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.