Soil available P, soil organic carbon and aggregation as affected by long-term poultry manure application to Andisols under pastures in Southern Chile

Andisols, rich in minerals like allophane, imogolite and iron- (Fe) or aluminum- (Al) oxides have high phosphorus (P) sorption capacity and require annual P additions to ensure plant productivity. It is known that the use of composted poultry manure (PM) increases soil labile P and carbon concentration, although the mechanisms controlling P availability and soil organic carbon (SOC) sequestration are not fully understood. The aim of this study was to investigate the relationship between soil available P and particles size distribution, SOC, and iron and aluminum complexes on Southern Chilean pastures with long-term PM amendment. Soil samples, were collected from 4 pastoral farms where PM had been applied annually at 3 tons ha⁻¹ for 5 and 10 years. We analyzed for elemental concentrations, P forms through a modified P fractionation methodology and aggregation through particle size analyses and scanning electron microscopy. Moreover, we determined oxalate and pyrophosphate extractable Fe and Al. Andisols receiving long-term PM amendment had 22–65% higher SOC contents than unfertilized control likely due to improved aggregation capacity as indicated by greater soil particle size. Moreover, the readily available inorganic P increased by 56 to 286% between control and PM amended soils. Andisols amended with PM also showed lower oxalate extractable Al and Fe and higher pyrophosphate extractable forms. These parameters correlated with SOC, labile P concentrations and particle size distribution. We concluded that increased P availability under pastures growing on Andisol with long-term PM amendment is regulated by three mechanisms related to input of readily available P, release of native soil P and decreased availability of sorption sites.