Graphic courtesy of the IPNI
By: Anna Cates, Extension soil health specialist
Predicting the amount of nitrogen (N) that will be available to a crop in the upcoming year is difficult. Crops need N at different physiological stages. The timing of those stages depends on growing degree days and available moisture and nutrients. Soil microbes transform organic N into plant-available nitrate (NO3—) or ammonium (NH4+). The pace of those transformations is also dependent on the weather. Most microbes need both water and oxygen, so overly dry or moist conditions slow things down. (Saturated soils will lose N as nitrogen dioxide (NO2) because a different group of microbes that do not need oxygen takes over.) So predicting N availability comes down to being able to predict the weather, and taking into account both plant and microbial responses to temperature and moisture.
The tools we use now for predicting crop-available N are based on measuring how much N is in the soil. This makes sense: the source of N that becomes available to plants over the growing season is soil N. Nitrate is a plant-available pool of nitrogen that is easy for researchers to measure. The amount of NO3— in your soil prior to planting, or pre-plant nitrate test (PPNT), is correlated with how much N your soil will supply to a crop. You can get a credit and decrease N application if you have high enough PPNT values. However, since NO3— dissolves in water, it washes through the soil with rain or irrigation. The PPNT provides a snapshot of NO3— in soil, but cannot account for how NO3— might be lost if you get a lot of moisture.
Nitrogen from soil organic matter provides at least 50% of the N to a corn crop (up to 83% when only 50 lbs. N/acre was applied!). If you go back to your nitrogen cycle notes, you will see that microbes immobilize organic N in their own bodies, but since microbes are short-lived, a portion of that organic N becomes NO3— and NH4+ that plants can use. The process of transforming organic N to NO3— and NH4+ is something that many different kinds of microbes can do, so no particular group of microbes (that we can measure yet!) is responsible. That means that while it is fascinating to explore different groups of soil microbes via tests like PLFA or DNA profiles, we cannot use that information to predict N availability to crops.
New tests for N try to measure just the pool of organic N that is currently immobilized, or predict the rate at which that organic N will be transformed to NO3— and NH4+. The ACE-protein N test, which is offered in the CASH assessment from Cornell University, is an example of measuring just the organic N pool. Potentially mineralizable N, or PMN, measures the rate at which microbes release N in the lab. The Solvita burst test, or any test of soil respiration, estimates how fast microbes are mineralizing organic matter. Since organic N is part of total organic matter, soil respiration is correlated to how much N microbes will release.
These tests for rates of N or organic matter mineralized give you a good sense of how active your microbes are, and whether they have access to enough organic matter. More organic N as measured by ACE-protein tests will definitely be good for your crops down the road: the more soil organic N, the more potential there is for plant-available N later. These tests are generally well-correlated with total organic matter. However, since microbial activity is tightly linked with temperature and moisture conditions in the soil, knowing the quantity of protein N or rate of organic matter mineralization early in the season does not always predict how much will be mineralized later in the season, when crops need N. As we continue to use these tests and compile larger datasets, we may be able to calculate an N credit for PMN or Solvita like we do with PPNT. At this point, we do not have enough data to include this in our N recommendations.
Source: University of Minnesota Extension: Minnesota Crop News