estimation of the fertiliser value of pig slurry by physicochemical models: 2 Usefulness and contraints. Biosystems Engineering, 111 (2): 206-216

In the European Union, the maintenance of soil quality is a key point in agricultural policy. The effect of additions of dairy cattle (Bos taurus) manure (DCM) during a period of 11 years were evaluated in a soil under irrigated maize (Zea mays L.) monoculture. DCM was applied at sowing, at wet-weight rates of 30 or 60 Mg ha À1 yr À1 (30DCM or 60DCM). These were compared with a mineral-N treatment (300 kg N ha À1 , MNF), applied at six to eight emerged leaves and with a control (no N, no manure). Treatments were distributed in a randomized block design. Factors analysed were stability against wetting stress disaggregation, porosity, soil organic carbon (SOC) fractions and earthworm abundance, studied eight months after the last manure application. The application rate of 30DCM increased aggregate stability and the light SOC fraction, but not the pore volume, nor the earthworm abundance, compared with MNF. The DCM rates did not result in unbalanced agronomic advantages versus MNF, as high yields (12-16 Mg ha À1 yr À1 ) were obtained. In Mediterranean environments, the use of DCM should be encouraged mainly because of its contribution to the light SOC fraction which protects dry macro-aggregates from implosion (slaking) during the wetting process. Thus, in intensive agricultural systems, it protects soil from physical degradation.

Organic fertilizers (manures and slurries) applied repeatedly over many cropping seasons favourably influence nutrient recycling, maintenance of soil organic matter (SOM), and improve soil quality parameters such as soil aggregation and porosity. These aspects are particularly relevant in Mediterranean environments characterized by low SOM. This study was set up in a subhumid Mediterranean area where two different trials, devoted to winter cereals, were fertilized with dairy cattle manure (DCM) or pig slurry (PS) for a period of 12 years. One objective of this research was to evaluate the impacts of these fertilization practices on aggregate stability and SOM fractions, when compared with a mineral N fertilizer and a control (no-N) treatment. Porosity and pore shape were also studied in PS plots. The use of DCM significantly increased water stable aggregates by up to 16.4%-18.0%. Slurry addition did not affect aggregation but it increased the area occupied by pores >65µm. Soil organic carbon (SOC) and light organic fraction (0.05-0.2mm) increased with DCM incorporation but in PS treatments the SOC increment was non-significant. Data from DCM and PS together showed a positive and significant linear relationship between SOC (p<0.05, R 2 =0.60), SOC light fraction (p<0.01, R 2 =0.75) and SOC light fraction at 0.05-0.2 mm size (p<0.01, R 2 =0.83), with water-stable aggregate. The use of animal residues (DCM or PS), applied according to an N criterion, increased available phosphorus and potassium soil content while improving yields. The enrichment of soil nutrients with DCM and PS use requires further research in order to avoid potential environmental impacts.

Effects of applications of pig slurry on soil aggregate stability are not well understood in dryland agriculture. This research aims to (i) identify aggregate stability tests that give a reliable description of the soil's behaviour when pig slurry (PS) is applied to calcareous soil and (ii) interpret them in terms of chemical, biological, morphological and physical soil properties for soil quality assessments. Soil samples from eight fertilizer treatments (mineral fertilizers and PS), applied over seven growing seasons were analysed. We applied five methodologies to examine different mechanisms of aggregate breakdown. Porosity was characterized by image analyses. There was minimum resistance to the mechanical breakdown of aggregates when slurries had been applied 12 months before analysis. Recent applications of slurry (3 months before the analysis) improved resistance to implosion caused by the penetration of water into dry aggregates (slaking), although the opposite result can occur if the method of evaluation is not chosen properly. Recent applications of PS also enhanced soil respiration and increased soil porosity in the 25–100‐µm size range (packing pores between aggregates) and in the 100–400‐µm size range (interaggregate or faunal pores). In dryland systems and in the winter cereal cropping season, the resistance of dry aggregates to slaking is improved temporarily if PS is applied at N rates equivalent to around 1.7 Mg OM ha−1 year−1. Highlights How does soil aggregate strength change in response to pig slurry (PS) applications? Improve understanding of PS as a fertilizer and its effect on soil quality assessments Changes in aggregate stability are controlled by time since pig slurry application The effects of PS on the increase in packing pores between aggregates are transient

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.