What on Earth is a Rhizosphere Priming Effect? Weixin Cheng Environmental Studies University of California Santa Cruz Email: wxcheng@ucsc.edu
A Brief Introduction About My Research 1. Root dynamics and coupling of aboveground and belowground processes (Minirhizotron, digital camera, buried Scanner) 2. Rhizosphere processes: How much rhizo-microbial respiration? ( 14 C) (NSF) Rhizosphere priming effect ( 13 C) (USDA) Rhizosphere responses to climate change (DOE) 3. Temperature sensitivity of SOM decomposition ( 13 C) (USDA) 4. Climatic controls of 15 N natural abundance (Inner Mongolia) 5. C & N cycling under elevated CO 2 (Nevada, Oak Ridge) 6. Soil C and N processes in arctic tundra (San Diego)
The rhizosphere is a key interface in the Earth system. Releases 3-10 times more CO 2 than fossil fuel burning Uses 30-60% of global primary production on land Regulates virtually all aspects of biogeochemical cycles Rhizosphere Atmosphere Biosphere Lithosphere Pedosphere Hydrosphere 程
The term rhizosphere was proposed by Prof. Dr. Lorenz Hiltner of Germany over 100 years ago.
Cryo-scanning electron micrograph of the rhizosheath surrounding a buckwheat (Fagopyrum esculentum) root growing in the field. Root hairs can be seen crossing a macropore. (Courtesy of M. E. McCully) (Source: P.J. Gregory 2006, European J. Soil Sci.)
Aug. 15 Sept. 9 Oct. 11 Nov.17 A progressive change of Thelephoroid mycorrhizal rootlets under a mixed conifer stand near Blodgett Forest Station.
How important is rhizosphere CO 2 flux in the global carbon budget?
610 Land plants GPP - 121 R p - 60 830 + 5/ Yr Atmosphere Fossil fuels 9 N P P - 6 0? R s - 6 0 9 0 9 2 92-90 = 2 1,580 39,200 Soils Ocean A 15% change of global soil CO 2 efflux is equivalent to the amount of CO 2 emission from total fossil fuel burning) The C sequestration potential of global soils was estimated at 42-78 Pg C. (Ratten Lal 2004. SCIENCE)
A conceptual definition of a rhizosphere priming effect on SOM decomposition Assuming total soil CO 2 efflux =100% Soil Basal Respiration Root Respiration SOM-derived Plant-derived Rhizosphere Priming Rhizo-Microbial Respiration The RPE is defined as the alteration of soil organic matter decomposition by plant roots and rhizospheric microbes.
The RPE is defined as the alteration of soil organic matter decomposition by plant roots and rhizospheric microbes. Here is an operational definition:
The Issue of Priming Effect Apparently, most roots grow in the soil. However, because of various methodological difficulties, a significant portion of our understanding of soil functions is based on measurements using rootless soil samples. A question inevitably arises: Does rootless soil functions in the same way as the soil with roots?
The Question: how important is the rhizosphere priming effect? Many studies of SOM decomposition have relied on laboratory incubations of soil samples without live roots and vegetation, implicitly assuming that the rhizosphere effect has no impact on the results. However, the assumption needs to be rigorously tested.
C a r b o n F l o w s i n S o i l S y s t e m s S o i l S u r f a c e P r i m a r y P r o d u c t i o n C O 2 R o o t R e s p L i v e R o o t s L i t t e r E x u d a t i o n R o o t T u r n o v e r R h i z o - D e p o s i t i o n D e a d R o o t s C O 2 R h i z o - R e s p M i c r o f l o r a F a u n a C O 2 C O 2 S o i l R e s p. S o i l C a r b o n
WHERE DO YOU ALL COME FROM? C O I am from roots C O C O C O C O C O I am from soil organic matter C O I am from the rhizosphere C O C O C O C O C O
Photosynthetic Pathways of CO 2 fixation in Higher Plants Characteristics C 3 C 4 CAM* CO 2 acceptor RuBP PEP In light: RuBP First product of photosynthesis C isotope ratio in photosynthate (δ 13 C) CO 2 -compensation level Photosynthetic capacity Dry matter production Our natural 13 C tracer method: C3 acids (PGA) C4 acids In dark: PEP In light: PGA In dark: malate -20 to -40%o -10 to -20 %o -10 to -35 %o 30-50 ppm <10 ppm In light: 0-200 ppm In dark: <5 ppm slight to high *CAM: Crassulacean Acid Metabolism high to very high In light: slight In dark: medium Medium High Low
C3 plants δ 13 C: - 27 C4 plants δ 13 C: - 12 Naturally Occurring C3 soil δ 13 C: - 25 C4 soil δ 13 C: - 14 C4 plants δ 13 C: - 12 C3 plants δ 13 C: - 27 Switched in Experiments C3 soil δ 13 C: - 25 C4 soil δ 13 C: - 14
The Natural 13 C Tracer Method If one grows C3 plants in a C4-derived soil, the following equation can be used to partition soil-derived C4-carbon from plant-derived C3-carbon: C 3 = C t ( δ t - δ 4 )/( δ 3 - δ 4 ) C t =C3+C4, total carbon from belowground CO 2 ; C 3 : carbon derived from C3 plants; C 4 : carbon derived from C4 soil; δ t : δ 13 C value of the C t carbon, δ 3 : δ 13 C value of the C3 plant carbon, δ 4 : δ 13 C value of the C4 soil carbon.
