Long-term upland-paddy rotation effect on the soil C and N stocks in the middle and lower reaches of the Yangtze River

Abstract Observations of the effects of long-term paddy-upland rotations on soil carbon (C) and nitrogen (N) stocks are scarce. This experiment used four paddy-upland rotation systems with summer rice as the main crop: rice-winter fallow (RF), rice-potato with rice straw mulch and return (RP), rice-wheat (RW), and rice-Chinese milk vetch (RC) with the return of winter crop residues (from 2003 to the present) to examine the soil C and N stock characteristics in the 0–50 cm soil layer. The results showed that in the topsoil at 50 cm, compared to RF, both RP and RC increased the soil organic carbon (SOC) stock by 19.67% and 15.29%, the total nitrogen (TN) stock by 20.40% and 18.54%, and the available nitrogen (AN) density by 61.54% and 34.44%, respectively; in particular, the increases in the TN and AN at 20–30 cm were likely to increase the rice yield. RW increased only the AN density by 30.24% compared with RF. Furthermore, the highest SOC, TN, and AN densities were all observed in RP. Except for TN at 30–40 cm, the densities of SOC, TN, and AN in RP were slightly greater than those in the other treatments due to the high C and N concentrations at 0–40 cm; thus rice-potato with rice straw mulch was the best choice for increasing soil C and N accumulation and improving rice yield. In addition, SOC, TN, and AN had obvious stratification and surface aggregation characteristics at 0–50 cm, and 72.17%–83.37% of the total SOC stock, 75.70%–80.48% of the total TN stock and 75.69%–81.08% of the total AN density were concentrated in the top 30 cm. However, the characteristics were dependent on the residues.