BO-R-1: Humus content of arable land – case study
2019 Monitoring Report on the DAS
2019 Monitoring Report on the DAS
On average, the humic soil contents in permanent monitoring areas in Bavaria have not changed to any relevant extent since the mid-1980s. It must be said, however, that depending on site and use, developments can vary substantially.
Soils are affected by climate change in a number of ways. Only healthy, vital and resilient soils can withstand the risks of desiccation and erosion caused by wind and water, enabling them to continue their multifarious positive functions crucial for maintaining a landscape’s water and mineral regimes.
Humus plays a major role in the resilience of soils, as it influences nearly all soil properties and functions in complex ways. Humus is the entirety of organic matter in the soil, composed of all dead vegetable and animal remains as well as their organic conversion products. Humus is an important storage medium for nutrients and water. It ensures favourable soil structure thus benefiting the air and water regime of soils. Moreover, humus reduces summer-related dessication and furthers the activity of soil organisms as well as the development of stable soil structures. The latter provides effective protection from soil compaction and soil erosion.
Maintaining site-adapted humus contents and, where necessary, also augmenting the amount of humus in the soil, therefore are important adaptation measures in safeguarding the health of soils. In that context, it is important to remember that climate change has a direct bearing on the humus contained in soils. Higher temperatures can accelerate mineralisation processes in the soil and consequently also the decomposition of organic matter. At the same time, it is possible for climate change to have humus-enhancing effects, e.g. increased temperatures enable plants to produce more biomass thus also increasing the amount of material available in the soil for conversion to organic matter. These relationships appear even more complex when also taking into account the prevailing precipitation conditions, because both humus decomposition and humus formation require water in adequate quantities, i.e. not too much and not too little.
In view of these complex relationships of climate change and humus formation or decomposition, it is currently not possible to make any reliable statements on changes regarding contents and reserves of organic matter. Besides, changes in soil use such as ploughing up grassland for use as arable land, have much stronger impacts on humus content than any long-term climate change.
The contents of organic matter in soils are essentially dependent on site-typical circumstances (type of soil, groundwater levels etc) and it therefore cannot be augmented wholesale by adding organic materials. However, management tools such as adaptation of use can serve a useful purpose. On agricultural soils options include grassland regimes, application of farmyard manure, cultivation of catch crops or leaving harvest and root remnants on site for the maintenance and accumulation of humus. Many trends which are currently focused on intensifying agriculture actually counteract any efforts to stabilise or even augment the humus contents of soils. In forests and woodlands the composition of tree species and the amount of remnants from timber harvesting are crucial factors in the formation of humus.
Regular surveys of humus contents in soils are carried out by the Länder within the nationwide network of permanent monitoring areas (BDF). The Thünen Institute has established a homogeneous system of agricultural soil condition surveys (BZE-LW/Agricultural Soil Inventory) in more than 3,000 locations nationwide. A nationwide evaluation of 171 agricultural surveys found that there were statistically significant changes to humus contents in a total of 39 BDF locations. Essential humus changes were controlled on the basis of the humus starting level at each site21.
The influence of longer-term climate change on the development of humus cannot be ruled out and requires further research. As shown by results from Bavaria, the mean contents of the most important humic components such as organic carbon (Corg) and total nitrogen (Ntot) in soil depths of 0 to 15 cm remained almost constant for the past few years. The reason being that changes in humic cotents, in particular humus increase, take place very slowly resulting in measuring inaccuracies. Depending on the type of use practised in the sites monitored, different developments have been observed: In Bavaria a significant decline in humus contents since the end of the 1960s was observed in the agricultural sites monitored where the biggest proportion of crop rotation consisted in maize and root crops, as against a lower proportion of cereal, rapeseed and fodder legumes22.
Likewise, it is difficult to achieve an unambiguous evaluation of results, because an appropriate conclusion as to what amounts of humic content are optimal, depends on a site-specific assessment. Fundamental guidance on how to derive site-specific humic contents is available from BZE-LW23. This publication has made nationwide data on Corg in agricultural soils available and is to be reviewed in ten years time, in order to facilitate the evaluation nationwide of changes in humus reserves of mineral soils in relation to site and management type.
21 Marx M., Rinklebe J., Kastler M., Molt C., Kaufmann-Boll C., Lazar S., Lischeid G., Schilli C., Körschens M. 2016: Erarbeitung fachlicher, rechtlicher und organisatorischer Grundlagen zur Anpassung an den Klimawandel aus Sicht des Bodenschutzes – Teil 3: Bestimmung der Veränderungen des Humusgehalts und deren Ursachen auf Ackerböden Deutschlands. UBA-Texte 26/2016. Dessau-Roßlau, 90 pp.
22 Capriel P. & Seiffert D. (Bayerische Landesanstalt für Landwirtschaft) 2009: 20 Jahre Boden-Dauerbeobachtung in Bayern. Teil 3: Entwicklung der Humusgehalte zwischen 1986 und 2007. Schriftenreihe 10 der Bayerischen Landesanstalt für Landwirtschaft LfL, Freising, 47 pp.
23 Jacobs A., Flessa H., Don A., Heidkamp A., Prietz R., Dechow R., Gensior A., Poeplau C., Riggers C., Schneider F., Tiemeyer B., Vos C., Wittnebel M., Müller T., Säurich A., Fahrion-Nitschke A., Gebbert S., Jaconi A., Kolata H., Laggner A., Weiser C., Freibauer A. 2018: Landwirtschaftlich genutzte Böden in Deutschland – Ergebnisse der Bodenzustandserhebung. Thünen Report 64, Braunschweig: 316 pp.
DOI: 10.3220/ REP1542818391000
Maintaining the site-typical humic contents of the soil, especially by means of adequate input of organic matter or by reducing the intensivity of cultivation. (BBodSchG, § 17 (2) 7)
Site-adapted cultivation and safeguarding the sustainable fertility of soil and long-term usability of fields (BNatSchG, § 5 (2) 1)
Protecting the ecological efficacy of soils by means of […] maintaining organic matter; intensification of soil protection in the light of the risks […] of declining humic contents; implementation of site-adapted land use strategies in order to curb negative effects resulting from changes in soil and humus formation which in turn affects C sequestration (DAS, ch. 3.2.4)