Soil carbon stocks, climate and land use changes. Dominique Arrouays

Transcription

1 Soil carbon stocks, climate and land use changes Some issues, research needs and the 4 0 / 00 French initiative Dominique Arrouays In collaboration with Nicolas Saby, Martial Bernoux and Titia Mulder

2 Stocks Yearly fluxes Atmosphere: 830 Vegetation: Topsoil: 830+? Soil 1-m: From Le Queré et al ,004x2400 = 9.6 9,8 + 0,9 0,004x830 = 3.32 All in GtC = Billion tc

3 What do we learn from this? - There is theoretically an enormous potential for carbon sequestration because a very small relative change could offset a large proportion of emissions. - So the «4 pour mille» initiative makes sense But - The uncertainties on present world s soil C stocks are very large - We do not have the proper data to estimate them correctly => Are we able to map C stocks and C changes?

4 Can we map soil C stocks? Organic Carbon (%) No Data > 35 World ressource institute, 2000 JRC, Jones et al.2005 Yes we can! INRA, Meersmans et al 2012

5 Can we map soil C stocks? - Yes. At a global level all maps are rather consistent and show gradients linked to temperature and rainfall - However, they do not give the same global estimates And moreover - When you refine the estimates at more local scales (e.g. National) - The main drivers of spatial variability may change - The quality and the quantity of the data changes

6 National vs global C stocks estimates for France (GtC) Depth layer GSM 90m SG 1km 0-5 cm 0,70 1, cm 1,30 1, cm 1,55 2, cm 1,58 3, cm 0,92 2, cm 0,86 1,67 Total stock 6,91 12,20 GSM90m: All French soil C data Mulder et al Geoderma. SG1km: All ISRIC soil data Available ancillary variates (France) 90-m resolution Calibration area: France Available ancillary variates (World) 1-km resolution, Calibration area: world

7 What do we learn from this? - The global estimate is nearly the double of the local one for France - The discrepancies increase with depth - The proportion of topsoil C (0-30cm) ranges from ca 50% (national prediction) to ca 30% (world prediction) Why do we get these differences? - Many more soil data at national level - Many more ancillary data on controlling factors at national level - A finer resolution is more adapted to local processes (e.g. erosion and lateral redistribution) and to land cover effects.

8 So what about these discrepancies? - One priority should be to produce reliable estimates of C stocks and their related uncertainties - The involvement of national stakeholders, agencies and local institutions is crucial to improve the spatial predictions - The prediction of the top-soil to deep-soil ratio remains very uncertain - Strategies for research and data collection should be coordinated

9 Two ways to improve global predictions? (not exclusive) - Feed a global system with soil C measurements point data and calibrate the predictions with globally available ancillary data? - Feed a global system with national predictions? Pillar 4

10 Are there other limitations to the spatial modelling of future carbon changes using current models? YES The uncertainty on many other data on soil properties, e.g.: - Clay and silt content - Soil depth - Available Water Capacity

11 Are there other limitations to the spatial modelling of future carbon changes? Should we improve soil C models? Yes, for instance: - Soil C saturation linked to clay or clay+silt content - Phosphorus and nitrogen availability

12 Are there other limitations to the spatial modelling of future carbon changes? Should we incorporate/couple other processes? Yes, for instance: - Erosion and soil and C redistribution - Soil sealing - Deep soil carbon dynamics

13 Are there ways to monitor changes in C stocks? The first questions are: What do you want to know? How much? (globally) Where? (spatially) How much and where? (both) At wich spatial resolution and time scale? What is the minimum change you want to detect?

14 Then you come into a mess between soil monitoring guys, Statisticians, and and policy makers Do you swear to tell the truth, the whole truth and nothing but the truth? For sure e 20% of the time Please tell me. My dear? Adapted from Lark, 2009; and Arrouays Cartoons from Mandryka (Dargaud)

15 Let s communicate! We want to be able to detect a change of 1% in SOC with 95% confidence! Yes, but Got it? With what statistical power? Adapted from Lark, 2009; and Arrouays Cartoons from Mandryka (Dargaud)

16 Let s communicate! We want to be able to detect any change with 95% confidence! Yes, but Got it now? You people are stupid! Adapted from Lark, 2009; and Arrouays, Cartoons from Mandryka (Dargaud)

17 And it ends it looks like this: We are turning round How should we do, then! But hopefully there are some practical solutions Adapted from Arrouays Cartoons from Mandryka (Dargaud)

18 Three ways to monitor soil at a national or global level using soil C measurements Re-use data that were not intended for this objective (e.g. : use and put into a DB legacy soil test data asked by the farmers; e.g. France, NL, Thaïland, Korea, NZ) - Re-sample sites that were not initially sampled for this objective (e.g. resample old profiles locations; ex: Belgium, EU Biosoil programme) Settle dedicated programmes (e.g. : a soil monitoring network e.g. UK, Switzerland, France, EU) Arrouays et al., 2012

19 Other ways to do this Settle numerous benchmark and long-term agroecological sites, including trials on practices and monitoring fluxes and stocks Use their results to better understand the processes and their controlling factors and further include them in modelling Monitor (and verify) land-use changes and practices

20 We should do both! Because the global benefit will be much larger!

21 Let s come back to the 4 0 / 00 initiative

22 4 0 / 00 : What does this initiative tell us? We need to get better global and spatial estimates of carbon stocks in soils and the potential of sequestration (link with GSP pillar 4 and GlobalSoilMap). We need to monitor them. We need to settle long-term experiments and researches to better understand and model the processes and their controlling factors We need to settle benchmark sites and demonstration sites to check and demonstrate the feasability of C sequestration We need to restore degraded soils for enhancing C sequestration and food security We need a common global research and development programme to achieve these goals It is as simple as this!

23 4 0 / 00 : What is it? An international programme of research and development A voluntary process From international bodies, nations, research and development institutions To be launched and discussed during the COP21 in Paris on the 1st of December 2015

24 4 0 / 00 : It s time to adopt it! UNCCD

25 Thank you for your attention Slide: courtesy of Martial Bernoux IRD & FAO

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27 Random or systematic sampling? Random/ design-based Unbiased estimates of mean and variance Few assumptions Suited to large scale estimates Can focus through stratification Systematic / model-based Many assumptions Large sampling requirement Need to validate If valid can map Flexible to queries Courtesy from Ben Marchant (pedometrics 2011) Brus & de Gruijter,1997) Geoderma

28 Consensus? It depends. Model-based for local mapping or flexibility Design-based otherwise (i.e. for broad-scale averages over regional or soil/land use units) Judgemental and paired to assess the effects of practices From Ben Marchant (pedometrics 2011) modified