Exogenous carbon addition soil mediated phosphorus dynamics under eight years continuous input of swine manure in a wheat-rice rotation

Abstract

The study investigated the prolonged impact of swine manure application on soil inorganic phosphorus (P) fractions over an 8-year continuous, randomized field trial involving rotating wheat (wet conditions) and rice (flooded conditions) crops. The trial comprised six treatments: triplicate control plots receiving only chemical fertilizers, and triplicate plots receiving chemical fertilizers and/or swine manure ranging from 150 to 1200 kg P2O5 ha−1. The continuous application of swine manure increased soil P content and availability. Initially, P primarily accumulated in the soil as Fe-P during the first four years of manure application, transitioning to Ca8-P over the following four years. The main driver of these changes in P fractions was soil total organic carbon (TOC) was identified as the main driver of these changes in P fractions, contributing 31.5% to the observed variations. TOC increased steadily throughout the trial, with a faster rising in years 1–4 compared to years 4–8. Laboratory-scale soil incubation experiments were conducted, involving the addition of glucose or cellulose as exogenous carbon sources to test their effects on soil P dynamics and mitigate environmental damage from P leaching. The addition of cellulose to soil that had received high quantities of swine manure for eight years resulted in increased the moderately labile and moderately resistant organic P fractions while decreasing the Ca8-P and Olsen-P fractions. This promoted adsorption of high-activity inorganic P to organic matter, consequently limiting the accumulation of Ca8-P. The long-term application of swine manure altered soil P fractions and enhances P lability, enhancing P lability and availability. This study identified an optimized the risk of leaching of labile P from soil under a fertilization regime applying 10,300 kg ha−1 yr−1 swine manure. To further mitigate the risk of P leaching, the inclusion of additional cellulose-based organic matter (e.g., straw) in swine manure fertilization regimes is recommended.