Nitrogen Transformations in Soil

Javed Iqbal Author

03/05/2021 Added

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From "Mind Your Manure Manners" portion of Land Application Training 2021

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[00:00:00.110]My name is Javed Iqbal, and I am a nutrient management
[00:00:03.720]and water quality specialist
[00:00:06.680]at the University of Nebraska-Lincoln.
[00:00:09.340]So in this presentation, I am going to share with you
[00:00:13.340]how the nitrogen moves and transforms
[00:00:16.360]into different nitrogen pools, into the soil,
[00:00:21.340]and this will give you a kind of sense,
[00:00:24.820]as which nitrogen sources or fertilizers
[00:00:30.840]are good for the soil, for your crop,
[00:00:33.660]and for (indistinct).
[00:00:37.150]So, to start this presentation,
[00:00:41.320]I will be sharing the nitrogen transformation
[00:00:44.100]when nitrogen from two different sources of fertilizer
[00:00:49.330]is applied into the soil.
[00:00:51.770]So in the first example,
[00:00:53.467](indistinct) when commercial fertilizer,
[00:00:55.900]in the form of ammonium nitrate, is applied into the soil,
[00:01:00.450]it provides nitrogen in the form of nitrate and ammonium.
[00:01:06.410]This uptake of nitrate is rapid,
[00:01:10.444]because of its mobility.
[00:01:13.820]However, ammonium uptake is slow,
[00:01:16.920]because ammonium is bound to the soil,
[00:01:19.260]and roots have to reach it.
[00:01:21.890]So, most of the plants, they prefer nitrate over ammonium,
[00:01:25.890]however this ammonium is, most of it,
[00:01:28.080]it is converted through soil bacteria to nitrate pool,
[00:01:34.630]which is then taken up by the plant.
[00:01:37.480]However, some of this nitrate
[00:01:39.660]also leeches down into the groundwater,
[00:01:43.160]especially when this nitrate pool
[00:01:46.420]is in excess of plant nitrogen needs.
[00:01:54.020]'Kay, we have different sets of nitrogen transformation
[00:01:57.050]when we applied manure into the soil.
[00:02:00.120]For example, when we are applying manure,
[00:02:02.120]this provides nitrogen in two forms,
[00:02:05.030]organic nitrogen and ammonium nitrogen.
[00:02:08.370]Organic nitrogen is not mobile,
[00:02:10.700]and is not directly taken up by plants,
[00:02:13.010]however it transforms into ammonium pool
[00:02:15.573]through the process of mineralization.
[00:02:19.000]This process speeds up as the soils get warmer,
[00:02:22.290]for example in July.
[00:02:24.790]In the next step,
[00:02:25.650]this ammonium is again nitrified into nitrate pool,
[00:02:29.840]and this process again speeds up
[00:02:31.500]as the temperature exceeds 50 degree Fahrenheit.
[00:02:34.920]Among these two pools again,
[00:02:36.550]ammonium uptake is slow,
[00:02:39.620]because it is bound to the soil,
[00:02:41.710]while nitrate uptake is faster than ammonium nitrogen pool.
[00:02:49.020]Here it is important to note
[00:02:50.380]that when you're applying manure,
[00:02:52.910]then this nitrate pool is comparatively small,
[00:02:58.310]when you apply nitrogen
[00:03:00.630]in the form of commercial nitrogen fertilizer.
[00:03:03.140]So this small nitrate pool
[00:03:04.910]is, though, replenished through mineralization
[00:03:07.800]and nitrification processes,
[00:03:09.740]and provides the (indistinct) nitrogen of the crop uptake.
[00:03:13.780]Because of small size,
[00:03:17.981]this reduces the nitrate leaching potential,
[00:03:21.680]compared to commercial fertilizer.
[00:03:25.495](man sings in background)
[00:03:29.590]All right, okay.
[00:03:31.440]So, in the previous slides,
[00:03:33.940]we have talked about the nitrogen transformation
[00:03:36.400]and the pools,
[00:03:38.050]but now we are going to see how these chemicals move
[00:03:41.230]within the soil profile.
