Pasture Degradation Modifies Soil Organic Matter Properties and Biogeochemical Functioning in Tibetan Grasslands.

Kobresiapastures of the Tibetan Plateau represent the world’s largest alpine ecosystem. Moderate husbandry is beneficial for the storage of organic carbon (OC), nitrogen (N) and other nutrients. However, undisturbed root mats are affected by freezing and thawing, which cause initial ice cracks. Decomposition of root mats is accelerated and increased grazing may additionally enhance root mat degradation. We studied the impact of the root mat layer on soil OC stabilization and microbial functioning depending on depths, to predict future changes by overgrazing and climate change.

We investigated the soil below Kobresia root mats along a degradation sequence from intact root mat (stage 1) to mats with large cracks (stage 4). Vertical gradients of δ¹³C values, neutral sugar, cutin and suberin contents as well as microbial community composition (PLFA profiles) and activities of six extracellular enzymes involved in the C, N, and P cycle were assessed.

Soil OC and N contents as well as C/N ratios indicate an increasing illuviation of topsoil material into the subsoil with advancing degradation status. This was confirmed by more negative δ¹³C values and increasing contributions of cutin to OC in the subsoils from degradation stages 1 to 4. PLFA profiles were surprisingly similar in the subsoils of degradation stages 1, 2 and 3 although OC contents and composition in the subsoil changed progressively from stage 1 to 4. Only the PLFA profiles of stage 4 differed from those of the other subsoils. These findings were confirmed by the enzyme activities which were highest in the subsoil of degradation stage 4, whereas degradation stages 2 and 3 showed low enzyme activities in the subsoil if related to soil OC amount. We conclude that pasture degradation decreases not only mechanical protection of soil surface by Kobresia root mats, but also changes their biochemical and microbial functions.