The effect of nutrient management on organic matter in subsoils of temperate and tropical agroecosystems

One of the aims of sustainable soil management in agriculture is to preserve soil organic matter (SOM) because it relates to important ecosystem services such as carbon storage and soil fertility. Soil organic carbon (SOC) storage can contribute to climate regulation, while soil fertility is essential to provide food to a steadily growing population. Most studies focus on the topsoil (i.e., 0 to 30 cm depth). Nevertheless, studying only the topsoil SOC is not sufficient to estimate the effect of management practices on the organic carbon (OC) storage capacity of soils. Indeed, an important quantity of OC is present in the subsoil.

In this project, soil samples were collected in two long-term agricultural field experiments in temperate (Switzerland) and tropical (Kenya) climates. The Kenyan trial (the SOM trial) was established in 2002 and maize has been cultivated in monoculture in every growing season (two per year). The different treatments include the application of organic residues with different carbon to nitrogen ratios, and manure, with and without mineral nitrogen. Samples have been collected down to 70 cm depth in increments of 5 cm. The Swiss trial (the DOK trial) was established in 1978. Different crops (wheat, potato, soy, grass, and maize) are grown in rotation. The treatments include different fertilizer and crop protection regimes representing conventional and organic agricultural practices. Samples have been collected down to 90 cm depth in increments of 5 cm.

In the DOK trial, mean SOC concentration decreased from ca. 1.8 % in the 0-5 cm to ca. 0.5 % in the 85-90 cm depth layer. Soils under organic and biodynamic treatments had a higher SOC content over the whole depth profile compared to soils under conventional practices, pointing out the potential effect of these practices to increase SOC stocks over time. In the SOM trial, the mean SOC concentration decreased from ca. 2.2 % in the 0-5 cm to ca. 1.2 % in the 65-70 cm depth layer. Organic inputs had a positive effect on the SOC content over the whole depth profile, while the addition of mineral N without organic input had a negative effect. As the information on the effect of nutrient management practices on subsoil SOC content is limited, specifically in the tropics, these data help to improve our knowledge about these effects, while providing guidelines to farmers and farm advisors on how to maximize the SOC content of soils. In addition, these data provide a valuable resource for the formulation and calibration of model simulations of SOC dynamics in agroecosystems.