Evaluating crop response and environmental impact of the accumulation of phosphorus due to long-term manuring of vertisol soil in northern China

The availability of soil phosphorus (P) is one of the key factors that regulate crop productivity. Fertilization practices with P fertilizers carry a high risk of non-point environmental pollution due to water run-off and leaching. The present work discusses data from a 29-year (1982-2011) fertilization experiment with wheat-soybean rotation. Its aim was to quantify and evaluate the dynamic of soil P availability in relation to P accumulation, crop yield, and environmental safety in northern China. This study included six treatments with four field replicates: CK (no fertilizer), NPK (mineral fertilizers), 1/2SNPK (mineral fertilizers plus 50% wheat straw return), SNPK (mineral fertilizers plus 100% wheat straw return), PMNPK (mineral fertilizers plus pig manure), and CMNPK (mineral fertilizers plus cattle manure). Continual additional application of farmyard manure (i.e., PMNPK and CMNPK) produced significantly (P < 0.01) better soil total P and Olsen-P than NPK treatment, and both factors showed increasing trends. However, straw incorporation (i.e., 1/2SNPK and SNPK) had no effect on soil P or Olsen-P. There were significant positive correlations between P budget and increase in Olsen-P and total P. These correlations indicated that, with each 100 kg ha−1 of P budget, there were about 1.0-1.1 and 22.5-26.0 mg kg−1 increases in Olsen-P and total P for the straw incorporation treatments, whereas there were 5.7-5.9 and 26.5-30.8 mg kg−1 increases in Olsen-P and total P for use of manure. Average P activation coefficients for soil Olsen-P in the PMNPK and CMNPK treatments increased sharply by 87.2% and 121.3% compared to the NPK treatment (P < 0.01). Two segment regression analyses indicated there to be observable changes in the relationships between Olsen-P and relative crop yield, and in Olsen-P and soluble-P, indicating critical Olsen-P levels of 11 mg kg−1 for crop yield, and 18 mg kg−1 for soluble P, respectively. In conclusion, much more P is available in the soil under long-term farmyard manure than crop straw incorporation when the amount of P accumulation is uniform. Long-term, continuous, excessive use of farmyard manure to increase crop yield is not sustainable and has a high risk of P environmental pollution. The incorporation of crop straw is a recommended means of managing which decreases the risk of P environmental pollution without compromising crop productivity of vertisol soil in northern China.