Material and Methods

1.4.1 Study sites

Most of the studies were conducted in the Nagqu Prefecture of the Tibetan Autonomous Region (TAR) on sites of the Kobresia Ecosystem Monitoring Area (KEMA) (Fig. ES-1). The KEMA research station was established in 2007 by the University of Marburg and the University of Lhasa with support from the Volkswagen foundation, and is currently managed by the Institute of Tibetan Plateau Research (ITP). The sites are located in the core area of the Kobresia pygmaea distribution at 4510 m a.s.l., close to the village Kema, on a gentle north-west exposed slope (92°05`39``E, 31°16´17´´N). The village was founded in the 1980´s resulting in a strong increase of the grazing intensity since then (He & Richards 2015).

Grazing is mainly by Yak and sheep but, a large number of small mammals (Plateau pikas, Ochotona curzoniae) compete for with livestock forage. The soil is classified as a Stagnic Eutric Cambisol (Humic, IUSS Working Group WRB 2014) and developed from a Holocene loess layer. The mineral soil consists of loam (mainly fine sand and silt) and has neutral pH values (pH in H2O: 6-7). The topsoil is strongly rooted and forms a dense mat roughly 15 cm thick (Kobresia turf). It protects the soil from mechanical degradation such as trampling and erosion.

According to the Nagqu meteorological station (4507 m a.sl.) the mean annual temperature (1971-2000) is -0.9°C with 15.6°C (monthly mean) in the warmest month (July). The mean annual precipitation is 430 mm but can range between 210 and 440 mm. The rainfall strongly depends on the onset and end of the summer monsoon (Miehe et al. 2008).

Accordingly, most of the rainfall occurs in the summer months between May and August (Babel et al. 2014; Ingrisch et al. 2015). Together, the precipitation and the temperature control the length of the vegetation period, which normally lasts from May to mid-September (Miehe et al. 2008).

1.4.2 Research approaches

Investigations were primary implemented on two sites of the Kobresia Ecosystem Monitoring Area KEMA (Fig. ES-1) between June and September in 2012 and 2013. Grazing exclosure plots were established in 2009 in a spring pasture and 2010 in a year-round pasture for yak, sheep and goats (sites are termed KP-2009 site and KP-2010 site). These sites were selected according to the following criteria: (a) a predominance of Kobresia pygmaea, (b) homogeneous root mat and soil characteristics, (c) appropriate relief position enabling footprinting for eddy-covariance measurements, (d) accessibility and (e) grazing regime. This chapter gives a general overview of the field sites, sampling design and experiments, with a more detailed description in each individual study.

Research sites

The KP-2009 site is mainly used as spring pasture from April to June, with exceptional winter grazing in years with heavy snowfall, and therefore has a low grazing intensity throughout the growing season. The site is characterized by very dense coverage of Kobresia pygmaea (average about 61%, maximum 81%), interspersed with perennial herbs such as Lagotis brachystachya, Lamiophlomis rotata, Lancea tibetica, Potentilla bifurca, Potentilla plumosa, Potentilla saundersiana and Thalictrum alpinum and monocotyledons such as Carex ivanoviae, Carex spp., Festuca spp., Kobresia pusilla, Poa spp., Stipa purpurea, Trisetum spp.

(Seeber et al. 2016). The limited presence of other plant species enabled a species-specific investigation of Kobresia pygmaea nutrition and other plant traits (investigated by cooperation partners, see previous chapter 1.2). In contrast, the KP-2010 site is characterized by a shallow soil and less precipitation and is thus drier. Thus, although the total species set is comparable to KP-2009, the vegetation community contains a higher proportion of grasses such as Stipa purpurea and Elymus spec. (E. Seeber, pers.

communication). The site has a high grazing intensity throughout the year and therefore was used to identify grazing effects on N and C dynamics using stable isotope approaches.

Isotope studies

Stable isotopes (in this study, ¹³C and ¹⁵N) provide a powerful tool in soil biogeochemistry.

Methods can be classified as tracer or natural abundance approaches (Fry 2007). Both approaches were used in this PhD study. ¹³C tracer studies were carried out using enriched

13CO2 inside labeling chambers. The ¹³C label was chased through several plant and soil compartments to identify how C allocations changed depending on grazing regime (chapter

3.1). ¹⁵N tracer, in the form of ¹⁵N enriched urea, nitrate, ammonia or ammonia-nitrate, was mainly due to kinetic fractionations, preferential losses or changing C and N inputs (Högberg 1997; Fry 2007; Werth & Kuzyakov 2010). Thus the changing isotope ratio is a suitable proxy to reflect relevant biogeochemical processes (chapter 2.2; 2.3, 3.1) .

Degradation sequences

Based on theoretical concepts, experimental approaches and field surveys, various degradation sequences were selected on the Kema research sites (KP-2009 and KP-2010).

Here, the soil- and root-mat characteristics (i.e. soil type, soil texture, root mat thickness, above- and belowground biomass, bulk density, SOC and nutrient contents) had relatively low variability and offered a good opportunity for investigating degradation-induced impacts on SOC and nutrient losses and its driving processes. Chapters 2.3 and 2.4 provide a comparison of intact Kobresia turf vs. Kobresia turf with extending polygonal cracks vs. bare soil patches (S0 to S5); in Chapters 2.5 and 2.6: intact Kobresia turf vs. dying Kobresia turf vs.

crust-covered Kobresia turf (living, dying, dead); in Chapters 3.5 and 3.6: intact Kobresia turf vs. crust-covered Kobresia turf; and in Chapter 3.7 the most common surface patterns of the Kobresia pastures: intact Kobresia turf vs. crust-covered Kobresia turf vs. bare soil patches.

Laboratory incubation experiments

Research in the Autonomous Region of the Tibetan Plateau is challenged by unfavorable conditions: calculability, accessibility of research sites, physical demands at high altitude, intermittent supplies of clean water and electricity, and poorly equipped laboratory facilities.

To overcome these restrictions, samples were taken in situ (mainly from the KP-2009 site), transported to Göttingen and investigated with incubation experiments (Chapters 4.5, 4.6 and 5.4). This allowed the investigation of more sensitive and dynamic parameters such as microbial characteristics or quantities of organic and mineral N forms (i.e. DON, NO3-, NH4+).