Acceleration in N cycling controlled by aggregate size, moisture, substrate quality and phosphorus fertilization in floodplain soil

Dr Muhammad Riaz1,2, Dr. Pascal A Niklaus3, Dr. Beat  Frey4, Dr. Beat  Stierli4, Dr. Joerg Luster2

1Department of Environmental Sciences & Engineering, Government College University Faisalabad, Pakistan, Faisalabad, Pakistan, 2Forest Soils and Biogeochemistry, Swiss Federal Research Institute WSL, Birmensdorf, Switzerland, 3Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland, 4Rhizosphere Processes, Swiss Federal Research Institute WSL,, Birmensdorf, Switzerland

Semi-terrestrial soils, such as floodplain soils, undergo a characteristic changes in their physico-chemical environment which is generally conducive for nitrous oxide (N₂O) production. Being hotspots of N₂O emissions, the floodplain soils can become an important source of terrestrial N₂O flux. Processes of nitrogen (N) cycling are strongly influenced by the availability of water, oxygen and substrate, and these processes may differ between small and large aggregate. We performed a microcosm incubation experiment using soil from restored floodplain section of the Thur River in NE Switzerland to investigate the effects of aggregate size, moisture, carbon (C) sources and phosphorus (P) fertilization on N cycling. Experiment included aggregate size (<250 µm; 250 µm – 4.00 mm), moisture level (60% WHC; submerged conditions), C source (glucose; litter; litter-derived DOC) and P as experimental factors. Head space gas samples were collected for N₂O gaseous analysis and flux calculation. After 28 days incubation, the soil samples were analysed for N species (KCl-extractable NH₄-N, NO₃-N, dissolved organic N total N, water-extractable total N), water-extractable organic C, microbial biomass N,  leucine aminopeptidase activity (LEU), denitrification enzymatic activity (DEA), bacterial abundance (16S-qPCR), fungal abundace (ITS- qPCR) and some N cycling pathway functional genes (nirS, nosZ & nxrB). We found strong effects of experimental factors on N cycling processes. N₂O flux was generally higher in larger aggregates and soil treated with litter-derived DOC and after P addition. Mineral N concentrations were many-fold higher for litter-derived DOC and litter treated soil amended with P under submerged conditions. Dissolved organic N varied dramatically between the treatments. LEU activity was significantly higher in soil treated with litter and P in smaller aggregates. Bacterial and fungal diversity and functional gene abundance varied significantly among the treatments. Results suggested that strong heterogeneity in environmental factors could control hotspots and hot moments of N cycling.


Dr. Muhammad Riaz is working as an Associate Professor in the Department of Environmental Sciences & Engineering, Government College University Faisalabad (GCUF), Pakistan. He earned his PhD in Environmental Sciences from the University of York, UK. His research is mainly focused on soil biogeochemistry, CNP cycling in agroecosystems, biochar as a tool for soil C sequestration and soil quality management, and dynamics of soil organic matter cycling and recycling in semi-arid and arid agroecosystems.

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