Soil labile organic carbon fractions and soil organic carbon stocks as affected by long-term organic and mineral fertilization regimes in the North ChinaPlain.

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Summary 1.Reproductive biomass (RB) is a key component of plant life history. Because RB is allometric, it remains unclear whether the observed variations of RB in response to land use are driven only by a size dependency effect or if the partitioning of plant biomass may also shift independently of plant size. This question has not been addressed for all the main component species of a given plant community. 2. Here, we report the results of a 3-year experimental study conducted in Tibetan alpine grasslands. The above-ground vegetative biomass (VB) and the RB of 32 species were measured during 2 years in control, grazed and fertilized plots. We estimated the allometric slope and intercept for 301 VB-RB bivariates, using type II regression analyses in order to disentangle size-dependent and size-independent effects of treatment on RB. 3. Our results indicate significant changes in the VB-RB allometric slope for 26 out of 102 cases in grazed plots, and 16 out of 88 cases in fertilized plots. Moreover, in cases where slopes were not affected by treatment, a significant change in the allometric intercept was found in 50% of the models. For these cases, intercepts -which are indicative of the biomass invested in reproductive parts for a given VB -generally increased in grazed plots and decreased in fertilized plots. 4. Grazing and fertilization triggered a significant decreases or increases in the plant size for species, especially graminoids. However, we found no relationship between changes in allometric coefficients and magnitude of the biomass response. 5. At the interspecific level, the intercept, but not the slope, of the allometric VB-RB relationships was significantly affected by land use. Graminoids and forbs responded similarly to treatment. 6. Synthesis . We conclude that (i) size-dependent effects did not explain all the changes observed in RB following grazing and fertilization in alpine grasslands, and that (ii) changes in allometric intercept explained most of the size-independent variations both at the intra-and interspecific level. We discuss why size-independent changes in RB could be important for plant performance in changing environments.

Whether grazing response of species abundance can be predicted by plant functional trait remains a challenge untested in a specific ecosystem. We measured species abundance and relevant functional traits for 30 common component species in a moderately grazed and a control community over 3 years (2005, 2006 and 2007) in a Kobresia capillifolia dominated alpine meadow on the Tibetan Plateau. Our objective was to examine species response to grazing and to test the relationship between the response of species abundance and functional traits in grazed and control habitats. Our results showed: i) in terms of species relative abundance and saturated height (the maximum height of an adult individual), most component species significantly decrease in response to moderate grazing and the effect differed among species and between functional groups.ii) The response of species abundance was significantly negatively correlated with saturated height, but not correlated with specific leaf area or seed size. We concluded that the response direction of species abundance to grazing can be predicted by functional traits, whereas it is a weak predictor of the extent of grazing response in species abundance.

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Results are summarized for the first 15 yr of an eight‐site, long‐term experimental network in China designed to assess the sustainability of cropping systems in environments representing 70% of Chinese cropland. Systems were wheat–maize double cropping (two crops per year) at four sites, wheat–rice double cropping, rice‐based triple cropping, and wheat or maize single cropping. Without fertilizers, wheat yields were mainly ≤1 t ha−1, and maize yields were ≥2 t ha−1 With NPK fertilizer (rates averaging 154, 33, and 54 kg ha−1 per crop of N, P, and K, respectively), wheat yields mainly ranged from 5 to 7 t ha−1, and maize yields ranged from 6 to 9 t ha−1 Without P fertilizer, yields declined (up to 4 t ha−1 less than with NPK), and Olsen‐P values in soil declined, although rates differed between sites. Decreasing yields from withholding K usually emerged more slowly. The results emphasize the value of long‐term experiments to reveal trends in soil fertility not apparent within a few years and the need for research in these environments to define “critical concentrations” of plant‐available P and K in soil for maintaining maximum crop yields. Results with manures show the risk of overfertilization and water pollution with N and P if inorganic fertilizer applications are not decreased to take account of nutrients from manure. At two sites, there was evidence of significant N and P inputs from irrigation water. At one site, the addition of N fertilizer gradually caused soil acidification; this caused inefficient utilization of nutrients and led to crop failure.

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