature and rate of N mineralization, but this occurred only on poor and intermediate-quality substrates. Visually sharp, paddock-related geometrical patterns of such communities in satellite images should allow large-scale tracking of these probably irreversible ecosystem disturbances.