Long‐term effects of nitrogen and phosphorus fertilization on soil aggregate stability and aggregate‐associated carbon and nitrogen in the North China Plain

Soil aggregates and carbon storage are important in soil conservation, nutrient supply, and climate change mitigation. The long-term responses of aggregate-associated organic C (OC) and nitrogen (N) in surface versus subsoil to N and P fertilization remain unclear. We examined the effects of different N and P fertilization rates on aggregate stability and the associated soil OC (SOC) and N in the North China Plain through a 35-yr double-crop field experiment, hypothesizing that higher rates of mineral fertilizer would promote aggregate stability and increase C/N ratios in large macroaggregates. The OC and N were highest in the silt + clay fraction, accounting for 34 to 48% of bulk SOC and 28 to 47% of bulk soil N at 0 to 20 cm, and 38 to 62% of bulk SOC and 40 to 62% of bulk soil N at 20 to 40 cm. After 35 yr, N + P fertilization increased SOC (16-35%) and N (21-27%), especially the 540 kg N + 67.5 kg P treatment; the N + P fertilizer also increased the OC and N of the macroaggregate, microaggregate, and silt + clay fractions at 0 to 20 cm. The N + P fertilizer treatments increased bulk soil C/N ratios; the C/N ratios of large macroaggregates decreased. Nitrogen and P fertilizer did not affect aggregate stability. Long-term N and P fertilization increased SOC concentrations, and altered OC and N distributions in aggregates. Quantifying the impacts of long-term fertilization strategies on organic matter sequestration and soil stabilization is important.