BO-R-1: Humus contents in soils of arable land and grassland soils | Umweltbundesamt

BO-R-1: Humus contents in soils of arable land and grassland soils – case study

On average, the humic soil contents in permanent soil 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 land use, developments can vary substantially. In grassland, humic contents are generally higher. In arable fields, an increased cultivation of catch crops and the additional organic fertilisation promote the build-up of humus.

Humus strengthens soil resilience

An important adaptation strategy aiming at soil protection consists in making soils more resilient towards hazardssuch as desiccation and / or erosion. Humus plays amajor role in the resilience of soils, as it influences nearlyall soil properties and functions. 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. It is an important storage medium for nutrients and water and facilitates a favourable soil structure. This has a beneficial effect on the air and water balance in the soil and reduces soil compaction and erosion. Moreover, humus reduces desiccation in summer and furthers the activity of soil organisms. Therefore, conserving and augmenting site-adapted humic content, are important adaptation measures in safeguarding the health of soils.

With a view to the diverse relationships between climate change and humus, it is important to remember that also climate change has a direct bearing on the humus contained in soils. On one hand, higher temperature can accelerate mineralisation processes in the soil and the decomposition of organic matter, while on the other hand, it can also have humus-increasing effects, for example when plants form more biomass owing to the higher temperature, thus increasing the availability of more material for conversion to organic soil matter (cf. Indicator BO-I-4). Changed precipitation scenarios also affect humus formation. For the time being, it is not possible to make any statistically-assured statements on climate-change dependent alterations in the contents and reserves of organic matter. However, it is assumed that weather patterns and climate play a subordinate role in terms of humus condition. The most severe changes are induced by changes in land use¹⁰⁰.

The contents of organic matter in soils are essentially dependent on site-typical circumstances (such as type of soil and the distance from groundwater levels) and therefore they cannot be augmented wholesale by adding organic materials. However, a useful control mechanism is the choice of land use practices. In forests and woodlands, the composition of tree species and the amount of residues from timber harvesting are crucial factors in the formation of humus (cf. Indicator FW-R-3). On agricultural soils, options include utilisation as grassland, application of farmyard manure, cultivation of catch crops or incorporating / leaving harvest and root remnants on site for the conservation and accumulation of humus. However, doubts have been raised in respect of any long-term humus-enriching effects from reduced ploughing and tilling of the soil. Long-term trials have demonstrated that humus reserves hardly change, and that it is merely a matter of humus being distributed differently in the soil profile¹⁰¹.

With the ‘Greening’ stipulations contained in the European Direct Payments Regulation, the funding within the framework of the CAP from 2014 to 2022 was made conditional on agricultural businesses contributing towards environmental and climate protection. Thanks to those stipulations, it was possible to achieve a significant increase in the cultivation of catch crops in Germany in recent years¹⁰². Humus enrichment benefits from catch crop cultivation, not only thanks to the formation of biomass above ground, but also from the soil-borne input of organic matter. In the course of the new CAP funding period which started in 2023, the conditionality in relation to Greening has been expanded. The standards in respect of good agricultural and ecological condition of surface areas (GLÖZ) that have been in force since 2023, include measures such as the conservation of permanent grassland (cf. Indicator BO-R-2) and a minimum of ground cover at the most sensitive times, which also serves the purpose of humus enrichment. Furthermore, EU member states are beholden to offer voluntary eco  egulations (ER). The ER catalogue includes the ‘retention of an agroforestry management approach in respect of arable land and permanent grassland’. This combined use of land for trees and arable fields or grassland is another method of contributing to humus enrichment. Furthermore, several Länder are in the process of preparing targeted funding programmes for agroforestry.

As far as restricting the decomposition of humus is concerned, organic soils are of utmost relevance. Substantial amounts of carbon are sequestered in moorlands. In cases where moorlands are under agricultural and forestry management, drainage is practised, thus allowing oxygen to penetrate the peat which causes a strong acceleration of microbial conversion and leading to decomposition of organic matter. In terms of the national greenhouse gas inventory, drained moorlands in the agricultural and forestry sectors account for roughly 40 % of emissions and roughly 7.5 % of the total of national greenhouse gas emissions in Germany.¹⁰³ By means of moorland renaturation or the transformation to wet management regimes by means of paludiculture, it is possible to halt humus decomposition, thus ideally leading to the moorland reverting to peat replenishment.

Regular surveys of humus contents in agricultural soils are carried out by the Länder within the nationwide network of BDF. As shown by results from the Bavarian BDF, the mean contents of the most important humic components such as organic carbon (Corg) and total nitrogen (Nt) in soil depths of 0 to 15 cm remained almost consistent for the past few years. Throughout the Bavarian BDF areas, decrease and increase figures remained roughly in balance over the past 35 years. As far as arable land is concerned, the mean value indicated a slight decline in Corg contents by 3 % compared to the baseline condition. However, most of the sites examined did not indicate any changes. In BDF areas where a deviation in terms of an increase in Corg contents occurred, it was possible to provide evidence for a connection with the transformation to ecological farming and organic fertilisation as well as the retention of harvest residues in the fields. However, more apparent is a general tightening of the C / N relationship as Nt contents tend to increase at the same time as Corg contents are slightly declining. The situation is similar in respect of grassland BDF. In this case too, a major part of the sites examined did not indicate any striking Corg changes, and the mean value for the observation period did not indicate any significant change in Corg contents.¹⁰⁴ Generally speaking, humus replenishment proceeds very slowly, and there are measuring inaccuracies. Hence, long time series would be required to draw any statistically assured conclusions.

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104 - Wiesmeier M., Burmeister J. 2022: 35 Jahre Boden-Dauerbeobachtung landwirtschaftlich genutzter Flächen in Bayern. Band 4: Humus. Schriftenreihe der Bayerischen Landesanstalt für Landwirtschaft. Freising-Weihenstephan, 82 pp. https://www.lfl.bayern.de/publikationen/schriftenreihe/337020/index.php.