Quantifying gross mineralisation of P in dead soil organic matter: Testing an isotopic dilution method

Assessment of gross and net mineralization rates of soil organic phosphorus – A review

“…Calculating the mineralization of non-living organic P from soil respiration in the soils studied by Achat et al (2009a) confirmed that gross organic P mineralization was mainly related to microbial immobilization (93%). This proportion was even higher (97%) when estimated using the change in the specific activity of the combined pools of water-extractable phosphate and microbial P over time (Achat et al, 2010a). Bünemann et al (2012) likewise found a dominance of microbial P immobilization when measuring gross and net organic P mineralization rates in a non-P-fertilized grassland soil with a large fumigant-labile P pool (36 mg P kg À1 ) and a very low concentration of phosphate in the soil solution (0.1 mg P kg À1 ).…”

Plant and microbial uptake of nitrogen and phosphorus affected by drought using 15N and 32P tracers

“…Soil samples were spiked with 0, 5, 10, 15 and 20 mg P kg À1 soil (as K 2 HPO 4 , replicated 3 times) and recovery of P was measured in the extracts colorimetrically using the ammonium molybdateestannous chloride reagent (Olsen and Sommers, 1982). We used linear or power equations to describe the relationship between P added and P recovered in the extracts (Morel et al, 1996;Achat et al, 2010).…”

“…For microbial P we did not correct for extraction efficiency, but accounted for P adsorption of microbial P during fumigation and extraction using the P-addition-recovery relationships measured for both soil types (Morel et al, 1996). An increase in microbial P in the extracts also affects the level of radioactivity of the extractable inorganic P (iP) because of sorption of P (including 32 P) to the solid phase and because of a change in the equilibrium between the radioactivity of iP and P bound to the solid phase (Achat et al, 2010). To correct for the net effect of both processes, 10 g of fresh soil was spiked with 4 KBq carrier-free 32 P and 0.12 mg unlabelled P (as K 2 HPO 4 , 12 mg kg À1 , replicated 3 times) mimicking microbial P release during fumigation (Achat et al, 2010).…”

“…An increase in microbial P in the extracts also affects the level of radioactivity of the extractable inorganic P (iP) because of sorption of P (including 32 P) to the solid phase and because of a change in the equilibrium between the radioactivity of iP and P bound to the solid phase (Achat et al, 2010). To correct for the net effect of both processes, 10 g of fresh soil was spiked with 4 KBq carrier-free 32 P and 0.12 mg unlabelled P (as K 2 HPO 4 , 12 mg kg À1 , replicated 3 times) mimicking microbial P release during fumigation (Achat et al, 2010). Soils were extracted immediately after spiking.…”

“…In general, N/P ratios in microbial biomass are lower than in plants or SOM (Cleveland and Liptzin, 2007) suggesting a higher microbial P requirement relative to N. Microbial P limitation was shown in a tropical rainforest where soil microbial activity was stimulated by P additions while plant growth was enhanced by N additions (Cleveland and Townsend, 2006). Furthermore, microbes showed a large capacity to immobilise 33 P added to a P-poor grassland soil (Bünemann et al, 2012) and a sandy forest soil (Achat et al, 2010), indicating a large microbial demand for P that could negatively affect plant growth. However, microbes could also facilitate plant P uptake in P-limited systems by extending plant roots through mycorrhizal hyphae and by enhancing P solubilisation and mineralisation in the rhizosphere (Marschner et al, 2011).…”