Landscape Heterogeneity of Differently Aged Soil Organic Matter Constituents at the Forest-Alpine Tundra Ecotone, Niwot Ridge, Colorado, USA

One of the major remaining obstacles to understanding how ecosystems process carbon (C) and nitrogen (N) within soil organic matter (SOM) is landscape heterogeneity. While many studies have investigated landscape heterogeneity in total SOM C and N, less information exists on landscape patterns for differently aged constituents within SOM. These differently aged constituents can show distinct landscape-level patterns and levels of heterogeneity that contribute to our understanding of the production and decomposition processes that create SOM. Using field measurements from an alpine-subalpine ecosystem and geostatistical analyses, I show here that C and N in the older more recalcitrant SOM of mineral soil have more defined spatial patterns and are less heterogeneous than C and N in the newer more labile SOM of mineral soil at the forest-alpine tundra ecotone (SOM C: CV = 45% in older, 59% in newer; partial sill [sill minus nugget, i.e., percent of variation explained by spatial autocorrelation] = 38% in older, 11% in newer; SOM N: CV = 50% in older, 48% in newer; partial sill = 6% in older, 44% in newer). I also demonstrate that C:N ratios show better spatial patterns and reduced landscape heterogeneity when compared with their constituent C and N concentrations (CV of total SOM C = 41%, total SOM N = 31%, total SUM C:N = 20%; partial sill of total SOM C = 15%, total SOM N = 18%, total SUM C:N = 64%). The reduced heterogeneity and strong relationships between C and N in older SUM suggest that landscape variation in the chemical composition of the SUM in mineral soils converges over time, possibly as a result of greater chemical variation in plant inputs relative to the products of decomposition reactions.