Accumulation and depletion of soil Phosphorus pools

Abstract

In this thesis, three experiments were carried out to assess the effect of straw or sewage sludge on soil P pools and plant P uptake. To assess the impact of straw and inorganic P amendment on P pools and wheat growth, barley straw (C/P 255) was added to a loamy sand alone or with inorganic P to reduce the C/P ratio to 127 or 25 (straw treatments). Other treatments included inorganic P alone at the same rates as in the straw treatments. P pools increased with P addition rate and were higher with inorganic P alone than straw treatments except MBP which was greater in the straw treatments. Wheat growth for 5 weeks reduced HCl P, phosphatase P, citrate P and CaCl2 P. Barley straw was added into another set and after three weeks incubation. Straw addition also reduced HCl P and citrate P but increased phosphatase P, CaCl2 P and MBP. It can be concluded that P pools are affected mainly by P addition rate whereas the form in which P is added is less important. To assess the effect of longer term P fertilisation on P pools and the influence of straw addition and plant growth, two experiments were carried out with silt loam which had been amended with 0, 10 and 20 kg P ha-1 a-1 for 7 years (referred to 0P, 10P and 20P). In the first experiment, soil was incubated for 4 months without or with 5 g kg-1 barley straw (C/P 255). After 4 weeks of wheat growth in 10P and 20P, citrate P was 25% higher and HCl P 25% lower than before planting. Plant P uptake was higher with straw which indicates that microbial biomass P is an important P source for plants. In the second experiment, wheat was grown for 5, 10 and 15 weeks in unamended soils from the field trial. After 10 and 15 weeks, HCl P and citrate P in 10P and 20P were reduced by 20% compared to the start. In the third experiment, a loamy soil was amended with the same amount of N and P (43 mg P kg-1 soil and 60 mg N kg-1 soil) as sludge and inorganic nutrients. Sludge or inorganic N and P were added either alone (100S and 100F), or 75% S+25% F, 50% S+50% F and 25% S+75% F. Before planting, HCl and citrate P was about four-fold higher in amended treatments than the control. Leachate N increased with proportion of F and leachate P with >50% sludge was higher than in the control. After 5 weeks of growth compared to the control, shoot P uptake was about 50% higher in 100S and 75S25F and three-fold higher in treatments with ≥50% F. HCl P in amended treatments was lowest in 100F. Citrate P was higher in treatments with ≥50% sludge than in 100F. Leachate inorganic P was lowest in 100S and increased with proportion of F. Available N and leachate inorganic N were very low after harvest.