Rice paddies reduce subsequent yields of wheat due to physical and chemical soil constraints

Yields of wheat crops that succeed rice paddy crops are generally low. To date, it has been unclear whether such low yields were due to rice paddies altering soil physical or mineral characteristics, or both. To investigate this quandary, we conducted field experiments in the Jianghan Plain to analyze differences in the spatial distribution of wheat roots between rice-wheat rotation (RW) and dryland-wheat rotations (DW) using a range of nitrogen treatments. Dryland wheat crops were preceded by either dryland soybean or corn in the prior summer. Biomass of wheat crops in RW systems was significantly lower than that of DW for all N fertilizer treatments, although optimal nitrogen management resulted in comparable wheat yields in both DW and RW. Soil saturated water capacity and non-capillary porosity were higher in DW than RW, whereas soil bulk density was higher in RW. Soil available nitrogen and organic matter were higher in DW than RW irrespective of N application, while soil available P and K were higher under RW both at anthesis and post-harvest stages. At anthesis, root length percentage (RLP) was more concentrated in surface layers (0–20 cm) in RW, whereas at 20–40 cm and 40–60 cm, RLP was higher in DW than RW for all N treatments. At maturity, RLP were ranked 0–20 > 20–40 > 40–60 cm under both cropping systems irrespective of N fertilization. Root length percentage and soil chemical properties at 0–20 cm were positively correlated (r = 0.79 at anthesis, r = 0.68 at post-harvest) with soil available P, while available N (r = −0.59) and soil organic matter (r = −0.39) were negatively correlated with RLP at anthesis. Nitrogen applied at 180 kg ha−1 in three unform amounts of 60 kg N ha−1 at sowing, wintering and jointing resulted in higher yields than other treatments for both cropping systems. Overall, our results suggest that flooding of rice paddies increased bulk density and reduced available nitrogen, inhibiting the growth and yield of subsequent wheat crops relative to rainfed corn or soybean crops.