The impact of over a century of different organic fertilization on the properties of soil organic matter and water holding capacity

The long-term experiment was established in 1921 at the Experimental Station of the Warsaw University of Life Sciences in Skierniewice (central Poland) on sandy loam Luvisol, which received mineral fertilization (Ca, N, P, K). The treatment included: (1) control with arbitrary crop rotation; (2) manure applied every five years at a rate of 30 t ha–1; (3) legumes (Trifolium pratense L.); and (4) manure with legumes. Soil samples from A horizon (0-20 cm) were collected during the 2022 midgrowing season and analyzed for total organic carbon (TOC), fractional composition of soil organic matter (SOM) for humic acids (HA), fulvic acids (FA) and humin (HUM), as well as spectroscopic properties of bulk soil (TC-GC/MS) and isolated HUM (UV-Vis and fluorescence). In addition, plant available water, soil moisture, soil temperature, and CO2 emission were measured in the field during the 2022 and 2023 growing seasons from treatments 1 and 2 only. The greatest impact on SOM content was observed in plots where manure was applied, which resulted in an increase in TOC by 48% and 68% in variants (2) and (4), respectively. The application of legumes alone caused an increase in TOC by 32%. The analysis of the fractional composition of HS revealed that the transformation of organic matter under fertilization with manure led to a significant increase in HUM amount and decrease in the HA/FA ratio concerning control. Different management caused also changes in the spectroscopic properties of HUM, which indicated an increase in UV-Vis absorbance and fluorescence emission in legume applied soil. Thermochemolysis and gas chromatography/mass spectrometry showed that HUM was enriched in carbohydrates in almost all pairs of soil and HUM. Manure fertilization and application of legumes resulted in enrichments in carbohydrates in bulk soil and HUM samples as well as in decrease in lipids. Changes in the amount and properties of SOM were accompanied by changes in the water-holding capacity. Manure fertilization increased plant available water by 20% and 10% in variants 2 and 4, respectively, while legumes alone decreased plant available water by 11%. Based on the two years of monitoring, CO2 emissions were affected by soil temperature only. The results confirmed that appropriate cultivation can significantly increase C resources in the soil, contributing to the mitigation of climate change. This is particularly effective when using legumes supported by fertilization with manure. Furthermore, the long-term different soil management not only altered the SOM contents and properties but surprisingly also the chemical composition of HUM which is considered as particularly stable and a long-term sink of atmospheric carbon. The greatest impact on SOM content was observed in plots where manure was applied, which resulted in an increase in TOC by 48% and 68% in variants (2) and (4), respectively. The application of legumes alone caused an increase in TOC by 32%. The analysis of the fractional composition of HS revealed that the transformation of organic matter under fertilization with manure led to a significant increase in HUM amount and decrease in the HA/FA ratio concerning control. Different management caused also changes in the spectroscopic properties of HUM, which indicated an increase in UV-Vis absorbance and fluorescence emission in legume applied soil. Thermochemolysis and gas chromatography/mass spectrometry showed that HUM was enriched in carbohydrates in almost all pairs of soil and HUM. Manure fertilization and application of legumes resulted in enrichments in carbohydrates in bulk soil and HUM samples as well as in decrease in lipids. Changes in the amount and properties of SOM were accompanied by changes in the water-holding capacity. Manure fertilization increased plant available water by 20% and 10% in variants 2 and 4, respectively, while legumes alone decreased plant available water by 11%. Based on the two years of monitoring, CO2 emissions were affected by soil temperature only. The results confirmed that appropriate cultivation can significantly increase C resources in the soil, contributing to the mitigation of climate change. This is particularly effective when using legumes supported by fertilization with manure. Furthermore, the long-term different soil management not only altered the SOM contents and properties but surprisingly also the chemical composition of HUM which is considered as particularly stable and a long-term sink of atmospheric carbon.