Pig slurry and mineral fertilization strategies' effects on soil quality: macroaggregate stability and organic matter fractions.

Applying pig slurry to the land as fertilizer at appropriate agronomic rates is important to close nutrient cycles and optimize the value of organic matter. However a long-term discussion has taken place about its effects on soil quality. In the north-east of Spain, eight fertilization strategies were evaluated on the soil quality parameters' aggregate stability, soil organic matter (SOM) physical fractions and soil microbial biomass (SMB). Six strategies used different pig slurries (PS) which provided organic matter from 1.7 to 2.6 t ha(-1)yr(-1), the rest (mineral N fertilization and a control) did not. Pig slurries were applied at sowing and/or at cereal tillering, as sidedressing. Field experiments were maintained for an 8-year period, in a silty loam soil devoted to a rainfed winter cereal. Soil samples were taken once, before the last sidedressing in 2011. Aggregate stability was quantified using the standard water-stable aggregate method but including a modification which meant that pre-wetting was avoided (WSA(MOD)). When using the WSA(MOD) method, we found a tendency for the percentage of water-stable aggregates to increase due to PS application (differences of up to 74% in the increment) and it was more marked the nearer they were measured to the application time (3 months vs. 12 months). The strategies which include PS show a positive effect on the SOM amount, mainly in the 0.05-0.2 mm light fraction, which increased by up to 34% with every 10 t ha(-1) organic C applied, and on SMB (up to 53% increment). There is a positive and significant linear relationship (p < 0.05, R(2) = 0.75) between the SOM light fraction (%) and the water-stable aggregates soil content (%, WSA(MOD)). Thus, the introduction of PS in fertilization strategies improves soil quality parameters. However, the soil quality benefits need to be balanced with any other potential environmental impact.