Open Access
Effects of long-term sewage sludge addition to a calcareous soil on soil organic C fractions and soil functions
(Elsevier, 2024) Simoes da Mota, Ana Claudia; Barré, Pierre; Baudin, François; Poch, Rosa María; Bruni, Elisa; Antón Sobejano, Rodrigo; Enrique Martín, Alberto; Virto Quecedo, Íñigo; Ciencias; Zientziak; Institute on Innovation and Sustainable Development in Food Chain - ISFOOD
Soil organic matter (SOM) is a recognized carbon reservoir and paramount in soil functioning and agrosystems productivity. Different management strategies have been established to enhance SOM in arable soils, and one is the application of exogenous organic matter (EOM). Despite significant efforts in recent years, the consequences of EOM addition on increasing soil organic C (SOC) storage and stability in different pedoclimatic contexts remain incompletely understood. In this study, we evaluated the effect of long-term (28 y) sewage sludge (SS) addition to a calcareous soil supporting rainfed extensive crops in a Mediterranean sub-humid area in terms of SOC stabilization and distribution among functional fractions. To that end, we studied total SOM storage and soil fractions in a long-term experiment comparing 4 different doses of SS with mineral fertilization and no-fertilization controls. We compared the concentration, storage and distribution of C using two different fractionation methods (particulate vs. mineral-associated, defined by granulodensimetric fractionation, and active C vs. stable C defined by Rock-Eval® thermal analysis coupled to the machine learning PARTYsoc v2.0 model) on the tilled layer (0–30 cm). Three soil functioning indicators (crops yield, soil microbial biomass C and aggregate stability) were also quantified. We found that SS application, which slightly increased SOC concentration when SS was added, resulted in a net SOC stock gain only with the highest dose used (80 tons/ha), compared to mineral fertilization, suggesting that most of the C added was mineralized. An uneven response of soil fractions was however detected. The coarsest heavy physical fractions > 250 μm and 50–250 μm in size were the most enriched in their C concentration with SS addition, whereas SOC was mostly accumulated as mineral-associated C in the silt-size (2–50 μm) and the 50–250 μm fractions. Regarding thermal fractionation, SS treatments showed C gains between 57 % and 35 % in the active C pool, compared to mineral fertilization. This can be explained considering that the accumulated SOM in mineral-associated fractions corresponded to mean-residence times in the order of 20–40 years. The consequences on soil functioning indicators were not directly related to the amount of SOC stored in the soil, as the highest SOC gains corresponded to the highest dose, but not the highest yields, and some negative correlations were observed between SOC fractions and soil structural stability and microbial biomass. This can be related to some deleterious effects of excessive SS application reducing yields, the soil biological activity and soil structure, and represents an example of the need to decouple the assessment of soil health from that of SOC storage for certain soil management practices. The overall evaluation of the net consequences of long-term SS application indicated that low doses (10 tons/ha) seem a better choice, as they resulted in the highest efficiency in C incorporation, and in a slightly greater increase in SOC concentration than intermediate doses, and equal yields than mineral fertilization, but did not have the negative effects observed in soil functioning with higher doses.
Open Access
Soil quality assessment after 25 years of sewage sludge vs. mineral fertilization in a calcareous soil
(MDPI, 2021) Simoes da Mota, Ana Claudia; Poch, Rosa María; Enrique Martín, Alberto; Orcaray Echeverría, Luis; Virto Quecedo, Íñigo; Institute on Innovation and Sustainable Development in Food Chain - ISFOOD
The aim of this work was to identify the most sensitive soil quality indicators and assess soil quality after long-term application of sewage sludge (SS) and conventional mineral fertilization for rainfed cereal production in a sub-humid Mediterranean calcareous soil. The treatments included six combinations of SS at different doses (40 t ha−1 and 80 ha−1) and frequencies (every 1, 2 and 4 years), plus a control with mineral fertilization, and a baseline control without fertilization. Twenty-five years after the onset of the experiment, 37 pre-selected physical, chemical and biological soil parameters were measured, and a minimum data set was determined. Among these indicators, those significantly affected by treatment and depth were selected as sensitive. A principal component analysis (PCA) was then performed for each studied depth. At 0–15 cm, PCA identified three factors (F1, F2 and F3), and at 15–30 cm, two factors (F4 and F5) that explained 71.5% and 67.4% of the variation, respectively, in the soil parameters. The most sensitive indicators (those with the highest correlation within each factor) were related to nutrients (P and N), organic matter, and trace metals (F1 and F4), microporosity (F2), earthworm activity (F3), and exchangeable cations (F5). Only F3 correlated significantly (and negatively) with yield. From these results, we concluded that soil quality can be affected in opposite directions by SS application, and that a holistic approach is needed to better assess soil functioning under SS fertilization in this type of agrosystem.
Open Access
Effects of long-term sewage sludge application to a calcareous soil structure
(British Society of Soil Science, 2022) Simoes da Mota, Ana Claudia; Virto Quecedo, Íñigo; Poch, Rosa María; Zientziak; Institute on Innovation and Sustainable Development in Food Chain - ISFOOD; Ciencias
Soil degradation is a growing challenge to global agriculture and the United Nations' Sustainable Development Goals (SDGs). This has prompted calls for less use of mineral fertilizer and greater reliance on organic fertilizers. However, we need to understand better the long-term effects of organic fertilizer usage on soil structure to guide soil management practice, as many soil functions are sensitive to pore morphology and connectivity. In this study, we characterized topsoil (0–30 cm) pore architecture in relation to soil physical properties in a long-term experiment (LTE) site where calcareous soil had received 25 years of sewage sludge application. Two dosage rates (SSa, 20 and SSe, 80 Mg ha−1) were compared to mineral fertilization treatment and a control (no fertilization) in a random factorial block design. Soil microstructure and the types of pores were characterized using micromorphological methods and image analysis, in soil thin sections. Long-term sewage sludge SSa application improved soil microstructure (crumb and sub-angular-blocky type) and increased the presence of biopores, while mineral fertilized soil showed a platy to apedal microstructure, with more elongated pores and lower faunal activity. Mineral fertilized soil had the lowest total porosity values, with differences found in the aspect ratio of pores of equivalent diameter 100–200 μm. These findings suggest a relation between the different types of fertilization and soil pore shape and network. Further exploration of these changes in soil functioning is needed for a complete assessment of the consequences of SS application.

Simoes da Mota, Ana Claudia

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