Maize rhizosphere priming: field estimates using 13C natural abundance
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Root-mediated changes in soil organic matter (SOM) decomposition, termed rhizosphere priming effects (RPE), play crucial roles in the global carbon (C) cycle, but their mechanisms and field relevance remain ambiguous. We hypothesize that nitrogen (N) shortages may intensify SOM decomposition in the rhizosphere because of increase of fine roots and rhizodeposition.
RPE and their dependence on N-fertilization were studied using a C3-to-C4 vegetation change. N-fertilized and unfertilized soil cores, with and without maize, were incubated in the field for 50 days. Soil CO2 efflux was measured, partitioned for SOM- and root-derived CO2, and RPE was calculated. Plant biomass, microbial biomass C (MBC) and N (MBN), and enzyme activities (β-1,4-glucosidase; N-acetylglucosaminidase; L-leucine aminopeptidase) were analyzed.
Roots enhanced SOM mineralization by 35 % and 126 % with and without N, respectively. This was accompanied by higher specific root-derived CO2 in unfertilized soils. MBC, MBN and enzyme activities increased in planted soils, indicating microbial activation, causing positive RPE. N-fertilization had minor effects on MBC and MBN, but it reduced β-1,4-glucosidase and L-leucine aminopeptidase activities under maize through lower root-exudation. In contrast, N-acetylglucosaminidase activity increased with N-fertilization in planted and unplanted soils.
This study showed the field relevance of RPE and confirmed that, despite higher root biomass, N availability reduces RPE by lowering root and microbial activity.
KeywordsC3/C4 vegetation change Soil CO2 SOM decomposition Enzyme activities Microbial biomass N-fertilization
The authors thank Thomas Splettstößer and Yue Sun for field assistance and Dirk Böttger for his help in constructing the closed-circulation system. We also would like to thank Karin Schmidt and Anita Kriegel for laboratory assistance and Reinhard Langel at the Center for Stable Isotope Research Analysis (KOSI) at the University of Göttingen for isotopic analyses. We gratefully acknowledge the German Academic Exchange Service (DAAD) for a scholarship award to Amit Kumar. This study was supported by the German Research Foundation (DFG) within project PA 2377/1-1.
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