European survey shows poor association between soil organic matter and crop yields

Nutr Cycl Agroecosyst https://doi.org/10.1007/s10705-020-10098-2 (0123456789().,-volV) ( 01234567 89().,-volV) ORIGINAL ARTICLE European survey shows poor association between soil organic matter and crop yields Wytse J. Vonk . Martin K. van Ittersum . Pytrik Reidsma . Laura Zavattaro . Luca Bechini . Gema Guzmán . Annette Pronk . Heide Spiegel . Horst H. Steinmann . Greet Ruysschaert . Renske Hijbeek Received: 26 March 2020 / Accepted: 20 October 2020 Ó The Author(s) 2020 Abstract A number of policies proposed to increase soil organic matter (SOM) content in agricultural land as a carbon sink and to enhance soil fertility. Relations between SOM content and crop yields however remain uncertain. In a recent farm survey across six European countries, farmers reported both their crop yields and their SOM content. For four widely grown crops (wheat, grain maize, sugar beet and potato), correlations were explored between reported crop yields and SOM content (N = 1264). To explain observed Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10705-020-10098-2) contains supplementary material, which is available to authorized users. W. J. Vonk (&)  M. K. van Ittersum  P. Reidsma  R. Hijbeek Plant Production Systems, Wageningen University and Research, Wageningen, The Netherlands e-mail: L. Zavattaro Department of Agricultural Forest and Food Sciences, Università Degli Studi Di Torino, Turin, Italy L. Bechini Department of Agricultural and Environmental Sciences, Università Degli Studi Di Milano, Milan, Italy G. Guzmán Institute for Sustainable Agriculture-CSIC, Cordoba, Spain variability, climate, soil texture, slope, tillage intensity, fertilisation and irrigation were added as covariables in a linear regression model. No consistent correlations were observed for any of the crop types. For wheat, a significant positive correlation (p \ 0.05) was observed between SOM and crop yields in the Continental climate, with yields being on average 263 ± 4 (95% CI) kg ha-1 higher on soils with one percentage point more SOM. In the Atlantic climate, a significant negative correlation was observed for wheat, with yields being on average 75 ± 2 (95%CI) kg ha-1 lower on soils with one percentage point more SOM (p \ 0.05). For sugar beet, a significant positive correlation (p \ 0.05) between A. Pronk Agrosystems Research, Wageningen University and Research, Wageningen, The Netherlands H. Spiegel Institute for Sustainable Plant Production, Austrian Agency for Health and Food Safety, Vienna, Austria H. H. Steinmann Centre for Biodiversity and Sustainable Land Use, GeorgAugust-Universität Göttingen, Göttingen, Germany G. Ruysschaert Flanders Research Institute for Agriculture, Fisheries and Food Research (ILVO), Merelbeke, Belgium 123 Nutr Cycl Agroecosyst SOM and crop yields was suggested for all climate zones, but this depended on a number of relatively low yield observations. For potatoes and maize, no significant correlations were observed between SOM content and crop yields. These findings indicate the need for a diversified strategy across soil types, crops and climates when seeking farmers’ support to increase SOM. Keywords Soil organic matter  Crop yield  Europe  Arable farming  Survey Introduction Agricultural science has a long history of searching for correlations between soil organic matter (SOM) content and soil fertility (Russell 1977). SOM is found to affect soil water retention (Nyamangara et al. 2001; Zebarth et al. 1999), nutrient availability and the suppression of pests and soil borne diseases (Asirifi et al. 1994; Darby et al. 2006). While all these processes are beneficial for crop yields, the size of these benefits remains uncertain. More recently, a number of studies attempted to quantify the direct benefits of SOM on crop yields. Several studies found a significant positive correlation (de Moraes Sa et al. 2014; Lucas and Weil 2012; Oldfield et al. 2019, 2020). However, others indicate that no significant effect of SOM on crop productivity could be found (Hijbeek et al. 2017a; Loveland and Webb 2003; Schjønning et al. 2018). These diverging findings call for a deeper search to understand the conditions under which SOM may contribute to improved soil fertility and crop yields. Most of the mentioned studies used field or pot experiments. Findings in controlled experiments may however deviate from farmers’ experience in the field with more varying circumstances and less controlled management. The inclusion of farmers’ experiences would add a valuable dimension to the available data on SOM and crop yields. Even more so, because the benefits of SOM depend on farm management, as more intensive management and reliance on technical means reduce dependence of crop yield on SOM functions (van Noordwijk et al. 1997). Next to management, the beneficial effect of SOM on crop yields depends on climates and soil types. 123 Increase in SOM content may also have potentially negative effects: slow nutrient mineralisation by organic matter, for example, might not supply nutrients at the precise moments when the crop needs those nutrients leading to potentially larger nutrient losses (Chen 2006). In a recent large-scale farm survey across Europe, farmers were asked to report their average SOM content and crop yields. Analysis of these data could give further insight in the relationship between SOM and crop yields under actual farming conditions, taking into account the variation in climates, soil types and cultivated crops. Using these data, we aim to answer the following three research questions: 1. Can correlations be found between SOM content and crop yields under current European farming practices, based on farmers’ observations? 2. What is the influence of climate, slope, soil texture and crop type on the correlation between SOM and crop yields? 3. How is the correlation between SOM and crop yields affected by farm management such as irrigation, tillage intensity and fertiliser use? Material and methods Study area The relationship between SOM content and crop yields was analysed based on a farm survey conducted in 2013 as part of the European Catch-C project. Farm survey data for the following six countries was used: Austria, Belgium, Germany, Spain, Italy and the Netherlands. For each country, details on climates and respondents are listed in Table 1. Methodology related to the findings presented here is described below; more details about the Catch-C farm survey are described by Bijttebier et al. (2015). Analysis of survey data Main variables The two main variables used for our analysis were observed yield (tonnes ha-1) from the crops of interest and the reported average SOM content (%) across the whole farm. Representative crop yields for the last Nutr Cycl Agroecosyst Table 1 Climate zones, number of survey respondents, average SOM content and reported crop yields per country Country Climate zone # Farmers (...truncated)