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Influence of grazing pressure on the carbon isotope composition of the grasslands of China: Spatio-temporal variations at multiple scales

Subject Area Soil Sciences
Term from 2007 to 2011
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 5470836
 
Final Report Year 2011

Final Report Abstract

Summary of results The project was based on the idea that grazer tissue, especially wool, is a superior material than vegetation itself, to retrieve the ratio of C3 and C4 plants because it allows easy sampling; it integrates over the whole year and thus considers the asynchronous growth of both components; it levels out small-scale variations within the grazing ground and thus reduces the scatter when analyzing large-scale pattern; it provides an archive that allows retrospective views. This idea depends on several preconditions that had to be scrutinized before the central research topic could be treated. 1. It is necessary that selection by the animals or difference in digestibility between the ingested vegetation components do not alter the isotopic relation between grazer tissue and vegetation. The grazing experiment provided an excellent tool to study this. In cooperation with subproject P3 and P4 it could be shown that selection and digestibility have no effect on the isotopic signal laid down in grazer tissues (Wittmer et al. 2010a, b). This was true for δ13C and δ15N, which capture different plant functional groups (e.g. δ13C mainly differentiating between C3 and C4 photosynthetic groups, δ15N mainly differentiating between legumes and non-legumes but also between C3 and C4) and which follow different metabolic pathways within the animal. This finding was true for wool reflecting long-term (months) effects and faces reflecting short-term effects (days). 2. It is necessary to know the isotopic shift between feed and a specific tissue. These shifts could also be obtained from the grazing experiment with high statistical accuracy due to the large number of samples (Wittmer et al. 2010a, b). Furthermore, the calculation of shifts (or enrichments) can be subject to mathematical or experimental artefacts, which have to be avoided. This was extensively treated by Auerswald et al. (2010). 3. A simple mixing model is applied when retrieving the ratio of C3 to C4 plants from the isotopic composition of a bulk material like grazer tissue. As a prerequisite, the isotopic signatures of both end-members have to be known. They may be subject to spatial and temporal variation that has to be considered. a. It could be shown that the C3 end member is influenced by altitude (Männel et al. 2007; preceding project) and moisture availability (Wittmer et al. 2008). Both effects P10 - Influence of grazing pressure on the carbon isotope composition of the grasslands of China: Spatio-temporal variations at multiple scales. 162 can be predicted from available information although the prediction of moisture availability makes a rather sophisticated geostatistical approach necessary (Wittmer et al. 2008) due to the sparse meteorological network. b. For time series it has to be proven that the end-members do not change in time differently to the Suess effect. It was intended to analyze this for the C3 component of the Mongolian grassland based on herbarium samples (see hypothesis 3). This analysis could not be finished because our Chinese cooperation partner could not convince enough owners of such herbariums to provide samples. We are optimistic that this can be achieved in the near future. Furthermore, during the course of the project we were able to do such analyses for C3 components in other areas than the Mongolia plateau, which show that the Suess effect dominates the temporal change but changes in water vapour deficit and stomatal opening modify the response (Köhler et al. 2010; Barbosa et al. 2009, 2010). c. A constant C4 end-member is usually assumed and this was also assumed in all our past publications. Our data, however, suggest that this assumption is not true for the C4 community on the Mongolian plateau due to predominance of Cleistogenes squarrosa, which is characterized by a high leakiness (mean of own data: 0.59). The high leakiness causes its discrimination to vary with grazing intensity, precipitation and possibly other factors. Furthermore, the C4 community discrimination varies with the contribution of C. squarrosa. This contribution mainly depends on the proportion of C4 annuals, which differ in their proportion depending on the degree of disturbance. Two publications on both aspects are presently under preparation. It is important to note, however, that the findings and conclusions of our other papers, which assumed a constant C4 end-member, are not biased by these findings because the effects only contribute to the scatter but not to the pattern (due to the deviating spatio-temporal scale on which they operate). After having solved the methodological issues mentioned above we were able to analyze the spatio-temporal pattern of the C3/C4 ratio on the Mongolian plateau. The major findings were: I. The pattern of the present C3/C4 ratio in Inner Mongolia follows closely the 22 °C isotherm of the warmest month (Auerswald et al. 2009). This is interesting as this temperature is the predicted crossover temperature for the present CO2 concentration and for open stands, where the crossover temperature indicates equal photosynthetic efficiency of the C3 and the C4 photosynthetic pathways. P10 - Influence of grazing pressure on the carbon isotope composition of the grasslands of China: Spatio-temporal variations at multiple scales. 