Assessing soil health: Soil nitrogen cycling
The following article is the third in a five-part series on assessing soil health. It focuses on soil nitrogen cycling. It is part of a larger Soil Science Society of America webinar series produced in partnership with The Soil Health Institute and sponsored by The Walton Family Foundation.
When it comes to managing soils to provide plant-available nitrogen, it is all about organic matter. Organic matter provides a source of nitrogen as well as the fuel for the microbes to recycle the organic matter into plant-available forms of nitrogen.
Welcome to the third installment of the “Assessing Soil Health” series, which is all about measurements of soil nitrogen cycling. This article will discuss insights on soil nitrogen measurements from general soil science literature and from the recent Soil Health Institute project titled, “The North American Project to Evaluate Soil Health Measurements (NAPESHM).” Evaluations of the measurements of total nitrogen, soil protein, potentially mineralizable nitrogen, enzyme activity, and several forms of extractable nitrogen will be assimilated to help understand each measurement in the context of soil health management practices and drivers of soil’s capacity to cycle nitrogen.
Total Nitrogen
Total nitrogen includes all forms of N in soil, organic plus inorganic. Total nitrogen is well correlated with soil organic carbon. Total nitrogen is accurately measured by the dry combustion method (Nelson & Sommers, 1996), which simultaneously provides the most accurate measure of total soil carbon. Note in Table 1 the coupling of total nitrogen in soil with organic carbon, especially in perennial systems. In row crop systems, the relationship between total nitrogen and organic carbon is more variable. In the long-term trials measured in NAPESHM, total nitrogen consistently increased with most soil health management practices.
Table 1. Spearman correlation matrix of nitrogen measurements assessed in The North American Project to Evaluate Soil Health Measurements (NAPESHM)
| Row crop means | Perennial means | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| SOC | TN | ACE | PMN | NAG | WEON | SOC | TN | ACE | PMN | NAG | WEON | ||
| TN | 0.93 | 0.99 | |||||||||||
| ACE | 0.65 | 0.68 | 0.83 | 0.84 | |||||||||
| PMN | 0.65 | 0.68 | 0.56 | 0.87 | 0.88 | 0.87 | |||||||
| NAG | 0.39 | 0.41 | 0.37 | 0.44 | 0.51 | 0.56 | 0.42 | 0.51 | |||||
| WEON | 0.52 | 0.51 | 0.21 | 0.40 | 0.59 | 0.70 | 0.72 | 0.50 | 0.66 | 0.54 | |||
| NO3 + NH4 | 0.10 | 0.05 | -0.03 | 0.06 | 0.13 | 0.32 | 0.05 | 0.09 | 0.31 | 0.13 | 0.07 | -0.12 | |
Autoclaved Citrate Extractable Protein
Autoclaved citrate extractable (ACE) protein uses a strong extracting solution of sodium citrate followed by autoclaving to quantify a diverse pool of soil proteins (Gillespie et al., 2011; Rosier et al., 2006). Proteins extracted with this method include the fungal hyphal protein, glomalin, and bacterially exuded proteins (Redmile-Gordon et al., 2014), both of which are expected to contribute to the correlation between ACE protein and aggregate stability (Costa et al., 2018). This measurement is correlated to potentially mineralizable nitrogen, but the proteins measured in this test have not been shown to be more readily mineralized than other forms of soil nitrogen. The ACE protein measurement is found in Cornell’s Comprehensive Assessment of Soil Health.
Potentially Mineralizable Nitrogen
Potentially mineralizable nitrogen (PMN) is estimated by incubating soil for seven days in an anerobic environment and measuring the amount of ammonium (NH4+–N) at the end of the incubation. While not a direct measure of N mineralization, this test has been well correlated to laboratory incubations of aerobic N mineralization (Schomberg et al., 2009) as well as buried bag field tests of N mineralization (Sullivan et al., 2020). It is thought that this test is well related to the amount of N that becomes plant available in a growing season because a significant portion the of the aerobic microbes responsible for N mineralization die under anaerobic conditions. This death causes the cells to break open, and cellular nitrogen contents are a good representation of the potential of certain organisms to mineralize N (Schomberg et al., 2009). This measurement is not recommended in any popular soil health tests but was chosen by a panel of scientists advising the NAPESHM project as a standard way to estimate mineralizable N.
