Effects of rice cropping intensity on soil nitrogen mineralization rate and potential in buried ancient paddy soils from the Neolithic Age in China's Yangtze River Delta

Rice cropping density, rice cropping duration, and fertilization can affect soil nitrogen (N) supply, but rice cropping intensity (RCI) on soil N fertility is not fully understood, particularly for ancient paddy soils without N fertilization. Eight buried ancient paddy soils from the Neolithic Age in China's Yangtze River Delta, and its parent material, and seven present paddy soils in the same fields were used to investigate the effects of RCI on soil nitrogen mineralization rate and potential. In the present study, concentration of phytolith of rice in soils was used to indicate the RCI. Soil N content was obviously greater in the buried Neolithic paddy soils than in the parent material. Total soil N increased with increasing phytolith from 5,200 to 60,000 pellets g(-1), but tended to decrease with increasing phytolith from 60,000 to 105,000 pellets g(-1). A possible reason for RCI-induced increase of soil N was due to biological N-2 fixation in the rice field because there was a significant negative relationship between total N and delta N-15 in the buried Neolithic soils. The mineralization rate constant (k) ranged from 0.0126 to 0.0485 d(-1) with an average of 0.0276 d(-1), which was similar to that of the parent material, but lower than those in the present paddy soils. The k value increased with increasing RCI in the Neolithic paddy soils. There was a significant positive relation between RCI and the percentage of cumulative mineralizable N in the 14 d of that within 103 d incubation. Soil N content tended to increase with the increasing intensity of rice cropping and then decreased under the high intensity of rice cropping; the excessive high intensification of rice cropping could facilitate fast N mineralization (labile N) fraction in the cumulated mineralized N. The unfertilized paddy field could only meet soil N supply under the low intensification of cropping rice in the Neolithic Age. The N fertilization is necessary in order to improve soil fertility for sustaining the present high-yield rice production.