Continuous 13 C-labeling Greenhouse at UCSC [CO 2 ] = 400 ppm δ 13 C = -17
Measuring total soil CO 2 efflux Sealant Mylar balloon NaOH trap Pump Water trap
Magnitude of the rhizosphere effect on SOM decomposition measured by isotope methods (Based on Cheng and Kuzyakov 2005). Plant Type Treatment Soil Type 1 PGC 2 %Priming 3 Time 4 (d) Reference Wheat CLO GC -37 16 Cheng 96 Wheat Ambient CO 2 CLK GC 44 28 Cheng & Johnson 98 Wheat Elevated CO 2 CLK GC 17 28 Cheng & Johnson 98 Wheat Ambient CO 2, +N CLK GC 42 28 Cheng & Johnson 98 Wheat Elevated CO 2, +N CLK GC 73 28 Cheng & Johnson 98 Sunflower Ambient CO 2 CLK GH 55 53 Cheng et al. 00 Sunflower Elevated CO 2 CLK GH 31 53 Cheng et al. 00 Wheat 12/12 hrs light/dark CLK GC 100 38 Kuzyakov & Cheng 01 Wheat 12/60 hrs light/dark CLK GC -50 38 Kuzyakov & Cheng 01 Soybean Growing season mean CLK GH 70 120 Fu & Cheng 02 Sunflower Growing season mean CLK GH 39 120 Fu & Cheng 02 Sorghum Growing season mean SLC GH -9 120 Fu & Cheng 02 Amaranthus Growing season mean SLC GH -5 120 Fu & Cheng 02 Soybean CLK GH 3 35 Cheng et al. 03 Wheat CLK GH 7 35 Cheng et al. 03 Soybean CLK GH 382 68 Cheng et al. 03 Wheat CLK GH 287 68 Cheng et al. 03 Soybean CLK GH 312 89 Cheng et al. 03 Wheat CLK GH 130 89 Cheng et al. 03 Soybean CLK GH 254 110 Cheng et al. 03 Wheat CLK GH 60 110 Cheng et al. 03 Soybean Growing season mean CLK GH 164 119 Cheng et al. 03 Wheat Growing season mean CLK GH 96 119 Cheng et al. 03 %priming is calculated as: (planted - unplanted)/unplanted X 100.
A summary: Based on results from 16 papers over 20 years of research, both positive and negative rhizosphere effects on SOM decomposition have been found in numerous experiments. The magnitude of the effects varies widely, from -50% to as high as +380% above the unplanted control. Many factors (e.g., timing, plant phenology, N, atmospheric CO 2, light, temperature, soil types, and plant species) influence the magnitude of the rhizosphere priming effect.
Dijkstra & Cheng 2007 Ecology Letters
More rhizodeposition leads to more priming. (Dijkstra & Cheng, 2007, Ecology Letters)
Implications to soil carbon modeling Most soil C models: M R=kM Our evidence suggests: M R=kM The rhizosphere link Or is it like this?
What are the mechanisms of the priming effect? The following hypotheses have been given (Cheng & Kuzyakov 2005): Microbial activation/ faunal grazing Water fluctuation Preferential substrate usage Competition for nutrients Aggregate destruction Uptake of labeled-c by roots
Mechanism-1: The priming effect is negatively correlated to microbial biomass turnover time, or positively correlated to turnover rate (Cheng, SBB2009). SOM-Derived CO 2 (mg C pot -1 d -1 ) 180 160 140 120 100 80 60 40 20 0 No-plant Wheat Soybean 85 62 31 SOM-Derived CO2 165 61 42 Microbial Turnover Time 180 160 140 120 100 80 60 40 20 0 Microbial Biomass Turnover Time (Days)
Mechnism-2: Transpiration-induced wetting-drying cycles accelerates microbial turnover, SOM decomposition, and nitrogen mineralization through microbial dehydration and osmotic shock upon re-hydration (Cheng, SBB 2009). 140 SOM-derived CO 2 (mg C pot -1 d -1 ) 120 100 80 60 40 20 Unplanted control 0 0 100 200 300 400 500 600 ET (ml water pot -1 d -1 )
What happened to the soil nitrogen dynamics if so much soil C has been primed by roots?
100 Soil N recovered 2.5 2.0 % initial soil N 80 60 40 20 No_Plant Soybean Wheat g N pot -1 1.5 1.0 0 No-Plant Soybean Wheat 0.5 0.0 Shoots Roots NH4+NO3 N-fix-1 N fix-2 Net Min By 15 N By N budget
Then, there is one more question: How could roots get soil N by giving available C to soil microbes?
