What do humus balances really tell about soil organic matter?

Abstract

Humus balance methods (HBM) are applied as tools for decision support and environmental impact assessment in arable farming. For reasons of applicability in practice, several HBM avoid a demand for comprehensive site data and are only sensitive to management. As a consequence, these methods, from a methodological point of view, cannot quantify SOM changes. Still, we show that such HBM can be used to assess the relative impact of different management scenarios at a defined site. To do so, we calculated humus balances for treatments in two long-term field experiments according to the well-recognized VDLUFA (2014) method, and compared the variation of treatments in humus balances to the variation in SOM level changes by applying analysis of correlation. In fact, the variables were positively correlated, even though the absolute deviation between balances and SOM changes was considerable. The application of another HBM that considers management impact on SOM as a site-dependent process (HU-MOD-2) showed that the ability to predict absolute SOM changes actually is dependent on sensitivity to environmental drivers of OM turnover in soils.