Long-Term Field Experiments (LTEs)—Importance, Overview, Soil Organic Matter

Abstract

In the context of climate change, at the end of the last century, humus became the centre of attention. Long-term field experiments (LTEs) are indispensable for practical soil research and for solving current problems. The evaluation of 79 long-term experiments worldwide leads to the following statements: All considerations on humus require a distinction between permanent and nutrient humus. Permanent humus is long-term stable. It is defined as humus content that is not undershot under field conditions in the absence of any fertilization and cultivation of humus-eating crops (bare fallow). In the European LTEs, the content of permanent humus C is mainly between 0.3% C_org (sandy soil) and 1.5% C_org (black earth). 90% of the organic primary substances fed to the soil, based on the starting material of vegetable biomass, are re-mineralized and returned to the atmosphere. The contents of total humus C are between 0.15% C_org and 2.29% C_org over 68 LTEs. In 31 out of 68 LTEs, the C_org content of the optimally organic and mineral fertilized variants was less than 1% C_org (= 1.724% humus). The nutrient humus C is 0.3% on average for 68 LTEs and often accounts for less than 0.2% of C_org, i.e. about 10 t/ha (at 4500 t/ha soil in the processing (A) horizon). On average across 68 LTEs, mineral fertilization increases the C_org content by 0.06 to 0.08% compared to “unfertilized”. On average across 42 LTEs, 11 t/ha of farmyard manure (FYM) annually increases the C_org content compared to “unfertilized” by 0.24% C_org. The mineral fertilizer ensures a high nutritional yield for the people and is also the basis of a sufficient humus supply. The results of 350 comparisons quantified the effect of combined organic-mineral fertilization compared to only optimal mineral fertilization, indicating that there was a 6% yield increase. Data indicate that models of organic carbon cycling in soils require permanent verification and calibration by representative LTEs. Data also show that current political goals for climate change mitigation via carbon sequestration on cropland, such as the 4 per 1000 initiative, are too optimistic. An annual increase in carbon content by 4‰ (or 1.2 billion tonnes of carbon every year) is practically impossible under the conditions of the twenty-first century, which means high yields and optimal organic and mineral fertilization.