Complex sentence: The overall excess of silicic acid utilization over nitrate is reinforced because the rate of biogenic silica is lower than the rate nitrogen degrades at, and when ammonium is present nitrate will not be utilized as much because ammonium is more reduced than nitrate.
Split into simpler sentences: The overall excess of silicic acid utilization over nitrate is reinforced because the uptake of biogenic silica is lower than the rate at which nitrogen degrades. When ammonium is present, nitrate may not be utilized as much because ammonium is more reduced than nitrate.