Practical ways of using RothC to quantify soil C stock changes caused by land use change

Transcription

1 Practical ways of using RothC to quantify soil C stock changes caused by land use change Andy Whitmore, Kevin Coleman, David Powlson Agriculture and Environment Division Rothamsted Research

2 Land use change causes changes in total soil organic C Examples :

3 Broadbalk wilderness Data modelled by RothC-26.3 (Solid lines) 100 Organic C in soil (t C ha -1 ) Woodland Arable Year

4 RothC: Morrow Plots, Illinois 80 SOC (t/ha to 15cm) Bluegrass Border Corn-oats -clover Continuous Corn Year

5 All soil organic carbon is not the same! Some parts are much more labile than others turnover times ranging from days to thousands of years. Most soil C models recognise this through multiple pools, e.g. RothC, Century, DNDC Sometimes, for simplification, all soil C is treated as a single pool but risk of error! e.g. land carbon module in HadCM3LC.

6 Structure of the Rothamsted Carbon Model Organic Inputs DPM Decay CO 2 RPM BIO CO 2 HUM Decay BIO IOM HUM Decay RPM : Resistant Plant Material DPM : Decomposable Plant Material BIO : Microbial Biomass HUM : Humified OM IOM : Inert Organic Matter

7 Modelled change in soil C (kg C m -2 ) between 1860 and 2100 Assumes constant land use. HadCM3 climate model with land C feedbacks included IS92a Business as usual scenario HadCM3LC for soil RothC for soil C loss 80 Gt C loss 54 Gt Positive feedback to climate change : soil C atmospheric CO 2 Jones, McConnell, Coleman, Cox, Falloon, Jenkinson, Powlson (2005), Global Change Biology 11,

8 UK land uses considered Arable Pasture Forest Semi-natural

9 Selected land use transitions (Soil C trend) Arable to Forest (increasing) Pasture to Arable (decreasing) Pasture to Forest (increasing) Pasture to Semi-natural (increasing)

10 Changes in soil C stock caused by LUC RothC simulations for single soil type and climate Plant C inputs for each land use as used in previous work Pasture to Semi-natural Pasture to Forest Arable to Forest Pasture to Arable Arable_2_Forest Pasture_2_Arable Pasture_2_Forest Pasture_2_Semi-natural

11 Possibilities for calculating UK soil C changes under LUC Simple soil C transition rates Use RothC on each grid square Meta-models (exponentials function approximating model simulations) - tried various meta-models for selected land use transitions - compared statistically against RothC simulations

12 Simple (parallel) exponentials 1 exponential C = (A1(1-exp(-K1*t))) 2 exponentials C = (A1(1-exp(-K1*t))) + (A2(1-exp(-K2*t))) 3 exponentials C = (A1(1-exp(-K1*t))) + (A2(1-exp(-K2*t))) + (A3(1-exp(-K3*t))) 4 exponentials C = (A1(1-exp(-K1*t))) + (A2(1-exp(-K2*t))) + (A3(1-exp(-K3*t))) + (A4(1-exp(-K4*t))) Where: C is the soil carbon A1, A2, A3 and A4 are coefficients representing the C stock K1, K2, K3 and K4 are coefficients representing the rate of that change

13 Single exponential Soil C Time

14

15 Sequential single exponentials Exponential with a single break point C = A0 + A1(1-exp(-B1*t)) C = A0 + A1(1-exp(-B1*T1)) + A2(1-exp(-B2(t - T1))) t < T1 t T1 Exponential with two break points C = A0 + A1(1-exp(-B1*t)) C = A0 + A1(1-exp(-B1*T1)) + A2(1-exp(-B2(t - T1))) C = A0 + A1(1-exp(-B1*T1)) + A2(1-exp(-B2(T2 - T1))) + A3(1-exp(-B3*(t - T2))) t < T1 T1 t < T2 t T2 Where: C is the soil carbon A1, A2 and A3 are coefficients representing the C stock B1, B2 and B3 are coefficients representing the rate of that change T1 and T2 are the times where the break points occur

16 Break point exponentials Exponential 2 Exponential T1 Soil C Time

17

18 Differential form exponential Single exponential C = Cf (Cf - Co)*exp(-K1t) Double exponential C = Cf (Cf - Co)*[M1*exp(-K1t) + M2*exp(-K2t)] Triple exponential C = Cf (Cf - Co)*[M1*exp(-K1t) + M2*exp(-K2t) + M3*exp(-K3t)] where: Co Cf Initial carbon Final carbon M1, M2, M3, K1, K2 and K3 are parameters to be estimated.

19 Differential form C f Soil C C o Time

20

21 Conclusions Some of the meta-models emulate RothC very well - not necessarily the most complex. Danger of over-parameterisation. But meta-models giving good fit have a larger number of parameters than RothC. Require different parameters depending on whether C is increasing or decreasing. Seems little benefit in meta-models compared to using RothC!

22 Can RothC be linked to data in Excel? Yes interface between RothC and Excel can be made. Will discuss with RM exact requirements. Possible problem - time to do large numbers of runs there may be better approaches.