Carbon sequestration in soil aggregates under different crop rotations and nitrogen fertilization in an inceptisol in southeastern Norway
Effects of crop rotation and fertilization (nitrogen and manure) on concentrations of soil organic carbon (SOC) and total soil nitrogen (TSN) in bulk soil and in soil aggregates were investigated in a long-term field experiment established in 1953 at Ås, Norway. The effect of these management practices on SOC sequestration was estimated.
The experiment had three six-course rotations: (I) continuous spring grain, (II) spring grain for 3 years followed by root crops for 3 years, and (III) spring grain for 2 years followed by meadow for 4 years. Three fertilizer treatments compared were: (A) 30–40 kg N ha–1; (B) 80–120 kg N ha–1; and (C) a combination of B and 60 Mg farmyard manure (FYM) ha–1.
All plots received a basal rate of PK fertilizer. Soil samples from these treatments were collected in autumn 2001 and analyzed for aggregate size, SOC and TSN concentrations.
There were significant increases in 0.6–2 mm and < 0.6 mm aggregate size fractions, and reduction in the 6–20 mm and the > 20 mm sizes for rotation III only. There were also significant differences among rotations with regard to water stable aggregation. The order of increase in stability was II < I < III.
Fertilizer treatment had no effect on aggregation or aggregate size distribution, but there was a slight tendency of increased stability with the application of FYM. Aggregate stability increased with increasing concentration of SOC (r2 = 0.53).
The SOC and TSN concentrations in bulk soil were significantly higher in rotation III than in rotations II and I. Application of FYM increased SOC and TSN concentrations significantly in the 0–10 cm soil depth, but there were few significant differences between fertility treatments A and B.
There was a trend of increase in concentration of SOC and TSN with decreasing aggregate size, but significant differences in these parameters in different aggregate size fractions were found only in few cases. The SOC and TSN concentrations were higher in >0.25 mm than in < 0.25 mm aggregates. The SOC sequestration rate was 77–167 kg SOC ha–1 yr–1 by increasing the N rate and 40–162 kg SOC ha–1 yr–1 by applying FYM. The SOC sequestration rate by judicious use of inorganic fertilizer was the greatest in the grain–meadow rotation, while that by application of FYM was the greatest in the all grain rotation.