Basics of Biosolids-Borne Nitrogen

Happy New Year and welcome back to the monthly biosolids library! The library this month is meant to ease you back into work after what was hopefully a relaxed and happy holiday season. We are taking it back to basics here -- no new exotic contaminants, no papers about the future of the world, not a mention of contaminated water in Flint MI.

We are focusing here on nitrogen in biosolids -- the primary reason we want to land apply the material in the first place. Nitrogen is the most critical element required for plant growth. Biosolids contain significant amounts of nitrogen, typically between 1% - 8%. Because of this, biosolids are used as an alternative to synthetic fertilizers. They are also most commonly applied at rates designed to provide the equivalent quantity of plant available nitrogen as those synthetic fertilizers. And that is the trick; figuring out how much of the total N in the biosolids will become available to plants and how quickly.

The first paper in the library is a new review paper sent to me by Ned Beecher from NEBRA. The authors give a general overview of types of biosolids and associated N concentrations. For all biosolids, the majority of the nitrogen is present as organic N. There is a small amount of ammonia, generally less than 10% of the total and only trace quantities of N as nitrate. The authors note that most variations on aerobic and anaerobically stabilized biosolids have similar total N. Biosolids that are from a lagoon, have been lime stabilized, or composted have much less total N.

The paper includes tons of tables of the range of studies available on N in biosolids. There are tables reporting total N and N species based on treatment type and country. The authors have a large section of how much of the total N will be plant available and the different types of tests you can use to estimate the N availability in your own material. There are two types of lab tests as well as field incubations.

Aerobically and anaerobically digested biosolids, in general, have similar N release the first growing season, between 25% and 40% of total N. All materials will have residual N available during the 2nd growing season as wel, typically about half the quantity that was available during the year immediately after application. The more stabile the materials to begin with (lagoon and composted biosolids), the lower the fraction of available N. The authors also note that the fraction of N that becomes available during one growing season will vary based on climate and soil factors. Warmer soils with better air flow will have greater microbial activity and faster transformation of N from organic to plant available forms.

One paper that these authors site is by Gilmour et al (2003). This paper is a classic on the topic and so is the 2nd paper in the library. This reports on a multi-location, multi-biosolids study that involved lab incubations, field trials, and computer modeling. N availability in biosolids is linked to how quickly the carbon in the biosolids is transformed. You are likely to recognize a few of the authors on this classic- -- Craig Cogger, Dan Sullivan and Greg Evanylo. The results are similar to what was reported in the first paper. But you might want to take a look, as many of the biosolids used in this study were produced by the subscribers to this library.

The next two papers present results from individual incubation N availability trials. The first by Cogger et al. (2004) is a follow up to the Gilmour study. Here you get more detail on the biosolids they used, their methods, and N availability over two growing seasons. Again, relatively consistent results from the first two papers are reported.

The second is out of New Zealand and reports on N availability in two biosolids and two pulp sludges for use in forest soils. The authors saw about double the mineralization rate from aerobically digested materials in comparison to anaerobically digested biosolids.

What you should be starting to get from these papers, if you read them in depth, is that determining N mineralization rates is more of an art than a science. Site and soil specific factors will influence rates. Different crops will have different N demands. Finally, different biosolids processing including upstream from the final treatment, can and do influence the rate of N mineralization. It is almost like you need to read a textbook on this to get the full picture.

For that textbook we turn to the last item in the library. Talk about classic! This is a Washington State Department of Ecology publication on nitrogen in biosolids written by Chuck Henry, Dan Sullivan, Craig Cogger, Bob Rynk, and Kyle Dorsey. I believe that this publication came out about the time that Captain Nitrogen made an appearance at Biofest.

The publication guides you through the nitrogen cycle in all of its complexity so that you can understand the difficulty in coming to an exact pathway or prediction for nitrogen transformations. It discusses the interrelationships between N and C, and explains how to calculate plant available N for the application year as well as subsequent years. It also discusses ammonia volatilization and how that can vary based on type of application and type of biosolids as well as soil factors such as pH. There are individual chapters for forest applications and agricultural applications. One thing that we talk about at NBMA meetings is how to pass on the critical knowledge to those new to the field. One easy answer to that is ‘Read this Guide’.

If you read some of the items in this library, you will develop or refresh your knowledge of nitrogen in biosolids. This is a critical topic. While research has not given us a precise number for availability, we have a solid understanding and a very reasonable range of values that we can use to maximize benefits associated with biosolids use and minimize any potential for excess N.

Sally Brown University of Washington