I have read many studies which use anaerobic incubation method to determine PMN. My understanding is that when soils are flooded with water, denitrification processes are enhanced because of lack or oxygen which result in some N lost as a gas and some leached. Are results from this method of incubation in determining PMN an accurate representation of the mineralization process? Does this method then encourage waterlogging in soils?
Portia,
The objective of this test is to determine NH4-N production after a 7-day anaerobic incubation. As incubation is done under anaerobic conditions, all NH4+ produced during the incubation remains as NH4+ since the nitrification is restricted by anaerobic conditions. It is true that any already existing NO3- in the soil sample can be denitrified; but, it does not matter, since the aim of the test is to measure potentially mineralizable N. It is the biological N availability index recommended by Keeney (1982) and the purpose and usefulness of the test is well explained in Keeney, D.R., 1982. (N availability indices. Page 711-733, Methods of Soil Analysis Part 2.Agron. Monogr. 9 ASA & SSSA).
I appreciate the good question of Ms.Portia.I agree with the reply of Dr. Susantha. Anaerobic incubation method was proposed as a good indicator of potentially available soil N.Only draw back of the method is, it takes one week time to get result.So it may not fit for routine soil testing.Also, the anaerobic method is preferred over aerobic method as one can control the soil environment better in the former compared to the later.In aerobic method, the moisture level, oxygen supply and redox conditions may be difficult to manage(may fluctuate) compared to anaerobic method underwater- logged condition.A few Indian studies indicated that this anaerobic method also serves as good indicator of available P and K in soil(correlated well with crop response).
The usefulness of any index of N mineralisation index is whether the method can be used to make any assessment of N that is available to plants. This method has been linked to plant production in the previous studies (although not many last time I looked). Nitrogen supply, when not directly from recent fertiliser inputs, depends on turnover of nitrogen of soil organic matter. It is probably important to also remember the incubation temperature for anaerobic N is often high e.g., 40C which likely leads to some microbial biomass turnover. In my experience anaerobic N is correlated to microbial biomass. This might suggest that anaerobic N is an index of microbial N release (although correlation is not causation). Measuring anaerobic N is easier than measuring microbial biomass.
With respect to denitrification - this would lead to an initial loss of nitrate in the soil sample but nitrate concentrations in a soil sample is likely quite low as a proportion of the total N. Because the incubation is flooded and anaerobic, ongoing nitrification and coupled denitrification will not occur. So N losses via denitrification probably not a big deal
Dr.Schipper,it is interesting information that anaerobic N mineralization in soil is related to microbial biomass turn over and N mineralization and release from microbial biomass.
Dear colleagues,We understand that the anaerobic incubation method provides us an estimate of mineralized ammmonium N which can serve as a measure of plant- available soil N.Similarly, from this method can we get mineralized P also? Will there be a problem of reduction of mineralized sulphate- sulphur under anaerobic conditions(one week)?Are there any attempts for using this method for availability of micronutriets like iron?