N-Acetyl β-Glucosaminidase
The N-acetyl β-glucosaminidase (NAG) measurement represents the potential enzyme activity in the soil that catalyzes the terminal reaction in chitin degradation. Chitin is a primary component in both fungal cell walls and arthropod exoskeletons and is therefore a prevalent protein in soils. This enzyme represents one of many crucial steps in the cycling of N from organic matter to plant-available N (Schimel & Bennett, 2004). Results from NAPESHM show a clear increase in the NAG enzyme with the addition of cover crops, but the reaction between treatments can be either positive or negative with the addition of organic matter or a reduction in tillage. This differential response is thought to be related to microbial resource allocation (Allison & Vitousek, 2005). If it is already available, why spend the energy to make the enzyme? The NAG enzyme is one of the four enzyme measurements recommended by the USDA-NRCS Soil Health Division for monitoring Soil Health (Stott, 2019).
Haney Soil Health Nitrogen Test
The Haney Soil Health Nitrogen Test uses two extraction procedures, the H3A extract and a water extract. The H3A solution contains citric, malic, and oxalic acid and is meant to mimic the acids exuded by plant roots that aid in the liberation of nutrients from soil minerals (Haney et al., 2010). The soil extract is analyzed in conjunction with results of the water extraction from the soil sample. The difference between the total N in the water extraction and inorganic nitrogen from the H3A extraction is the water extractable organic nitrogen (WEON). Data from NAPESHM revealed total water extractable nitrogen was highly correlated to inorganic N measured with the H3A extraction and soil clay content. Changes in water extractable nitrogen and inorganic nitrogen were not consistently related to changes in management practices, other than recent N fertilization. However, WEON was similarly consistent with other soil health nitrogen measurements and was related to mineralizable N.
Relationship among Measurements
The NAPESHM project measured all N-related measurements listed above at the same time of year in order to look at correlations among these measurements. Soil organic carbon, ACE protein, and PMN were all very strongly correlated with each other and are listed in greatest to least correlation strength with total nitrogen (0.96, 0.81, and 0.81, respectively). Correlations were noticeably stronger when only perennial systems were included. Potentially mineralizable nitrogen values were predictable by using both total nitrogen and 24-hour respiration, along with clay and sand (R2 = 0.85 and 0.72, perennial and row crop systems, respectively). The inorganic measures of nitrogen and the water extractable nitrogen had the lowest correlation with total N. Mineral N is ephemeral in soil systems, and while it can be used successfully with locally indexed fertilizer recommendations, it is not indicative of soil health.
Nitrogen Measurement and Soil Health Management Systems
Table 2 shows the relationship between soil health management systems of decreasing tillage, adding organic amendments, using cover crops, retaining residue, and increasing the number of cash crops in a rotation with the seven N-cycling measurements described above. While N-cycling measurements were not responsive to diversification of cropping rotations, research has shown increasing cropping diversity can disrupt disease and pest cycles and promote additional off-farm benefits. Management practices such a decreased tillage, adding organic amendments, retaining residue, and adding cover crops showed a consistent pattern among most indicators except water extractable N and inorganic N, which were not well associated with management changes.
Table 2. Significant responses of soil nitrogen-cycling indicators to management from the data set in The North American Project to Evaluate Soil Health Measurements (NAPESHM). “Yes” means that the indicator changed significantly (α = .05) as a result of the treatment listed
| Measure | Decreased tillage | Organic additions | Use of cover crops | Residue retention | Increased crop count |
|---|---|---|---|---|---|
| Total N | Yes | Yes | Yes | Yes | No |
| ACE protein | Yes | Yes | Yes | Yes | No |
| PMN | No | Yes | Yes | Yes | No |
| NAG | Yes | No | Yes | Yes | No |
| WEON | Yes | Yes | No | Yes | No |
| WEN | No | No | No | No | No |
| H3A inorganic N | No | No | No | No | No |
Summary
- Most soil N is bound to soil organic matter.
- Soil organisms are responsible for soil nitrogen cycling.
- Inorganic N is highly ephemeral in soil.
- Nitrogen indictors associated with organic N respond similarly to soil health management changes.
- Total N and ACE protein most consistently reflected changes in soil health management.
- Total N and 24-hour respiration were the best predictors of potentially mineralizable N, which has been related to how much N is made plant available in a growing season and has been used to drive management decisions.
Dig deeper
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