Introduction

Current nutrient application recommendations for Wisconsin indicate that when manure is applied to land, the amount of available nitrogen (N) in manure should be accounted for in the total N applied. In cases where the available N in manure is not sufficient to meet crop needs, the manure available N should be subtracted from the total recommended N rate to determine the remaining N needs (Laboski and Peters, 2012). However, estimating the available N from manure can be challenging as not all the N is immediately available in the first season after application. This is because manure contains both organic and inorganic forms of N (where inorganic forms are plant-available). Ammonium-N is plant-available but organic N needs to be converted to a plant available form, a process called mineralization. In this process microbes break down organic N and convert it to ammonium and nitrate (Figure 1). This process takes time and depends on favorable environmental conditions, and characteristics of manure and soil. The inorganic form of N in manure is primarily present in the form of ammonium. The presence of ammonium combined with a relative high manure pH can lead to loss of N to the atmosphere in the form of ammonia. In the soil, ammonium can also be transformed to nitrate in a process called nitrification. Nitrate can easily dissolve in water and can be lost through the soil profile in leaching. If soil becomes anaerobic, nitrate can also be reduced to gaseous forms of nitrogen such as nitrous oxide and lost to the air.

In current guidelines for Wisconsin, N availability from manure varies based on animal species, the manure dry matter content (i.e., liquid or solid), and time between application and incorporation. Additionally, the total N content of manure varies with animal species (median values in Table 1) and is affected by the animal’s diet, manure storage and management, and land application technique. This variability is apparent in samples analyzed in Wisconsin from 2015 to 2021 where, for example, the N content in semi -solid dairy manure ranged from 0.2 to 48 lbs/ton and in solid beef manure from 2.2 to 44.4 lbs/ton.

Manure N content also varies within the same operation. For example, Dou et al. (2001) found dry matter and total N varied by 22 and 30%, respectively, in a manure storage system. While agitation can create a more homogeneous manure composition, a recent assessment of 16 Wisconsin dairy farms measured persistent high nutrient variability in agitated manure. The variability of total Kjeldahl N (TKN) and total ammoniacal N compositions during one season of emptying manure storage ranged from 7 to 48% and 4 to 54%, respectively (Aguirre-Villegas et al., 2018). This is why sampling and testing manure before application is recommended and more accurate than using book values.

Equations for Determining Plant Available Nitrogen from Manure

Whether you use typical “book” values or collect and analyze manure at a certified laboratory, you will need to determine how much of that N is likely to be available to a crop to know its fertilizer value. Plant available N (PAN) from manure is the amount of N that can be taken up by the plant (Bundy and Meisinger, 1994) and knowing this value is important for planning N fertilizer applications.  

There are various methods that can be used to calculate PAN from manure, some of which are presented below. Each method has sound justification for its use, however specific circumstances including data availability may favor one method over others.  

Plant available N can be calculated by multiplying the manure N analysis by an availability coefficient (Equation 1).

This equation accounts for all manure N losses and mineralization in a single availability coefficient. In Wisconsin, the availability coefficient in Equation 1 is provided in the Nutrient Application Guidelines for Field, Vegetable, and Fruit Crops in Wisconsin (publication A2809) as a function of manure type and time to incorporation. Time to manure incorporation is important because the longer the time between application and incorporation, the more ammonia is lost to the atmosphere, reducing the amount of N that remains in the soil and thus lowering estimated credits. Although, not necessarily more accurate, there are equations that include additional N components and coefficients, in calculating PAN from manure (Schmitt et al. 1996).

Equation 2 requires manure analysis of ammonium (NH₄) and organic N. In using this equation, the assumption is that all the ammonium-N is available, and a mineralization coefficient is used to determine how much organic N becomes available. The mineralization coefficient in this equation is a function of animal species and method of storage and handling (Schmitt et al. 1996).

Equation 3 includes an estimation of the organic-N that will be mineralized and an estimation of the loss of ammonium as affected by the method of application (Schmitt et al. 1996). In this equation, ammonium availability is calculated using a loss coefficient and the assumption is that ammonium availability varies with manure application method. Like Equation 2, this equation requires ammonium analyses of manure which is not offered as part of a routine manure analysis of certified labs in Wisconsin but is available upon request.

Again, while each of these approaches can be equally valid, each requires coefficients or components that might not always be easy to obtain or estimate. The reason to choose one over the other is a matter of which coefficients and components are available for given circumstances. In research different methods have been used to estimate these coefficients. Below is a highlight of some common approaches used to estimate N availability coefficients and N recovery from applied manure. Specifically, these include: 1) the Fertilizer Equivalence approach; 2) mineralization or decay series; 3) the Difference Method; and 4) the ¹⁵N approach.

Fertilizer Equivalence

The most common approach to estimate PAN from manure is referred to as the Fertilizer Equivalence approach. This approach determines equivalent rates of commercial fertilizer N and applied manure N that result in similar levels of crop yield or N uptake (Klausner and Guest, 1981). This is accomplished by plotting the yield or nutrient uptake in the fertilizer treatments against the rate of fertilizer N and generating a regression model (Figure 2). Yield or N uptake values from the manure treatments are then entered into the equation and the fertilizer N rate that would have produced the same N uptake or yield as a manure N rate is the fertilizer equivalent value.

