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

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. [ABSTRACT FROM AUTHOR]