Soil P availability, inorganic P fractions and yield effect in a calcareous soil with plastic-film-mulched spring wheat

Plastic film mulching (PFM) has been extensively used for crop production since the 1990s in the semi-arid Loess Plateau, Northwest China. The availability of soil inorganic P under PFM is poorly understood and hard to be predicted in this area. This paper determined the effects of PFM on P availability, soil P budget and topsoil P balance, and inorganic P fraction transformation on a calcareous soil with plastic-film-mulched spring wheat. This study was conducted in Zhonglianchuan, Yuzhong County, Gansu Province over two growing seasons in 2009 and 2010. There were two treatments of spring wheat either mulched with plastic film (M) or non-mulched/bare (B). Individual plots, 10m long×6m wide, were replicated three times in a randomized complete block design. Compared to the B treatment, grain yield in the M treatment increased by 94.5% and 73.4% in 2009 and 2010, respectively; and straw biomass increased by 62.6% and 53.6%. P mass in aboveground spring wheat in the M treatment was significantly higher (P≤0.05) than in the B treatment in both years. Moreover, P mass was significantly higher (P≤0.05) in 2010 than the corresponding treatments in 2009. As a result, P remaining in the M treatment was significantly lower (P≤0.05) than in the B treatment in both years. In the 2009 growing season with less rainfall, the balance of available P was positive, but negative in 2010 with more rainfall. This can be explained on the basis that high crop biomass requires more soil available P in wet years than in dry years, which may decrease actual soil available P content in the wheat-growing period. In this trial, inorganic P fraction contents increased in both treatments after two consecutive growing seasons, but decreased for O-P (occluded Fe/Al bound P). In the M treatment, a significant positive correlation existed between Olsen-P and Ca ₂-P (dicalcium bound P), Al-P (aluminum bound P), Fe-P (iron bound P), Ca ₁₀-P (phosphorite bound P) in 2009, and Ca ₂-P in 2010, but no significant correlation occurred in the B treatment. Therefore, we suggest that PFM is beneficial for reducing P remaining in soil and promoting inorganic P fraction availability to crops in semi-arid areas. Future research should focus on the mechanism for the effect of PFM with other crops on soil P availability and inorganic P fractions.