Desertification in Central Argentina: Changes in ecosystem carbon and nitrogen from imaging spectroscopy

Many arid and semiarid (dryland) regions are subject to desertification from intensive land-use pressures such as cattle ranching. However, the lack of quantitative approaches required to assess desertification has slowed our understanding of how vegetation and soils are changing in dryland regions. Using airborne high-fidelity imaging spectroscopy and field measurements, we developed the first regional assessment of vegetation structural and soil biogeochemical properties in the Monte Desert biome, Argentina. We evaluated the long-term impacts of grazing on vegetation cover and soil carbon (C) and nitrogen (N) storage, which are core indicators of biogeochemical status and change in drylands. A comparative analysis was carried out on vegetation and soil properties in four major Monte Desert plant communities, as well as within the U.N. Nacunan Man-and-Biosphere Reserve and adjacent areas subjected to long-term grazing. The four dominant plant communities differed substantially in vegetation cover, leaf area index, foliar N concentration, and soil organic C and N stocks. Imaging spectroscopy with Monte Carlo spectral mixture analysis provided accurate estimates of fractional photosynthetic vegetation (PV), nonphotosynthetic vegetation (NPV), and bare soil cover at <5-m spatial resolution throughout a 763-km² region. Hotspots of grazing management, (e.g., ranch centers, water sources) have undergone some woody vegetation encroachment, but the spatial patterns were localized and variable. More clearly, a widespread NPV decrease and bare soil increase was common outside of the reserve. Soil organic C and N stocks were highly correlated with PV + NPV cover fractions. Soil organic C and N storage was 25-80% lower in areas subjected to long-term grazing, as compared to protected ecosystems within the Nacunan Reserve. Long-term grazing has depleted stores of C and N in soils and dramatically altered vegetation structure in the Monte Desert. A persistent lack of NPV due to grazing has likely impaired C and N cycles considered central to the biogeochemical functioning of the region. This study demonstrates a novel approach to remotely measure both surface and soil properties most indicative of progressive land degradation and desertification in dryland regions.