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

Highlights

• Andisols managed with PM enhance SOC and available P.

• PM release native P through dissolution of Fe and Al complexes.

• PM decrease P adsorption potential due competition for sorption sites and/or less available surfaces.

Abstract

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.

Introduction

Dairy and meat production in Southern Chile is based on the use of permanent pastures grown on Andisols (Demanet et al., 2015). Andisols have specific andic properties such as high content of short-range order minerals, favoring phosphate immobilization through sorption and/or precipitation with cations such as Al and Fe (Borie and Rubio, 2003; Mora et al., 2006; Redel et al., 2016). In these soils, organic matter and inorganic and organic forms of P accumulate under pasture (Redel et al., 2016; Velásquez et al., 2016). Consequently, pasture soils require high levels of P fertilizer inputs to enhance and maintain plant development and animal productivity. In the current context, the challenge is to implement sustainable intensification with new strategies to increase yields using fewer resources (limited agricultural area and fertilizers) and waste recycling (Calabi-Floody et al., 2018)

Enhancement of plant available P may be achieved by using P-rich organic waste, such as poultry manure with a low N:P ratio (Shepherd and Withers, 1999). Poultry manure has high P availability (He et al., 2008; Pagliari and Laboski, 2012; Waldrip et al., 2011) and is abundant globally due to broiler meat production (FAO, 2018). Composted poultry manure (PM) may be a suitable P fertilizer for soils with high P fixation capacity, such as Andisols. Although PM is widely used as soil amendment, mechanisms by which it increases P availability and controls soil organic carbon sequestration are not fully understood. PM could increase P nutrition on Andisols through (1) direct contribution of available P, and (2) slow release of other P pools, and (3) activation of native soil P pools. In this study, we investigated Andisols under pasture at four farms in Southern Chile. The aim of the study was to assessed composted PM amendment effect on C sequestration and soil P availability. We hypothesized that increased P availability in Andisols receiving long-term PM amendment is controlled by its impact on native soil P pools stabilized in Al and Fe complexes. Moreover, we hypothesized that PM could affect SOC sequestration though its effect on aggregate formation. In order to test these hypotheses, we investigated the relationship between soil available P forms, SOC concentration, soil particle size distribution, and Al and Fe forms in Southern Chilean pastures with long-term amendment of PM. The specific objectives of this study were to: i) assess the PM amendment effect on soil available P on pastures growing on Andisols, ii) investigate the effect of long-term PM amendment on soil particle size and, iii) to elucidate the relationship of available P and SOC with Al and Fe soil complexes.