Effects of Nitrogen and Phosphorus Additions on Aggregate-associated Soil Carbon and Interactions with Phosphorus Fractions in Evergreen Broad-leaved Forest

Abstract

[Objective] To explore how phosphorus input regulates the effects of atmospheric nitrogen deposition on soil aggregate organic carbon content and the relationship between carbon and phosphorus. [Methods] A long-term monitoring test platform for nutrient addition was established in evergreen broad-leaved forest soil for 6 years (2015—2021), including 4 treatments: Control [P0+N0, P 0 kg/(hm2·a)+N 0 kg/(hm2·a)], nitrogen addition [P0+N100, P 0 kg/(hm2·a)+N 100 kg/(hm2·a)], phosphorus input [P50+N0, P 50 kg/(hm2·a)+N 0 kg/(hm2·a)] and nitrogen and phosphorus were simultaneously input (P50+N100, P 50 kg/(hm2·a)+N 100 kg/(hm2·a)], and each treatment was repeated 3 times, and a total of 12 plots were obtained. 0—10 cm soil samples were collected from the sample site in August 2021 to determine the basic physical and chemical properties, soil particle size distribution, phosphorus components of soil aggregates of different particle sizes, and organic carbon (SOC) content. [Results] (1) Under P0 treatment, nitrogen addition significantly increased the proportion of large aggregates, decreased the clay and powder contents, and increased the SOC content in each aggregate particle size. Nitrogen addition significantly decreased and increased the content of labile phosphorus (LP) and residual phosphorus (RP) in aggregate clay and powder, respectively. (2) Under P50 treatment, nitrogen addition significantly increased the geometric mean diameter (GMD) of soil aggregates, but had no significant effects on phosphorus components and SOC contents in aggregates of different particle sizes. (3) Under P0 treatment, soil aggregate SOC was positively correlated with refractory phosphorus. There was no significant correlation between SOC of soil aggregates and functional phosphorus components under P50 treatment. It was concluded that under P0 treatment, nitrogen addition could increase soil organic carbon retention by increasing the refractory phosphorus in soil aggregate clay and powder particle size of evergreen broad-leaved forest. Under P50 treatment, the effect of nitrogen addition on organic carbon in each particle size may be regulated by biological factors and has nothing to do with the availability of phosphorus. [Conclusion] The effect of nitrogen deposition on the internal relationship of carbon and phosphorus in soil aggregates in evergreen broad-leaved forests was regulated by phosphorus. The results of this study provide data supported for forest soil carbon cycling in response to global climate change.

Keywords

• phosphorus input
• nitrogen deposition
• soil aggregate
• phosphorus component
• soil organic carbon
• evergreen broad-leaved forest