Print PageThe boreal forest is the second largest biome in the world, and phosphorus (P) is an important macronutrient in these forests, influencing and sometimes limiting plant growth. Phosphorus, however, occurs as numerous compounds with very different degrees of bioavailability, so that detailed characterization of these compounds and their behavior is necessary to understand the role of P in plant productivity and microbial decomposition, and how these will in turn respond to global environmental change. This knowledge is also important to develop viable alternatives to rock phosphate for agriculture, given that global rock phosphate supplies are expected to collapse in the near future. We used solution and solid state 31P Nuclear Magnetic Resonance (NMR) spectroscopy to determine soil organic P composition along two short (90 m) groundwater recharge and discharge gradients in Fennoscandian boreal forest, which are also gradients in plant productivity, and in P sorption capacity due to differences in aluminium (Al) and iron (Fe) concentration in the humus.
Results/Conclusions
Phosphorus composition changed sharply along the gradients. Labile phosphate diesters and their degradation products, as well as polyphosphates, were proportionally more abundant in the unproductive, low Al and Fe sites, whereas phosphate monoesters such as myo-, scyllo- and unknown inositol phosphates dominated in productive, high Al and Fe soils. Whereas in high Al and Fe sites the composition of P seemed to be closely associated with stabilization processes, in low Al and Fe sites it more closely reflected inputs of organic P, given the dominance of diesters which constitute the bulk of organic P inputs to the soil, and of polyphosphates which are abundant in fungal tissues. Finally, the near absence of inositol phosphates in low P sorption sites was unexpected given the widely assumed recalcitrance of these compounds. We hypothesize that low sorption capacity, combined with ecto-and ericoid mycorrhizal dominated soil microbial communities and low soil pH create conditions more conducive to inositol phosphate mineralization. This is the first time that inositol phosphate concentrations are reported to change so sharply over distances as short as 90 m, and in relation to sorption sites in the humus. Our study gradients encompass the variation in soil properties detected in the Fennoscandian boreal forest landscape, as such our findings provide insight into the factors controlling P biogeochemistry in the region, but should be of relevance to boreal forests elsewhere.