[00:03:42.910]And actually, soil does a great job
[00:03:46.050]of absorbing all different kind of chemicals,
[00:03:49.520]which are added to the soil profile.
[00:03:52.990]However, not all those chemicals
[00:03:55.190]can be absorbed by the soil particles.
[00:03:58.350]Some of those chemicals usually can move
[00:04:02.600]through the soil profile along with the water,
[00:04:06.045]and contaminate groundwater.
[00:04:08.890]As an example, nitrate, when it's not intercepted
[00:04:13.260]by the crop roots or by soil bacteria,
[00:04:17.050]it usually leeches down into the groundwater
[00:04:22.250]through irrigation water or through rainfall,
[00:04:27.293]which results in contamination of groundwater.
[00:04:30.200]And this problem is usually severe
[00:04:32.950]where we have sandy soils
[00:04:35.280]or we have shallow groundwater table,
[00:04:38.110]and then some evidences have also shown that
[00:04:41.940]this high nitrate contamination also occurs in wells
[00:04:47.590]which are near to the animal feeding operations,
[00:04:51.770]or at home septic tanks,
[00:04:55.960]or the fertilizer dealerships.
[00:04:58.300]Another question is, why does nitrate move into the soil
[00:05:02.060]and ammonium, which is another form of nitrogen,
[00:05:04.470]doesn't move into the soil?
[00:05:06.470]And where does this ammonium nitrogen end up,
[00:05:09.870]and where does this nitrate nitrogen move?
[00:05:13.720]So to see this chemical movement,
[00:05:16.630]we did an experiment to illustrate
[00:05:19.700]what happens when different chemicals
[00:05:24.570]which are water-soluble are added into the soil,
[00:05:28.580]with different ionic charges.
[00:05:30.730]So here in this example, you will see,
[00:05:32.910]we filled up the soil columns, our tubes, with sandy soil,
[00:05:37.210]and so there are some holes at the bottom of these columns
[00:05:42.380]to allow the percolation of the water into the container
[00:05:45.950]that you are seeing below these tubes.
[00:05:48.870]In the left soil column,
[00:05:52.500]we added a red solution containing nitrate nitrogen
[00:05:56.720]which has negative charges.
[00:05:59.520]And then, on the right column,
[00:06:03.530]we added a blue solution
[00:06:05.620]containing ammonium form of nitrogen,
[00:06:08.360]with positive charges.
[00:06:11.310]In the middle column,
[00:06:14.230]we added a mixed solution of nitrate and ammonium
[00:06:17.100]containing both negative and positive charges.
[00:06:20.750]Now we will see how these solutions
[00:06:23.940]containing nitrate and ammonium
[00:06:26.890]move through this soil profile
[00:06:29.830]into the container below.
[00:06:32.110]So, as you have seen,
[00:06:34.590]the solution moved into the container below,
[00:06:37.840]and as you see on the left column,
[00:06:40.620]all the nitrate that was poured into the soil column
[00:06:44.505]has moved into the container,
[00:06:46.426]as you can see the color, which is red.
[00:06:49.422]So this indicated that nitrate
[00:06:51.994]didn't stay into the soil profile,
[00:06:53.940]and it moved through the soil into the container.
[00:06:57.153]However, when you see on the right side column,
[00:07:00.980]the blue solution containing ammonium form of nitrogen
[00:07:06.000]was retained by the soil,
[00:07:08.250]that didn't move into the container.
[00:07:10.360]So now the question is, why did nitrate move
[00:07:13.810]through the soil profile into the container below,
[00:07:17.270]and ammonium did not move?
[00:07:19.100]And then you can see also in the middle column
[00:07:21.970]that nitrate also moved through the soil
[00:07:26.210]into the container below.
[00:07:28.360]So this nitrate moved through the soil
[00:07:32.450]because this nitrate had negative charges,
[00:07:35.210]and these negative charges were repelled
[00:07:37.640]by the negative charges on the soil.
