The decrease in soil organic matter (SOM) and soil total nitrogen (TN) in the agricultural regions of Uruguay over the last 100 years depends on the intensity of agricultural activities, soil N supplying capability and soil potential productivity. The objective of this study was to investigate the use of ¹⁵N natural abundance (d¹⁵N), as an index of perturbation of agricultural soils.  Soil samples were collected from 20 sites located in the south west of the country, both from within agricultural areas and from under permanent fences, an area which has supposedly been less disturbed than the rest of the field. Samples were analyzed for SOM, TN, ¹⁵N natural abundance (d¹⁵N), mineral nitrogen (NH₄⁺ and NO₃^-), and anaerobic mineralizable N (Nmin). Also, field experiments were established in 8 of these sites to evaluate barley response to N, with the N fertilizer applied at different timings of the crop growth cycle. The results showed that in samples collected from the agricultural area a strong positive relationship (R²=0.83, P=0.01) existed between the soil TN content and soil d¹⁵N value, but only after these samples were classified in groups of similar soil types. Also, soil d¹⁵N showed a high correlation with other indexes related with the degree of site perturbation, such as Nmin, mineral N, SOM, and with the degree of barley response to N. In soil samples collected from under the fence, however, no relationship was observed between d¹⁵N and TN, and neither was a relationship between d¹⁵N and any of the other indexes of site perturbation. Also, lower TN and d¹⁵N values were observed in samples collected from the agricultural area. These results could be explained by considering that the gross N mineralization and immobilization rate would tend to decrease when the degree of soil perturbation increases. Concurrently, the N lost from this cycle would be relatively more impoverished in ¹⁵N than the bulk of the soil. On the other hand, the N inputs, coming mainly from N fertilization and N symbiotic fixation, would tend to have even lower d¹⁵N values. Thereby, in the long term the permanency of this cycle would determine an impoverishment in the ¹⁵N content of the soil, which would be more accentuated in the most perturbed areas. Although our results are preliminary, they seem to indicate that soil d¹⁵N could be used as an index to quantify the degree of perturbation of agricultural sites, and that this index could also help to define in a more objective way the concept of soil quality.

Keywords: ¹⁵N natural abundance, soil organic matter, soil perturbation, gross N mineralization and immobilization, soil ¹⁵N content, mineralizable. N