Mechanisms of environmental factors affecting uptake and utilization of N and P by vegetable crops

This project was implemented by Zhejiang University. In 2012, soil samples from a Chinese Cambisol under three cropping systems were collected vertically to a depth of 200 cm and then analyzed to evaluate the effect of manure application on soil P quantities, forms and the potential environmental implications under different land use patterns.

Results showed that calcium-bound P was the most abundant P fraction in the soil, followed by residual P. Organic P only accounted for less than 5% of total P in most of the soils. Manure application increased the levels of inorganic P (Pi), with higher proportions of Pi in labile forms than stable forms. After manure applications for 8 to 5 yr, available P (Olsen P) and degree of P saturation (DPS) values of the 0 to 20 cm layer at all sites exceeded the threshold for Olsen P (60 mg/kg) and DPS (30%), and the risk of P loss was significantly increased. The DPS values were generally lower than 30% below 20 cm, indicating a minimal risk of P loss via leaching from deeper soil. The on-going practice of manure application not only increased the size of each of the labile and non-labile P pools, but also caused a shift in the relative sizes of different pools, regardless of the cropping systems. Long-term application of organic manure(s) might cause P accumulation in top soil, thereby, increasing the risk of P loss through surface runoff and leaching.

Results also showed that P loading in all experimental soils, irrespective of the different soil extractions used for analysis, were low below the 40 cm depth, with most P concentrated in the 4 to 6 cm depth. High simulated rainfall led to lower soil water extractable P (Pw) concentrations, and higher leachate dissolved reactive P (DRP) and total P (TP) in comparison with low simulated rainfall. Based on the relationships between Pw and corresponding degree of P saturation (DPS) indices, the critical levels of DPS indicating P leaching could be identified at 19% DPSM3(Ca) and 13% DPSM3(Ca+Mg). The DPSM3(Ca) and DPSM3(Ca+Mg) values were above the thresholds in the top 30 cm of soil, but below the thresholds at a soil depth of 30 to 40 cm. Intensive rainfall can significantly increase P leaching, which can also be enhanced by manure application in the fallow season. Zhejiang-25