[00:07:40.520]But when you see on the right column,
[00:07:43.480]ammonium nitrogen, which has positive charges,
[00:07:46.270]was attracted and retained by the soil profile,
[00:07:49.670]which had negative charges.
[00:07:53.920]So, let's look into this concept by another example,
[00:07:59.760]so here we have a play with magnetic characters in the soil.
[00:08:07.160]So in this cast today we have six characters,
[00:08:11.430]our first character is ammonium nitrogen,
[00:08:14.400]which has positive charge.
[00:08:16.970]The second character which is nitrate nitrogen
[00:08:21.140]has negative charges.
[00:08:23.120]Our third and fourth character,
[00:08:26.790]which is clay particle and organic matter,
[00:08:28.900]which are found in the soil,
[00:08:30.660]both of them, they have negative charges.
[00:08:33.330]Our fifth character is ammonium nitrate,
[00:08:37.060]which contains both ammonium and nitrate,
[00:08:39.120]but it has together, these have neutral charge.
[00:08:42.780]Our last character, which is water,
[00:08:45.060]also has a neutral charge.
[00:08:48.990]Now, let's start this play and see our first character,
[00:08:52.170]which is ammonium nitrate,
[00:08:54.320]you can see that ammonium and nitrate,
[00:08:55.960]they are bonded together by their ionic strength.
[00:08:59.890]So this ammonium nitrate, it's just like a magnet,
[00:09:03.600]and it has a neutral charge.
[00:09:06.450]However, when this ammonium nitrate
[00:09:09.570]comes in contact with water,
[00:09:11.930]water dissolves this ammonium nitrate
[00:09:15.260]into two different ions, nitrate nitrogen ion
[00:09:19.400]containing negative charges,
[00:09:21.650]and ammonium nitrogen ion containing positive charges.
[00:09:27.410]So let's look into our other two characters,
[00:09:31.960]those are found into the soil,
[00:09:33.470]clay particle, and organic matter,
[00:09:35.300]and both of these are negatively charged.
[00:09:39.110]So which form of nitrogen that we saw in the previous slide
[00:09:44.090]is going to be attracted
[00:09:46.220]by these negative charges in the soil?
[00:09:50.630]The answer is ammonium.
[00:09:53.870]So this ammonium is just like,
[00:09:55.830]you know you might have heard, opposites attract,
[00:09:59.000]so this ammonium nitrogen form
[00:10:02.070]contain positive charges,
[00:10:04.570]so that's why it is attracted by the negative charges
[00:10:08.360]on the soil particles.
[00:10:10.470]So because of this form,
[00:10:11.830]this ammonium nitrogen does not move into the soil profile
[00:10:15.650]until it is taken up by the plant
[00:10:18.370]or until it is nitrified by the soil bacteria.
[00:10:22.670]But what happens with the nitrate nitrogen form?
[00:10:27.870]So this nitrate nitrogen form also has negative charges.
[00:10:32.660]So, will this be attracted by this clay
[00:10:34.997]and organic matter particle?
[00:10:37.910]Probably not,
[00:10:38.800]because those clay and organic matter particle,
[00:10:41.750]they have negative charges,
[00:10:43.570]and negative charges on the nitrate nitrogen,
[00:10:46.760]so this nitrate nitrogen
[00:10:48.210]is repelled by these negative charges
[00:10:53.650]on the clay particle and organic matter,
[00:10:56.760]which results in the movement of this nitrate nitrogen down
[00:11:00.940]through the soil profile into the groundwater,
[00:11:03.820]if this nitrate nitrogen is not taken up by the plants.
[00:11:07.520]So in this play, finally, the ammonium nitrogen is adsorbed,
[00:11:13.967]or held by the soil,
[00:11:15.680]while nitrate nitrogen,
[00:11:20.200]it keeps moving down into the soil profile
[00:11:22.500]and it leeches down through the soil profile
[00:11:25.050]into the groundwater, again,
[00:11:27.770]this happens only if this nitrate nitrogen
[00:11:30.360]is not uptaken by the crops,
[00:11:34.470]which is the case when excess nitrate nitrogen
[00:11:37.400]is present into the soil.
