A Properly Chosen Rate of NPK Fertilizers Has a Positive Effect on C Sequestration in Sandy Soils in the Conditions of a Changing Climate

Soil organic carbon (SOC) plays a significant role in climate change. Its content can be modified by soil management practices, however, the effect of mineral fertilization on SOC is not clear. For this reason, a long-term effect of gradually increasing rates of NPK fertilizers on changes in soil organic carbon (SOC) in bulk soil and in water-stable aggregates (WSA) in soils with sandy loam and loamy sand texture at two experimental sites (Skierniewice, Poland, and Dražovce, Slovakia) was quantified. In both sites, soil samples were collected from the following treatments: NF – no fertilization, NPK1 and NPK2 – 1^st level and 2^nd level of NPK fertilization, respectively. The results showed that 100-year long application of NPK1 increased total carbon (TC) and SOC content by 24%, while NPK2 decreased it by 5% compared to NF at the Skierniewice site. The content of water-stable macroaggregates (WSAma) increased because of NPK application. In NPK1, the content of WSAma was higher and the content of water-stable microaggregates (WSAmi) was lower than in NPK2 or NF. However, as a result of NPK application, the content of agronomically favorable WSAma in size fraction 0.5–3 mm was reduced by 8 and 24% in NPK1 and NPK2, respectively, compared to NF. Overall, SOC in WSAma was lower than in bulk soil. The SOC in WSAma in NF, NPK1 and NPK2 treatments was 6.51, 7.77 and 5.89 g.kg⁻¹, respectively. Similar tendency of SOC in WSAma 0.5–3 mm was observed (NF: 6.12 g.kg⁻¹, NPK1: 7.35 g.kg⁻¹, and NPK2: 6.88 g.kg⁻¹). The SOC in WSAmi in NF, NPK1 and NPK2 was 8.33, 7.39 and 7.24 g.kg⁻¹, respectively. At Dražovce site, TC content decreased significantly due to the graded rates of NPK, not because of SOC mineralization but as a result of carbonate dissolution for a period of 14 years. The carbonate content decreased from 20 g.kg⁻¹ in NF to 6.5 g.kg-1 in NPK1 and 3.0 g.kg-1 in NPK2, while SOC did not change significantly: (NF: 23.8 g.kg⁻¹, NPK1: 25.9 g.kg⁻¹, and NPK2: 23.4 g.kg⁻¹). In NPK1, the WSAma content was reduced significantly when compared to NPK2 and NF treatments. No significant difference was observed between NF and NPK2. On the contrary, the content of WSAma 0.5–3 mm significantly increased when compared to NF and NPK1. No difference was observed between NF and NPK1. Lower SOC content was found in WSA than in the bulk soil. Overall, higher SOC content was observed in WSAma when compared with WSAmi. The application of NPK1 and NPK2 increased SOC in WSAma as well as in WSAma 0.5–3 mm. The effect was more significant in NPK1 than NPK2 treatments when compared to NF.