How does phosphorus fertilizer improve the stability of soil aggregates? Evidence from a decade fertilization experiment

Phosphorus (P) fertilizer inputs can increase soil P availability, which improves soil carbon (C) cycling and microbial community structure. However, the potential mechanisms via which P drives soil organic carbon (SOC) and microbial regulation of aggregates formation and stabilization are still unclear.

A 10-year field experiment was conducted, including (1) CK, no fertilization; (2) NK, N and K fertilizer addition; (3) NP1K and (4) NP2K, NK with 28 and 56 kg P ha⁻¹ addition, respectively.

Relative to NK treatment, long-term P fertilizer application significantly increased the proportion of >0.25 mm aggregates and mean weight diameter (MWD), which were increased by 16.4% and 18.0%, respectively. Scanning electron microscopy further confirmed that P addition resulted in better soil structure. Meanwhile, compared with NK treatment, the content of soil exchangeable Ca and SOC (especially stable C=C chemical speciation) was increased with P fertilizer addition, which could form organic-Ca complexes to improve aggregate stability. And compared with NK treatment, the relative abundance of copiotrophic bacteria (i.e., Actinobacteriota) involved in aggregate formation and stability was increased by 11.3% and 8.4% in NP1K and NP2K, respectively. Additionally, redundancy analysis indicated that the main factor for bacterial diversity was available P (AP).

Taken together, P fertilizer addition can increase the content of soil exchangeable Ca and SOC (especially C=C) to form organic Ca complexes, while AP improves the microbial community structure, thereby improving the stability of aggregate structure in saline alkali soil.