[00:11:41.340]All right, so, we have seen two forms of nitrogen,
[00:11:45.040]ammonium and nitrate nitrogen in our previous slides.
[00:11:49.210]So the question here is,
[00:11:51.400]what happens to the organic nitrogen,
[00:11:53.770]does the organic nitrogen move in the soil profile?
[00:11:59.410]And actually, this organic nitrogen does not move,
[00:12:02.260]because in this pool,
[00:12:04.160]nitrogen is attached to the organic matter particles,
[00:12:07.720]that doesn't move in the soil profile,
[00:12:10.423]neither it is taken up by the plant,
[00:12:13.167]nor it leeches down into the groundwater,
[00:12:16.500]rather it is transformed into ammonium and nitrate pool,
[00:12:21.000]which is then taken up by the plants.
[00:12:23.517]And again, this transformation speeds up
[00:12:26.510]as the soil gets warm.
[00:12:29.150]So otherwise, you can also say that this organic nitrogen
[00:12:33.040]also acts as Mother Nature's nitrogen inhibitor.
[00:12:36.520]So you may have heard about nitrogen stabilizers,
[00:12:38.950]which are used to prevent nitrogen loss
[00:12:41.630]with commercial fertilizer.
[00:12:43.380]So, this organic nitrogen, or manure,
[00:12:47.520]is an alternative product that acts
[00:12:49.960]as Mother Nature's nitrogen inhibitor.
[00:12:54.180]Okay, so now to wrap up,
[00:12:56.740]here I have a summary of 141 studies,
[00:13:02.340]so in this study, they compare the effect of manure
[00:13:07.150]and commercial fertilizer on nitrogen losses.
[00:13:11.540]So what they found,
[00:13:13.480]that when you substitute commercial fertilizer with manure,
[00:13:22.100]it provides some benefits.
[00:13:25.030]For example, it adds nitrogen into the soil profile,
[00:13:28.920]it increases nitrogen pooled by 22%.
[00:13:33.210]And this substitution decreases
[00:13:35.880]ammonia volatilization by 27%,
[00:13:39.700]it also decreases nitrogen runoff by 26%,
[00:13:43.360]decreases nitrate leeching by 29%.
[00:13:46.430]From crops perspective,
[00:13:49.320]this substitution increases nitrogen uptake by 4%,
[00:13:53.940]and crop yield by 7.8%,
[00:13:57.640]which indicates that this replacement
[00:14:00.100]is really a win-win situation,
[00:14:02.340]not only for decreasing the nitrogen lost,
[00:14:05.330]but also enhancing the crop's yield.
[00:14:08.260]So now we'll just wrap up with a take home message,
[00:14:11.370]that these ammonium-based nitrogen fertilizers,
[00:14:13.830]they are good for retaining nitrogen into the soil,
[00:14:18.670]which ultimately increases the nitrogen uptake,
[00:14:21.700]when there is a better synchronization
[00:14:23.640]between nitrogen uptake and nitrogen availability.
[00:14:27.890]And we have seen some bad years in Nebraska,
[00:14:31.560]which increases the nitrogen loss,
[00:14:35.410]so when you are using this ammonium-based nitrogen sources,
[00:14:40.010]these provide good insurance
[00:14:41.790]under heavy precipitation events,
[00:14:43.700]which eventually reduces nitrate
[00:14:45.460]leeching into the groundwater,
[00:14:47.020]and as I mentioned earlier,
[00:14:49.430]these ammonium-based fertilizers of organic nitrogen,
[00:14:52.510]especially this manure-based fertilizer,
[00:14:56.010]they are good alternative to nitrogen stabilizers
[00:14:58.810]that are used with commercial fertilizers.
[00:15:02.240]'Kay, so with this I will end my presentation.
[00:15:04.530]If you have any questions,
[00:15:05.990]you can reach out to me through my email or phone.

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Nitrogen Transformations in Soil

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