Factors Affecting Dissolved Silica Concentrations, and DSi and DIN Stoichiometry in a Human Impacted Watershed (Po River, Italy)

Data on dissolved (DSi) and particulate (PSi) silica concentrations, along with DSi and DIN (dissolved inorganic nitrogen) stoichiometry in surface waters were reviewed for the Po river watershed. DSi in the cascade river and lake ecosystems followed clear upstream-downstream gradients. The DSi (77-178 kt Si y-1) and PSi (879-1,486 kt Si y-1) loadings from the Po river to the Adriatic Sea display strong inter-annual variability, related with the river discharge. In the lowland river reaches, the DSi to DIN ratio highlighted frequent potential Si limitation, especially in summer during diatom blooms. Since the Po river watershed is heavily inhabited and exploited with agriculture (~43 % of the total surface) and livestock husbandry (~3. 4 × 106 cattle heads), agriculture likely interferes with the natural silicon cycle. We present a preliminary assessment of the biogenic silica (BSi) which was fixed in and harvested with the main crop biomass. In the period 2000-2010, the total BSi in crops was 270-386 kt Si y-1. Three main cereals (maize, wheat, rice) accounted for 70 % BSi, of which 89 % was accumulated in straw. The quantity of BSi that was annually accumulated in the cereal biomass increased 2-4 folds from 1950 to 2010. We estimated that a great part of the BSi accumulated in straw and fodder was processed by the cattle stock and returned to soil as manure. A large part of the cropland is exposed to erosion and PSi can be exported to canals and rivers at particularly high rates during flash flood events. Recent transformations of either agricultural practices or crop typology probably perturb the Si cycling.