Soil organic matter has long been recognized as one of the most important components in maintaining soil fertility, soil quality, and agricultural sustainability. The soil zone strongly influenced by plant roots, the rhizosphere, plays an important role in regulating soil organic matter decomposition and nutrient cycling. Processes that are largely controlled or directly influenced by roots are often referred to as rhizosphere processes. These processes may include exudation of soluble compounds, water uptake, nutrient mobilization by roots and microorganisms, rhizosphere-mediated soil organic matter decomposition, and the subsequent release of CO2 through respiration. Rhizosphere processes are major gateways for nutrients and water. At the global scale, rhizosphere processes utilize approximately 50% of the energy fixed by photosynthesis in terrestrial ecosystems, contribute roughly 50% of the total CO2 emitted from terrestrial ecosystems, and mediate virtually all aspects of nutrient cycling. Therefore, plant roots and their rhizosphere interactions are at the center of many ecosystem processes. However, the linkage between rhizosphere processes and soil organic matter decomposition is not well understood. Because of the lack of appropriate methods, rates of soil organic matter decomposition are commonly assessed by incubating soil samples in the absence of vegetation and live roots with an implicit assumption that rhizosphere processes have little impact on the results. Our recent studies have overwhelmingly proved that this implicit assumption is often invalid, because the rate of soil organic matter decomposition can be accelerated by as much as 380% or inhibited by as much as 50% by the presence of live roots. The rhizosphere effect on soil organic matter decomposition is often large in magnitude and significant in mediating plant-soil interactions.
Our new USDA-funded project (Cheng & Firestone 2006-09) aims to validate two key mechanisms behind the rhizosphere effect on soil organic matter decomposition: (1) accelerated soil microbial turnover rate, and (2) transpiration-induced drying-rewetting cycles. As a central part of the new project, we will also investigate the role of rhizosphere carbon fluxes in shaping the temperature sensitivity of soil organic matter decomposition. This is related to the current debate about the potential positive feedback mechanism between global warming and the rate of soil organic carbon decomposition. The positive feedback mechanism informs that, if warmer environment accelerates the release of CO2 from the decomposition of soil organic carbon, the global environment will get even warmer because of the extra CO2 from soil carbon pool will increase the atmospheric CO2 concentration and intensifies the greenhouse effect, and this even warmer condition will further lead to more CO2 release from soil organic carbon pool, therefore forming a viscous circle. Although there are clearly controversies about temperature sensitivity of soil organic matter decomposition among published reports, one issue is widely recognized: the lack of understanding about rhizosphere carbon fluxes and how they may respond to temperature changes is at the heart of the current debate.
Key methods: Natural 13C tracers, 13C continuous labeling, and dynamic temperature and moisture control.
Bader, N.E. and W. Cheng. Rhizosphere priming effect of Populus fremontii obscures the temperature sensitivity of soil organic carbon respiration. Soil Biology and Biochemistry . In press.
Cheng, W. and F.A. Dijkstra. Theoretical proof and empirical validation of a continuous labeling method using naturally 13 C-depleted carbon dioxide. Journal of Integrative Plant Biology. In press.
Dijkstra, F.A., W. Cheng and D.W. Johnson. 2006. Plant biomass influences rhizosphere priming effects on soil organic matter decomposition in two differently managed soils. Soil Biology and Biochemistry 38: 2519–2526.
Cheng, W. and Y. Kuzyakov. 2005. Root effects on soil organic matter decomposition. IN: R. Zobel and S. Wright (eds.) Roots and Soil Management: Interactions between Roots and the Soil, Agronomy Monograph no. 48. American Society of Agronomy, Madison, WI, USA. pp.119-143
Cheng, W., S. Fu, R. B. Susfalk, and R. J. Mitchell. 2005. Measuring tree root respiration using 13 C natural abundance: rooting medium matters. New Phytologist, 167: 297-307.
Fu, S. and W. Cheng. 2004. Defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter under a sunflower species: Helianthus annuus . Plant and Soil , 263: 345-352.
Kuzyakov, Y. and W. Cheng. 2004. Photosynthesis controls of CO 2 efflux from maize rhizosphere. Plant and Soil , 263: 85-99..
Cheng, W., D.W. Johnson, and S. Fu. 2003. Rhizosphere effects on decomposition: controls of plant species, phenology, and fertilization. Soil Sci. Soc. Am. J. 67:1418-1427.
Fu, S., W. Cheng, and R. Susfalk.2002. Rhizosphere respiration varies with plant species and phenology: A greenhouse pot experiment. Plant and Soil, 239:133-140.
Fu, S. and W. Cheng. 2002. Rhizosphere priming effects on the decomposition of soil organic matter in C 4 and C 3 grassland soils. Plant and Soil, 238:289-294.
Kuzyakov, Y. and W. Cheng. 2001. Photosynthesis controls of rhizosphere respiration and organic matter decomposition. Soil Biology and Biochemistry , 33: 1915-1925.
Cheng, W. 1996. Measurement of rhizosphere respiration and organic matter decomposition using natural 13 C. Plant and Soil 183(2): 263-268.
Cheng, W. and D.C. Coleman. 1990. The effect of living roots on soil organic matter decomposition. Soil Biology and Biochemistry 22:781-787.