Compost protects soil carbon. But climate change is rewriting the rules

Compost helps agricultural soils accumulate carbon, but global warming could gradually undermine some of these benefits. This is suggested by a study published in the scientific journal Geoderma, which examined changes in soil organic carbon over 29 years of agricultural experimentation in Germany. The findings show that applying organic soil amendments can significantly increase soil carbon stocks, but also that these stocks may decline even when fertilization practices are maintained.

The research, led by David I. Schubert and conducted by a team including scientists from the Technical University of Munich, the Bavarian State Research Center for Agriculture and the Thünen Institute, examined a long-term field experiment carried out between 1994 and 2023 in Puch, Bavaria. The aim was to understand how different agricultural management strategies affect the soil’s ability to store carbon over time and what role climatic conditions may play.

Twenty-nine years of compost application and fertilization

The researchers compared agricultural plots subjected to different treatments: green manure, three types of compost, and varying levels of mineral nitrogen fertilization. The findings confirmed the importance of organic inputs in maintaining soil organic matter.

Even the highest level of mineral nitrogen fertilization produced an increase of between 7% and 9%, but its effect was considerably smaller than that of organic amendments.

One significant finding concerns crop productivity. Although nitrogen fertilization substantially increased yields and, consequently, the amount of crop residues returned to the soil, this greater input of biomass did not translate into a comparable increase in carbon stocks.

The amount of organic material entering the soil is therefore not the only determining factor. The properties of soil amendments and the processes governing the decomposition and stabilization of organic matter also play a crucial role.

Why warming can alter the soil carbon balance

Soil organic carbon is the result of a dynamic balance. On one side are inputs of plant material, roots, crop residues and organic amendments; on the other are the biological processes that break down organic matter and release some of its carbon into the atmosphere as carbon dioxide.

Higher temperatures can accelerate the activity of microorganisms responsible for decomposition when sufficient moisture is available. Drought, on the other hand, can temporarily slow this activity, while subsequent rewetting may trigger renewed bursts of microbial activity.

The overall effect therefore depends on the interaction between temperature, water availability, soil characteristics and agricultural management.

In the Bavarian experiment, the findings indicate that compost applications helped mitigate carbon losses compared with less favourable treatments, but were unable to prevent the overall reversal of the trend.

This is also an important consideration for carbon farming strategies, which aim to increase carbon stocks in agricultural soils through sustainable management practices. Carbon accumulated over a given period cannot automatically be considered permanent: its persistence must be monitored over time, taking environmental conditions into account.

Compost remains a valuable resource, but it is not enough on its own

The study’s findings do not call into question the benefits of compost. On the contrary, they confirm that organic amendments can significantly increase carbon levels in agricultural soils compared with management practices that do not include organic inputs.

However, the research highlights a limitation of strategies based solely on adding organic matter: in a changing climate, maintaining or increasing soil carbon stocks may become progressively more difficult.

For this reason, the authors emphasize the need to combine organic amendments with additional soil management practices, tailored to soil characteristics and local climatic conditions.

The experiment was conducted at a single site in southern Germany, meaning that its findings cannot be directly extrapolated to all agricultural regions of Europe. Nevertheless, it offers valuable insights for Mediterranean regions, where rising temperatures and increasingly frequent droughts are already placing additional pressure on soils.

The key conclusion is that preserving soil carbon depends not only on how much organic material is added, but also on the ability to protect it over time. Climate change could make this challenge even more difficult, just as agricultural soils are becoming increasingly important in strategies to mitigate greenhouse gas emissions.