Factors controlling decomposition and C cycling in deep soil horizons

Transcription

1 Factors controlling decomposition and C cycling in deep soil horizons Cornelia Rumpel CNRS Campus AgroParisTech Thiverval-Grignon, France

2 Subsoil 0 50 OC content (mg/g) Subsoil Depth (cm) Subsoil : soil below A horizon low OC content few organisms

3 Why is organic matter in subsoils important? World stock of soil OC 14 C age (years BP) Depth increment (cm) Gt of OC (10 15 g of OC) Depth (cm) Baisden et al., 2002 Jobbagy and Jackson, 2000 Batjes, 1996 Eusterhues et al., 2003 Krull and Skjemstad, 2003 E.A Paul et al., 1997 Subsoil > 50% of the SOC stock Subsoil = high level of OC stability

4 Origin of subsoil C 1. Dissolved Organic Matter 2. Root input 3. Bioturbation Relative importance of the three sources is unknown Depend on climate, soil- and, vegetation type

5 Why is organic matter in subsoils apparently so old? Strongly stabilised compounds Accumulation of recalcitrant compounds such like BC Reduced microbial activity

6 Acid subsoil horizons Steinkreuz (Steigerwald) Dystric Cambisol Vegetation: Fagus sylvatica Humus form: moder Waldstein (Fichtelgebirge) Haplic Podzol Vegetation: Picea abies Humus form: mor C content g kg -1 Ah 82.6 Bv 9.8 SwdBv1 3.0 SwdBv2 1.4 III Cv 1.1 IV Cv 0.5 C content g kg -1 Aeh 29.0 Bh 92.8 Bvs 52.0 BvCv 7.7 Cv1 1.7 Cv2 1.9

7 Quantity and radiocarbon age of organic matter in subsoils Steinkreuz, Dystric Cambisol carbon storage 8.1 kg C/m² % of total organic carbon Waldstein, Orthic Podzol carbon storage 11.4 kg C/m² % of total organic carbon Ah Bv mod. mod. Aeh Bh SwdBv1 655 Bvs 745 SwdBv BvCv 1570 IIICv IVCv Cv1 Cv2 } 3840 Radiocarbon age (year B.P.) 47 to 76 % of total organic carbon is stored below the A horizon. Radiocarbon age is increasing with depth in both soils, showing that high concentrations of stabilised carbon are present below the A horizon

8 How is SOM in subsoil horizons stabilised? Demineralisation with 10 % hydrofluoric acid Steinkreuz (Steigerwald) Dystric Cambisol Waldstein (Fichtelgebirge) Haplic Podzol Ah Bv SdBv1 SdBv2 IIICv IVCv1 Carbon loss (% of total C) Aeh Bh Bvs BvCv Cv1 Cv2 Carbon loss (% of total C) Carbon loss after demineralisation increases below the A horizons Rumpel et al., 2002, Org. Geochem.

9 Interaction with the mineral phase C loss after HF C in heavy fraction Joung C 14 C activity (pmc) y = -0.42x r 2 = y = -0.37x r 2 = Old C % of initial C % of initial C Carbon in subsoils is intimatly associated with soil minerals Rumpel et al., 2002, Org. Geochem. Eusterhues et al., 2007, Org. Geochem.

10 Adsorption in acid subsoils Good correlation with iron oxides suggest that they are stabilising agents in acid subsoils Eusterhues et al., 2005, EJSS

11 Why is organic matter in subsoils apparently so old? Strongly stabilised compounds Accumulation of recalcitrant compounds such like BC Contribution of 14 C depleted geogenic C Reduced microbial activity

12 13 C NMR spectra of plant biomass Plant biomass ppm 0-45 ppm C Alkyl C: lipides, wax, aliphatic biomolecules ppm O-alkyl C: polysaccharides, proteines, lignines ppm aryl C: lignines, tannins, black carbon ppm carboxyl C: acides, proteines

13 13 C NMR spectra of plant material at different stages of degradation Moder overlying Dystric Cambisol Mor overlying Haplic Podzol alkyl/o-alkyl alkyl/o-alkyl L Of Oh ppm ppm Degradation of plant material leads to accumulation of alkyl C and loss of O- alkyl C

14 Is there a stabilisation of specific compounds in subsoil horizons? Dystric Cambisol alkyl C/O-alkyl C Haplic Podzol alkyl C/O-alkyl C Alfisol alkyl C/O-alkyl C A 0.84 Aeh 1.57 A 0.58 Bv 1.27 Bh 0.91 B 0.42 SdBv 1.60 Bvs 0.74 C ppm ppm ppm The chemical composition of OM in subsoils is dependent on soil-inherent pedological processes

