Ecological intensification increases soil C stocks via changes in crop residue traits

Pablo García-Palacios, Andreas Gattinger & the Eco-Serve consortiumUniversidad Rey Juan Carlos; FIBL Research Institute of Organic

Agriculture; University of Coimbra; University of Wageningen; University of Lund; University of Grenoble

Ecological intensification (EI)practices such as organic farming(OF) enhance soil C stocks.However, increased C inputstypical of OF are not enough toexplain why OF outweighs the Csequestration capacity ofconventional farming (CF). Highersoil C stocks are also found inorganic farms with low manurerates, and C inputs via plantresidues from crop production arelikely lower under OF. Thus,altered soil C losses

(e.g. decomposition rates ofresidues) between farming systemsmight play a role.

Here we assess whether changesin crop residue traits that are keyfor decomposition, and thusinfluence soil C loss, drive soil Csequestration responses to OF.Specifically, we tested thehypotheses that (1) crop residuetraits are as important asfertilization inputs and climate todetermine soil C storageresponses to organic farming; and(2) that EI enhances soil C

storage by reducing crop residuequality for decomposers (low Nconcentration).

Using a global meta-analysis oncase studies reporting OF vs CFpaired farms, we found supportfor OF increasing SOC (Fig 2).Also, we found that the positiveOF effect on soil C sequestrationwas dependent on the leafnitrogen (N) concentration ofeach crop species. The positiveOF effect on soil C sequestrationat global scales was dependent onthe crop leaf N concentration (Fig3). Crop residue changes withmanagement were furtherinvestigated across six Europeansites comparing ecological (e.g.organic) vs. conventionalintensification (Fig 1). Increasedcrop residue lability (e.g. high leafN) promoted higher and lowerSOC stocks, respectively, inconventional and in ecologicalintensification (Fig 4). Thus,intraspecific changes towardscrop residue lability inconventional intensification wererelated with a higher response ofSOC stocks to OF.

We have shown that thephenotypic traits of crops andtheir residues modulate theresponse of SOC to agriculturalpractices. Therefore, a trait-based framework of cropresidues is fundamental whenevaluating the ecosystemservices delivered by ecologicalintensification practices such asorganic farming.

Fig. 2: Effects of organic compared withconventional farming (OF-effects) on soil Cand respiration in global meta-analyses. Dataare standardized mean differences (Cohen’s d)in soil respiration, SOC stocks and SOCsequestration (seq) between OF and CF acrossfarms worldwide. The bars around the meansare bias-corrected 95%-bootstrap confidenceintervals. If they do not overlap with zero, themeans are significantly different from zero.Analyses were conducted separately for allcase studies (black circles) and forcomparisons where organic farms applied lowmanure rates (LMR, grey circles). Number ofstudy cases are shown in brackets.

Fig 4: Members of the Eco-Serveconsortium at Frick (Switzerland). 17th

January 2017

Fig. 3: Structural equation models testing the importance of crop resource economics traits on the effects of organic farming (OF-effects)on SOC stocks (a) and sequestration rates (b) in global meta-analyses. The Cohen’s d (standardized mean difference between OF and CF) iscomputed as the response variable (in brown). The model also includes the influence of fertilization intensity (annual C and N inputs, ingrey), mean annual temperature (MAT, in red), and crop R:S ratio (in dark green). Continuous and dashed arrows are positive and negativerelationships, respectively. The widths of the arrows are proportional to the strength of the path coefficients. Non-significant (P > 0.05) pathcoefficients are softened. Goodness-of-fit metrics for each model are: SOC stocks (Bootstrap P = 0.251; RMSEA = 0.035, P = 0.476; GFI =0.997) and SOC sequestration (Bootstrap P = 0.472; RMSEA = 0.042, P = 0.426; GFI = 0.496). R2SOCstocks = 0.23, R2SOCseq = 0.24

Fig. 1: Conventional vs Ecological Intensification trials and experimental fields in fivedifferent types of agroecosystems across Europe

Fig. 4: Changes in crop residue traitsaccount for the effect of ecologicalintensification (EI) on SOC stocks acrosssix European sites. Relationships betweenthe effect sizes of EI (lnRR, the ratio ofecological intensive to conventionalfarming) on SOC stocks and on crop leaflitter N concentration. The predicted-relationship (dashed line) and the 95 %confidence intervals are shown (dottedlines). P < 0.01, n = 31.

Montados under ecological or conventional intensification, Portugal

Arable lands with different rotation practices, Sweden

Long-term DOK experiment, Switzerland

Extensive and intensive grasslands, France

Organic vs convential farming trials in The Netherlands

INTRODUCTION OBJECTIVES MAIN RESULTS

CONCLUSION