Soil organic matter balance as a practical tool for environmental impact assessment and management support in arable farming

Abstract

Soil organic matter (SOM) is considered a key factor in sustainable agriculture (FAO, 2005). The reason is that SOM is involved in most ecosystem services provided by soil, such as nutrient transfer and supply to crops, water regulation, resilience against mechanical stress, or pest regulation (e.g. Fageria, 2012). From an agronomic perspective, the transformation, storage and supply of nutrients, improvement of soil structure in the root zone and regulation of soil-borne pests and diseases are of basic interest for crop production. Furthermore, SOM is relevant for the prevention of environmental hazards, such as erosion and flood events in particular, by improving the hydraulic properties of soil and its resilience against mechanical stress. Ecosystem services to crop production (Zhang et al., 2003), including adequate soil functions, are of greater relevance in low-input agriculture. However, the improvement of soil structure, its physical resilience and improved water regulation are of basic interest in high-input farming systems also. Last but not least, the sequestration of carbon in agricultural soil has been given much attention in the context of climate change mitigation in recent decades (Freibauer et al., 2004). However, the potential of organic matter supply to arable soils to offset C emissions from fossil fuels is naturally limited (Smith, 2004). In principle, farmers have always been aware of the relevance of organic matter in soil, and have developed a multitude of approaches to supply it with sufficient amounts to ensure that the soil functions adequately for crop production. The management of organic matter in soil is complex, however, because measures that improve nutrient supply to crops might at the same time increase nutrient losses, resulting in adverse effects on the environment. Against this background, soil organic matter balances (also termed ‘humus balances’) have been developed as practical tools for environmental impact assessment and management support in arable farming. To be applicable in practice, SOM balances have to perform with a limited selection of input data, in particular soil data. This is a considerable methodical challenge because the sensitivity and insensitivity of a method to various factors has an effect on the availability and reliability of output information (Brock et al., 2013). For example, the most widely recognized method of SOM balance in the German-speaking countries that has been applied in peer-reviewed papers, the VDLUFA (Association of German Agricultural Analytic and and Research Institutes) method (see Brock et al., 2013), does not consider site properties (including initial SOM contents) and management history. Therefore, by its nature it cannot quantify changes in SOM contents at a site. Erroneous interpretation of the method and related approaches, and improper application, even in scientific assessments, has resulted in a bad reputation for SOM balance methods in the scientific community in German-speaking countries. Nevertheless, the improper application or interpretation of SOM balance methods does not impair their general value and the value of the information provided. In 2013 the first international SOMpatic workshops at Rauischholzhausen, Germany, aimed to review the concepts and application of SOM balance methods and to reclaim the tools for scientific research. The second workshop was organized in Rauischholzhausen in 2015, dedicated to the database for the parameterization of factors and processes in SOM balance methods. Both workshops have been funded by the German Research Foundation (DFG). We are pleased to present five papers based on contributions to the second international SOMpatic workshop in this special section of the European Journal of Soil Science, and we gratefully acknowledge the support of the reviewers and journal's editors. Three papers address the issue of estimating organic inputs to soil from roots and above-ground crop residues. Keel et al. show that the choice of allometric equations or fixed coefficients for the estimation of organic matter or C inputs from roots and residues has considerable effects on organic matter balances calculated by different methods. Their results emphasize the urgent need for a suitable parameterization of organic matter or C inputs to soil from roots and above-ground crop residues in models, which in turn requires an adequate database. In their survey, Poeplau & Kätterer studied the effect of nutrient availability and soil texture on shoot:root ratios of barley (Hordeum vulgare L.) in a pot experiment. The results indicate that soil texture might be an important factor in root production that has been overlooked in SOM balance methods and related models. They suggest that texture should be considered in the parameterization of organic inputs with roots. This recommendation now needs to be validated under field conditions. Weiser et al. present the results of a comprehensive survey of above-ground crop residues left on the fields after the harvest of winter wheat (Triticum aestivum L.). There is an urgent need to use this information in the parameterization of organic inputs with crop residues in SOM balance methods and related models; this has been very limited so far. The authors show that insufficient parameterization of wheat residues can lead to inaccurate estimates of C input to soil: in their study, C input was underestimated by up to 24% if standard values were applied for the quantification of wheat residues. Another important issue in modelling soil organic matter balances is that organic matter turnover is not restricted to the topsoil, but sampling in field experiments used for the parameterization and validation of models usually is (see Baker et al., 2007). Against this background, Knebl et al. analysed changes in organic matter (indicated by changes in C and N) in the soil under a long-term field experiment at different depth increments to 90 cm. They found that the evaluation of treatments was significantly different if changes in C and N in the upper subsoil (30–60 cm in their study) were or were not considered. This difference occurred for both factors in the split-plot design of the experiment, ‘farm type’ and tillage. Dannehl et al. address C gains in the soil from green manure and straw application. The parameterization of these substrates in models has been based, for some time, on chemical properties and related recalcitrance against microbial decomposition. This perception has changed in the last two decades, and recalcitrance is now considered a retarding factor in decomposition that is most relevant in the early stages of organic matter turnover, but less important in terms of long-term C fate (von Lützow et al., 2008). Following Schimel & Weintraub (2003), Dannehl et al. examine whether the retention of C from straw and green manure in soil depends on N supply required for the build-up of microbial biomass. The authors were able to confirm this hypothesis for fine-textured soil types (Fluvisol and Luvisol in their study), but not for a coarse-textured Arenosol. They hypothesize that recalcitrance might have been more relevant under the less favourable environmental conditions in the Arenosol during the survey period. The five papers, thus, give some impression of the challenges in the parameterization of SOM balance methods, and we hope to encourage researchers to improve the database further for this purpose. We consider that SOM balance methods are a promising example of the transfer of scientific knowledge to practice, and should not be underestimated in their potential role and value as decision support tools for environmental impact assessment and management support in arable farming.