15 Nature of mineral associated polysaccharides Bulk soil (% of total C) Dense fraction Dystric Cambisol Bv SdBv1 SdBv Ah GM/AX Haplic Podzol Aeh Bh Bvs BvCv n.d Subsoil horizons and dense fractions: higher contribution of microbial-derived sugars Rumpel et al., 2010, SBB

16 Contribution of microbial derived polysaccharides in mineral interaction to C stabilisation in two subsoils Microbial-derived Plant-derived GM/AX Bulk soil Dense fraction >2 g cm -3 r 2 = C activity (pmc) old young microbial-derived polysaccharides are persisting at mineral surfaces most probably through effective stabilisation by mineral interactions. Rumpel et al., 2010, SBB

17 What about nitrogen? 0 C/N ratio Woodland (Jenkinson et al., 2009, EJSS) broadbalk arable andosol (Jenkinson et al., 2009, EJSS) Soil depth (cm) Decreasing C/N ratio partly due to increasing proportion of soil NH 4+ -N fixed in minerals

18 C/N versus 14 C activity Joung C 14 C activity (pmc) Old C y = 9.2x C/N ratio r 2 = 0.79 Stabilised carbon compounds are found in subsoil horizons with low C/N ratios enrichment of microbial-derived compounds Data from Jenkinson et al., 2009, EJSS

19 Conclusions What are the mechanisms that lead to stabilisation of OM in subsoils? interaction with soil minerals (especially amorphous iron oxides) Is there a high contribution of recalcitrant compounds in subsoil horizons? bulk chemical composition of SOM is soil-type specific stabilised subsoil OM is associated with high soil nitrogen concentrations microbial material seems to be preferentially stabilised

20 Why is organic matter in subsoils apparently so old? Strongly stabilised compounds Accumulation of recalcitrant compounds such like BC Reduced microbial activity

21 May subsoil SOM be able to participate in C and N cycles? 3 hypothesis: Subsoil C turns over slowly because of unfavourable abiotic conditions low microbial biomass and diversity nutrient and/or energy limitations of microbial biomass

22 Incubation experiment with 13 C and 15 N labeled roots 0-30 cm cm cm 100 g soil 2 g wheat roots (labeled with 13 C et 15 N) Litterbag (100-µm) 10 cm Length 120 cm and 7.5 cm 10 cm

23 Incubation experiment with 13 C and 15 N labelled roots R-1 R-2 R-3 50 cm 1 m 1 m 0-30 cm cm cm 0-30 cm cm cm 0-30 cm cm Adjacent soil cm Litterbags (3 for each depth) were extracted after 6, 12, 20, 29 and 36 months of incubation. Monitoring of C and N decomposition and physical conditions (temperature and humidity)

24 Abiotic parameters Volumetric Water content [%] Temperature [ºC] (6/2006) Months 30cm 60cm 100cm Sanaullah et al. 2011, Plant and Soil

25 Microbial biomass C MBC [mg / g C] * a * a * a a a b ab a b * b a b a a * a Soil MBC labelled MBC 30cm 60cm 90cm * a a a Months MBC increase in all depths Sanaullah et al. 2011, Plant and Soil

26 Bacteria:fungi ratio a 16S/18S ratio a a ab b ab ab * ab b b b b b b b b b b * 30cm 60cm 90cm Monate More fungi in surface soil Contribution increased in all depths after root addition

27 Microbial communities Automated intergenic spacer analysis (ARISA) 30cm 60cm 90cm Eigenvalues d = d = d = Bacteria Eigenvalues Eigenvalues d = d = d = 0.1 Fungi Eigenvalues Eigenvalues Eigenvalues 6 Months 20 Months 36 Months Microbial communities seem to be quite different in top- and subsoils

28 Decomposition dynamics in top- and subsoil horizons Labeled C remaining Labeled N remaining % of initial C, N cm 60 cm 90 cm Time (months) Time (months) There are distinct carbon decomposition dynamics in top- and subsoil horizons Similar amounts of C and N remaining at the end of the incubation and similar stabilisation mechanisms operating Sanaullah et al. 2011, Plant and Soil

