25 years of continuous sewage sludge application vs. mineral fertilizers on a calcareous soil affected pH but not soil carbonates

Abstract

Inorganic and organic fertilizers have been widely used to maintain crop yields. However, several studies have demonstrated that the dissolution of carbonates in agricultural soils by the acidification induced by N-fertilizers can result in their total or partial loss in the tilled layer of some agricultural soils. The effect of inorganic and organic fertilization on carbonates in calcareous semiarid Mediterranean soils has been less studied and is still unclear. Based on a 25-year field experiment, we investigated the effects of different fertilization strategies on the soil pH, inorganic C content, and in the proportion of pedogenic carbonates in the topsoil (0-30 cm) of a carbonate-rich soil in Navarre (N Spain). Five treatments were compared: no amendments as a control (SC), mineral fertilization (MF), and the application of sewage sludge at different doses (80 Mg ha-1 every year (80-1), 40 Mg ha-1 every year (40-1) and 40 Mg ha-1 every four years (40-4). Results showed a decrease in soil pH values by increasing the amount of sewage sludge and a small alteration in the calcite structure particularly in 40-4. However, no significant differences between treatments were found in the total content of carbonates nor in the proportion of pedogenic carbonates. The high concentration of total carbonates in the soil (16.09 ± 0.48%), and of the proportion of pedogenic carbonates (40.21 ± 1.29%, assuming a δ13C of primary carbonates = 0 ‰) seemed elevated enough to compensate for the observed acidification in the studied soil. In the case of MF, no changes were observed in the pH values, nor in the carbonate content (total and pedogenic). This suggests that the use of sewage sludge could induce changes in the future at a faster rate and of greater dimension than the use of mineral fertilizers such as the ones used in this field (46% urea, superphosphate and ClK).

Sewage sludge treated with bentonite, vermiculite or biochar can improve soil properties and enhance growth of grasses

“…However, the duration of sludge's liming effect on soils can vary, depending on frequency and rate of sewage sludge application, soil properties, cultivated crops and agricultural management practices. The fact that the pH of alkaline soil treatments with sludge remained also unchanged after the experiment is consistent with the findings of De Soto et al (2022), who reported that even a continuous application of sewage sludge (25 years) to an alkaline soil did not cause major changes to its pH because of the CaCO 3 buffering capacity.…”

Evaluation of the Use of a Material with Struvite from a Wastewater Treatment Plant as N Fertilizer in Acid and Basic Agricultural Soils

“…Due to the high concentration of carbonates in the study soil (21.6%), changes in the carbonate concentration of the bulk samples were not detected. Similar results have recently been observed by de Soto et al [45] after 25 years of continuous sludge and mineral fertilizer application on a Mediterranean calcareous soil. This suggests that the initial carbonate content is a key factor for observing and detecting these changes in bulk soil samples with a high content of carbonates.…”