Long-term organic fertilization with high carbon input improves pore geometry and functionality of no-till sandy soil

Soil structure governs the functions of soil in many ecosystems, including those dominated by agriculture, such as water and carbon storage, biomass production, and physical stability. Specifically in tropical and subtropical soils, the long-term impacts of different fertilizers on soil functionality and stability in no-till crops are poorly understood. Under subtropical climate conditions, we evaluated how 17 years of continuous fertilizer application (organic vs. inorganic) on no-till crops affected structure of a sandy loam texture, in terms of the pore size distribution and pore functionality for water storage and aeration, and the intra-aggregate pore geometry. The investigated long-term experiment was implemented in a randomized block design with four repetitions in Santa Maria, Brazil. Treatments were pig slurry (PS), cattle slurry (CS), pig deep litter (pig manure with rice husk; PDL), mineral fertilizer (MF), and an unfertilized control (CL) applied in a no-till system. Soil sampling was done in two depths (0–5 and 5–15 cm) to analyze (i) soil pore size distribution, soil water retention, air permeability and pore continuity indices in core samples (± 98 cm³); and (ii) the intra-aggregate pore system using X-ray computed tomography in macroaggregate samples (± 5 cm³). The treatments had different impacts on soil pore functionality and intra-aggregate pore geometry. Only PDL application increased field capacity by around 34 % and the plant available water by about 36 % (compared to all other treatments). Soil air-filled porosity was not affected by fertilizer management in any of the layers. However, in the 0–5 cm layer, fertilizer management had a significant effect on soil air permeability which increased at −6, −10, and −33 kPa matric potential from 6.6, 14.4, and 16.1 µm² in CL treatment to 29.5, 34.2, and 43.6 µm² in PDL, respectively. The PS and PDL treatments increased the intra-aggregate porosity and provided a continuous, connected pore network. These fertilizers provided increased biomass productivity (PS and PDL) and soil organic matter content (higher in PDL only). Therefore, continuous application of fertilizer with higher carbon input, such as PDL, improved soil structural conditions and crop yield of sandy soil under subtropical climate.