Soil organic nitrogen fraction and sequestration in a buried paddy soil since the Neolithic age

Abstract

Purpose Soil organic nitrogen (SON) biochemistry trends in paddy soils are poorly understood on a long-term scale. Methods To explore the effect of land use on SON sequestration, SON and amino acid (AA) fractions were investigated in soil profiles comprising recent and buried paddy soil (BPS) and buried non-paddy soils (BNS). Two ancient paddy soils from Chuodun ruin site, China, were distinguished based on colour and rice phytolith abundance. ¹⁴C abundance in soil organic carbon was used to estimate the age of carbonized rice and ancient paddy soil via a liquid scintillation analysis method, dating to 3800–5500 and 960–4000 BC. Results The proportions of D-AAs and acidic AAs in BPS, up to 6.13% and 7.73%, respectively, were higher than those in modern paddy soils. D-alanine (and the D-/L- ratio), aspartate, and glutamate increased with soil depth in BPS, and the amount of D-aspartate was linearly and significantly positively correlated with soil depth (p < 0.05). Based on phytolith stability and abundance, the N sequestration rate (NSR: residual N content as a proportion of initial N content) was proposed to indicate the residual N content varied with time. The NSR was estimated as 10.8–91.2% in BPS with a phytolith stability factor of 0.5–0.9. Conclusion These data suggest that intermittent continuous high-intensity rice cultivation could increase soil N sequestration potential over the long term, and that N sequestration is not only associated with AA aging in the organic N fraction, but also with biogeochemical processes in BPS and paddy management. In addition, high-intensity rice cultivation can increase N loss risks, and in turn result in large fluctuations in N sequestration.