Long-term conversion of upland to paddy increased SOC content and N availability in a sand dune of Japan

Land use change driven by anthropogenic activities has greatly affected terrestrial C and N cycling; however, it remains unclear how dynamics of soil organic carbon (SOC) and nitrogen (N) availability respond to long-term land use change in sand dunes. Here, we investigated the changes in SOC, total nitrogen (TN), and their aerobic and anaerobic decomposition or mineralization in a vinyl film paddy, compared with dry land use and management changes (LUMCs) (original upland, greenhouse, and agricultural forest (tree area)) in Shonai sand dunes, northeast Japan. The vinyl film paddy had been singly cultivated for 54 years. Soil samples were collected from five soil depths (0–10, 10–20, 20–30, 30–40, 40–50 cm) to compare the effects of depth and LUMC among the four treatments. The C decomposition (Dec-C) was assessed as the aerobic CO2 production and anaerobic methane and CO2 productions. Meanwhile, N mineralization (Min-N) was determined as aerobic NH4+ and NO3– production and anaerobic NH4+ production. The results showed that the vinyl film paddy increased SOC and TN contents, Dec-C and Min-N than upland and greenhouses at 0–30 cm depth but lower levels than the tree area at all depths. The arithmetic mean for all depths showed the aerobic Dec-C in the paddy (216.0 mg kg−1) was higher than the corresponding anaerobic Dec-C (192.8 mg kg−1). Conversely, the aerobic Min-N (35.9 mg kg−1) was lower than the anaerobic Min-N (75.7 mg kg−1). There were significantly positive correlations among SOC, TN, aerobic and anaerobic Dec-C and Min-N, whereas the paddy had steeper slope than other three LUMCs. Conclusively, our results indicated that long-term conversion of upland to vinyl film paddy increased SOC and TN accumulation and N availability (Min-N) in Shonai sand dunes, which can be applied as an effective agricultural practice to improve fertility of coastal sandy soils.