Plant δ¹⁵N reflects the high landscape‐scale heterogeneity of soil fertility and vegetation productivity in a Mediterranean semiarid ecosystem

Abstract

We investigated the magnitude and drivers of spatial variability in soil and plant δ N across the landscape in a topographically complex semiarid ecosystem. We hypothesized that large spatial heterogeneity in water availability, soil fertility and vegetation cover would be positively linked to high local-scale variability in δ N. We measured foliar δ N in three dominant plant species representing contrasting plant functional types (tree, shrub, grass) and mycorrhizal association types (ectomycorrhizal or arbuscular mycorrhizal). This allowed us to investigate whether δ N responds to landscape-scale environmental heterogeneity in a consistent way across species. Leaf δ N varied greatly within species across the landscape and was strongly spatially correlated among co-occurring individuals of the three species. Plant δ N correlated tightly with soil δ N and key measures of soil fertility, water availability and vegetation productivity, including soil nitrogen (N), organic carbon (C), plant-available phosphorus (P), water-holding capacity, topographic moisture indices and normalized difference vegetation index. Multiple regression models accounted for 62-83% of within-species variation in δ N across the landscape. The tight spatial coupling and interdependence of the water, N and C cycles in drylands may allow the use of leaf δ N as an integrative measure of variations in moisture availability, biogeochemical activity, soil fertility and vegetation productivity (or 'site quality') across the landscape.

Heterogeneous environments shape invader impacts: integrating environmental, structural and functional effects by isoscapes and remote sensing

“…3d). A comparable enriching effect of vegetation cover on foliar δ 15 N was also found in Brazilian white-sand vegetation 42 and, based on NDVI, in a semi-arid ecosystem 35 . Vegetation cover and root systems modify physical conditions in the soil, such as humidity and temperature, as well as organic matter content, nutrient availability and microbial activity 4, 36, 37, 48 .…”

“…As increasing rates of N transformations and N losses are typically linked to soil moisture, and as topography is an important control of hydrological processes, correlations between topographic position and δ 15 N of non-fixing plants across landscapes in different ecosystems have been attributed to moisture-related patterns of mineralization, nitrification and denitrification (e.g. refs 35, 40 and 41). In our study, we used a landform classification to characterize the topographic position relative to its surrounding for each individual plant as well as a topographic wetness index as a proxy for soil moisture.…”

“…Therefore, patchy vegetation can increase spatial heterogeneity through multiple feedbacks on soil properties and processes, such as protection from erosion, higher soil infiltration and water-holding capacity, larger soil C and N pools and higher microbial activity as well as by constituting sinks for runoff, sediment and nutrients (ref. 35 and references therein).…”

“…In turn, these factors can influence N cycling and therefore, δ 15 N values of plants 24, 38, 39 . Accordingly, landforms and associated soil moisture patterns 35, 40, 41 , as well as vegetation cover 35, 42 have previously been shown to correlate with plant or soil δ 15 N. This indicates that variation of δ 15 N in heterogeneous landscapes can potentially be predicted from LiDAR-derived land-surface attributes, thereby allowing us to disentangle spatial influences of invasive species on native species δ 15 N from background variation.…”

“…For instance, airborne LiDAR (light detection and ranging) yields spatially explicit data that can capture land-surface attributes, such as site topography or vegetation cover 33 , which potentially govern ecosystem processes. Topography is a major control of physical soil conditions, often correlating with N transformation rates 34, 35 . Similarly, vegetation cover can modify physical conditions of the soil, such as humidity and temperature, as well as organic matter content, nutrient availability and microbial activity 4, 36, 37 .…”