Above and belowground net primary productivity of grassland influenced by supplemental water and nitrogen in Inner Mongolia
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While water availability determines grassland productivity in semiarid regions, nutrient availability is the main limiting factor under wet conditions. An experiment was conducted in 2008 at two sites in Inner Mongolia with histories of heavy grazing (HG) and moderate grazing (MG) to study the interactive effects of water and nitrogen on above- and belowground net primary productivity (ANPP and BNPP), biomass partitioning, and plant species composition. The study comprises two water treatments (no irrigation and irrigated when soil water content was below 70% of the field capacity), and two nitrogen (N) levels (0 and 100 kg N ha−1). Mean values of ANPP at the peak biomass time reached 1,028 ± 95 SD g m−2 at the HG site and 568 ± 32 SD g m−2 at the MG site in irrigated and fertilized treatment. Nitrogen use efficiency (NUE) was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites. Water use efficiency (WUEt) based on total water input and ANPP decreased with irrigation at the HG site. Meanwhile, N application significantly increased WUEt, WUEp (based on precipitation), and WUEi (based on irrigation water) at both sites. BNPP was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites, and it tended to decrease with N addition. However, the fraction of belowground to total biomass (f BNPP = BNPP/(ANPP+BNPP) decreased with the addition of supplemental resources and exhibited a negative correlation with ANPP. Species diversity remained lower at the HG site compared to the MG site; it decreased with the addition of supplemental resources at the latter site. The annual Salsola collina contributed the most to the total biomass under irrigation. Based on global climate models, more frequent extreme climates are predicted in the future, which can result in changes in resource availabilities. Therefore, our research results have important implications for predicting the production and other properties of grassland ecosystems.
KeywordsANPP BNPP Water/nitrogen use efficiency Biomass allocation Species diversity Grazing history
We would like to thank the National Nature Science Foundation of China (NSFC, 41071207/30821003) and the Deutsche Forschungsgemeinschaft (DFG, project DI 546/3-2). We also extend our gratitude to the Inner Mongolia Grassland Ecosystem Research Station of Botany Institute, Chinese Academy of Science, for providing the working facility. We thank Dr. Chengci Chen of the Central Agricultural Research Center of Montana State University Central, USA, for his constructive comments and linguistic revision. Three anonymous reviewers and Dr. Klaus Butterbach-Bahl are thanked for their constructive comments for the improvement of the manuscript.
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