Coniferous mediterranean forest soil dynamics

Black layer is a condition where a darkened stratum exists in the root-zone of golf putting greens. It was the number one malady of greens in the 1980s because its nature was unknown and severe turf injury was associated with it. Researchers at Michigan State University (MSU) hypothesized that black layer was an accumulation of metal sulfides (MeS) linked to release of hydrogen sulfide (H2S) by sulfur-reducing bacteria (SRBs) in response to low redox potential. Turf injury likely resulted from H2S toxicity. Elemental sulfur (S0) was thought to contribute to black layer, and nitrate (NO3-) could be a control. Researchers at MSU sought to test this hypothesis. The presence of MeS was verified in in situ black layers from putting greens at 26 golf courses by spot testing with a solution of sodium azide (NaN3-) and iodine (I2). Black layer and H2S were linked when H235S and Me35S were produced after 35SO42- was injected into intact soil cores from an in situ black layer. Adding molybdate (MoO42-) with the label demonstrated involvement of SRBs. MeS-based black layers resulted and redox potential as pe + pH was lowered when S0 was applied to waterlogged sand. Nitrate (NO3-) poised redox high enough to avert release of H2S. Black layer was MeS. Applying S0 contributed to black layer because it reduced redox and stimulated H2S. The release of H2S by SRBs was controlled by poising redox with NO3- and limiting S0. Fertilizing with NO3- and limiting S0 were effective controls for black layer.