The aim of this study was to determine the effect of soil amendments on P pools and their depletion by plants and microbes. The experiment was divided into two parts. In Part A, barley straw (C/P 255) was added alone or with inorganic P to reduce the C/P ratio to 127 or 25 (straw treatments). In three other treatments, the same amount of P was added as in the straw treatments as inorganic P (fertilizer treatments). The soil was incubated for three weeks. Then (Part B), wheat was grown for five weeks in one set of soils. Barley straw was added to another set to increase microbial growth and incubated moist for three weeks. P pools were measured at the end of Part A and Part B. At the end of Part A all P pools increased with P addition rate. P pools were greater in the fertilizer than the straw treatments except for microbial biomass P (MBP) which was greater in the straw treatments. After wheat growth, HCl-P, phosphatase-P, citrate-P, CaCl 2 -P were depleted compared to Part A, MBP was higher. Addition of barley straw in Part B induced depletion of HCl-P, and citrate-P, but increased phosphatase-P, CaCl 2 -P and MBP. It is concluded that the size of the P pools is mainly influenced by P addition rate not form in which P is added. The impact of growing plants on P pools differs from that of microbes stimulated by high C/P straw addition.

Little is known about the effect of long-term P fertilisation on soil P pools and how they are influenced by straw addition and plant growth. Two experiments were carried out with soil which had been amended with 0, 10 and 20 kg P ha −1 for 7 years (referred to 0P, 10P, 20P). In experiment 1, soil was incubated moist for 4 months without or with 5 g kg −1 barley straw (C/P 255). Then wheat was grown for 4 weeks. In experiment 2, wheat was grown for 5, 10 and 15 weeks. HCl P, citrate P, resin P and microbial biomass P (MBP) were twofold higher with 20P and 10P than 0P. After 4 months compared to 10 days, HCl P and citrate P were 25% lower with straw, but unchanged without straw. Resin P and MBP were twofold higher. With 10P and 20P after 4-week wheat growth, citrate P was 25% higher and HCl P 25% lower. Plant P uptake was higher with straw, In experiment 2 after 10 and 15 weeks compared to the start at 10P and 20P, HCl P and citrate P were reduced by 20%. Plant P uptake was smaller than the decrease in HCl and citrate P. The greater P uptake by wheat in soil with straw indicates that microbial biomass P is an important P source for plants. In P amended soil, wheat mobilised HCl and citrate P, but only took up a fraction of the mobilised P.

Compost application can increase plant nutrient availability. But the effect of compost on nutrient availability may depend on a number of factors. In this study, the effect of application method (mulch layer or mixed into the soil) and combination with inorganic fertiliser on soil P pools and available N was investigated. Soil was filled in microcosm with six treatments, including control, vermicompost layer with or without fertilisers (CL, CL/F), bulk soil mixed with inorganic fertiliser alone (F), vermicompost alone (CM) and both of inorganic fertiliser and vermicompost (CM/F). The microcosms were incubated in the dark for 3 weeks. Citrate P, HCl P and resin P were the highest in F, but MBP was higher in CM and CM/F. Citrate P and HCl P were about three-and six-fold higher in CM and CM/F than in CL and CL/F. Available N was the highest in CL/F and 20% higher in CL than in CM. Vermicompost mixed into soil slightly increased soil nutrient availability compared to unamended soil but had little effect when placed on the soil surface. Vermicompost mixed into soil with inorganic N and P could be used to minimise loss of N and P after inorganic fertiliser addition and thereby provide a longer-lasting nutrient supply for plants.