Source: landwirt.com
Crop residues and farmyard manure increase humus content and reserves in the soil. However, incorporating crop residues also increases greenhouse gas emissions.
Humus comprises all dead organic matter from the soil and consists of approximately half (approximately 58%) organic carbon. Plant residues are the starting material for the formation of stable humus compounds. These residues consist, on the one hand, of readily degradable components such as carbohydrates and nitrogen compounds, and, on the other hand, of more slowly degradable woody components such as lignocellulose. After microbial decomposition, the non-mineralized portion is bound as humus.
According to the guidelines for proper fertilization, 0.5–2.0 t of CaO (= pure lime)/ha can be applied to non-alkaline arable land every four to six years, depending on soil gravity. This promotes the formation of the particularly important clay-humus complexes in the soil. Clay-humus complexes are soil particles consisting of both organic (humus) and inorganic (clay minerals) substances. Clay minerals and humus combine to form soil particles, forming humus-rich topsoils and improving the soil structure. This enables good aeration and living construction of the soil.
It also promotes the activity of soil life. Humus also plays an important role in nutrient and water balance. Due to its dark color, the soil warms up very quickly in spring. Furthermore, humus has a nitrogen potential, which can be made available to plants for growth through mineralization. Temperature, water content, and pH have the greatest influence on its availability.
Reduce greenhouse gases
Carbon dioxide (CO2) and nitrous oxide (N2O) are among the most important greenhouse gases responsible for climate change. The most significant sources of CO2 are deforestation, clearing of grassland for agriculture, and the drainage of wetlands. Increasing the humus content in agricultural soils can counteract this CO2 loss.
Heavy machinery is known to cause severe soil compaction on wet soils. This significantly impairs soil structure and aeration. The result is increased nitrous oxide emissions and nitrogen losses, which lead to a reduction in nitrogen utilization capacity. This increases nitrogen fertilizer costs and worsens farm performance.
The most important organic fertilizers for increasing soil humus content are farmyard manure, slurry, compost, and crop residues. Although farmyard manure can enrich more humus, incorporating crop residues is considered an ecologically sustainable and cost-effective soil management method for improving soil fertility. This method allows livestock-free farms, in particular, to improve their humus balance.
Crop residues are sustainable
The EU research project "CATCH-C" investigates which factors are relevant for farmers regarding soil use. The goal is to develop sustainable soil management practices. "Sustainable" means the long-term preservation of soil fertility, mitigation of climate change, and ensuring high yields. In addition, experiences from good agricultural practices are made available to policymakers and consultants for the further development of soil and climate protection measures.
Field trial results across Europe show that carbon levels and stocks increase when crop residues are incorporated into the soil. Clayey soils have higher organic carbon levels and stocks. Soils with lower clay contents, on the other hand, generally have lower organic carbon levels and stocks. With increasing trial duration (ten to 20 years) of incorporating crop residues, organic carbon levels increase by 6% to 11%. Likewise, soil carbon stocks only increase after an average trial period of ten years. Consequently, only the long-term incorporation of crop residues enables improvements in physical, chemical, and biological soil quality. Climate zones have an influence in that carbon stocks increase by 7% in temperate latitudes compared to the Mediterranean climate zone (+3%).
Results from field trials across Europe also show that incorporating crop residues into soils with low clay contents (<18%) increases CO2 emissions sevenfold. N2O emissions show a similar picture, with average increases of up to 13fold expected. The apparent contradiction (higher organic carbon contents/stocks – higher greenhouse gas emissions) can be explained by the higher microbial activity associated with incorporating crop residues. In particular, greenhouse gas emissions increase with the first use of the soil management technique.
The results of carbon dioxide and nitrous oxide emissions from agriculture are becoming an increasingly important topic for advisory institutions and farmers, but these are only measured in a few field trials.
Conclusion
Incorporating crop residues into the soil is an important tool for farmers to improve physical, chemical, and biological soil quality and safeguard the yield potential of agricultural soils. Carbon content and stocks benefit from this, although rising CO2 and N2O emissions must not be overlooked. Leaving crop residues in the field leads to the careful use of resources and reduces costs. A comprehensive assessment of the climate impact would require year-round CO2 and N2O measurements in long-term field trials.
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