No relationship between outputs of simple humus balance calculators (VDLUFA and STAND) and soil organic carbon trends

Simple humus balance calculators were developed for farmers and consultants to determine the best crop rotation and amount of organic fertilizer required to improve soil quality and prevent nutrient leaching in croplands. Although the potential of these tools to infer the impact of different agricultural practices on soil organic carbon (SOC) dynamics in croplands is not well studied, they have been integrated in several farm-level climate or environmental impact assessment calculators. Here we examine the correlation between humus balance values estimated with two different tools developed in Germany/Central Europe and observed changes in SOC content at 14 long-term sites in Switzerland. The first tool was developed by the Association of German Agricultural Investigation and Research Institutes and is referred to as the VDLUFA. The humus balance calculator STAND is a descendent of the VDLUFA that accounts for pedoclimatic factors in Central Europe. Crop rotations were distinguished based on cultivation practice, whereby those with mixed fertilization were supplied with mineral fertilizer alone and in combination with organic materials, while those with organic fertilization include unfertilized and organic fertilizer treatments. An analysis of 133 short-term observations (i.e. individual crop rotations of five and 6-year duration) and 26 long-term observations (i.e. several crop rotations with a total duration of ≥10 years) showed that humus balance values (kg C ha⁻¹ year⁻¹) of short-term crop rotations were not or only poorly correlated with the observed change in SOC content (%) (R² = 0.06 in STAND and R² = 0.05 in VDLUFA for crop rotations with organic fertilization, and R² < 0.01 for crop rotations with mixed fertilization). The correlation did not improve when the humus balance values of long-term observations with mixed fertilization were compared with decadal SOC development (R² = 0.04 for STAND and R² = 0.06 for the VDLUFA). Stronger correlations were found only for long-term observations with organic fertilization (R² = 0.68 for STAND and R² = 0.64 for the VDLUFA). These findings underline that while the studied humus balance calculators are able to distinguish the effect of different fertilizers (organic vs. mineral) on a farm's humus supply on the longer term, neither are suited for predicting SOC trends over single crop rotations. Although this study was carried out in Switzerland, the results should apply to any region with temperate climate and similar soil properties.