Enhanced microbial turnover is a key competition mechanism for mineral nitrogen between plant roots and soil microbes. Higher Turnover Rate Lower Turnover Rate Microbial Biomass Mineral Nitrogen Plant Roots Microfauna Signal/hormonal compounds Microbial Loop
Implications to the positive feedback hypothesis of global warming
Temperature sensitivity of SOM decomposition and the positive feedback hypothesis Atmospheric CO 2 Accelerated Decomposition Surface Temperature Soil Organic Carbon
Which component of the total soil respiration is more sensitive to warming? Soil Basal Respiration Root Respiration SOM-derived Plant-derived Rhizosphere Priming Rhizo-Microbial Respiration
CO 2 -C (mg C kg -1 soil d -1 ) 20 15 10 5 The rhizosphere priming effect is more sensitive to warming. * * 0 Total CO2 efflux Plant-derived SOM-derived Unplanted Primed soil C Q =1.5 10 Q =1.2 10 Q =2.2 10 Q <1.0 10 Q =7.3 10 Responses of different components of belowground CO 2 to 3 C warming. Open bars are for the ambient temperature treatment, and crossed bars are for the ambient+ 3 C treatment. * indicates significant (P<0.05) difference between the two treatments. (Zhu & Cheng, Global Change Biology 2011)
National Science Foundation funded project: Rhizosphere priming under field conditions.
Summary and Implications (1) Both positive and negative rhizosphere effects on SOM decomposition have been found in various experiments. The magnitude of root effects varies widely, from -50% to as high as +380% above the unplanted control. Several factors were found to be important for the assessment of the root effects on SOM decomposition, e.g., timing, plant phenology, N, CO 2, light, temperature, soil types, and plant species.
Summary and Implications (2) The dynamic pattern associated with rhizosphere processes is more like a pipeline system with a high through-flow than a reservoir system. More rhizosphere input stimulates more SOM decomposition. Accelerated turnover of microbial biomass is identified as one of the key mechanisms operating in the rhizosphere. Transpiration-induced drying-rewetting cycles in the rhizosphere can be another key mechanism responsible for the rhizosphere effect.
Summary and Implications (3) Rhizosphere priming effect can be more sensitive to temperature change (implications for global warming) These findings challenge the reliability of existing assessments on the rates of soil organic matter decomposition because they primarily came from soil incubations without the presence of plant roots and vegetation.
My Research on Inner Mongolian Grasslands
De-Convoluting Isotope Signals in Plants and Soils in Inner Mongolian Grasslands
11 9 7 5 3 Elevation precip temp S15N2 1-1 -1 1 3 5 7 9 11 Longitude (E) Cheng et al., 2009
8 6 Soil δ 15 N (0-20 cm) 4 2 0-2 Soil_15N0-20 soil_15npredict -4 110 112 114 116 118 120 122 124 Longitude (E) Cheng et al., 2009
Digitizing Root Dynamics in Inner Mongolian Grasslands Leymus chinensis
Root Turnover of Leymus chinensis Grassland in Inner Mongolia 6.0 Root Turnover Rate (times yr -1 ) 5.0 4.0 3.0 2.0 1.0 Coring 2004 Rhizotron 2004 Coring 2005 Rhizotron 2005 Root turnover rates in temperate grasslands are 5-fold higher than traditional assessments using soil coring method. This provides the major missing piece to the puzzle why grasslands store so much soil carbon with relatively low NPP compared to forests and other ecosystems. 0.0 Control N-addition (Bai et al., in preparation)
As we were traveling on the grasslands, I thought about The Tragedy of the Commons by Gary Hardin published in 1968, and his stark prognosis (socialism or privatism), and the paper of David Sneath (1998), based on satellite image analysis, he showed that grassland degradation occurred to a much greater extent in IMAR than in comparable regions in neighboring Mongolia. According to Sneath, nomadic pastoralists in Mongolia have better maintained their grasslands than the state-own settled collectives established in IMAR in the middle of the 20 th century. Is it really the settlement or the commons causing the degradation?
Appropriate policies for managing common resources are dependent on the combined levels of natural and social resource densities. Social Resource Density Japan Survival levels China Western Europe Many African Countries USA Russia Pre-historical Communities Natural Resource Density
So my newly found research question is: How natural capital and social capital of resource-poor communities in Inner Mongolia interactively influence grassland degradation and recovery under the fast changing natural and social environment? Or How will they struggle out of the trapped Tragedy of the Commons and revert the trend of grassland degradation?
Acknowledgements Collaborators & Co-authors: Nick Bader, Feike Dijkstra, Shenglei Fu, Dale Johnson, Yakov Kuzyakov, Biao Zhu John Blair provided great help on obtaining the valuable Tallgrass prairie soil. David Harris analyzed all samples for isotopes. Many undergraduate students helped on sample processing. USDA NRI and Kearney Foundation provided the funding.
Thank you! Any questions?