Unlocking organic phosphorus and carbon in soil aggregates under long-term P and manure inputs

Phosphorus (P) availability and soil organic carbon (SOC) dynamics within soil aggregates play critical roles in regulating nutrient uptake and crop productivity. However, the interactive effects of mineral P, organic manure, and soil P status on organic P pools remain poorly understood. This study evaluated how long-term mineral P fertilization and cattle manure (CM) influence aggregate-associated P pools, SOC retention, and wheat productivity in soils with high (HGP) and medium (MDP) Olsen-P status.

A long-term field experiment consisted of mineral P fertilization rates of 0, 13, 26, and 39 kg P ha^- 1, CM alone, and integrated P + CM treatments. Soil samples were fractionated into aggregate-size classes to quantify aggregate-associated organic P fractions and SOC, while soil pH, Olsen-P, and wheat yield were also determined to assess treatment effects under contrasting soil P status.

Labile organic P (LBPo) increased with mineral P up to 26 kg ha^- 1, particularly in macroaggregates (> 2.3 mm). Integrated P26 + CM further increased organic P fractions by 18% (HGP) and 23% (MDP) compared to CM alone. Similar, but to a lesser extent, increases in LBPo and SOC were observed in aggregates of 1.18-0.6 mm and 0.3-0.075 mm. Aggregate-associated SOC increased by 18.6% (HGP) and 61.5% (MDP). While P fertilization lowered soil pH, it significantly raised Olsen-P. Maximum wheat yields were achieved at 13 kg P ha^- 1 in HGP soils and 26 kg P ha^- 1 in MDP soils. Integrated P + CM treatments produced the highest wheat yields under both Olsen-P status soils. In HGP soils, P13 + CM and P26 + CM resulted in statistically comparable yields, whereas P26 + CM produced the maximum yield in MDP soils.

Soil Olsen-P status and aggregate size regulate organic P stabilization and SOC retention. Integrating CM with mineral P is the most effective strategy for enhancing aggregate-associated SOC and sustaining wheat productivity in soils with contrasting P status.