29 Decomposition dynamics in top- and subsoil horizons % of initial C, N % Labeled of root-c C remaining Labeled N remaining in fpom 30 cm 60 cm 90 cm root-c in <50 µm fraction: 34 7% of initial Root-C in opom fraction: 30 ± 3% of initial Time (months) Time (months) There are distinct carbon decomposition dynamics in top- and subsoil horizons Similar amounts of C and N remaining at the end of the incubation and similar stabilisation mechanisms operating Sanaullah et al. 2011, Plant and Soil

30 Conclusions There are distinct carbon decomposition dynamics prevailing in subsoil horizons carbon remaining after three years and operating stabilisation mechanisms similar Abundance, activity and community structure is different between the three layers and evolves differently after root litter addition Same job was performed despite lower amounts and potential activity of microbial biomass in the subsoil

31 The importance of available nitrogen Subsoil Topsoil Chloroform labile C Potentially mineralizable N In subsoil horizons, N availability may be a limiting factor for C mineralisation Garcia-Pausas et al., 2008, SBB

32 Energy limitation of the microbial biomass MRT Mineralbound OM Cambisol 320 +/- 27 yr 50 % /- 74 yr 58 % m ppm m unlikely in this soil But input of fresh organic matter greatly reduced Fontaine et al., 2007, Nature

33 Fontaine et al., 2007, Nature Energy limitation of the microbial biomass Subsoil at m depth proportion of mineralised SOM Age of mineralised SOM CO 2 total = 1329 ±154 yrs CO 2 control = 222 ±119 yrs CO 2 released due to priming = 2567±226 yrs Subsoil C may be able to participate in the soil C cycles if an energy is provided

34 Effect of wastewater irrigation Subsoil was more prone to SOM loss compared to topsoil after wastewater irrigation Jueschke et al., 2008, Water Science and Technology

35 Main difference between topsoil and subsoil horizons roots In deep soil horizons the spatial distribution of SOM and the soil microbial biomass is stuctured and related to those of preferential flow pathway and of soil pores (Bundt et al. 2001; Nunan et al. 2003). Preferential flow

36 Study site : Long-term observatory sampling points 120 cm long and 18 mm

37 Variability of carbon and nitrogen in deep soil horizons Normalised semi-variance 2 1 Carbon 0-30 cm cm cm 2 1 Nitrogen Distance (m) Distance (m) In deep soil horizons: high small scale variability

38 Carbon in subsoils is not homogenously distributed Lp S SFe Cv Tongues C = 5,7 ± 2,1 mg g -1 ** Matrix C = 2,7 ± 0,7 mg g -1 Carbon is horizontally stratified Chabbi et al., 2009, SBB

39 Stabilisation of C by interaction with soil minerals 55 ± 2 pmc HF soluble C (% of initial C) yrs BP 105 ± 12 pmc modern *** 0 Matrix Tongues Stabilised carbon in subsoil horizons is located in spatially distinct parts of the soil profile Chabbi et al., 2009, SBB

40 Preferential flow of dissolved organic matter Preferential flow pathways are biological hot spots Remain stable for decades Bundt et al., 2001, SBB Hagedorn and Bundt, 2000, Geoderma

41 Earthworm burrows Anecic earthworms build burrows down to 5 m depth transport of fresh organic matter from the soil surface into the borrow. 14 C activity (pmc) 10 cm Burrow Matrix cm Burrow Matrix cm Burrow Matrix 79.0 Don et al., 2008, SBB

42 Incorporation of root derived carbon in subsoil C3-C4 chronosequence Les Closeaux, Versailles wheat C3 plant maize C4 plant δ 13 C = 28 δ 13 C = 12 Wheat Maize SOC content δ SOC (t=0) δ C4 δ C3 new SOC old SOC 0 t years of C4 after C3 after Balesdent and Mariotti, 1996 Mendez-Millan, PhD thesis

43 Incorporation of root derived carbon in subsoil depth (cm) δ 13 C ( ) Root markers (di-acids) soil under wheat soil under maize (12 yrs) Important incorporation of maize root markers in all the soil profile Root colonize preferentially ancient root channels (Rasse et al., 1999) Mendez-Millan et al., 2012, Organic Geochemistry

44 Conclusions Does subsoil OM participate in C and N cycles? Subsoil OM may participate in C and N cycles provided that fresh litter is available for microbial activity microbial biomass has access to SOM Horizontal stratification is extremely important: SOM which participates actively in C and N cycles is located next to passive SOM; this should be taken into account in soil carbon models The processes which would lead to changes of active and passive SOM contribution within the soil profile need further investigations