Let’s consider the following example from Figure 2: Manure is applied at a rate of 168 lbs N/acre (manure N rate is determined by multiplying the manure application rate by the total % N in manure). This manure N rate results in an N uptake of 170 lbs N/acre.

Using the regression equation fitted to N uptake vs. fertilizer N rate (orange line), we can solve for the N fertilizer rate that would have resulted in 170 lbs N/acre of uptake (which is what we observed with the 168 lbs N/ac from manure). In doing so, we find that an N fertilizer rate of 71 lbs N/ac would have resulted in the same N uptake (170 lbs/ac) as 168 lbs N/ac from manure. Thus, the N availability coefficient is determined as follows: Availability Coefficient = [(71 lbs N/ac from fertilizer) / (168 lbs N/ac from manure)] x 100% = 42%.

Using the N Fertilizer Equivalence approach, Laboski et al. (2013) found that the average N availability is 51% for injected manure, consistent with Wisconsin’s nutrient application guidelines (A2809). However, the N availability was calculated at 36% when manure was incorporated less than one hour after spreading, which is less than the 50% N availability outlined in A2809 (Table 2). It is interesting to note the high N variability from this study from year to year. Muñoz et al. (2008) found first year N availability of 57, 53, 14, and 4% for fresh poultry, dried poultry, composted poultry, and composted cow manure respectively.

A shortcoming of this method is that it is highly dependent on the equation selected to describe crop response to N and this equation might change as the crop response changes from year to year. In addition, this approach discounts crop response to non-fertility benefits from manure such as the improvement of soil physical and chemical characteristics (Beegle et al. 2008).  

Incubation and Litter Bag Technique

Another approach to estimating PAN from manure is by determining mineralization coefficients (Equations 2 and 3). A mineralization coefficient is a number that shows how much of the organic N in manure is converted to PAN over a certain amount of time (ammonium N is immediately available and does not need to be mineralized). These coefficients are determined through incubation studies in the lab or in the field using litter bags.  For example, Wisconsin’s manure N crediting guidelines use a mineralization coefficient of 25% of organic N to estimate first- year N availability from liquid manures with less than 2% dry matter (DM) and more than 75% of total N as NH-N (Laboski and Peters, 2012).

When mineralization coefficients are determined over multiple time intervals, the resulting values form a decay series. One common method for determining these coefficients is through incubation studies. These involve mixing soil with the manure amendment, maintaining specific temperature and moisture conditions, and measuring the release of inorganic N over time.  A challenge with this method is that the pre-treatment of the soil and the amendment (e.g. grinding, drying and sieving) might cause an initial flush of mineralization potentially skewing early results (Beegle et al., 2008). In an incubation experiment where the contribution of individual manure components to mineralized N using ¹⁵N were evaluated, Cusick et al. (2006) found that on average 30% of manure ¹⁵N mineralized over the 168 days of this trial with the highest rate of mineralization occurring in the urine (Table 3). In this study manure was labeled with ¹⁵N isotope and then ¹⁵N recovery was measured in the soil at the end of the study. The stable isotope ¹⁵N has been used for decades as a tracer to study N dynamics in soil and N uptake in above ground crop biomass.

The use of buried litter bags is a common way to measure decomposition or decay of organic compounds under field conditions. Mineralization is then determined by the loss of material weight as a function of time. There are limitations with this method however, some of which are the isolation of the manure in the bag from the surrounding soil, protection from the manure in the bag from the soil microbes and the soil microenvironmental condition (Beegle et al., 2008). Lentz and Lehrsch (2012) developed a 5-year decay series for solid dairy manure using litterbags in which 23% of manure total N was mineralized in year 1. Subsequently, 12%, 10%, 9%, and 8% of total manure N was mineralized to plant available N in years 2 through 5, respectively (Figure 3).  

Nitrogen Recovery

Nitrogen recovery is the amount of N taken up by the plant relative to what has been applied. Although not the same as N availability, it can help us understand the fate and efficiency of applied manure N (and trace where the applied manure N is going). In the next section we discuss two methods that are used to estimate N recovery: the Difference Method and the ¹⁵N method.

The Difference Method

The Difference Method calculates the apparent N recovery by subtracting N uptake in a control (unamended) plot from the N uptake in an amended plot. The difference is then divided by the amount of applied N. Below is an example of calculating N recovery using the difference method.

In cases where the original background soil N levels are high, the N uptake in the plots that are unamended (control plots) might be high and sometimes even higher than the treated fields which could result in low to negative numbers with this approach. In a study conducted in Wisconsin an N recovery of -31% was reported (Table 4). The Difference Method assumes that that the soil provides the same amount of N to all plots and that the N taken up by the crop in fertilized or manured plots is coming from the supplied N (Muñoz et al. 2004). Another assumption is that N mineralization is not affected by the applied soil amendment which is contrary to the priming effect that other research has shown where amendments increase the mineralization rate of the original soil N (Beegle et al. 2008).

¹⁵N Isotope Recovery