Elevational gradients and distributions of aggregate associated organic carbon and nitrogen and stability in alpine forest ecosystems

Abstract

Abstract Distribution and elevational controls on soil aggregate‐associated organic carbon (OC) and nitrogen (N) in alpine soils are not well known, but may be critical to the stabilization of soil C and N pools in alpine areas. In this study, we determined the variability in aggregate associated OC and N concentration and aggregate‐stability in 0 to 20 cm layers along the elevation gradient in an alpine forest (2,600–3,200 m) of the Qilian Mountains. The results showed that 1–2 mm macroaggregate accounted for the largest proportion (33.9%) of all aggregate fractions, and also contributed greater proportions (33.2 and 32.6%) to OC and N in bulk soils. Concentrations of OC and N, and C to N ratio increased with increasing aggregate size across all elevations. With increasing elevation, the proportion of >1 mm macroaggregates, mean weight diameter (MWD) and geometric mean diameter (GMD) increased significantly ( p < .01). Aggregate OC and N concentration and C to N ratios, and the contribution of > 1 mm aggregate OC and N to OC and N in bulk soils also increased significantly ( p < .01), while the contribution of 0.25–1 mm and 0.053–0.25 mm aggregates decreased with increasing elevation. In addition, MWD, GMD, OC and N concentration in aggregate decreased with mean annual temperature (MAT) and increased with mean annual precipitation (MAP). Our results demonstrated that the stability of soil structure improved, and accumulation of OC was mainly due to increases in the contribution of OC in the >1 mm macroaggregate with increasing elevation. Given that higher elevations in this alpine forest support large concentrations of SOC and macroaggregate OC, which may be vulnerable to climate warming.