Relationships between root density of the African grass Hyparrhenia diplandra and nitrification at the decimetric scale: an inhibition-stimulation balance hypothesis

Previous studies have shown that Lamto savannah exhibits two different type s of nitrogen cycle with high and low nitrification sites and suggested tha t the perennial grass Hyparrhenia diplandra is responsible for this duality at a subpopulation level, with one ecotype being thought to be able to inh ibit nitrification. The present work aimed to investigate the relationships between nitrification and the roots of H. diplandra at two scales. (i) Sit e-scale experiments gave new insight into the hypothesized control of nitri fication by H. diplandra tussocks: the two ecotypes exhibited opposite infl uences, inhibition in a low nitrification site (A) and stimulation in a hig h nitrification site (B). (ii) Decimetric-scale experiments demonstrated cl ose negative or positive relationships (in sites A or B, respectively) betw een the roots and nitrification (in the 0-10 cm soil layer), showing an une xpectedly high sensitivity of the nitrification process to root density. In both soils, the correlation between the roots and nitrification decreased with depth and practically disappeared in the 20-30 cm soil layer (where th e nitrification potential was found to be very low). Therefore, the impact of H. diplandra on nitrification may be viewed as an inhibition-stimulation balance.