163 II. The pattern of the C3/C4 ratio as derived from soil samples deviates pronounced from the pattern of present vegetation insofar as the proportion of C4 in present vegetation is higher and the areas of low and high proportion differ between vegetation and soil (Wittmer et al. 2010c). Again the pattern stored in the soil agreed well with the predictions from crossover theory, when the CO2 concentration 50 years ago and the temperature pattern of the last normal period (1961 – 1990) were used. The difference between the soil pattern and the present vegetation pattern indicated that C4 have increased especially by spreading to the north and to high (> 1000 m) altitudes. III. Our hypothesis 2 implied that C4 have increased in areas of high grazing pressure. This hypothesis was based on several publications, which mostly speculate about this. This speculation is based on the fact that the dominant C4 species, C. squarrosa, is grazing tolerant and the co-dominant C4 species, Salsola collina, is an annual and hence also promoted by disturbance like heavy grazing. Furthermore, the increase in C4 over time is parallel to the increase in stocking rates in Inner Mongolia. To answer this question and to break the correlation inherent in the comparison of two time series we had proposed in a preceding project to compare Inner Mongolia with Mongolia (formerly Republic of Mongolia). However no publication resulted from this (small) preceding project due to the surprising fact that the results did not corroborate the expectations. This was even more surprising as Mongolia not only has lower stocking rates than Inner Mongolia but also nomadic grazing is still practised in Mongolia, which is thought to be better adapted to the climatic conditions of the region, while Inner Mongolia switched to sedentary grazing in the 1960s. This surprising result hence had to be carefully examined for possible artefacts. After having included more data from this project and after careful consideration of all methodological issues (see above) we are now certain that grazing pressure has no influence on the spatial pattern. In fact, again temperature describes best the C3/C4 pattern on the Mongolian plateau, with no apparent break along the Mongolian border (Fig. 4.11.1), which led us to submit this surprising finding now (Wittmer et al., submitted). Presently we are preparing a manuscript based on the MAGIM grazing experiment, which – under better controlled conditions – also shows that grazing pressure has no influence on the C3/C4 ratio but the temperature increase even during the short time span of MAGIM’s second phase was able to significantly alter the C3/C4 ratio also in the grazing experiment. P10 - Influence of grazing pressure on the carbon isotope composition of the grasslands of China: Spatio-temporal variations at multiple scales. 164 Fig. 4.11.1: Regional percentage of grazed C4 biomass estimated by ordinary block (5 × 5 km²) kriging, derived from wool originating from 1996 – 2007. The white lines denote the 21, 22 and 23 °C isotherms of the mean July temperature, averaged for the years 1996 – 2007. Towns are UB = Ulaanbaatar, SH = Sainshand, E = Erenhot & Zamny-Uud, XH = Xilinhot, D = Duolun and HU = Haliut. IV. The increase in the proportion of C4 species will reduce the amount of biomass early in the growing period (April/May) due to the preference for higher temperatures of C4 plants. This seasonal shift could also change the water use, which was analyzed in the publication by Yang et al. (2010), which was only partly funded by this MAGIM subproject and partly by Chinese funds. Yang et al. (2010) evaluated the water sources of six dominant species (three perennial C3 grasses, one C3 sedge, one C3 shrub and C. squarrosa) in measuring the hydrogen stable isotope composition of stem water, soil water and precipitated water as well as the leaf water potential. Only the C3 shrub exhibited a complete access to the (winter precipitation recharged) deep soil water, while the C4 grass entirely depended on summer precipitation. The C3 grasses showed a resource-dependent water use strategy, utilizing deep soil water when it was well available and shifting to rain water when subsoil water had been exploited. The C3 component thus considerably increases the resilience towards summer draughts. In consequence, the relative increase in C4 plants due to the temperature rise will cause a more pronounced, summer rain related, interannual fluctuation in primary production with severe consequences on the available feed and overgrazing in summer-dry years. In addition to the manuscripts under preparation, which were mentioned above, our future work until the end of the MAGIM project will establish a time course of the C3/C4 ratio, P10 - Influence of grazing pressure on the carbon isotope composition of the grasslands of China: Spatio-temporal variations at multiple scales. 165 deducted from δ13C woollen samples originating from back to the 18th century and to relate this time course to time courses of climatic, socio-economic and biotic parameters.

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