Stratification of soil organic C, N and the C : N ratio as affected by different plastic film mulching modes in a semiarid region of China

The soil degradation caused by plastic film mulching tillage in rain-fed areas of north-west China is known to affect sustainable and stable crop yields because of major losses of soil organic carbon (SOC) and nutrients. To evaluate the effects of different plastic film mulching modes on SOC and total nitrogen (STN) sequestration capacity in loessic soil, we investigated the effects of different plastic film mulching on SOC, STN, and the soil C : N ratio in semiarid areas of southern Ningxia for a 4-year period (2013–2016). Five treatments were tested: (i) the control, conventional flat planting without mulching (CK); (ii) alternating mulching and bare rows without ridges and planting in mulched rows (P); (iii) furrow planting of maize, separated by consecutive plastic film-mulched ridges (S); (iv) furrow planting of maize, separated by alternating large and small plastic film-mulched ridges (D); and (v) furrow-flat planting of maize with a large plastic film-mulched ridge alternating with a flat plastic film-mulched space (R). In the final experimental year (2016), the results showed that the mean soil bulk density at 0–60 cm depth had decreased with film mulching treatments by 2.82%, 5.90% (P R/D > P > CK; however, there was no significant increase with the storage of SOC and STN. The mean soil C : N ratio was higher with mulching treatment, i.e. 2.91% (P > 0.05) higher than CK in 0–60 cm depth. Mulching treatments significantly (P < 0.05) increased the stratification ratio (SR) of SOC and soil C: N ratio from the surface (0–20 cm) to all depths compared with CK, i.e. the SR of SOC at the 0–20 : 20–40 cm depth significantly (P < 0.05) increased with D, R, S, and P by 14.81%, 9.47%, 14.18%, and 9.51% respectively, compared with CK.