A gronomy J our n al • Volume 10 0 , I s sue 3 • 2 0 0 8 517 ABSTRACT Th e improved soil N min -based N management is a promising approach to precision N management, which determines the optimum side-dress N rates based on N target values and measured soil nitrate N content in the root soil layer at diff erent growth stages. A total of 148 on-farm N-response experiments, in seven key summer maize (Zea mays L.) production regions of North China Plain (NCP) from 2003 to 2005, were conducted to evaluate the N min -based N management compared to traditional farmer's N practices. Th e recommended N rates based on the improved soil N min method were not signifi cantly diff erent ( ≤31 kg N ha -1 ) from those determined by yield response curves (n = 13). Th e average N rate determined with the soil N min method (157 kg N ha -1 ) was signifi cantly lower than farmer's practice (263 kg N ha -1 ), while maize grain yield was 0.4 Mg ha -1 higher than farmer's N practice (8.5 Mg ha -1 ) across all sites (n = 148). As a result, the improved soil N min -based N management signifi cantly increased net economic gains by $202 ha -1 , reduced residual nitrate N content and N losses by 44 kg N ha -1 and 65 kg N ha -1 , respectively, and improved recovery N effi ciency, agronomic N effi ciency and N partial factor productivity by 16%, 6 kg kg -1 and 36 kg kg -1 , respectively, compared with farmer's N practice. We conclude that the improved soil N min -based N management can be applied for summer maize production in NCP for improved N use effi ciency and reduced environmental contamination.
The critical value of soil Olsen-P is the point above which the probability of crop yield response to fertilizer P is small or nil. Determining this critical value is fundamental when making appropriate P fertilizer recommendations. In this study, the critical values were determined for continuous maize (Zea mays L.)-winter wheat (Triticum aestivum L.) cropping systems from a 15-year field experiment across three sites in China using linear-linear, linear-plateau and Mitscherlich models. The mean critical values for maize using the three models ranged from 12.1 to 17.3 mg P kg −1 (average 15.3 mg P kg −1 ) and for winter wheat from 12.5 to 19.0 mg P kg −1 (average 16.3 mg P kg −1 ) among study sites. The mean critical value for maize was approximately 7% lower than that for winter wheat across all sites based on the three models. Critical values identified by the Mitscherlich model were 1.4 to 2.1 times those from linear-linear and 1.3 to 1.9 times of those from linear-plateau and were crop and site dependent. There was a significant negative correlation (P<0.05) between the mean critical value from the three models and the observed P uptake by either maize or wheat. Our study shows that the critical values can vary with sites, crops and models used, and thus caution should be taken when selecting the most appropriate one when making P fertilizer recommendations for agronomic return and to minimize chances of negative environment impact from overfertilization.
We determined the historical change in soil organic carbon (SOC) stocks from long-term field trials that represent major soil types and climatic conditions of northern China. Soil carbon and general circulation models were validated using these field trial data sets. We then applied these models to predict future change in SOC stocks to 2100 using two net primary production (NPP) scenarios (i.e., current NPP or 1% year À1 NPP increase). The conversion rate of plant residues to SOC was higher in single-cropping sites than in double-cropping sites. The prediction of future SOC sequestration potential indicated that these soils will be a net source of carbon dioxide (CO 2 ) under no fertilizer inputs. Even when inorganic nutrients were applied, the additional carbon input from increased plant residues could not meet the depletion of SOC in parts of northern China. Manure or straw application could however improve the SOC sequestration potential at all sites. The SOC sequestration potential in northern China was estimated to be À4.3 to 18.2 t C ha À1 by 2100. The effect of projected climate change on the annual rate of SOC change did not differ significantly between climate scenarios. The average annual rate of SOC change under current and increased NPP scenarios (at 850 ppm CO 2 ) was approximately 0.136 t C ha À1 yr À1 in northern China. These findings highlight the need to maintain, and where possible increase, organic carbon inputs into these farming systems which are rapidly becoming inorganic fertilizer intensive.
Zebrafish embryogenesis is a rapid process driven by a myriad of gene products and small molecules. As previous studies have detailed the relevant transcriptional and proteomics changes, here we assess the metabolomic changes that occur at different stages of embryogenesis (4, 8, 12, 24 and 48 hours post fertilization). Metabolite levels were detected using GC-MS and LC-MS, following which multivariate analysis (OPLS-DA) was applied to identify metabolites that were differentially regulated throughout embryogenesis. From the two robust OPLS-DA models that were generated (Q(2)(cum) = 0.940 and Q(2)(cum) = 0.894), a total of 60 detected metabolites (20 from GC-MS, 40 from LC-MS) were identified and found to be important in discriminating between developmental stages. Hierarchical clustering analysis was applied to the dataset and metabolite classes such as amino acids and lipids were shown to be differentially regulated. Biologically relevant transcriptomic and proteomic data were associated with metabolites to provide a more holistic systems perspective of embryogenesis. In summary, the metabolic profiles of different developmental stages highlight the dynamic changes occurring during embryogenesis. These data could serve as a basis for future toxicological or developmental studies.
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