Optimum soil water for soil respiration before and after amendment with glucose in humid tropical acrisols and a boreal mor layer

The measurements of water content most often used by soil microbiologists are gravimetric water content (Wg), volumetric water content (Wv), percentage of water holding capacity (%WHC), percentage of water filled porosity (%WFP), and water potential (Wp). This study investigated the relationship of these water measurements to soil respiration, before and after substrate addition (glucose, nitrogen and phosphorus), focusing on the water contents at which the exponential respiration rates after amendments (μ) were at their maximum. Three tropical Haplic Acrisols and one boreal mor layer were used. One Acrisol had high loss of ignition (LOI 17.4% w/w) and high clay content. The second Acrisol had high clay content and low LOI (3.9%), and the third had low clay content and low LOI (5.7%). The mor layer had a LOI of 95.6%. The maximum μ was found at −15 kPa, (50% WHC), irrespectively of the soil type. The decrease in μ above −15 kPa was not as pronounced for the Swedish mor layer as for the Acrisols. At water potentials above −2 kPa the respiration rate was no longer exponential for the Acrisols, making it impossible to define μ. For Wg, WFP, and Wv the maximum differed greatly between the soils. The maximum for basal respiration was found to be at higher water content than the maximum for μ. However, the water contents used were not high enough to specify optimum water content for basal respiration except for the humus rich Acrisol which was at 66% of the WHC or about −5 kPa. It is suggested that respiration measurements of the Acrisols, after substrate addition, should not be made using water contents exceeding −15 kPa or 50% WHC. Because of the dependency of method, WHC must be defined and complemented by water potential. The use of WFP, Wv, and Wg is not recommended for adjusting the water contents of these soils.